November 2013

Working Reports contain information on work in progress

or pending completion.

David Dixon

Golder Associates Ltd

Johanna Hansen

Posiva Oy

Leena Korkiala-Tanttu

Aalto University

Taina H. Karvonen,

Nuria Marcos, Ursula Sievänen

Saanio & Riekkola Oy

Working Report 2012-09

Underground Disposal FacilityClosure Design 2012

 

UNDERGROUND DISPOSAL FACILITY CLOSURE DESIGN 2012 ABSTRACT Posiva has developed a detailed design for construction of a KSB-3 type disposal facility for spent nuclear fuel on Olkiluoto Island. The disposal facility design calls for construction of five shafts and an access tunnel that will serve to connect the disposal level to the surface. Routes to the underground disposal facility would be via the underground rock characterisation facility, ONKALO, which is currently under construction. The positioning of the underground disposal facility at Olkiluoto Island has been done by taking into account the main geological structures and available geological information on the site. Conditions actually encountered on reaching the potential disposal level will be assessed and the final layout selected at that time. The closure of the underground facility shall complete the isolation of the spent nuclear fuel and contribute to restoring and maintaining the favourable natural conditions in the bedrock. Beyond the deposition tunnels there remain underground openings associated with central and access tunnels, shafts and technical rooms or other spaces that comprise by definition the closure of the underground disposal facility. The underground disposal facility’s general closure design described in this document deals specifically with the backfill and plugs installed in regions beyond the deposition tunnels and how the local geosphere may affect their performance. These tunnels, rooms and shafts represent approximately 60 % of the volume of the underground disposal facility openings and will intersect a variety of geological and hydrogeological features. Hence, backfilling and plugging of the disposal facility excavations in these regions will need to be approached in a flexible manner with the ability to modify the materials used to reflect the properties of the features intersected. Based on relevant available information the basis for flexible and general closure plan has been developed. The general closure plan takes into consideration the changes with depth, hydrogeological features, hydrological parameters, geochemical properties and bedrock characteristics in different underground openings at Olkiluoto Island. In addition to presenting a safe closure solution for long-term time scale the closure design is also needed before the implementation of the closure, because it is used e.g. as a background data for stray material estimation and for performance assessment. The general closure design will be updated before detailed planning. The general closure plan works as background information for underground disposal facility closure production. Keywords: Closure, backfill, plug, design, component, disposal facility, underground, bentonite, crushed rock, concrete, clay.

MAANALAISEN LOPPUSIJOITUSLAITOKSEN SULKEMISEN SUUNNITELMA 2012 TIIVISTELMÄ Posiva on kehittänyt yksityiskohtaisen suunnitelman KBS-3 tyyppisen käytetyn ydin-polttoaineen loppusijoituslaitoksen rakentamiseksi Olkiluotoon. Loppusijoituslaitoksen suunnitelmassa loppusijoitustasoa ja maanpintaa yhdistävät viisi kuilua ja ajotunneli. Reitit loppusijoitustiloihin kulkisivat maanalaisen tutkimustilan, ONKALOn, kautta, jonka rakentaminen on käynnissä. Loppusijoituslaitoksen asemointi Olkiluodon saarelle on tehty ottaen huomioon tärkeimmät geologiset rakenteet ja saatavilla ollut tieto alueen geologiasta. Lopullinen loppusijoitustilojen asemointi tullaan päättämään loppusijoitus-tason saavuttamisen jälkeen kun viimeisimmät tiedot olosuhteista ovat käytettävissä. Maanalaisen loppusijoituslaitoksen sulkeminen toiminnan aikaisesti ja sen jälkeen tulee viimeistelemään käytetyn ydinpolttoaineen eristämisen ja ylläpitämään alueen suotuisia kallioperän ominaisuuksia. Loppusijoitustunneleiden ulkopuolelle jäävät tilat ovat keskustunneleita ja ajotunneli, kuilut ja tekniset tilat, sekä näihin liittyvät pienemmät tilat. Maanalaisen loppusijoitustilan yleispiirteinen sulkemisen suunnitelma, joka esitellään tässä työraportissa, käsittelee erityisesti loppusijoitustunneleiden ulkopuolelle jääviin tiloihin asennettavia täyttömateriaaleja ja tulppia, ja kuinka paikallinen geosfääri vaikuttaa niiden sijoitteluun. Nämä tunnelit, tilat ja kuilut ovat noin 60 % koko maan-alaisen loppusijoituslaitoksen tilavuudesta ja leikkaavat erilaisia geologisia ja hydroge-ologisia piirteitä. Tästä syystä laitoksen tilojen täyttömateriaalien ja tulppien suunnitteleminen tulee tehdä niin, että ne ovat muokattavissa paikallisten piirteiden tuottamien vaatimusten mukaisesti. Joustava perussuunnitelma sulkemiselle on kehitetty oleellisten tutkimustulosten ja tietojen perusteella. Suunnitelma ottaa huomioon muu-tokset syvyydessä, hydrogeologisissa muuttujissa, geokemiallisissa ominaisuuksissa ja kallioperän ominaisuuksissa Olkiluodon loppusijoituslaitoksen tiloissa. Sulkemisen suunnitelmaa tarvitaan turvallisen pitkäaikaisen sulkemisen lisäksi jo ennen sulkemisen ajankohtaa, sillä sitä käytetään esim. loppusijoituslaitokseen jäävien vierasaineiden määrien arvioimisessa ja toimintakyvyn arviointiin liittyvässä työssä. Yleisluontoinen sulkemisen suunnitelma toimii myös tausta-aineistona sulkemisen tuotantolinjalle. Avainsanat: Sulkeminen, täyttö, tulppa, suunnitelma, komponentti, loppusijoituslaitos, maanalainen, bentoniitti, murske, betoni, savi.

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TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ DEFINITION OF TERMINOLOGY .................................................................................. 3 

PREFACE ....................................................................................................................... 7 

1  INTRODUCTION .................................................................................................... 9 

1.1  Scope ............................................................................................................ 9 

1.2  General description of KBS-3V disposal facility .......................................... 10 

1.3  Materials and options for closure of a disposal facility ................................ 12 

2  LAYOUT OF A DISPOSAL FACILITY .................................................................. 13 

2.1  Disposal facility layout ................................................................................. 13 

2.2  Access tunnel .............................................................................................. 15 

2.3  Shafts .......................................................................................................... 15 

2.4  Technical and other rooms at the -420 to -457 m level ............................... 16 

2.5  Central tunnels, central tunnel connections and vehicle connections ......... 16 

2.6  Repository for low and intermediate level waste ......................................... 17 

2.7  Tunnel sections between deposition tunnel plugs and central tunnels ....... 17 

3  CONDITIONS AND PREMISES FOR DESIGN BASIS ........................................ 19 

3.1  Performance targets for closure .................................................................. 19 

3.2  Design requirements for closure ................................................................. 20 

3.3  Olkiluoto-specific conditions influencing the closure design basis .............. 21 

3.3.1  Bedrock composition and fracture features ..................................... 21 

3.3.2  Strength, stress state and thermal properties of rock ..................... 25 

3.3.3  Hydrogeology .................................................................................. 28 

3.3.4  Hydrological evaluation of Olkiluoto site ......................................... 32 

3.3.5  Hydrogeochemistry ......................................................................... 34 

3.4  Estimation of water inflow to ONKALO and disposal level .......................... 37 

3.5  Climatic evolution ........................................................................................ 39 

3.6  Design specifications for closure ................................................................. 40 

4  CLOSURE CONCEPTS FOR A DISPOSAL FACILITY ........................................ 43 

4.1  Closure concepts ........................................................................................ 43 

4.2  Closure components ................................................................................... 45 

4.3  Backfill materials and installation methods ................................................. 46 

4.3.1  Background ..................................................................................... 46 

4.3.2  Backfill material composition and production .................................. 47 

4.3.3  Other alternative backfill materials and installation methods .......... 55 

4.4  Plugs within the disposal facility .................................................................. 55 

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4.4.1  General ........................................................................................... 55 

4.4.2  Plug functions ................................................................................. 56 

4.4.3  Specific locations requiring plug installation .................................... 58 

4.5  Laboratory and large-scale tests on the closure of a disposal facility ......... 60 

4.5.1  Laboratory tests .............................................................................. 60 

4.5.2  Field tests ........................................................................................ 62 

5  APPLYING THE CLOSURE CONCEPT TO OLKILUOTO ................................... 69 

5.1  Description of closure concept for Olkiluoto site ......................................... 69 

5.1.1  Surface down to HZ19 (0 to -100 m) ............................................... 70 

5.1.2  Depth between HZ19 and HZ20 (~ -100 to ~ -300 m) .................... 70 

5.1.3  Access tunnel below HZ20 structure (~ -300 to -420 m) ................. 71 

5.1.4  Disposal depth - all excavations except deposition tunnels and holes (below ~ -400 m) ................................................................... 71 

5.2  Proposed Olkiluoto specific closure materials ............................................. 71 

5.2.1  Closure backfills .............................................................................. 72 

5.2.2  Plugs at Olkiluoto site...................................................................... 81 

5.2.3  Composition and number of plugs required for Olkiluoto site ......... 82 

5.3  Discussion about knowledge with uncertainties for the closure of the disposal facility designed to be constructed at Olkiluoto ............................. 84 

6  SUMMARY ........................................................................................................... 87 

REFERENCES ............................................................................................................. 89 

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DEFINITION OF TERMINOLOGY In the course of discussing the design, construction, operation and sealing of a KBS-3V disposal facility there are many terms that need to be defined in order to avoid ambiguity within the text. This is particularly important with regards to backfilling and sealing activities and materials. As this document focuses on backfilling and sealing of those regions beyond the tunnels used to facilitate deposition of the canisters a number of very specific terms are used. These are based on the terminology defined in Hansen et al. (2009) with additions as required to cover key items discussed in this document. The key terms used in this document include the following:

Access tunnel: The inclined tunnel used to provide access to the disposal level from the surface.

Andra: The National Radioactive Waste Management Agency (France).

Äspö HRL: Hard Rock Laboratory in Äspö, Oskarshamn, Sweden.

Backfill - deposition tunnel: Backfill is the material or materials that is/are used for backfilling of tunnels or shafts. When discussed in connection to deposition tunnels the term is used for the engineered barrier that comprises the deposition tunnel backfill and the deposition tunnel plug.

Backfill - of other openings (closure backfill): The materials used to fill the excavations beyond the deposition tunnels as part of the closure process.

Bentonite: Commercial name for smectite-rich materials (often used as a generic term for these materials). The most common smectite mineral is montmorillonite.

BFZ: Brittle fault zone.

Buffer: Densely compacted bentonite blocks used to surround the copper canister in the deposition hole.

Canister: A container with a copper overpack and cast iron insert used to contain the spent fuel assemblies.

Central tunnel: Tunnels situating at disposal depth and from where the deposition tunnels merge.

Central tunnel connection: Central tunnels are constructed to be parallel and short tunnels connecting a pair of parallel central tunnels are called central tunnel connections.

Closure: Closure consists of backfilling of other openings (than deposition tunnels and holes) and sealing structures emplaced mainly after operating phase is over. (The borehole closure is considered with the closure of the underground disposal facility.)

Closure component: Certain material or material combination installed with a specified method, e.g., clay-aggregate backfill installed in situ, mechanical plug.

Concrete: product consisting of aggregate, cement and water, and possibly of additives and admixtures. Aggregate is e.g. of crushed rock, of which the grain size and its distribution effects on properties of concrete. Cement reacts with water, which causes the hardening of product. By using additives and admixtures, one can affect on the properties of concrete.

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Deposition hole: The hole in the floor of the deposition tunnel where canister and the surrounding buffer are installed in KBS-3V concept.

Deposition tunnel: The tunnel where deposition holes are located in KBS-3V concept.

DFN model: Discrete fracture network.

Dry density: Used to describe the mass per unit volume of the solids component present (water component is not included), sometimes referred to as dry unit weight.

EDZ/EdZ: Excavation damaged zone and/or excavation disturbed zone. The original state or properties of the bedrock have changed permanently in this zone.

EBS: Engineered Barrier System (refers to canister, clay buffer, backfill of the deposition tunnels and closure of the underground disposal facility).

ESP: Enhanced Sealing Project: a full-scale shaft plugging demonstration in Canada.

Foundation layer: The material installed as a permanent backfill component on the tunnel floor as part of backfilling of the deposition tunnel as well as other locations where a floor-leveling material is installed to allow for installation of precompacted backfill blocks.

Grout: The material (typically cement-based or e.g. colloidal silica) injected under high pressure into rock fractures via borehole or core drilled hole in order to hinder water leak into a tunnel.

Host rock: geological formation that acts as a natural barrier supporting the EBS and underground disposal facility system.

HZ: Hydraulic zone.

Initial state: The initial state of different EBS components is defined as the as-placed state right after placement of the components. Inflow of groundwater into the materials in the components (e.g. backfill) and its impact is not accounted for in the initial state.

JAEA: Japan Atomic Energy Agency.

KBS-3 (Kärnbränslesäkerhet): The concept for implementing the deep geologic disposal based on multiple barriers (as required in Sweden and in Finland). KBS-1 and KBS-2 are former variations of this method.

KBS-3H: Disposal design based on a multi-barrier system, where the canister is placed horizontally in the bedrock (H = horizontal).

KBS-3V: Disposal design based on a multi-barrier system, where the canister is placed into a vertical deposition hole in the bedrock (V = vertical).

Layout: An area plan that shows the physical configuration of the facility in a manner that forms a functioning complex. A functioning complex compiled of the components of the disposal facility or ONKALO.

Over-excavation: Over-excavation, expressed as %, represents the excess volume excavated during tunnel construction compared to the theoretical tunnel volume. Over-break is also used to describe the same item.

Plug: A mechanical construction built to isolate one region within the underground disposal facility from another. Its intended purpose may also include a hydraulic

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isolation function. Intended lifespan of a plug with a specified function will vary from a few years for operationally-required structures to thousands of years for structures intended to provide long-term isolation.

POSE: Posiva’s Olkiluoto spalling experiment.

Repository: Deposition tunnels and deposition holes, including deposition tunnel plug.

Shafts: Vertical excavations intended to provide access to the disposal level or ventilation to the repository. Their dimension and method of construction will vary depending on their function.

Shotcrete: Sprayed concrete on tunnel surfaces for rock support and water inflow control purposes.

SKB: Swedish Nuclear Fuel and Waste Management Company.

Stripa mine: Stripa mine in Sweden is a location where several field tests concerning material behavior and technical features have been performed for disposal of spent nuclear fuel. It is a closed iron mine with a depth of approximately 450 m.

TDS: Total dissolved solids, often used interchangeably with porewater salinity to describe chemical characteristics of the groundwater.

Technical rooms: In Olkiluoto the technical rooms refer to all underground openings at or below the disposal depth other than central tunnels, deposition tunnels or holes, investigation holes or access tunnel.

TSX: Tunnel sealing experiment.

URL: Underground research laboratory.

Vehicle connection: In Olkiluoto underground disposal facility the vehicle connections are tunnels at disposal depth that connect technical rooms to each other or to central tunnels.

Weichselian glacial cycle: Term refers to last glacial period in Scandinavia. Weichselian occurred 115,000–11,500 years ago and had several warmer and colder periods.

WIPP: Waste Isolation Pilot Plant in Carlsbad, New Mexico (USA), an underground facility to permanently dispose of transuranic waste that is left from the research and production of nuclear weapons.

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PREFACE This report is the first general design for the closure of the disposal facility. The first draft was written already during the outline design phase. It was known then that the requirements and layout were still to evolve and this delayed the publication of the closure design report. During 2011 the report was updated and more background data was added and during 2012 the report was finalised, but final review was delayed until 2013. Following persons have participated in the development of closure of underground openings outside deposition tunnels, and in the production of material for this report and in writing: Leena Korkiala-Tanttu (working today at Aalto University), David Dixon (working AECL Canada during his contribution and currently at Golder Associates Ltd.), Ursula Sievänen (working at Saanio & Riekkola Consulting Engineers during her contribution), Taina Karvonen and Nuria Marcos (working at Saanio & Riekkola Consulting Engineers), and Johanna Hansen (working at Posiva Oy). Due to several edition phases and revisions of text during the writing process the authors are listed in alphabetical order on the cover page. Valuable contribution to the closure development and to the report has been received from Kari Koskinen and Jukka Tanskanen (Posiva Oy). The report has been commented by Jukka-Pekka Salo (Posiva Oy) and Heini Laine (Saanio & Riekkola Consulting Engineers) and based on expert comments and criticism to the design the report has been modified several times before it was published. This report has been the starting point to the Closure Production Line 2012 – Design, production and initial state of the underground disposal facility closure.

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1 INTRODUCTION 1.1 Scope This document is intended to provide an overview of how closure of Posiva’s proposed disposal facility at Olkiluoto Island would be approached and accomplished. The closure is designed specifically for the crystalline rock environment of Olkiluoto site. In order to develop an understanding of how the proposed closure methods have been developed this report provides a review of the KSB-3V repository method, which defines the basic geometric basis for the facility. Materials proposed to be used in backfilling and closing the facility are briefly described since they provide some technological and environmental constraints to what can be undertaken at the facility. An example of a possible layout for Olkiluoto disposal facility is presented and site-specific geological and hydrogeological conditions are reviewed. The performance and functional requirements and hence the design basis for disposal facility closure are discussed with reference to how they will be achieved and what effects local conditions will have in terms of what will be needed in the way of backfilling and plugging in the regions other than the deposition tunnels in the repository. Building on the background information available, options for effective closure are presented. This report details the following aspects of the disposal facility closure: generic layout of the disposal facility at Olkiluoto (Chapter 2), design basis for the closure of the facility including the requirements and site

conditions, discussing which site and environmental features are expected to have the greatest influence on a facility located at Olkiluoto (Chapter 3),

a brief introduction of options for disposal facility closure (Chapter 4), including materials and technologies for the production of the closure, with a review of experiences in different countries, and

description of the facility closure concept as it would be applied at Olkiluoto (Chapter 5).

The closure of investigation boreholes is excluded from this report. A general plan for the closure of investigation holes is presented in Karvonen (2012). This Posiva’s Working Report acts as a background report for Posiva’s Safety Case portfolio (TURVA-2012) reports. TURVA-2012 was compiled for the construction licence application of the disposal facility. Safety Case reports are published as Posiva reports and in this report they are referred to in Italics , by a short name of a report (e.g. Design Basis), following the style used in TURVA-2012 reporting, to systematically refer in a similar manner in all these different reports that are partially compiled concurrently. TURVA-2012 portfolio reports are listed separately in the beginning of the references and they are then followed by the Safety Case supporting documents and then the other references in alphabetical order. TURVA-2012 portfolio is described in more detail e.g. in the Synthesis report.

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1.2 General description of KBS-3V disposal facility In a KBS-3V method repository each canister containing the spent nuclear fuel is placed vertically into a deposition hole drilled in the floor of deposition tunnels as shown in Figure 1-1. The canister is surrounded by a buffer material composed of highly compacted bentonite clay. In the deposition tunnels above the deposition holes backfill, which limits the upward expansion of the buffer and resists formation of transport pathways, will be installed. This ensures that disturbance to the natural groundwater flux at the repository site is minimized, thereby limiting any eventual radionuclide transport to close to what would occur in an undisturbed site. An alternative for KBS-3V method is KBS-3H method, in which the spent fuel canister would be installed into horizontal deposition drifts (Figure 1-1). The buffer in the deposition holes and deposition tunnel backfill (and any plugs installed in these regions) are not considered as parts of the disposal facility closure design described in this document. Detailed discussion of backfilling approach for the deposition tunnels is provided by Keto et al. (2009) where the results of the joint Posiva/SKB backfill development project are presented. Subsequently the deposition tunnel backfill has been presented by Hansen et al. (2009) and Autio et al. (2012). Design, production and initial state of the deposition tunnel backfill and plug are presented in the Backfill Production Line report. Closure of the disposal facility includes backfill and plugs in access and central tunnels, vehicle connections, central tunnel connections, shafts, technical rooms, miscellaneous excavations and investigation holes. Different types of closure components will be used in different parts of the disposal facility volumes.

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Figure 1-1. The KBS-3V (left) and KBS-3H (right) are alternatives of the KBS-3 spent fuel disposal method. (Autio et al. 2008.)

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1.3 Materials and options for closure of a disposal facility This report will establish the engineering approaches, materials and technologies that would be appropriate for closure of a disposal facility. The degree to which this closure plan meets the performance and design requirements (see Section 3.2 and 3.3) and how well it can deal with the anticipated variations in conditions within the disposal facility (e.g., geologic, hydrogeologic, chemical or thermal) are also outlined. Determining if the disposal facility closure methods and materials proposed for use are: 1) implementable, 2) practically achievable, and 3) convincing in a technical and safety-case sense, is of great importance. The closure of the disposal facility will require a range of backfill and plug materials to be utilized following completion of the backfilling of deposition tunnels. Backfill in the context of closure refers to the materials utilized to backfill underground openings outside the deposition tunnels and holes as described in the Design Basis report. Backfill will consist of natural materials that are compatible with other EBS components. Plugs in the context of the closure refer to structures utilized for one or more than one of the following purposes (Design Basis): for isolation of different facility sections during the operational phase, for avoiding the formation of transport routes through the tunnels and other

excavated openings over the long-term, for obstructing inadvertent human intrusion into the repository through existing

tunnels and shafts after closure over the long-term, and for stabilizing sections of investigation holes that intersect water-bearing fracture

zones. Plugs will hamper inadvertent human intrusion (along with closure backfill), act as sealing structures and physically support backfill material during operational stage and closure. The safety functions of the closure backfill and plugs of the underground openings complement those of the other EBS components and the host rock.

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2 LAYOUT OF A DISPOSAL FACILITY With the identification of the Olkiluoto site as a possible location for a deep geological disposal and during construction of ONKALO, the site has been extensively characterized. A summary of some of the key site characteristics and processes that may affect the closure of the designed facility and post-closure evolution are presented in Chapter 3. Based on the characterization work that has been done at the site, a tentative repository construction layout for a 9000 tU quantity of spent nuclear fuel has been developed (Kirkkomäki 2012). With the development of a detailed, yet still flexible, design for the disposal facility, a range of excavation sizes, shapes and degrees of excavation roughness that are likely to be encountered in the deposition tunnels and the regions other than those tunnels have been identified. The greatest variation in excavation size, shape and hydrological and geological conditions likely to be encountered in the disposal facility is expected to be found in the regions outside the deposition tunnels. These regions will therefore pose a challenge with respect to their backfilling and plugging. Using the information and design layouts developed it has been possible to begin evaluation of what approaches to backfilling and closing of these excavations are best suited for use. Based on this background information, this Chapter briefly describes the general layout of the disposal facility. 2.1 Disposal facility layout A spent fuel disposal facility at the Olkiluoto site will consist of deposition tunnels and holes (the repository), and central tunnels, central tunnel connections, vehicle connections, access tunnel, shaft connections and technical rooms (other underground openings) (Figure 2-1). Deposition and central tunnels, as well as central tunnel connections, will be located at an approximate depth of -420 m. The technical rooms locate at a slightly lower level, about -437 m (Saanio et al. 2012). Five shafts and the access tunnel connect the disposal level to the ground surface (the underground rock characterisation facility ONKALO is part of this connected system). The positioning of the repository within the Olkiluoto Island surface footprint and in relation to the main geological structures is presented in Figure 2-2. These structures are the primary constraints to the disposal facility layout. The layout presented here is one of the possible alternatives developed for situating the repository to the site. The low and intermediate level waste repository for the waste produced during the encapsulation and disposal phase can be seen in Figure 2-1 branching from the access tunnel at the depth of about -180 m. Depending on the function of tunnels, other spaces or shafts, a wide range of tunnel and shaft profiles will exist. The volume of the disposal facility (without deposition holes and tunnels for 5440 tU layout) is approximately 1.3 million m3 and approximately 60 % is associated with the regions other than the deposition tunnels (Kirkkomäki 2012).

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Figure 2-1. A generic layout of a disposal facility (KBS-3V) for spent fuel from Olkiluoto and Loviisa nuclear power plants (9000 tU). Deposition tunnels and holes are not displayed but only the spaces considered in this report.

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Figure 2-2. Example of repository layout adapted to Olkiluoto site between levels -400 m and -420 m. Red ovals represent borehole sites at disposal level. The shaded features are layout determining features that restrict the disposal facility design. (Saanio et al. 2012.) 2.2 Access tunnel The access tunnel constructed for research, characterisation and training purposes also allows for vehicular travel from the surface to the disposal level. It consists of four loops as shown in Figure 2-1. The cross-sectional area of the access tunnel varies from about 34.5 m2 to 53.8 m2, in straight tunnel sections and curves, respectively. The cross-sectional area of shaft connections varies from 33.0 m2 to 97.7 m2. In some tunnel crossings the access tunnel height is even 10.4 m, which results in an area of tunnel cross-section of 83.8 m2. The volume of the access tunnel (including shaft connections) is approximately 245,000 m3. The inclination of the access tunnel is 1/10 except in the areas of shaft connections where the inclination is not so steep. 2.3 Shafts Three shafts are being constructed at the ONKALO-stage and two more during the construction of the rest of the disposal facility when the construction application has been approved. The personnel shaft is 4.5 m wide and the canister shaft is planned to be 5.5 m diameter. Inlet air shaft and both exhaust air shafts 1 and 2 are 3.5 m in diameter (Kirkkomäki 2012). When using the given diameters and depths of -455 m a volume of 31,179 m3 can be calculated for the five disposal facility shafts.

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2.4 Technical and other rooms at the -420 to -457 m level Technical rooms, demonstration tunnels, sedimentation pool and pumping station are predominantly located between the levels -420 m and -455 m of the disposal facility (Figure 2-1). They represent challenging volumes to fill due to their size and various dimensions but are generally not along a direct flow path from the deposition tunnels and the shafts or access tunnels. Plan views and the dimensions of the currently identified volumes are presented in Saanio et al. (2012) and Kirkkomäki (2012). 2.5 Central tunnels, central tunnel connections and vehicle connections A central tunnel layout is shown in Figure 2-1. The central tunnels are occasionally connected to each other by central tunnel connections. The vehicle clearance in the tunnels is 3.8 m and in straight sections the width is 6.4 m and in curves 8.5 m. Central tunnel cross-sections are about 45.7 m2 and 59.6 m2 in straight tunnel sections and in curves, respectively (Kirkkomäki 2012). Vehicle connections are routes connecting central tunnels, technical rooms and access tunnel to each other. Their height varies between 6.7 m and 9.7 m and the area of a cross section between 34.0 m2 and 76.6 m2. Theoretical volume of the vehicle connections is approximately 40,700 m3, when calculated according to reference layout used in this work (for 9000 tU). (Saanio et al. 2012, Kirkkomäki 2012.) The disposal facility will be excavated, put into use and closed in a stepwise manner, as shown schematically in Figure 2-3. Operational sequencing will provide challenges with regards to co-ordination of activities to ensure safe and efficient operations (excavation, disposal of spent fuel and closure of areas already used for disposal) and it will be developed prior to the start of the disposal facility operations. The general closure concept is described briefly in Chapter 4.

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Figure 2-3. An illustration of an example of a stepwise implementation of the spent fuel disposal resulting in progressive closure of the deposition tunnels. Closure of the central tunnels proceeds afterwards also in a stepwise manner. Current design for the stepwise implementation is in Kirkkomäki (2012). 2.6 Repository for low and intermediate level waste A repository for low and intermediate level waste that is generated in the encapsulation plant and during the disposal operation is designed to be excavated to a depth of approximately -180 m. It is designed to branch out from the access tunnel and in future be a part of the disposal facility (Figure 2-1). The repository for low and intermediate level waste is thus considered to be closed during the closure of the rest of the underground disposal facility also. (Saanio et al. 2012.) 2.7 Tunnel sections between deposition tunnel plugs and central

tunnels Backfilling of deposition tunnels and deposition tunnel plugs are considered separately from the closure of the underground disposal facility, and are not discussed in this report. An exception is the short deposition tunnel sections that remain on the central tunnel side after installation of the deposition tunnel plug (Figure 2-1). These remaining short sections of tunnels will be backfilled using the same materials and techniques as in the central tunnels.

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3 CONDITIONS AND PREMISES FOR DESIGN BASIS The design basis for the disposal facility proposed for the Olkiluoto site is the result of a detailed assessment of the requirements for the safe construction, operation and closure of a deep geologic disposal facility for spent nuclear fuel and its long-term, post-closure performance. All of these requirements need careful evaluation of the site, its past, present and likely range of future conditions as well as evaluation of what processes will control any future evolution of the repository components and in case of any releases of radionuclides their transport within the EBS and the host rock.

A description of the performance and design requirements defined for the overall system and closure components in a disposal facility and how these influence a generic design for the closure is provided below. The Olkiluoto-specific conditions during repository evolution that will influence closure are briefly discussed with the intent to select an appropriate approach to repository closure and to identify suitable materials for use in accomplishing this. At the end of this Chapter design specifications are presented. They are derived from the Olkiluoto-specific conditions and estimated climate evolution and are set to fulfill the performance targets and design requirements. 3.1 Performance targets for closure The safety functions of closure plugs and sealing structures have been discussed in detail in the Design Basis report. According to these safety functions and the currently identified performance targets (Table 3-1) the closure components have well-defined roles. The backfill and plugs installed throughout the disposal facility are to ensure that underground openings of the disposal facility do not form preferential pathways for groundwater flow and thus also possible radionuclide transport. The backfill of the central tunnels also acts to prevent deposition tunnel backfill from swelling into the central tunnels, ensuring that its function will not be compromised through density reduction. Further away from the disposal level, backfill and plug components should be designed in such a way that they provide a substantial impediment to any future attempts to access the closed disposal facility. There is also the additional need for the closure materials to avoid having chemical or other components that could adversely affect adjacent materials or the rock. (Design Basis.)

20

Table 3-1. Performance targets for closure (Design Basis).

Performance targets

Unless otherwise stated, the closure materials and structures shall fulfill the performance targets listed below over hundreds of thousands of years in the expected repository conditions except for incidental deviations.

Closure shall complete the isolation of the spent nuclear fuel by reducing the likelihood of unintentional human intrusion through the closed volumes.

Closure shall restore the favorable, natural conditions of the bedrock as well as possible.

Closure shall prevent the formation of preferential flow paths and transport routes between the ground surface and deposition tunnels/deposition holes.

Closure shall not endanger the favorable conditions for the other parts of the EBS and the host rock.

Retrieval of the spent nuclear fuel canisters shall be technically feasible in spite of repository tunnel and closure structures.

3.2 Design requirements for closure In order to have confidence that the basic performance targets are met, there is a need to understand a wide variety of geological, chemical, thermal, hydraulic, mechanical and microbial processes that may be active in the disposal facility and the surrounding geosphere. Additionally, the selection and installation of closure backfill and plugs also needs to be undertaken with an understanding of the engineering realities associated with construction, operation and closure of the underground. The generic performance targets presented in Table 3-1 identified what will be necessary in terms of basic behavior of the closure system in a disposal facility. Performance targets for the closure are based on estimates of how the backfill and plugs may affect the transport (migration), mechanical and chemical properties of the system. In particular these materials and structures are required to not compromise the isolation of the deposition tunnels. From these comes the development of the design requirements for the closure system. The design requirements for the closure are presented in Table 3-2. (Design Basis.) Many of the design requirements deal directly with how closure will be undertaken and what is needed in terms of operational safety. How these will be accomplished will be part of an ongoing process of developing materials, equipment and operation-related techniques. Design specifications for the requirements have been derived from the Olkiluoto-specific circumstances. These design specifications are presented at the end of this Chapter in Section 3.6 after presentation of the site characteristics, water inflow estimate and climate evolution estimate.

21

Table 3-2. Design requirements for closure backfill and plugs (Design Basis).

Design requirements

The ground surface of the disposal area shall be landscaped to resemble its natural surroundings.

Structures and materials that considerably obstruct unintentional intrusion shall be utilized in the closure of the uppermost parts of the facility and investigation holes extending to the ground surface.

Structures and materials of the closure components shall be selected in such a way that the isolation functions of closure can be provided despite possible loadings related to glacial cycles, such as permafrost or changing groundwater chemical conditions.

Rock materials shall be used increasingly as backfill when moving from the disposal depth up to the ground surface due to the increasing risk of clay erosion.

Closure as a whole shall be so designed that the hydraulic connections from the disposal depth to the surface environment through the closed tunnels, shafts, and investigation holes are not better than through existing natural fractures and fracture zones.

Sections in the underground openings intersected by highly transmissive zones such as the HZ20 structure shall be hydraulically isolated from other facility sections.

The closure as a whole shall be so designed that short-cuts from the deposition tunnels/deposition holes to existing significant groundwater flowpaths are prevented.

The closure components shall keep the backfill and plugs of the deposition tunnels in place.

The amount of chemical species harmful for canister/buffer/deposition tunnel backfill/host rock in closure components shall be limited.

3.3 Olkiluoto-specific conditions influencing the closure design basis The environment for the closure and its components will be affected by the characteristics of the host rock, temperature, water chemistry and water flow rates. These Olkiluoto-specific conditions constrain both the design and construction of the disposal facility as well as determine how closure can be accomplished and what the post-closure evolution will be. In order to provide a general outline of the key mechanisms affecting the closure design basis a summary of the key features and processes are provided below and are briefly described in the following sections (3.3.1-3.3.5). The key site-related features interacting with the closure system are; bedrock composition and fracture features, strength, stress state and thermal properties of the rock, hydrogeology and hydrogeochemistry. 3.3.1 Bedrock composition and fracture features The bedrock of Olkiluoto Island will largely define the mechanical and hydraulic characteristics to be accounted for in closure engineering. The main rock types in Olkiluoto are migmatitic gneiss, tonalitic-granodioritic-granitic gneisses and pegmatitic granites (Posiva 2009a). Each of these rock-types has slightly different thermal, hydraulic and mechanical properties and their lithology has an influence on the distribution of fracturing at site. Fracturing of the bedrock encountered in the disposal facility is either naturally-occurring or excavation induced, both of which can affect mass transport at the disposal facility site and hence the closure design, e.g. the location of the plugs. It will be necessary from the construction and functional point of view to ensure that fractured

22

rock sections that are encountered will be avoided whenever possible when selecting the location of the plugs. From the plug design and positioning point of view, the presence and location of fracture features are critical and careful attention needs to be paid to the site geology. Also hydraulic properties of fractures as well as other fracture properties (density, width etc.) will have to be taken into account in the closure design process. A description of the main geological zones and fracturing in the intact rock mass between the major fracture zones has been developed and used also as input into a geological model that is briefly described in Posiva 2009aa. In the Olkiluoto geological model the brittle deformation zones are classified into two categories: site scale and local scale brittle deformation zones. Site scale zones that result in large lateral features extending over the entire site are intersected by numerous drill holes. As would be expected, fracturing is denser near the fault core (zone of influence). The compilation of all of this geological information, showing the main geological features (Brittle Fault Zones: BFZ018, BFZ056, BFZ098 and BFZ080) are presented in Figure 3-1. There is a clear decrease in the intensity of all fracture sets with increasing depth (Figure 3-2) (Posiva 2009a). The fracturing is the densest in the uppermost 100 m of bedrock and it is suspected to be due to the glacial unloading and stress release. Information used to develop the geological pattern was used to generate the Olkiluoto discrete-fracture network (DFN) model. This is based on a statistical analysis of fracture orientations, sizes and intensities (Posiva 2009a). The DFN model describes the fracturing outside of the deformation zones and the zone directly influenced by them. The DFN model identifies three fracture domains at Olkiluoto: upper fracture domain above deformation zone BFZ098, intermediate fracture domain representing an approximately 100 m thick section of rock between the two major deformation zones BFZ098 and BFZ080 and, a lower fracture domain beneath the second major deformation zone BFZ080. The fracturing properties of these domains are given in Posiva (2009).

23

a)

B

FZ

018

b)

B

FZ

056

c)

BF

Z09

8

d)

BF

Z08

0

Fig

ure

3-1

. Mai

n ge

olog

ical

str

uctu

res

in th

e O

lkil

uoto

Sit

e, a

) B

FZ

018,

b)

BF

Z05

6, c

) B

FZ

098

and

d) B

FZ

080.

(M

atti

la e

t al.

2008

.)

24

Figure 3-2. Terzaghi-corrected P10 (fractures/meter) for all fractures as a function of depth (Posiva 2009a). Curves with dots are measured values and without dots modelled. Effects of disposal facility construction on rock Construction of tunnels, shafts and other openings causes alteration in the properties of the host rock surrounding the underground openings. The alteration zones can be described as excavation damaged zones (EDZ), where irreversible deformation and fracture propagation and/or development of new fractures occur, and excavation disturbed zone (EdZ), where stress redistribution occurs with no new fracturing. (Mellanen et al. 2009.) When the rock is excavated, an EDZ is formed as a consequence of the mechanical changes related to rock removal during excavation. Besides the rock properties, the extent and properties of the additional EDZ depend on the excavation method. When excavated using the drilling and blasting method, the EDZ in the tunnel floor is usually assumed to be twice as deep as in the walls and the ceiling (Pursiheimo 2006). Field measurements have shown that the depth of the EDZ in the blasted ONKALO tunnel varies from 400 mm to 1000 mm. For the larger excavation areas, such as parking halls, the depth of the EDZ is expected to be higher than for the tunnels. In excavations made using the tunnel boring method the EDZ extends a smaller distance into the rock. According to Mellanen et al. (2009) the EDZ can be minimised through use of smooth rock excavation methods, like the raise-boring excavation used in the shafts. Locations, where plugs are to be installed in the disposal facility, may also benefit from use of techniques intended to minimise disturbance to the rock adjacent to the excavation.

Disposallevel

25

Temperature-related effects on rock The rock surrounding the disposal level of the facility will undergo slow heating due to radioactive decay of the spent fuel in the canisters. This causes the rock to expand and damage associated with the EDZ could be exacerbated and ultimately thermal spalling could occur (Pastina & Hellä 2006), if it is not adequately confined by the closure materials. This effect is estimated to remain small in the openings other than the deposition holes and tunnels due to the increased distance to the heat sources (canisters). The closer the underground opening is to the spent fuel, the larger the temperature changes are, and the greater is the potential for heat-induced disruption to the rock. The heat generation from the canisters is limited to such that the bentonite buffer will not see temperatures higher than 100 ºC, as it could result in mineralogical changes. A 100 ºC limit also limits the highest temperatures of the rock of openings other than deposition holes and tunnels to substantially lower values. The rock in the repository will at maximum be heated up to 65 ºC, (Performance Assessment, page 138) but in central tunnel the values are lower. These changes can start immediately after canister emplacement or after several decades, depending on the rock, hydraulic conditions and the backfill materials used. During the operational phase slight rock damage in the limited area around the excavation periphery due to stress concentrations and EDZ are possible, as are small rock movements along existing fractures (Pastina & Hellä 2006). Although not directly related to the disposal facility construction, glaciation-related processes have the potential to induce damage to the rock surrounding the disposal facility openings, especially in the regions closest to the surface. The isostatic land uplift at Olkiluoto is anticipated to be some tens of metres above the sea level during the post-closure period (up until next glaciations). Actual elevation change will depend on climate driven changes on sea level. During the glacial phase, isostatic loads increase as the result of ice sheets on the earth’s surface and the crust may be depressed by tens to hundreds of metres (Pastina & Hellä 2006). The advancing, melting and retreating of ice sheets erode the surface bedrock. Expected bedrock erosion rates in the Olkiluoto area are estimated to be at maximum 10 m per full glacial cycle and very likely will be only in the range of just few meters (Complementry Considerations, Table 7-1).Using either of these erosion estimates, the disposal level at Olkiluoto will remain physically distant from the surface for any climatic conditions evaluated for any time span of significance to the disposal facility. 3.3.2 Strength, stress state and thermal properties of rock Strength From the plug design point of view, rock strength, Young’s modulus, the state of stress, failure criterion (Mohr-Coulomb and Hoek-Brown) and thermal properties of the rock are of particular importance. When plugs are constructed the largest deformations in rock have already occurred as they are largely associated with first days following excavation. However, the mechanical properties of the rock remain important in the post-excavation period because of the interactions between the plug, backfill, hydraulic pressures and the surrounding rock. After water saturation of the backfill, the plug(s) must withstand the swelling pressure of backfill (and potentially at least a portion of the hydraulic head at that location). The need to sustain at least part of the hydraulic

26

pressure is the result of the plug’s function to hinder water leakage from one side of the plug to the other. The rock adjacent to the plug therefore has to sustain the mechanical loads transferred from the plug. Stress State The stress state of rock has been measured or predicted over the depth range of 29-985 m (Posiva 2009a). The measurements made of in situ stress show strong scatter, making development of a stress model problematic. The rock strain measurements obtained from shafts were used to provide field-consistent parameters for use in developing the stress model for the Olkiluoto site. There is, as yet, no clear understanding of how the main geological zones influence the state of rock stress at Olkiluoto and this will need to be developed as part of a more comprehensive site model. Although there are some limitations regarding the details of the in situ rock stress conditions at Olkiluoto it is possible to generate a general description of the site (Posiva 2009a). The following controlling site conditions have been developed: A thrust faulting stress regime applies; i.e., the maximum horizontal stress σH > the

minimum horizontal stress σh > the vertical stress σv. Also the principal stresses are oriented horizontally and vertically, respectively. In the stress model, vertical stress is taken to be equal to overburden stress, with an assumed variation of 10 %.

On the regional scale, the maximum horizontal stress component is oriented NW-SE. On the other hand, the data suggest a maximum stress orientation of N-S for the upper 300 m of the bedrock and E-W for the lower 300-900 m.

For the disposal depth the mean stresses and estimated lower (10 %) and upper limits (90 %) are σH (E-W) = 27 (21-33) MPa, σh = 16 (12-20) MPa and σv =12 (11-13) MPa.

The deformation and strength properties of rock type domains and the rock mass properties are presented in Posiva (2009a). The mechanical properties of rock are of particular importance when developing designs for plugs and assessing the potential for the rock surrounding an underground opening to undergo undesirable changes (e.g. rock spalling), if the backfill does not provide sufficient restraint. The stress above, which spalling of the metamorphic rocks occurs at Olkiluoto, is 52-99 MPa. The mean value is estimated to be 66 MPa (Posiva 2009a), which is an important value to be considered in the design of the openings and the closure system. Thermal properties and processes involving rock Since spent fuel generates heat and the heat output decreases with time as radioactive decay progresses, the temperature in the surroundings of the repository will change with time. The heat generation and the thermal characteristics of the materials surrounding the canisters (buffer, backfill, rock) results in a peak in rock temperature at about 100 years after starting the operation of the repository (Pastina & Hellä 2006). Following this peak, temperatures will gradually decrease but the time required for the temperature of rock to decrease to its initial level is in the order of 10,000 years.

27

Figure 3-3 presents the evolution of rock temperature at different depths immediately above and below the repository. In the period immediately after disposal facility closure phase (or during the glacial phase) the effects of surface temperature changes on the disposal facility are minor. However over the longer term (>10,000 years) the development of penetrating permafrost may affect the performance of the backfill and plugs, especially in the upper parts (shafts and access tunnel) of the disposal facility. This is being evaluated as part of the design process. As temperature increase within the disposal facility is dependent on the thermal conductivity of the materials surrounding the canister, and the mechanical properties of rock change with temperature, the thermal properties of the rock at Olkiluoto have been well characterised in order to ensure that the system performs as designed. The main mechanical changes anticipated in the bedrock during the operational phase of the disposal facility are associated with changes in the stress state, movements in sparsely fractured rock (readjustment in existing fractures) and thermal expansion of rock (Pastina & Hellä 2006). During post-closure temperate phase thermal loading will increase the principal stresses in the rock mass and thermal stresses in the deposition area are expected to range from 15-30 MPa. Thermo-mechanical experiments at ONKALO (POSE experiment, Posiva 2009b) will help to better quantify and improve the understanding of the consequences of thermal processes on the disposal facility.

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ROCK TEMPERATURE INCREASE AT ABOVE REPOSITORY CENTRE

0

5

10

15

20

25

30

35

40

1 10 100 1000 10000 100000

Cooling time (a)

delt

a T

(K

)

h = -400 m

h = -300 m

h = -200 m

h = -100 m

Figure 3-3. Rock temperature increase due to decay heat from the spent fuel above the centre of repository at 100 m level intervals as a function of time. The -400 m level temperature means rock temperature at repository level, but far from canisters. No climate change effects are taken into account (Raiko 2005, Pastina & Hellä 2006). 3.3.3 Hydrogeology In order to design the closure (backfill and plugs), determine their hydraulic properties and establish plug locations, the hydrogeological properties of the bedrock and fractures are of primary concern. Olkiluoto Island has been extensively characterised through the drilling of boreholes to various depths and identification of key hydrological features within the rock mass underlying the island. The hydrogeological model for the site is based on the information obtained from these boreholes. The hydrogeological model for Olkiluoto consists of a hydrostructure model and a flow model (Posiva 2009a). The hydrostructure model describes the geometrical distribution of the permeable features of the rock and is closely related to the geological model, but not identical. The flow model refers to the groundwater flow simulation model. From the hydrogeological site-scale modelling point of view, the bedrock consists of large volumes of sparsely fractured rock with low fracture transmissivities and hydrogeological zones with moderate or high fracture transmissivities (Posiva 2009a). The hydrostructure model also provides geometries for the hydrogeological zones (HZ) and the hydrogeological properties of these zones and bedrock.

29

There are three hydrogeological features that have been identified as dominating the bedrock of Olkiluoto. They are HZ19, HZ20 and HZ21 systems and consist of large subhorizontal features with significantly higher transmissivity than the intact rock. Each of these systems is composed of a few subzones (Vaittinen et al. 2009). Hydrogeological zone system HZ19 is formed of three subzones HZ19A, HZ19B and HZ19C of significant extent at a depth of about -100 m in the vicinity of ONKALO (Posiva 2009a) and is clearly seen in the access tunnel and shafts. These structures are intersected by a large number of drill holes. HZ19A is the uppermost and HZ19C describes the most extensive connections within the HZ19 system. HZ19B describes the hydraulic connections that cannot be explained with zones HZ19A and HZ19C. Hydrostructure HZ19A corresponds approximately to the geological brittle fault zone BFZ018 and HZ19C corresponds to BFZ056. An influence zone of 4-40 m has been determined for BFZ018. The measured transmissivities in the HZ19 system show a few remarkably high values, as high as 10-4 m2/s and several of them > 10-5 m2/s. The orientations and transmissivities of the hydraulically transmissive zones are presented in Table 3-3. HZ19 and subzones comprising it are presented in Figure 3-4. Hydrogeological zone system HZ20 is formed of two gently dipping zones HZ20A and HZ20B of significant extent, and it is located at a depth between -300 and -400 m in the vicinity of ONKALO (Posiva 2009a) and is clearly seen in the access tunnel. These structures are intersected by several drill holes. HZ20A is the upper surface of HZ20 system and HZ20B is the lower surface of the system. Hydrostructure HZ20A corresponds approximately to the geological brittle fault zone BFZ098 and HZ20B corresponds to BFZ080. A zone of influence of 15-72.5 m has been determined for BFZ098, and 7.5-82 m for BFZ080. The measured transmissivities in the HZ20 system show remarkably high values, as high as 10-5 m2/s, which will make this a particularly important feature. The orientations and transmissivities of the hydraulically transmissive zones are presented in Table 3-3. HZ20 and subzones comprising it are presented in Figure 3-5. Hydrogeological zone HZ21 is located beneath the planned disposal facility. In its vicinity, there are hydrogeological zones HZ099 and HZ001. The orientations and transmissivities of these zones are presented in Table 3-3. Besides of the geometrical and hydrogeological properties, the leakage water inflows into open tunnels without grouting are estimated and presented in Table 3-3. Note that the majority of the leakages are sealed when constructing the tunnels and sealing is supposed to sustain during the operation time of the repository. After closing the facility, the tunnels are saturated with groundwater and the gradient decreases significantly. As the gradient decreases, the water flow in hydrogeological structures slows down. Flow rates after saturation are not assumed to be as high as presented in Table 3-3. The hydrostructure model and the properties of the sparsely fractured rock between the hydrogeological zones have been the basis for the numerical flow modelling (Posiva 2009a). The groundwater flow is controlled by local variations in the topography and by the network of HZs (Posiva 2009a). The flow direction is mostly downwards below the

30

hills, whereas near the shoreline and below the areas of lower elevation, water flows horizontally and/or upwards (Posiva 2009a). In the vicinity of ONKALO the flow is dominated by the presence of the HZ19, HZ20 and HZ21 systems. The anisotropy assigned to the upper layer of the sparsely fractured rock tends to change the flow to be horizontal at shallow depths. Deeper in the sparsely fractured rock the water flows towards the more conductive zones, which are connected to the sea. The discharge areas for groundwater originating from the potential repository rock volume at a depth of -420 m are located at the coast (Posiva 2009a). Posiva has also created hydrogeological DFN model, in which the hydrostructures follow the hydrogeological structure model. The description of the sparsely fractured rock out of zones is based on the measurements by Posiva flowlog (PFL) made in drill holes and pilot holes. Hydrogeological DFN model is described in Posiva (2009a). Table 3-3. The properties of the main hydrogeological zones. T stands for transmis-sivity, g.mean for geometric mean value and Q for flow.

HZ-zone

Corresponding geological structure

Orientation Depth (m)*

T-range (m2/s)

T g.mean (m2/s)

Q-potential (l/min)**

Q-observed

(l/min)

HZ19A BFZ018 144º/5º ~ 100

Up to 10-5..10-4

8x10-6 72

~ 2 in total HZ19B 151º/16º 3x10-6 27

HZ19C BFZ056 139º/8º 4x10-6 36

HZ20A BFZ098 121º/13º -300 to -400

Up to 10-5

5x10-6 121 ~ 6 in total

HZ20B BFZ080 136º/18º 3x10-6 73

HZ21 162º/20º > 420 - 1x10-8 0

HZ099 164º/34º > 420 - 2x10-7 6

HZ001 165º/28º > 420 - 10-6 28

*Average depth in the vicinity of the ONKALO or disposal facility **Calculated according to Swedish National Road Administration 1994 using average depth, geometric means of transmissivities tunnel radius of 3 m and skin-value of 0. As the tunnels saturate with groundwater, gradient decreases and the flow rate in hydrostructures slows. Note that transmissivities in a structure vary and locally water inflow can vary substantially.

31

 

 

 

Figure 3-4. Hydrogeological zone HZ19A (yellow) and geological structure BFZ018 (black) (upper figure), HZ19C (blue) and BFZ056 (black) (middle figure) and HZ19B (lower figure). Transmissivity(T)-values: blue>1x10-8 m2/s, green>1x10-7 m2/s, yellow>1x10-6 m2/s and red>1x10-5 m2/s. (Posiva 2009a, Vaittinen et al. 2009.)

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Figure 3-5. Hydrogeological zones HZ20A (top) and HZ20B (bottom). T-values: blue>1x10-8 m2/s, green>1x10-7 m2/s, yellow>1x10-6 m2/s and red>1x10-5 m2/s. (Posiva 2009a.) 3.3.4 Hydrological evaluation of Olkiluoto site At Olkiluoto the water pressure is dependent on the depth below groundwater table and on the hydraulic connections. Currently the groundwater table is a few meters above sea level and the main zones, and via them single fractures are also somewhat connected to the sea. The general approach applied to the site is that groundwater pressure increases 1 MPa per every 100 m. Local variations in hydraulic connections and gradient lead to the situations that groundwater pressure near an open tunnel may be lower than the theoretical pressure, at least during the operation phase. In order to assess site conditions, behaviour for construction purposes and begin the process of evaluating what might be encountered in a disposal facility at the Olkiluoto site, Vaittinen et al. (2007) compiled and analysed the PFL results of transmissive fractures between the depths of -300 to -600 m. The data, measured from nearly vertical drill holes, was projected into horizontal tunnels and classified according to the fracture dip and transmissivity. Based on this type of analysis, the average distance of fractures

33

along horizontal tunnels at the disposal depth varies from 112 m (dip 0-30° and T < 4.7x10-9 m²/s) to 3350 m (dip 30-60° and T > 7.3x10-8 m²/s). Fractures of 7.3x10-8 m²/s in their transmissivity are so few that it should not be difficult to find dry rock sections to use as plug locations at the disposal depth. The hydrogeological information for sparsely fractured rock sections between the highly transmissive features at Olkiluoto is presented in Table 3-4 (Andersson et al. 2007). The hydraulic conductivity requirements for the closure of access tunnels and shafts can be derived with the help of these values for the Olkiluoto site. This is of importance, if the objective of closure is that the conductivity of the overall system will remain in the same level as before the construction of the disposal facility. It will be possible to subdivide the disposal facility into smaller entities each being designed to exhibit a specific hydraulic characteristics. The hydraulic conductivity of the EDZ is 2.25x10-8 m/s, which corresponds to a transmissivity of 1.0x10-8 m2/s for the fractures representing the EDZ (Site Description). Previous studies by Mellanen et al. (2009) concluded a lower hydraulic conductivity for the EDZ by water loss measurements and the interpreted K-values were mainly in the order of 1x10-11 to 1x10-10 m/s. With these acquired results the hydraulic conductivity of EDZ may vary from the values representing those of sparsely fractured rock to that of slightly more fractured, but do not display drastic rise in the hydraulic conductivity. The EDZ will need to be considered for example in plug sites and installation, though. Table 3-4. The hydraulic conductivity of the sparsely fractured rock. The values presented in the table are based on the data from the well characterised area (WCA), where the drill hole investigations have been focused. These values are the selected base values after calibration (after K2 values in Table 6-2 of Posiva 2009a).

Depth interval (m) Hydraulic conductivity, K (m/s)

0-50 KH = 1.0x10-7 * Kv= 1.0x10-8 *

50-100 K= 3.2x10-8 – 5x10-9 **

100-200 K= 5.0x10-9 – 1.3x10-10 **

200-300 K= 1.3x10-10

300-400 K= 1.3x10-10

400-500 K= 3.0x10-11

500-2000 K= 3.0x10-11

*For the depth interval 0-50 m the hydraulic conductivity is anisotropic; the horizontal component KH being 10 times higher than the vertical component KV ** The logarithm decreases linearly with depth

34

3.3.5 Hydrogeochemistry In addition to understanding the hydraulic characteristics and geology of the Olkiluoto site, also knowledge of the hydrogeochemistry of the site is needed in order to select and design the plugs and backfill materials. The hydrogeochemical properties of the Olkiluoto site have been investigated and a model that describes the main water types and certain key parameters has been developed. This model contains the available groundwater composition information for the site and an assessment of the processes controlling its evolution (Posiva 2009a). The site information comes from the water sample data obtained from drill holes and ONKALO excavation. The site model is subdivided into a salinity model and a model that describes hydrogeochemical and microbial interactions between water and rock. Current site conditions and active processes In general, the chemical groundwater conditions are stable at disposal depth at Olkiluoto and reactions and transport processes proceed very slowly (Posiva 2009a). If groundwaters with different chemical states are mixed (due to e.g., climate change, glaciations, land uplift or construction), or other materials are in significant disequilibrium with groundwater, chemical interactions may activate and reduce the buffering capacity of the bedrock. The hydrogeochemical site descriptive model is illustrated in Figure 3-6. As noted, the changes in climate and the geological environment have had a significant effect on local paleohydrogeological conditions and this is seen as considerably variation in hydrogeochemical signatures with depth, notably in salinity (Posiva 2009a). These processes are expected to reoccur in the future and will influence the salinity and other chemical conditions at the disposal facility site, particularly in regions closer to the ground surface.

35

Figure 3-6. Illustrative hydrogeochemical site model of baseline groundwater conditions with the main water-rock interactions at Olkiluoto. Changes in colour indicate alterations in water type. The hydrogeologically most significant zones (HZ) are represented. Blue arrows represent flow directions. Rounded rectangles contain the main sources and sinks affecting pH and redox conditions. KR means drill hole. Enhanced chemical reactions dominate the infiltration zone at shallow depths, and at the interface between Na-Cl-SO4 and Na-Cl groundwater types. The illustration depicts hydrogeochemical conditions in the water-conductive fracture system, not in the diffusion-dominated rock matrix (Site Description, Figure 7-86). The salinity of groundwater prior to construction of ONKALO varied considerably over the 1000 m depth that has been characterised, with 84 g/L TDS being the highest value yet measured (Posiva 2009a). In general terms the site can be described as follows: Fresh water with low total dissolved solids (TDS ≤ 1 g/L) is found only in the

uppermost tens of metres. These near-surface groundwaters are rich in dissolved carbonate (meteoric origin).

Brackish water (TDS 0–10 g/L) dominates between 30–450 m and high SO4 concentrations are found between 100–300 m (marine origin).

Saline groundwater (TDS ≥ 10 g/L) dominates at the depths of about 400 m.

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Most of the brackish Cl-type groundwater has been recovered in the volume between HZ20 and HZ21 (see Figure 3–6 for locations) and originate from relatively low transmissivity sections of drill holes, whereas other water types cover the whole range of transmissivities (Posiva 2009a). Although groundwaters from the chemical point of view are horizontally layered, the vertical extent of salinity and water types varies locally in the Olkiluoto Island. The measured pH value in groundwater varies in a non-systematic manner, between 7.2 and 8.5 in the rock below the ground surface (Pitkänen et al. 2004). In the overburden pH values ranging from 5 to >8 have been measured. This limited variability means that it is unlikely that pH will affect system performance unless some outside influence causes disruption to the regional conditions (e.g., large volumes of groundwater enriched with leachates coming from high pH concrete/grout). Organics in groundwater may affect the durability of the construction materials such as concrete and copper. Dissolved organic carbon (DOC) content shows a strong scatter and the results include uncertainties due to the sampling techniques and analytical methods (Posiva 2009a). Three microbiological layers have been observed in Olkiluoto: the upper (the overburden and the uppermost part of bedrock (depth of 3 to 16.5 m) represent an oxygenic environment with numerous microbial populations and the biomass is ten times higher than deeper. Below this layer there is anaerobic, reduced deep biosphere. The second zone ends at around 300-400 m depth (at sulphate–methane interface), where the methane concentration increases and sulphate concentration decreases. The bacterial activity is the highest near surface and at the depth of around 300–400 m. The main microbes are given in Posiva (2009a). Excavation of the tunnels could increase microbial activity within the rock adjacent to the openings and within the backfilled facility, but after closure of the disposal facility and recovery of the natural groundwater conditions (anaerobic, methanic and saline), microbial-related processes are expected to return to close to the pre-excavation rates. Site evolution and processes affecting groundwater composition Groundwater composition evolves with time and is influenced by various processes; e.g., by natural climate evolution driven processes and construction influenced mixing of water types, water-rock interactions, weathering and microbiological activity. A detailed description of the processes has been provided in Pastina & Hellä (2006). The evolution of groundwater composition has been described in terms of time: 1) operational period (0-100 years), 2) early phase of the evolution (up to 1000 years), 3) temperate period (until next glaciation), 4) glacial periods (beyond 10,000 years depending on the climate scenario) and 5) after glacial periods. In order to predict the evolution of the system for each time frame a current reference groundwaters are selected and their evolution modelled taking into account the hydrogeological evolution of the system (changing flow rates, density, etc.). This modelling is currently under work, and for the purpose of this report representative groundwater compositions for the appropriate depths have been selected, according to current understanding of the site. Salinity of the groundwater and its changes both in time and space must be taken into account in order to ensure that the backfill materials will perform as expected. Of special concern is the depth at which saline, fresh or diluted water is encountered and it

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is recognised that this is not constant; the salinity of the groundwater at a specific depth may/will change with time due to changing regional conditions. This means that the effects of salinity on the functional properties and behaviour of the plugs and backfill materials (concrete, steels, clays etc.) must be taken into account during closure design. At the Olkiluoto site it is assumed that down to 300 m depth the construction-related disturbances will not increase the TDS above 10 g/L at the 300 m level (Pastina & Hellä 2006 p. 238). Following a major glaciation event the groundwater at considerable depth is anticipated to be diluted due to the influx of glacial meltwater (SKB 2006, Pastina & Hellä 2006). According to the site report the depth of discernible glacial water infiltration has been limited to 100‐300 m depth (Posiva 2009a, p. 384). Therefore the lower limit to which future glacial meltwater-induced dilution needs to be considered is currently set at about -300 m. At the planned disposal depth in Olkiluoto (~420 m), the pH of groundwater is, as described earlier, typically in the range of 7.5–8.5 (Pitkänen et al. 2004, Posiva 2009a). Substantial changes in the pH conditions within the disposal facility are highly undesirable as they can adversely affect closure system performance and mineral stability of clay materials. Cementitious materials have great potential to affect the pH of the groundwater and so facility operations or materials installed should be selected such that they influence the pH conditions as little as possible. One means of reducing impact on the environment is the use of low-pH cements (e.g., cement containing silica rather than lime) for any cementitious materials that will be left within the disposal facility (either as operational residues or as part of plugs installed during closure). The compositional requirements of cementitious materials may change with location in the disposal facility (with increased distance from the deposition tunnels, depth or location of installation), or in response to the type of materials immediately adjacent to them (e.g., bentonite). 3.4 Estimation of water inflow to ONKALO and disposal level A key feature in the evolution of the disposal facility is the rate at which water will enter the underground openings (and also the backfilled volume). Based on the site-specific conditions described in Section 3.3 calculations of water inflow into ONKALO and the disposal facility have been made (e.g. Lokkila & Sievänen 2009). They are intended to provide an indication of what range of inflow conditions might be encountered prior to excavation and from these data estimate what might be necessary in terms of options for disposal facility operations and closure. During operation and closure period The total leakage-related water inflow into 4580 m of access tunnel, two shafts down to the depth of about 270 m and one shaft down to the depth of 437 m at ONKALO is approximately 35-40 l/min (at the end of 2012) (Vaittinen et al. 2013). The water inflow from these excavations is distributed such that the majority of the water leakage is attributable to the first 1500 m of the access tunnel (~19 l/min), from HZ20 zones (~6.5 l/min) and elsewhere from local transmissive zones and rock bolt holes. After tunnel chainage (excavated tunnel length) of about 1500 m (depth below surface of ~150 m), there are tunnel sections with dry tunnel surfaces or only slow, isolated

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dripping occurring. The three shafts leak 0-1.5 l/min per 100 m of shaft section (vertical distance between interconnections between shaft and access tunnel) in the uppermost ~290 m of depth. From the plug location design point of view, this means, that there are a very limited number of optimal locations for water-restraining plugs to be constructed in the near-surface region. Instead, below the depth of about 150 m, it seems that potentially suitable hydraulic plug locations are easier to find. There are a number of factors that will affect the rate and nature of the water entering the disposal facility, both locally and generally during the operational phase. These include the following: During the operational phase impacts of the tunnelling (groundwater inflow into the

tunnel) causes drawdown of groundwater table around the disposal facility. This may result in a decreasing rate of inflow with time as the local groundwater levels are lowered.

The presence of drained openings at depth can also result in upconing of saline water from below the disposal facility, which can affect the swelling and hydraulic behaviour of clay-based closure materials.

Both of these phenomena can be managed by sealing the rock by grouting. Rock grouting affects the rate of water inflow and so also dimensioning of the closure backfill and plugs, and the amount of inflow into the open portions of the disposal facility has been estimated for a situation where grouting has been utilised (Figure 3–7). These inflow estimates include the five shafts, the access tunnel as well as the disposal level excavations were developed using the approach described by Lokkila & Sievänen (2009). These estimated water inflow rates are based on the location of the disposal facility in relation to the known water-conducting features of the bedrock, i.e., in practice on the hydrogeological model for the area (Vaittinen et al. 2009) and on the statistical properties of intact rock (Vaittinen et al. 2007). The estimate also takes into account the flow rate of water already seeping into ONKALO, as well as the forecast for the leakage into the lower, as-then unexcavated section of ONKALO. Figure 3–7 shows the estimated contributions from the main subdivisions of the excavations and total inflow should be 103–118 L/min after rock grouting (depending on the proportion (5–25%) of deposition tunnels remaining open at any one time). The factors affecting the results are discussed in detail by Lokkila & Sievänen (2009). Without any grouting the leakage water inflow has been estimated to be in the order of 1140 L/min (600,000 m3/a (Vieno et al. 2003). This is approximately ten-times that of a facility where grouting has been done and so represents a considerable reduction in the challenge of how to handle water entering the disposal facility in such a manner that it does not adversely affect operations or closure activities. There additionally needs to be determination of what potential exists for grout degradation and the effect of the site returning to its original pre-grouting conditions.

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Figure 3-7. Estimated total flow rate to ONKALO and the disposal facility after sealing the bedrock by grouting. Calculation uses forecast and actual water inflows, taking place in the already constructed part of ONKALO. For the sections below the depth of about -300 m and the two future shafts, a forecast is used. The forecast for intact bedrock between levels -337 and -457 meters was calculated using the median value for transmissivity of fracture groups. The percentage shows the effect of the proportion of deposition tunnels open on total inflow. For calculation purposes it is assumed that 50% of the central tunnels are open. (Lokkila & Sievänen 2009.) Post-closure inflow In the post-closure phase of disposal facility evolution, the driving forces for hydrological and hydrogeochemical disturbances become weaker and the recovery of the local groundwater conditions will slowly progress (Pastina & Hellä 2006). As the tunnels are filled with water and backfill saturates, the inflow rate into the tunnels will decrease. After the disposal facility site approaches steady-state conditions with regards to the regional groundwater system, significantly slower groundwater flow occurs in the vicinity, and changes are the result of regional conditions rather than disposal facility conditions. An example of results brought on by regional condition changes would be the result of changes in water table elevation or surface uplift that drives the surface groundwater flow downwards, and brackish or fresh water will replace saline waters at depth. During the glacial and post glacial phase the groundwater pressure can vary between 4–20 MPa. Locally saline water can exist, but intrusion of diluted glacial meltwater to depth and the resulting groundwater chemistry needs to be evaluated. 3.5 Climatic evolution Although a long-term and likely relatively gradual process, the evolution of climatic conditions at the disposal facility site has the potential to substantially affect the closure

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components, particularly those closest to the ground surface. In order to at least semi-quantitatively assess the influence of climate change on the disposal facility and in particular the closure system it is necessary to evaluate the range of change that could potentially be experienced at the Olkiluoto site. The lines of climatic evolution for the Olkiluoto site have been evaluated in various ways (Pastina & Hellä 2006, Pimenoff et al. 2011a, b). Basically, the expected evolution paths are either based on the repetition of the last glacial cycle (Weichselian) or are based on modelling the effects of anthropogenic greenhouse gas emissions that will delay the onset of the next glaciation. Climatic changes may have a significant effect on the properties of bedrock (fracturing, rock mechanics, hydrogeology, hydrogeochemistry), and especially of the regions closest to the surface. Climate conditions may particularly affect the plugs hampering human intrusion in the near surface. The backfill in the near-surface regions of the access tunnels and shafts must perform also during and after permafrost and the associated freezing and thawing cycles. While the duration and extent of permafrost penetration into the backfilled openings closest to the ground surface is still a topic of active investigation and modelling, the effects of the permafrost period must be taken into account in the design of the backfill and plugs. A brief summary of current conditions at Olkiluoto is given in Posiva (2009a) and it is further detailed in Site Description report, which has been under preparation concurrently with this design report. The temperature extremes during the period 1993‐2007 have been -27 ºC to +32 ºC. The snow cover is usually less than 0.2 m and annual precipitation is in the order of 500‐600 mm of water equivalent. In the winters since 2002/2003 the frost layer has at most penetrated to a depth of 0.1 to 0.7 m which would not have any discernible effect on the closure backfill installed, even in the uppermost portions of the access tunnel and shafts, but in the future it is expected that frost penetration (and permafrost) will extend to greater depths. The near future (10,000 years after present) temperature estimations for the Olkiluoto site are currently being updated and the results will be used in updating the site evolution models. With respect to the effects of frost penetration at the Olkiluoto site, the maximum depth of permafrost penetration that could occur based on the climatic lines of evolution (Pimenoff et al. 2011a, b) is less than three hundred meters (Hartikainen 2012). This is well short of the disposal depth of ~420 m but has the potential to affect the closure system in the uppermost parts of the backfilled access ways and so the effects of frost needs to be considered, as do other hydraulic, mechanical and chemical changes induced by climatic change. 3.6 Design specifications for closure In order to fulfil the performance targets and design requirements (Table 3-1 and 3-2, respectively) for the closure, design specifications concerning closure backfill and plugs have been set and are provided in Table 3‐3. The specifications are derived from Olkiluoto-specific conditions and climatic evolution estimate. Table 3-3 presents the

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specifications concerning both backfill and plugs, then specification concerning only closure backfill and then the closure plug design specifications. Table 3‐3. Design specifications, which are needed to meet the performance and design requirements provided in Tables 3-1 and 3-2, for closure backfill and plugs.

Design specifications (for both closure backfill and plugs)

Crushed rock, boulders, stones, and concrete are the main material components in the closure of the uppermost parts of the facility and investigation holes extending to ground surface.

The materials for closure components shall be selected in such a way that the closure components limit the extrusion of the deposition tunnel backfill to acceptable level after deterioration of the deposition tunnel plug.

The amount of organics, oxidizing compounds, sulphur, and nitrogen compounds in the closure components shall be limited.

Design specifications (concerning only closure backfill)

Grain size distribution and mineralogy of the rock materials utilized in the backfilling shall be chosen so as to resist erosion.

The water conductivities of backfill in different parts of the facility and investigation holes shall be sufficiently low to enable natural groundwater flow characteristics to be restored after closure.

Compaction of backfill shall be taken into account in design for instance by using sufficiently uncompressible materials in underground openings.

Design specifications (concerning only closure plugs)

Glacial erosion shall be taken into account in the design of the intrusion obstructing structures and materials.

Hydraulic plugs are utilized in the hydraulic isolation of the highly transmissive zones.

Hydraulic isolation of the hydraulic plugs is mainly based on swelling clay materials.

Design specifications of the concrete parts of the plugs shall be defined once the locations of the plugs are known and the loads from adjacent backfill and host rock environment can be determined.

Low pH concrete mix is used in the closure components composed of concrete and located below HZ20.

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4 CLOSURE CONCEPTS FOR A DISPOSAL FACILITY 4.1 Closure concepts In order to accomplish effective disposal facility closure a series of requirements were developed by Posiva for the closure (Section 3.1). To be able to fulfill these requirements, the disposal facility beyond the deposition tunnels is divided into volumes that reflect the variability of the geosphere surrounding these openings. These volumes will have backfill installed that provides isolation that is compatible with the surrounding rock mass. These separate volumes rely on plugs to separate them from each other, and may utilize installation techniques that reflect the needs and requirements of each of these volumes individually. Given the longitudinal and vertical range for the non-deposition tunnel volumes it can reasonably be expected that conditions outside the deposition tunnels will vary more that within the repository volume. Hence a uniform, tight backfill might not perform as well (in disposal facility other than the repository) as a multi-component solution under certain circumstances. The basic approach utilised is to use plugs to separate adjacent backfilled areas where differing conditions and requirements exist. The components of backfilling, as well as concrete plugs, have been and continue to be tested in experiments and demonstrations undertaken at various laboratory and underground research laboratories. Section 4.5 summarizes some of the most-relevant results of demonstrations. The results of this work will be incorporated into the final design for closure to be prepared once the underground openings associated with Posiva’s disposal facility have been excavated and characterized. The closure concept calls for treating the disposal facility openings as a series of volumes, each separated by plug structures, shown in Figure 4-1. This divides the disposal facility into a series of essentially isolated volumes, each surrounded by rock of differing hydraulic, mechanical or hydrogeochemical characteristics. This approach requires that the backfill installed within each of the volumes have compatible thermal, hydraulic, mechanical and chemical characteristics to the surrounding rock mass. Each of these regions would be separated by a plug, which has long-term functions, that is intended to provide effective isolation to each of the compartments. It should be noted that Figure 4-1 is conceptual and more detailed closure design will be presented in Closure Production Line report, POSIVA 2012-19.

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Figure 4-1. Generic illustration of closure options for ONKALO and vehicle connections with potential plug locations, and potential backfill materials conforming to the closure requirements. More detailed closure design will be presented in Closure Production Line report, POSIVA 2012-19. This sectioning type of backfill concept means that a combination of backfill and plugs would be used to accomplish isolation of the disposal level from the biosphere. The main reasons why stepwise closure solutions are considered to be potentially appropriate are: If portions of the disposal facility need to be locally closed during the operational

phase, mechanical plugs are needed to separate closed areas from open ones. Volumes that include different underground openings or structures within them,

such as access tunnels, shafts or fracture zones can be divided in a manner that separates them from each other so that they do not connect or create potential groundwater flow paths through the disposal facility.

Hydraulic plugs isolate features with high hydraulic conductivity from the other underground openings and act to avoid possible hydraulic backflows through the disposal facility.

Human intrusion is discouraged by mechanical (concrete) plugs in the upper parts of the access routes together with difficult to remove boulder or cobble backfill. These components form a human intrusion obstructing plug.

SKB has considered an alternative approach to closure that relies more on the use of the deposition tunnel backfill type. In this approach the material does not alter from underground opening to another but the block and pellet method would be implemented more thoroughly (SKB 2010). Figure 4-2 shows the proposed approach for backfilling in the central tunnels and transport tunnels for the SKB repository (central and access tunnels respectively in terminology used by Posiva). The facility has been divided into the three main areas: 1) main tunnel and transport tunnels, 2) central area and access tunnel, and 3) shafts. The closure principles for access tunnel and shafts, as well as main and transport tunnels, are based on the deposition tunnel backfill solution.

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Figure 4-2. Outline of the SKB reference design for closure of main and transport tunnels and the central area (SKB 2010). The central area, excavations associated with underground operations but not directly connected to access tunnels, shafts or other openings that could provide a shorter transportation paths and the uppermost regions (-50 to -200 meters), are anticipated to be backfilled with crushed rock material using in situ compaction. In both the SKB and Posiva versions of the KBS-3V disposal facility method, means of discouraging unintentional intrusion to a closed disposal facility are required. From this need follows that the closure of the upper part of the access tunnels, shafts and boreholes shall significantly hamper unintentional intrusion into the facility and so different approach to backfilling that region is necessary. The surface connections (uppermost ~50 meters) will therefore be blocked with coarse rock blocks in order to obstruct the unintentional intrusion into the facility. 4.2 Closure components As described above, the closure concept for the disposal facility in Olkiluoto allows for the use of different backfill materials (and techniques) based on the site conditions and the results of safety assessments where differing fill materials (and structures) are used.

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Backfills may be installed as precompacted blocks and pellets or as in situ compacted fill. In regions beyond the disposal level and in isolated volumes alternative materials are considered, and are likely for the most part to be in situ compacted. A range of materials has been identified as being potentially usable for backfilling of the lower regions of a disposal facility (central and access tunnels), using both precompacted block – pellet and in situ compaction. The material options include, but are not limited to, clay-only and clay–aggregate mixtures, with the potential of using crushed rock in certain locations. Some of the fundamental characteristics of the backfill (clay and clay‐aggregate) materials considered for use are presented in Section 4.3. There are foreign materials that will be utilised as part of the closure system. These include grout, concrete or plugs that utilise both concrete and clay components. These structures will be expected to serve a variety of mechanical, hydraulic or hydraulic-mechanical purposes, depending on their locations. Closure structure locations and functional requirements are site dependant and will require careful evaluation of their roles, performance assessment requirements and design considerations. On a very basic level, there is a need to identify the locations, volumes, numbers and types of concrete-based closure structures (plugs) in order to estimate the quantity of concrete that remains in the disposal facility and thereby estimate their effect on the pH of the surrounding environment. The general approach to designing various concrete-based closure components for tunnel and shaft plugging is discussed in Section 4.4. 4.3 Backfill materials and installation methods 4.3.1 Background Based on the requirements outlined in Chapter 3 and the backfilling concept described in Section 4.2, the backfill materials outside of the deposition holes and tunnels need to be physically, chemically and mineralogically stable and have analogues of long-term behaviour in nature. This means that the available backfill materials are mainly limited to clays (including bentonites), rock material with moraine-type grain size distribution, sands, gravels and other possible crushed aggregates. The chosen materials, or suitable replacements must be available during the whole operating phase. There are a number of other important aspects associated with closure design, especially related to the technical feasibility. This includes consideration of the installation method(s) chosen and the geometry of the excavations to be backfilled. How the different backfill materials tolerate water inflow during their emplacement is also a key consideration in backfill design. Inflowing water may erode the already emplaced backfill and move the material around in the tunnel or into the fracture network. The quality of the blasting and rock conditions (irregularities in walls and ceiling and the depth of the EDZ zone) can also in some cases affect the method of backfilling that must ultimately be used. The logistics of the backfill operations and the equipment should also be compatible with those used elsewhere in the disposal facility in order to avoid availability and timing conflicts. Plugs installed in these regions must keep the backfill in place,

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withstand the loads due to the swelling of bentonite or bentonite-rich backfill as well as from hydraulic pressures developed, within isolated volumes. During closure activities these plugs must also hinder the water leakage from the backfilled volumes into regions that are still open. In underground openings, where low hydraulic conductivity is required (e.g., central tunnels, access tunnels, shafts), the backfill material should have enough swelling capacity to prevent the formation of hydraulic flow paths. Therefore, in these areas, swelling clay materials should be used. 4.3.2 Backfill material composition and production From information about backfill material studies for deposition tunnels (i.e. Hansen et al. 2009, Johannesson et al. 2008, Keto et al. 2006, Riikonen 2009, Sandén et al. 2008) it is possible to select materials and compaction options that will meet the performance requirements for backfills in various parts of the facility. Similarly, technological development and demonstrations associated with deposition tunnel backfilling have direct application to closure of other openings. For regions beyond the central and access tunnels, there is expected to be a reduced hydraulic requirement for the backfill. As a result, the composition of the backfill can be altered and in some regions, crushed rock or a mixture containing very low clay content may prove to be suitable. For such materials, existing literature information regarding compaction and hydraulic behaviour can be utilised. As part of the work to develop options for deposition tunnel backfills, materials were identified that would meet the hydraulic and swelling pressure requirements for these regions (Gunnarsson et al. 2004; 2006), but not for deposition tunnels. Examples of crushed rock and low-clay content backfilling in disposal facility-like conditions are provided in Section 4.3.3. Backfill composition The composition of the materials to be used as backfill at the disposal level and elsewhere has not been fixed and a variety of compositional options are under consideration. Backfill composition may also vary depending on the location and geometry of the openings to be closed. As a result there is a need to have a means of quickly identifying the potential suitability of candidate materials and then confirming their performance. As the majority of the underground openings are produced using drill and blast techniques (producing uneven tunnels) it will be important to ensure that the backfill installed in a given location meets the performance requirements defined for the backfill. Although specifically intended to be applied to deposition tunnel backfilling, an example of work intended to identify potentially suitable materials for deposition tunnel backfill was presented by Johannesson and Nilsson (2006). Those calculations allowed for assessment of the effect of varying the over-excavation volume on the swelling pressure and hydraulic conductivity developed by deposition tunnel backfill exposed to a groundwater containing 3.5 % TDS. The same approach can be applied to the design of closure backfill in volumes beyond the deposition tunnels once performance criteria are established for those regions.

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Of key importance when considering the required degree of block filling or degree of in situ compaction needed in order for a particular backfill formulation to meet a specified set of performance properties. It needs to be remembered that behaviour is dependent on the density and smectite content of the component materials (blocks, pellet fill or in situ compacted backfill). Figure 4-3a and Figure 4-4a show some of the measured hydraulic conductivity and swelling pressure values for several candidate backfill materials in the range where they might be suitable for use in the central and access tunnels and lower regions of the shafts. These data indicate that material used, density achieved and groundwater composition will affect the swelling pressure developed and the hydraulic conductivity of backfill materials to varying degrees. Each material has a unique dry-density versus hydraulic conductivity (or swelling pressure) relationship, which makes generic design and material evaluation difficult. In order to try and develop a means of quickly estimating the likely behaviour of various backfill material compositions and clays means of normalising the swelling pressure and hydraulic properties against a generic material parameter. A means to produce such a generic relationship for general design purposes is the Effective Montmorillonite Dry Density (EMDD) parameter. EMDD makes the assumption that both the hydraulic and swelling pressure developed in materials containing a substantial swelling clay component (>~25 % smectite) will be controlled by the smectite component with the other clay, sand and aggregate components acting as inert filler (Dixon et al. 2005). The relationships developed using this parameter and pre-existing laboratory data for potential backfill materials are shown in Figure 4-3b and Figure 4-4b. The EMDD parameter (Dixon et al. 2005; 2011a, b) is a refinement of the relationship between clay content and material behaviour reported by Dixon (2000), where behaviour of bentonite-aggregate mixtures was described in terms of the dominant roles of the clay component in determining swelling and hydraulic behaviour, allowing direct comparison of clay-only and clay-aggregate materials. In Figure 4-3 and Figure 4-4 the originally unique relationships between density and swelling pressure or hydraulic conductivity for each material or admixture composition has moved towards a generic relationships where EMDD describes the behaviour of a range of materials for a given porewater composition. The data shown in these figures still retains a degree of scatter, especially for the clay-aggregate mixtures, which may be explained by uncertainties regarding the exact aggregate or smectite contents and the small database used. The data also shows that as expected, changing the porewater composition from a 3.5 % to a 7 % solution results in a slight increase in hydraulic conductivity and a reduction of swelling pressure.

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y = 2E‐07e‐0.011x

R² = 0.8349

y = 2E‐07e‐0.011x

R² = 0.7673

1.E‐12

1.E‐11

1.E‐10

1.E‐09

500 600 700 800 900 1000 1100 1200 1300

Hydraulic Conductivity, k (m/s)

Effective Montmorillonite Dry Density, EMDD (kg/m3)

3.5% TDS 7% TDS

1.E‐12

1.E‐11

1.E‐10

1.E‐09

800 1000 1200 1400 1600 1800 2000

Hydraulic Conductivity, k (m/s)

Dry Density, (kg/m3)

40/60 mix (3.5%) 40/60 mix (7%)50/50 mix (3.5%) 50/50 mix (7%)F‐clay (3.5%) F‐clay (7%)Milos B (3.5%) Milos B (7%)

(a) EMDD relationships (b) Dry density relationships

Figure 4-3. Examples of the effects of clay type, groundwater composition and achieved dry density on the hydraulic conductivity of various backfill materials (including clay-only and bentonite-ballast mixtures (data from Hansen et al. 2009)) and recalculated to show EMDD relationships. (%-values indicate the TDS in the fluid used in testing e.g. 3.5 %=35 g/l).

y = 1.4583x ‐ 755.05

y = 1.4012x ‐ 806.07

0

200

400

600

800

1,000

500 600 700 800 900 1000 1100 1200 1300

Swelling Pressure, Ps (kPa)

Effective Montmorillonite Dry Density, EMDD (kg/m3)

3.5% TDS 7% TDS

(a) EMDD relationships (b) Dry density relationships

Figure 4-4. Examples of the effects of clay type, groundwater composition and achieved dry density on the swelling pressure developed by of various backfill materials (including clay-only and bentonite-ballast mixtures (Hansen et al. 2009)) and recalculated to show EMDD – swelling pressure relationship for EMDD <1000 kg/m3. (%-values indicate the salinity of the fluid used in testing e.g. 3.5 %=35 g/l).

Backfill compaction techniques Backfilling concepts being considered for use in the central tunnels of Olkiluoto disposal facility use the same basic material formulation and emplacement techniques as are being considered for backfilling of the deposition tunnels. This involves use of precompacted blocks with surrounding pellets and a foundation layer or else in-situ compaction if the backfill can be placed reliably and efficiently. A third approach

0

200

400

600

800

1,000

1,200

1,400

1,600

800 1000 1200 1400 1600 1800 2000

Swelling Pressure, Ps (kPa)

Dry Density, (kg/m3)

40/60 mix (3.5%) 40/60 mix (7%)50/50 mix (3.5%) 50/50 mix (7%)F‐clay (3.5%) F‐clay (7%)Milos B (3.5%) Milos B (7%)

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involving blowing backfill into the openings is also a potential means for material placement in the regions beyond the deposition tunnels.

Use of block backfilling has advantages associated with consistency of placement and quality control of the as-placed materials. In-situ compaction is attractive for use in areas such as large chambers where block installation may be problematic or shafts where access is vertical and in-situ compaction can be readily implemented. Air-entrained placement (blowing) of backfill into the tunnels is also of potentially applicable in regions where stringent density specification is unnecessary (e.g. service areas that do not provide any connection between the deposition tunnels and the access tunnels or shafts. In addition to these backfill placement options, Posiva and SKB have examined a variety of other technologies for potential application in a disposal facility, but most had issues related to them that precluded their use. The results of studies intended to identify potentially suitable backfill materials are summarized by Gunnarsson et al. (2004, 2006), Dixon and Keto (2008), Keto et al. (2009) and Hansen et al. (2009).

Ultimately the choice of materials and the backfill method (blocks versus in situ backfill versus blowing) and the design specifications will need to be assessed based on the specific density requirements for the region(s) of the disposal facility to be backfilled. Knowledge of the practically achievable densification of each material and method will be needed in order to select a specific material for use in a particular area. Precompacted blocks and clay pellets Deposition tunnel backfilling using the block and pellet approach involves placement of precompacted blocks of swelling clay (or clay-aggregate blends) to fill the majority of the volume of a tunnel or other opening (Figure 4-5). The gaps remaining between these blocks and the surrounding rock are subsequently filled with precompacted pellets of swelling clay (Figure 4-6). Development of this approach and materials that are potentially suitable for use in deposition tunnel backfilling have been documented in a number of reports (Gunnarsson et al. 2003, 2004, 2006, Keto 2006, Keto and Rönqvist 2006, Dixon and Keto 2008, Keto et al. 2009, Hansen et al. 2009). From these studies comes a large body of information and experience that can be readily transferred to closure activities in the regions beyond the deposition tunnels. Much of the work associated with deposition tunnel backfilling using blocks has focused on use of high swelling capacity clay (bentonite) as the raw material in their production. If this type of block and pellet backfilling method is to be transferred to disposal facility areas beyond the deposition tunnels there are also other material options available, including low smectite content clays, and clay-aggregate mixtures (Keto et al. 2009). As the swelling pressure and hydraulic conductivity of backfill is very dependent on the smectite content of the clay and the density to which it is compacted, it is still possible to achieve low permeability and maintain a swelling capacity using these blended materials. The blocks could be compressed to required density using the uniaxial compression method, as it is a cost-effective and fast method for conventional swelling–clay block production (Pusch 2002a, b, Holt & Peura 2011, Koskinen 2012). The pellets could be manufactured for example by compaction or extrusion, from which extrusion is a potential method to be used in Olkiluoto disposal facility (Dixon et al. 2008a, b, 2011a, Riikonen 2009).

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The conduct of backfilling field trials and mock-ups has established that block and pellet backfilling will typically achieve an as-placed density that is sufficient to meet the requirements of the deposition tunnel backfill (Dixon et al. 2009, 2011b). The block and pellet technique has the advantage of being more readily quality-controlled with respect to in situ backfill density (Wimelius & Pusch 2008), a factor that is of particular importance in the deposition tunnels. From these studies comes information that can be transferred to the backfilling of underground openings beyond the deposition tunnels. The application of the block and pellet approach to deposition tunnel backfilling continues to be evaluated with respect to its ability to withstand severe hydraulic conditions (Dixon et al. 2011a,b, Keto et al. 2009) and will ultimately need to be demonstrated under repository conditions. Similar issues exist regarding the mechanical durability of some of the backfilled regions beyond the deposition tunnel and these should be addressed by the results of ongoing work on deposition tunnel backfilling. For areas beyond the deposition tunnels, installation of blocks having more modest swelling and hydraulic properties may be technically appropriate and cost-effective, especially for a backfilling concept that adopts an approach of matching backfill to surrounding rock characteristics.

Figure 4-5. Block and pellet backfilling and trials to test installation of components (Wimelius & Pusch 2008)

Figure 4-6. Examples of precompacted blocks and clay pellets.

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While the block and pellet backfilling technique has advantages with respect to as-placed quality control and achieving very high efficiencies of backfilling, there are some weaknesses associated with its use. The block and pellet backfilling technique involves a considerable level of technical complexity. The excavated floors need to be carefully levelled mechanically or using bentonite pellets/granules (essentially an in situ compacted layer), which must be carefully monitored so that subsequently installed blocks are physically stable and not subject of collapse (also the more tilted tunnel the more problematic levelling). The installation of these components involves use of specially designed handling equipment. Additionally, there is considerable cost in the manufacture of backfill blocks, an entire material production, storage and handling facility would also need to be set up and operated in order to provide pre-compacted backfill blocks. In situ technique The in situ compaction is especially attractive as a method of installing backfill, especially in vertical excavations (shafts) and other very large openings and may be also applicable in tunnels (especially in cases with considerable tilt). In situ compaction using clay-aggregate mixtures has been used in a number of large-scale mock-ups at the Äspö facility (Gunnarsson et al. 2003) and also in a full-scale shaft plug installed in the course of closing the Canadian URL (Dixon et al. 2009). There are challenges to use of in situ compaction in horizontal openings (e.g. central tunnel, service rooms, and access tunnel); especially with respect to ensuring adequate density in the regions closest to the roof. In situ compaction therefore provides a flexible installation method and utilises more conventional placement and compaction technologies but has some limitations with respect to achieving substantial as-placed density in the crown regions (as does the block and pellet method of backfilling). In situ compaction can be done as sloped or horizontal layers (Figure 4-7) and can be used in the installation of bentonite-aggregate mixtures or natural clay material. Pure clay materials are more challenging in achieving the adequate density and tend to be more susceptible to disruption by water flow. When using horizontal layers the ceiling area will need to be filled with some other method, like blowing pellets or other fill materials into it. The compaction method and materials will depend on the hydraulic conditions and performance requirements of the backfill in each region being filled. In situ compaction of bentonite-aggregate backfill has also been successfully demonstrated in a vertical shaft (Dixon et al. 2009) so it is reasonable to assume that this technique could be used in closure of the vertical shafts of a disposal facility. The in situ method has been tested in various field tests by SKB, most recently in the Prototype repository test performed at Äspö HRL, where the basic construction and compaction process was documented in Gunnarsson et al. (2004). In Finland, the field test at Riihimäki also examined in situ compaction with different compaction methods as a means of installing backfill (Korkiala-Tanttu et al. 2007). Börgesson et al. (2009) examined the ability of various clay materials and clay-aggregate mixtures to meet predefined, study-specific swelling pressure and hydraulic conductivity criterion (200 kPa swelling pressure and k<10-10 m/s) where tunnel backfilling was done using in situ or block and pellet approaches. It was determined that there were a variety of clay and clay-aggregate materials that could potentially meet those pre-determined

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performance specifications. From these results confidence can be gained that there are a variety of materials and installation options that could be workable in the backfilling of openings beyond the deposition tunnels. Compaction trials for materials containing <40 % clay content indicate that in situ compaction is a viable option for regions beyond the deposition, central and perhaps the lower access tunnels. In the regions beyond the disposal level, backfill behavior equivalent to what is needed in the deposition tunnels is not anticipated to be required. This will have a notable effect on backfill preparation, cost and installation. How changing backfill material composition may be applied to Posiva’s disposal facility concept for the Olkiluoto site is discussed in Chapter 5.

Figure 4-7. In situ compaction (Gunnarsson et al. 2004, Dixon & Keto 2008.) Air-entrainment (shotclay) method of backfill placement Placement of mixtures of bentonite and crushed rock, bentonite pellets or aggregate-only backfill can be achieved using conventional shotcrete (also known as gunnite) equipment. Trials completed using this technique produced promising results but development is needed in order to improve the consistency of the as-placed materials. This is however a conventional industrial method for rapid placement of backfill. This is of relevance at locations within the disposal facility where there may be difficulties in achieving adequate densification due to opening geometry or the volume to be backfilled does not require installation of anything more than a nominal fill that provides roof support, offers some nominal reduction in the permeability and occupies the volume to prevent adjacent backfill from swelling into it. Two spaces where nominal backfilling could be undertaken represent extremes with respect to surrounding rock quality:

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The first is a service room, at or near disposal level, which leads only to the access/central tunnel and is surrounded by tight rock. In such areas of the disposal facility the use of in situ compaction of clay-aggregate materials, while possible could be time consuming and the performance of the overall disposal facility would be unaffected by use of high-quality (swelling or low hydraulic conductivity) backfill. An approach that was developed for use in confined volumes is described by Dixon & Keto (2008) and involves blowing aggregate or aggregate-clay mixtures using a conventional gunnite/shotcrete technology (Figure 4-8). This technique is the same as proposed for use in placing the pellet component of the block and pellet backfill proposed for use in deposition, central and access tunnels. It has been demonstrated as being technically feasible for filling of tunnel mock-ups (Martino & Dixon 2007) and in field applications where tunnel plugs were constructed (Chandler et al. 2002, Martino et al. 2008). This technique has the potential to place large volumes of relatively stiff and hydraulically resistant backfill in a rapid manner.

The second type of region within the closure system where high swelling capacity of backfill is of limited importance is those locations where the rock is extremely conductive (e.g. a fracture zone). In such locations, placement of a low permeability/highly swelling backfill would not serve any particular function with respect to retarding mass transport since on contact with such a feature, the contaminant migration would become fracture-system controlled. Therefore the use of less-dense backfill is acceptable as long as the feature is then isolated from volumes needing high-quality, dense backfill.

In both of these locations it may be as effective in a performance sense to use a backfill which can be quickly placed, at a modest density rather than investing considerable effort (cost) to fill the volumes with low permeability, swelling backfill. The appropriateness of such an approach has been evaluated in preliminary hydraulic modeling done as part of initial evaluations of the closure concept for the Olkiluoto site (Section 5.1). SKB reference backfill As noted in Section 4.1, the backfilling method of SKB calls for the use of precompacted clay blocks placed such that they fill the majority of the excavation volume. Remaining volumes would be filled with precompacted clay pellets as described by Wimelius & Pusch (2008), Keto et al. (2009) and Hansen et al. (2009). The option also exists to use in situ compaction for installation of a compositionally similar backfill in tunnel and shaft openings (Gunnarsson et al. 2004, 2006).

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Figure 4-8. Installation of bentonite-aggregate backfill material into rough-surfaced mock-ups (Martino & Dixon 2007) and excavation at 420 m level at AECL URL (Chandler et al. 2002, Martino et al. 2008). 4.3.3 Other alternative backfill materials and installation methods The primary options for backfill and backfill placement in the Posiva and SKB disposal facility concepts have been briefly summarised in Section 4.1. Beyond these options a review of alternative approaches and materials was undertaken by Posiva in order to identify any other potentially viable means or materials that could be used in the backfilling of a disposal facility (Dixon & Keto 2008). Other evaluations associated with attempts to identify potentially suitable backfill materials are summarized by Gunnarsson et al. (2004, 2006), Dixon & Keto (2008), Keto et al. (2009) and Hansen et al. (2009). Although there are alternative materials, all the technically and environmentally feasible options thus far identified include use of some form of aggregate and/or clay for backfilling where low permeability and some degree of self-sealing was needed. 4.4 Plugs within the disposal facility 4.4.1 General Discussion of backfilling and sealing of a generic disposal facility provided in the preceding sections of this document has focussed on the backfill materials that will occupy the majority of the excavated volume in a disposal facility. Associated with the backfill materials used in closure, there will be rigid structures whose role will be to: volumetrically constrain backfill, transition from one backfilling region to the next, or provide tight hydraulic/mechanical seals at critical locations within the disposal

facility (e.g. adjacent to major hydraulic features). As can be seen in the description of the disposal facility backfilling concepts provided in Section 4.1, there will be a wide range of opening sizes and shapes present at the disposal level and in the remainder of the facility. Each of these openings will ultimately need to be backfilled and in key locations plugs installed, providing a

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challenge to the designers responsible for the closure. Adoption of an approach to deal with these openings that is both flexible with respect to design and installation is a necessity. Section 4.3 described materials and methods that could be used to accomplish backfilling of the majority of the openings associated with the repository. Those materials must then be matched with appropriate sealing structures (plugs). While the general layout of a disposal facility and the anticipated conditions that might be encountered were presented in Chapters 2 and 3, it is also necessary to consider the type of plug that will be needed at each critical location within a facility (see Chapter 5 for Olkiluoto-specific concept). From this information it will be possible to develop a general sequencing of the closure process and identify what the various materials and structures installed are required to do. 4.4.2 Plug functions In order to isolate key regions of the disposal facility a variety of plugs will need to be installed in association with the backfill(s). Based on the functional purpose of a plug, its depth and the local environment (current and future), the size, shape and design will vary. For some plug locations there may be a need to cut off the Excavation Damaged Zone (EDZ) where the hydraulic conductivity is discernibly higher than in the surrounding rock mass and hydraulic isolation is a primary need. The depth of the EDZ depends mainly on the excavation technique used, the geometry of the blasted area and the in situ rock stress conditions. The EDZ can be disconnected at the location of a plug by cutting the rock by using water jet, boring, sawing or other techniques to interrupt the feature, provided that the technology used for this is not itself damaging. Time- or stress-induced degradation of the exposed rock surrounding the plug could compromise the as-built performance of the plug. Excavation of each of the keys needs to have disconnected the EDZ at the location of plugs. Plugs will vary in their structure, form and dimensions depending on the environment where they are located. A general description of the closure plugs is provided below. More detailed discussion of how these and other plugs would be installed in an Olkiluoto-specific environment is provided in Chapter 5. For discussion and design purposes, closure plugs are divided into the following general functional categories: 1. mechanical plugs, 2. hydraulic plugs, and 3. intrusion obstructing plugs. Figure 4-9 shows how these plugs might be distributed in the regions beyond the central and deposition tunnels within the Olkiluoto disposal facility and this is discussed in greater detail in Chapter 5 where the site-specific aspects that will drive plug location and type are evaluated.

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Figure 4-9. Conceptual illustration showing types and locations of plugs in access tunnel and shafts at ONKALO after closure (deposition and central tunnels not shown). The long-term evolution of the disposal facility and geosphere will impact the performance and durability of the plugs installed within it. It is therefore necessary to evaluate what these effects will be and take these factors into account when designing and locating them. This is of particular importance in terms of the operational and immediate post-closure phases when most of the maturation process associated with cementitious materials occurs and a wide variety of thermal, hydraulic, chemical and mechanical processes will be active. Examples of some of the key processes over the short to long term are: Water and gas transport may occur through or adjacent to the plugs and at least

some highly alkaline porewater leaches out of the concrete. Prior to saturation of the regions adjacent to the plugs, air is present and this may

cause carbonation of cementitious materials. Once saturation is achieved and the system evolves towards an anaerobic

environment these processes will be substantially slowed. Microbes may also act on cementitious plugs, leading to local corrosion. Degradation of cementitious materials due to water flow through the cement matrix

of concrete under the dry rock conditions expected for much of the Olkiluoto site is assumed to be very slow. Once the site has reached hydraulic equilibrium following closure there will be only a very small hydraulic gradient present and hence little potential for hydraulically-driven concrete degradation.

Corrosion of steel components in steel-reinforced concrete structures. By the end of the post-closure thermal phase, the cementitious materials are

assumed to be gradually degraded via complicated chemical processes associated with reactions with the surrounding groundwater.

Additionally, during the glacial phase plugs located in the upper part of the disposal facility are subjected to freezing-thawing cycles and permafrost which is known to cause mechanical degradation of concrete materials.

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4.4.3 Specific locations requiring plug installation Preliminary geometries of the various plug options have been developed and generic sketches of the plugs have been developed for the following functions and they would be located as required during disposal facility closure activities. Mechanical plugs Mechanical plugs will be used to separate different parts of tunnels from each other during the operation of the disposal facility in order to cut water circulation routes and act as a supporting structure for the backfill behind it. The service life of these kinds of plugs is intended to persist as long as the volume on one side of it has not yet been backfilled (estimated to be ~100 years). After this the mechanical plug acts as tunnel fill and no voids will form due to it. Figure 4-10 shows how this mechanical plug is expected to function within the disposal facility tunnel. The material used in the mechanical plugs can be standard pH or low pH concrete or metal depending on the requirements in the particular environment where the plug will be used. In many respects these plugs have similar function as the compartment plugs proposed for use within the deposition drifts in the KBS-3H concept (Autio et al. 2008).

Figure 4-10. An illustration of two possibilities for the mechanical plug.

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Mechanical plugs may be used in central tunnels, technical rooms, access tunnels and shafts. These locations represent a large number of different shapes and dimensions and so their design needs to be very adaptable. They can be used at depths below surface ranging from a few tens of meters down to the deepest point of the facility. This means that they will be used in a wide range of environments, unless limitations or requirements for specific locations are identified. The properties, materials and dimensions of plugs may vary considerably based on their installed location and so they could be used in all environments of tunnel closure, except in the deposition tunnels. Hydraulic plugs Hydraulic plugs are meant to prevent or cut off groundwater flow in specific locations or along a particular flow route for periods well beyond the operational phase of the disposal facility. Their main purpose is to preserve the groundwater chemical conditions in a state that is favourable for spent nuclear fuel disposal and for the time required for spent fuel isolation. This maintaining of compatible groundwater conditions will enable the engineered barriers system to function as expected during the service life of the facility. Hydraulic plugs are anticipated to consist of a sequencing of swelling materials with very low hydraulic conductivity, standard pH or low pH concrete, other fill materials and filter layer(s) (Posiva 2009b). One generic concept showing how hydraulic plugs might be installed in the access tunnel is illustrated in Figure 4-11. They may be used to confine hydraulically conductive geological features. Similar concept is also considered for isolation of hydraulically conductive features intersected by shafts.

Figure 4-11. Conceptual illustration of hydraulic plug to prevent groundwater flow.

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Intrusion obstructing plugs The last of the plugs that will be installed are those intended to hamper unintentional intrusion into the disposal facility. These plugs are intended to provide a means of preventing easy, inadvertent intrusion into the closed disposal facility in some distant time period when record of the site’s content have been lost. These plugs will be used in mouth of the access tunnel and upper-most portions of the shafts as shown in Figure 4-9 (Posiva 2009b). An example of how such a plug might be installed in the entrance of the access tunnel is provided in Figure 4-12. These plugs should be constructed of natural and commonly occurring materials, for example rock boulders and concrete. The dimensions and mechanical properties of these plugs are dependent on the expected weathering rate due to the glaciations (freezing-thawing cycles, erosion due to melt waters) or climate changes. The size and shape of the materials installed also need to be such that they cannot be readily removed by any of these processes or by unsophisticated technologies.

Figure 4-12. Intrusion obstructing plug in the mouth of the access tunnel. 4.5 Laboratory and large-scale tests on the closure of a disposal facility While there have been no actual spent fuel disposal facility closure activities completed anywhere in the world, relevant information has been generated through laboratory-scale tests. There are also a large number of full-scale or nearly-full-scale simulations that have examined the closure. The largest volume of information has been associated with the near-field (buffer) or deposition tunnel regions of a repository but there are a number of tests that have examined materials and methods relevant to the regions beyond the deposition tunnel. 4.5.1 Laboratory tests Development of backfill and plugging materials of relevance to Posiva’s disposal facility method has been a subject of extensive study in Finland, Sweden and Canada. A brief listing of some of the most-relevant documents are provided in Table 4-1. While these references are not exhaustive, they provide evidence of the careful development process used to reach the current state of knowledge related to backfill design and engineering. Work is still ongoing to further refine material options and backfill installation techniques that are best suited to the Olkiluoto site.

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Table 4-1. Laboratory studies and fieldwork that have investigated the feasibility of aspects of Posiva’s disposal facility method.

Subject Details Reference

Backfill Material Evaluation and Design Studies

Design & selection Summary of work done as part of Posiva/SKB joint project Baclo III

Keto et al. 2009

Design & selection Evaluate deposition tunnel backfill options Hansen et al. 2009

Design & selection Buffer and Backfill process report (SR-Can) SKB 2006

Materials Review of potential alternative backfill materials and installation methods

Dixon and Keto 2008

Materials Material parameter effects on backfill compaction

Keto et al. 2006

Materials Material properties evaluation Johannesson 2006, 2008

Materials Wetting and homogenization of backfill Johannesson et al. 2008

Materials Erosion & sealing of backfill, lab testing Sandén et al. 2008

Materials Buffer and backfill materials properties Pusch 2002a,b, 2003

Concepts Evaluation of alternative materials and means of installation

Gunnarsson et al. 2003, 04, 06 Börgesson et al. 2009

Concepts Block option Keto & Rönnqvist 2006

Concepts In situ compaction option Keto 2006

Plugs Laboratory and other evaluations

Materials Effects of grouting, shotcreting and concrete on backfill geochemistry

Luna et al. 2006

Materials Concrete durability Martino 2006

Backfill Field-Scale Studies

Demonstrations Water uptake & flow through block-pellet mock-ups

Riikonen 2009

Demonstrations Water uptake and flow through block-pellet mock-ups

Dixon et al. 2008b,2011b

Demonstrations In situ compaction trials Korkiala-Tanttu et al. 2007

Demonstrations Half-scale mock up of block and pellet tunnel backfilling

Dixon et al. 2008a

Demonstrations Prototype repository SKB 2009

Demonstrations Backfill and Plug Test SKB 2009

Demonstrations Shaft backfilling Dixon et al. 2009 Martino et al. 2011

Demonstrations Deposition tunnel backfilling, block placement Wimelius & Pusch 2008

Plugs Field-Scale Studies

Concepts Low pH concrete plug Dahlström et al. 2009

Concepts Low pH self-compacting concrete plug Vogt et al. 2009

Concepts T-M analysis of plug for deposition tunnel Fälth and Gatter 2009

Demonstrations Tunnel Plug Test Stripa Mine Gray 1993

Demonstrations Plug II in Prototype Repository Äspö Dahlström 2009

Demonstrations Backfill and Plug Test SKB 2009

Demonstrations Prototype Repository SKB 2009

Demonstrations Tunnel Sealing Experiment Chandler et al. 2002 Martino et al. 2008

Demonstrations Enhanced Sealing Project Dixon et al. 2009 Martino et al. 2011 Holowick et al. 2011

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4.5.2 Field tests Field tests of particular relevance to the backfilling and plugging of a disposal facility in granitic rock have been completed by several organizations at a number of locations with a brief summary provided in Table 4-1. How some of these relate to Posiva’s disposal facility is briefly discussed below. Stripa Mine – Sweden, tunnel plug test (1982-1988) One of the first large-scale field tests to evaluate potential means and materials to seal a section of tunnel was completed at the Stripa Mine in Sweden. This test involved installation of an un-keyed plug composed of a composite concrete-highly compacted bentonite gasket in a section of tunnel within carefully-blasted tunnel section having a cross-section of 11 m2 (Gray 1993). This test did not use low pH concretes and was not as fully monitored as many of the later sealing tests completed elsewhere. The test did however identify the very substantial effect of local geology on the effectiveness of tunnel plugs. Despite of the evidence that the highly compacted bentonite did effectively seal the contact between the concrete and the rock, the seal was not particularly effective. In this test it was estimated that more than 90-95% of the water supplied to the upstream face of the plug escaped around the plug via fractures or other high-permeability features that intersected the tunnel (Gray 1993). The information gained at Stripa was valuable in planning later sealing and plugging demonstrations in other underground laboratories. It highlighted the need to carefully select the location and design of the plug. Installation of a very low permeability seal at Stripa did not sufficiently reduce flow out of the isolated region; it was the local geology that controlled mass transport. Hence installation of a backfill with a substantially lower permeability than the surrounding rock will not be particularly effective since contaminants will move through the higher permeability medium. Plugs that are expected to provide a long-term sealing function will need to be carefully chosen so that they work to reinforce the local hydrogeological system and are not just bypassed by regional or local flow paths. Äspö – Sweden, backfill and plug test The Backfill and Plug Test (BPT) shown in Figure 4-13 was constructed and put into operation in 1999 and is described by Gunnarsson et al. (2003) and Gunnarsson & Börgesson (2003). At the end of 2010 it was still functioning. This test was intended to examine the interactions between an in situ compacted backfill and the surrounding rock mass in a tunnel excavated using the drill and blast technique (SKB 2009). This also allowed for development of a technique to build tunnel plugs and test their functionality. The concrete plug is a cast structure installed in a 1.5 m-deep key and a gasket composed of highly compacted bentonite blocks.

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Figure 4-13. Backfill and plug test (BPT) at Äspö (SKB 2009). The 30/70 backfill installed in the BPT is a good demonstration of the advantages and disadvantages of in situ compaction of backfill as inclined layers, highlighting some of the issues related to inflowing water during backfill emplacement and also the potential for compartment backfilling. A higher permeability fill (crushed rock) was placed in a section where installation of a low permeability backfill would be unnecessary and then a carefully engineered concrete plug was installed in a location where two sections of tunnel having different hydraulic requirements meet. In the BPT the potential for a crushed rock fill to settle, and thereby generate an open gap at the crown of the tunnel was dealt with through use of a bentonite component at the upper part of the tunnel. Information gained from both of these backfill placement demonstrations has direct relevance to backfilling where in situ compaction is considered. Äspö – Sweden, Prototype repository experiment The Prototype Repository (PR) is installed in a 65-m-long section of tunnel at the -450 m level at the Äspö Hard Rock Laboratory (HRL) and it is intended to: test and demonstrate the integrated function of the final disposal facility

components under realistic conditions in full-scale and to compare results with model predictions and assumptions,

develop, test and demonstrate appropriate engineering standards and quality assurance methods, and

simulate appropriate parts of the disposal facility design and construction processes. (SKB 2009.)

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The PR consists of two sections of tunnel (Figure 4-14), isolated from each other by a concrete plug (Figure 4-15) intended to restrain the backfill as well as hydraulically isolate the internal sections. This allows for partial dismantling of the PR without adversely affecting the remaining inner region (Section I in Figure 4-14). It was intended at the time of construction that the inner portion of the PR be monitored for an extended period (up to 20 years, ~2020). The outer section was dismantled in 2011 and the results of this, once published, will provide valuable information regarding the evolution of not only the buffer, but also the backfill and plug. The backfill component in the PR is an in situ compacted mass of bentonite clay (30 % by dry mass proportion) and crushed rock (70 % by dry mass proportion).

Figure 4-14. Prototype repository test at Äspö: General layout (Dahlström 2009).

(Johannesson et. al 2004) (Dahlström 2009) Figure 4-15. Prototype repository test at Äspö showing concept and construction of a concrete plug.

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The self-compacting concrete plug installed in the PR is described in detail by Dahlström (2009). The design of the plug also included a concave shape on its downstream face in order to minimize its length and ensure uniform load transfer from the pressurized tunnel section to the rock. This required installation of an extensive steel reinforcing component so that its length could be minimized. URL – Canada The Underground Research Laboratory (URL) operated by Atomic Energy of Canada Limited from 1982 until 2006 and permanently closed in 2010, examined many of the aspects of backfilling and sealing relevant to Posiva’s disposal facility concept. Of particular note were the Tunnel Sealing Experiment, TSX (Figure 4-16), a joint AECL–JAEA–Andra–WIPP activity and the Enhanced Sealing Project, ESP (Figure 4-17), a joint NWMO–Posiva–SKB–Andra activity. The TSX involved installing two full-scale plugs/seals in a tunnel excavated so that there was a discernible stress-induced EDZ generated. Two structures were installed to contain a central, pressurized and heated chamber. Both were keyed into a section of essentially uniform rock, keying was done in an effort to cut off the EDZ. One structure was composed of mass-poured, unreinforced, low pH, low-heat, high-performance concrete. The other consisted of densely compacted blocks of bentonite clay and sand and it was mechanically restrained by a steel dome (Figure 4-16).

Figure 4-16. Tunnel Sealing Experiment (TSX) at Canadian URL.

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The movement of water into the chamber and also out the downstream face of these plugs was monitored over a period of several years (Chandler et al. 2002, Martino et al. 2008). Ultimately the central chamber was pressurized to 4.2 MPa and the water within it reached +85 ºC. Both of these seals proved capable of preventing substantial flow past them and demonstrated the ability to engineer and construct plugs that would perform effectively (Martino et al. 2008). This test has direct application to Posiva’s closure concept as it illustrates how effective a plug installed in competent rock can be in restricting flow from one compartment to the next. The Enhanced Sealing Project (ESP) at the URL was the result of an opportunity brought about by the closure of the Canadian URL. Full-scale shaft plugs of the types that might be used within Posiva’s disposal facility were completed at the URL in 2010 (Figure 4-17). These plugs spanned locations where the vertical excavations at the URL intersected a major water-bearing fracture (located at ~ -270 m depth), (Dixon et al. 2009). This is approximately the same depth as the HZ20 feature in ONKALO.

Figure 4-17. Joint NWMO–Posiva–SKB–ANDRA Enhanced Sealing Project (ESP) at Canadian URL. Installation of plugs bridging a major fracture feature at -270 m. In situ compacted shaft plug of 6 m length in a 5 m diameter main shaft and precompacted blocks in 1.8 m ventilation shaft.

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The plug in the main shaft at the URL consists of a 6-m long vertical section of in situ compacted backfill that is sandwiched between 2, 3-m-thick concrete segments that are keyed into the rock wall of the shaft. As part of the joint NWMO/Posiva/SKB/Andra ESP activities, water uptake and stress development are monitored (Dixon et al. 2009, Martino et al. 2011, Dixon et al. 2012). The backfill is a 60-40% aggregate-bentonite clay mixture. This material was installed to a density that exceeded that needed to ensure that the low-permeability (<10-10 m/s) and modest swelling pressure (<1000 kPa) defined for this installation. The backfill-type material installed in the ESP was mixed and compacted using conventional engineering techniques and equipment and was quality checked at all stages. Installation of backfill in the main shaft using in situ compaction was accomplished efficiently and achieved the target density without need of extraordinary quality control measures (Martino et al. 2011). The density achieved in the backfilled section of the shaft plug has a dry density of 1,750-1,850 kg/m3, corresponding to an EMDD of 1,030 kg/m3. This should result in a material that has a swelling pressure of >600 kPa and a hydraulic conductivity of <10-12 m/s. These properties would almost certainly meet and exceed the swelling pressure and hydraulic conductivity requirements of backfill for the lower portions of the shafts or disposal facility openings outside of the deposition tunnels. The installation also demonstrates the manner in which shaft plugs could be installed in Posiva’s disposal facility. After 3 years of monitoring the ESP, the initial curing of the concrete is complete, and the structures now have reached temperature equilibrium. The hydration of the clay component is progressing with saturation already achieved in the perimeter regions. Swelling pressures have started to develop and the groundwater table has begun to recover. Perhaps most importantly, hydraulic pressure monitoring shows that there is no open hydraulic connection between the top and bottom of the seal (Holowick et al. 2011, Dixon et al. 2012) with a hydraulic head difference of more than 20 m being maintained across the plug. The ventilation shaft plug identified in Figure 4-17 was constructed using precompacted blocks that were field fit to the opening rather than in situ compacted. Block installation was done in a very timely manner and it can be readily accomplished without the need for any specialized equipment. The evolution of this installation was not part of the ESP monitoring program, however given that the two shafts are connected at the 420 Level of the URL, it is reasonable to assume that it is functioning effectively since there is a pressure differential recorded across the main shaft plug. A leaky ventilation shaft plug would have meant that this pressure differential would not have persisted.

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5 APPLYING THE CLOSURE CONCEPT TO OLKILUOTO 5.1 Description of closure concept for Olkiluoto site Site-specific information collected in ONKALO and for Olkiluoto Island was presented in Chapter 3. It was used to identify the main bedrock domains in Olkiluoto and to identify a zone suitable for locating a repository. Each of these rock domains has relatively homogeneous hydraulic, geochemical and environmental properties and so can be handled as separate regions when assessing system performance. This information can also be used to develop backfilling and plugging approaches that best suit the site. This does not mean that the conditions within each domain will not change with time, but that each region should continue to show fairly uniform characteristics and this information can be used in closure planning and assessing potential future evolution of the site. The Olkiluoto site comprises three basic, depth-related areas and two that are related to environmental structures (HZ19 and HZ20), shown in Figure 5-1. Additionally, one area is operationally-related (e.g. intrusion obstructing plug). Closure is also defined by taking into account the conditions observed at Olkiluoto (Section 3) or derived from calculations intended to predict site evolution (e.g. chemistry and hydrogeology in the host rock change with depth, future potential permafrost penetration). These basic naturally occurring subdivisions in the rock volume at the Olkiluoto site provide a logical basis for selecting potential closure-related subdivisions at this site. The clearly differing zones at the Olkiluoto site also bring into question the usefulness of treating all the openings equally with respect to closure approach (e.g. filling them all using the same very low permeability backfill). By examining each of the natural zones within the site it is possible to subdivide the disposal facility into a number of regions and select backfill materials that are best suited for use in each of the regions and that are consistent with the adjacent geosphere. The effectiveness of closure components within each of these regions and how they meet the functional and other requirements of the closure system are summarized below. Evaluation of the closure system was based on the distance from the repository (deposition tunnels and deposition holes), host-rock properties and also taking into account the current hydraulic conditions and what is anticipated to exist in the future. They have also been used as the basis for a preliminary numerical modelling that examined the undisturbed and post-closure hydraulic characteristics at the proposed disposal facility location (Hartley et al. 2013). A brief description of the major subdivisions of the Olkiluoto site is provided below, additionally a preliminary estimate of the types of backfill that would be compatible with the local hydro-geologic conditions is provided.

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Figure 5-1. Distribution of regions with different backfills beyond the central tunnels in Posiva’s closure concept for Olkiluoto. HZ19 and HZ20 structures are presented to display their role in selecting backfill material. 5.1.1 Surface down to HZ19 (0 to -100 m) For the uppermost 100 meters the most important function of the backfill is to resist inadvertent human intrusion. Another important function for the upper part is to withstand the mechanical impacts of the glaciations and especially the erosion (upper 50 m). Features like giant’s kettles (small but very deep erosional excavation carved by glacier-derived boulders and fast-flowing water) could impact the uppermost regions of the disposal facility access tunnel or shafts and need to be considered when planning for closure. The overall bedrock surface in this region of Finland has been estimated to have been eroded an average of 8 m during the previous glacial period: i.e. about 8 m in 100,000 years (Okko 1964). According to Påsse (2004, p. 32) the erosion was only 1 m with 4 m local maximums. Based on this, the erosion at the Olkiluoto site needs to be taken into account when designing long-term intrusion resistant plugs and an average erosion of 10 m is conservatively assumed for the next glacial period (assumed to occur in the next 100,000 years). Material with good erosion resistance properties is the primary need for the near-surface fill materials since the near-surface rock is highly fractured and so provides little in the way of a hydraulic barrier. 5.1.2 Depth between HZ19 and HZ20 (~ -100 to ~ -300 m) The backfill in this region will need to have its hydraulic conductivity assessed such that it is able to accommodate the effects of permafrost as well as the changing hydraulic properties of the surrounding rock mass. It is also very important to ensure that the deeper, water-conducting feature HZ20 be effectively isolated from the underlying sealing system components. The shallower major feature HZ19 may be less critical in terms of its isolation from the access tunnel but this is to be definitively determined. The use of cement in sealing structures installed in this region needs to be controlled but

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according to Arenius et al. (2008), for the region above the HZ20 structure, the type of the concrete used is not critical. The reasoning for being able to use a wider range of cement types in sealing structures is that the areas above and below HZ20 are very different hydrogeologically. There is very limited natural interaction between these regions, and if no artificial flow routes exist or if they are closed and sealed effectively, this condition will be maintained. Ultimately the function(s) defined as being necessary for the closure components will determine the closure design adopted for this region. 5.1.3 Access tunnel below HZ20 structure (~ -300 to -420 m) At depth below the HZ20 feature, low hydraulic conductivity of the backfill is an important parameter. The backfill also needs to have sufficient swelling properties to resist development of preferential flow along tunnel or shaft walls. It has yet to be determined if the presence of substantial swelling capacity needs to be continuous throughout this volume or if it is only needed in certain key sections. Below the HZ20 the use of low pH materials for plugs is recommended since the presence of high pH materials may adversely affect the performance and longevity of the swelling clay component of the backfill. 5.1.4 Disposal depth - all excavations except deposition tunnels and holes

(below ~ -400 m) This lowermost level contains several regions where excavations have been made to accommodate a variety of operations: e.g. central tunnels, technical rooms and auxiliary facilities, connections to the shafts and pumping stations. The performance requirements of backfill and plugs installed in this region will vary depending on a number of factors, such as the distance to the deposition holes/tunnels and the presence of naturally-occurring (host rock fractures) or excavation-induced (EDZ) hydraulic connections between the regions of the disposal level. 5.2 Proposed Olkiluoto specific closure materials The layout and structure of the disposal facility proposed for the Olkiluoto site were provided in Chapter 2. A process of dividing the underground volume to regions is proposed for the site and this will involve installation of a range of materials and structures, which will be backfills and plugs as described in Chapter 4. From this general concept and the material evaluations, a generic evaluation of the closure concept was completed. Section 5.1 describes the results of initial concept evaluation and indicates that if several materials performance conditions can be met, the concept will be viable. How each of the regions in the underground disposal facility could be backfilled is described below. The details of the shape and construction of non-backfill structures (plugs) associated with the closure process were presented in Section 4.4. While the final definition of materials and methods will need to be formalized and adjusted as the properties of the underground openings and site-specific features are specified, a toolbox of potentially usable backfill materials and placement methods has been identified and is discussed below.

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5.2.1 Closure backfills Within each closure region there is a need to install backfill that is compatible with the hydraulic properties of the surrounding rock mass. Materials should be compatible also with any plugs they contact. In addition to backfill materials, the proposed approach for closing the disposal facility also requires installation of specialized plugs at key locations. Figure 5-2 shows the general locations of the plugs within ONKALO and the subsequently excavated disposal facility where they are expected to be needed. Based on this, different types of backfill that would be needed to accomplish disposal facility closure using Posiva’s closure concept are identified. The basic closure backfill types can be broken down into four types, each of which are briefly described in Table 5-1 and discussed below. The underground openings to be backfilled can be divided based on the depth as discussed in Section 5.1, but local deviations may also exist as the result of distance from the repository and the nature of the excavations. This might lead to practical limitations on backfill and its emplacement in those areas that do not affect the system performance. Therefore in addition to sectioning the disposal facility in relation to depth (and major hydraulic features), the backfill used in various areas may also be selected based on their function, as described below: central tunnel backfill (always at disposal depth), technical facilities backfill (isolated from repository area), access tunnel backfill (in different depth categories), and shaft backfill (in different depth categories).

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Figure 5-2. A general illustration of Posiva’s disposal facility layout option showing backfills and plugs for regions beyond the repository (lower figure including central tunnels), excluding deposition tunnels and holes. A detailed closure design will be presented in Closure Production Line report, POSIVA 2012-19. Table 5-1. Materials that may be used in backfilling the Olkiluoto underground disposal facility.

Location to be Installed

Backfill Type Compaction Method

Maximum Allowed Hydraulic

Conductivity (K) (m/s)

Surface above -50 m Boulders -

Above -200 m Crushed Rock In situ ~ 10-7

HZ20 to -200 m Clay-Crushed Rock mix In situ ~ 10-8

HZ20 Plugs and Crushed rock In situ, and

clay blocks and pellets ~ 10-6

-420 m to HZ20 Clay-Crushed Rock mix In situ ~ 10-8

Technical Rooms and Lower Shafts*

Crushed Rock In situ ~ 10-7

Central and Connecting Tunnels

Clay Blocks and Pellets Block and Pellet

installed in the tunnels ~ 10-9

* Lower shafts implies to shafts between technical rooms in two levels, from about -430 to -455 m.

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Central tunnel backfill The composition of the backfill installed in the central tunnels is currently based on the backfill used in the deposition tunnels (blocks and pellets). Alternative materials (e.g. mixture of clay and aggregate instead of swelling clay material selected for the deposition tunnels) could also be used. The central tunnel connections (Figure 5-2 and Figure 5-7) will need to be closed using mechanical plugs so that the system is divided during operation. The use of these mechanical plugs in the central tunnel connections also makes it possible to accomplish backfilling and closure in a stepwise manner. The first stage of central tunnel backfilling will occur in the central tunnels and deposition tunnel ends that remain on the central tunnel side of the deposition tunnel plug (Figure 5-2). This volume will be backfilled and closed during the operation period, after each deposition panel is ready to be isolated. The distance from the deposition holes and deposition tunnels to the central tunnels may result in slightly less stringent performance requirements for this backfill component relative to the deposition tunnels. For example, the requirement of the backfill to restrict the upward swelling/expansion of the buffer is not needed in the central tunnels and thermal gradients will be much lower. This means that it is reasonable to expect that some changes to the backfill composition used in this volume can be made without compromising system performance. Bentonite-aggregate blends or lower swelling capacity clay-only materials might be suitable for manufacturing precompacted blocks, depending on exactly how much swelling capacity is ultimately deemed to be necessary in this volume. These options are still being evaluated to determine what effects they will have on system performance, technologies required for placement, and disposal facility costs and schedules. An example of how backfilling using precompacted blocks and pellets could be accomplished for the central tunnel is provided in Figure 5-3. In this example, the over-excavation of the central tunnel was assumed to be the same (13.6 %), as has been encountered in ONKALO (Hansen et al. 2009). The theoretical cross section of the central tunnel is nominally 38.4 m2, but near the locations where it meets the technical services rooms the excavation cross section is approximately 39.6 m2 (Saanio et al. 2009). Despite the substantial volume associated with this size of tunnel and degree of over-excavation, a relatively high degree of block backfilling can be accomplished (77.8 %), because the tunnel cross section is so large. The central tunnel cross section is about 2.75 times larger than the largest anticipated deposition tunnel cross section (14 m2) in Posiva’s KBS-3V design. This large tunnel cross-section also means that the rate of backfilling of the central tunnel is expected to be slower than for the deposition tunnels if the same type of backfilling equipment is used in both volumes. Materials and methods potentially usable for this volume were presented in Section 4.4.

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Figure 5-3. An example for an assembly of block backfill for central tunnels, which is based on the assembly design of the deposition tunnel backfill presented in Hansen et al. (2009). Two common profiles are presented. Block sizes and assemblies will be designed in greater detail closer to implementation time. Technical facilities at or below disposal level As can be seen generally in Figure 5-2 and in detail in Figure 5-4, there are numerous excavations within the disposal facility that are not deposition or central tunnels or access tunnel or shafts. These miscellaneous excavations include the technical rooms, pumping station, and connections between shafts and access tunnels. All of these volumes will need to be backfilled and the manner they are backfilled and plugged will be based on determinations of their risk to provide connections between otherwise isolated volumes of the underground disposal facility. The backfill approach suggested for these excavations is as follows: The areas (i.e. vehicle connections) where shafts and access tunnel are connected to

the disposal area and provide a direct pathway from the central tunnels need to be backfilled in such manner that they do not provide preferred mass transport pathways. This will likely require installation of low-permeability backfill and plugs in key locations within the shafts and access tunnel.

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Where they are not on the pathway between the repository and large excavation areas, such as parking halls and connections to demonstration areas, these excavations are essentially isolated, occluded volumes that do not affect mass transport from the disposal level. It might be possible to backfill them with previously excavated rock, such as was demonstrated by Gunnarsson et al. (1996, 2001) in trials conducted at SKB’s Äspö laboratory. These volumes could then be separated from other tunnels with mechanical or hydraulic plugs. If needed a small amount of bentonite can be added to the crushed rock in order to; (1.) minimize the void space that would be present or (2.) maintain positive contact between the backfill and the chamber roof.

Areas like demonstration tunnels or areas later determined to be unsuitable for canister installation might be backfilled even earlier than operationally-related areas. The material(s) and method(s) of backfill installation used to fill these volumes will depend on what hydraulic connections exist to neighbouring underground openings or hydraulic features.

In those areas of the auxiliary spaces that do not require a particularly impermeable backfill or a swelling capacity, crushed rock recovered from the materials removed during disposal facility excavation is being considered for use in filling these spaces. Its ultimate use and where it is used will depend on the final layout of this region.

Figure 5-4. Example of possible approach to backfilling and plugging of technical areas at or below disposal level. More detailed closure design will be presented in Closure Production Line report, POSIVA 2012-19.

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Access tunnel backfill The access tunnel backfill will occupy volumes ranging from the surface to the disposal levels and so it will have changing requirements as closure proceeds. It can be divided in three depth categories as described in Section 5.1 and shown in Figure 5-1. Disposal depth - area below HZ20 At depths below the maximum potential permafrost level (Features, Events and Processes), the connections between the shafts and tunnels should be secured with plugs. The deeper sections of the access tunnel may be backfilled in the same manner as used in the central tunnels (either block/pellet or in situ compacted). Backfill installed between bulkheads will be designed such that it is compatible with the surrounding rock mass and hydro-geosphere, hence composition could be expected to vary with depth and location. The composition of this backfill may therefore differ from that in the central tunnels in order to maintain compatibility with the surrounding rock mass and ensure effective isolation of this region. For example, the area below HZ20 structure needs backfill, which cuts off the possible flow routes in the far future. The inflows in that area are minor and therefore processes such as erosion of the backfill are not key items of concern in backfill design in those regions. It is therefore desirable that lower part of the access routes have materials installed that exhibit self-sealing capacity and minor swelling pressure, thereby maintaining a positive contact between the backfill and the tunnel rock walls. The tunnel may have some localised regions where higher conductivities exist but they would not compromise system behaviour. Depth 300-100 (including HZ20 and HZ19) The area between structures HZ19 and HZ20 does not need very tight backfill according to the bedrocks hydraulic properties and distance from the disposal depth, but the material used there should not compress heavily with respect that the tunnel must remain closed and the backfill in it supports the upper closure components. Therefore material with optimal grain size distribution is used taking into account also the freezing and thawing resistance. Grain size optimisation will be conducted when the closure design will be finalised closer to the start of the closure operation. For the isolated HZ20 area the main target is to seal the structure area with permeable backfill, which still has a good grain pattern, in order to have uncompressed material within plugs. The section length of this backfill is ~100 meters. The HZ19 can have same type of isolation as HZ20 even it is not estimated to be necessary due to its location far from the disposal depth and above HZ20. Area above HZ19 up to the surface Above HZ19 the access tunnel should be backfilled with in situ compaction (Figure 4-7) using crushed rock or material with moraine-type grain size distribution. The material used as backfill should be selected to be easy to compact efficiently; it should not be frost susceptible or sensitive to erosion. This means that it should not have too much fine-grained material present (for example less than 10% of 0.074 mm grains) and the maximum grain size should not be more than about 100 mm. Such material has hydraulic conductivities in line with the hydraulic conductivity of the bedrock (Table 3-4 and Table 5-2).

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Table 5-2. Examples of hydraulic conductivities for different soil types and processed rock materials i.e. for different grain size distributions (Mälkki 1999, Niemi et al. 1994, Pusch 2008). Soil type / processed rock material Hydraulic conductivity (m/s)

Silt 10-5 - 10-9

Gravel moraine 10-4 - 10-7

Sand moraine 10-6 - 10-8

Silt moraine 10-7 - 10-10

Well graded aggregate 10-5 to 10-7

TBM-muck 10-7 to 2 x 10-10

The crushed rock/moraine type material backfilled region ends at approximately 50-100 m distance from the tunnel exit. A short (a few m in length) concrete plug would then be installed. Above the short concrete plug, backfill consisting of boulders and blast-excavated stones will be installed. This is used in order to withstand the erosive effect of ice and meltwater during glacial periods, as well as to prevent easy human intrusion into the underground disposal facility. Boulder backfill in the access tunnel should be continued for at least 50 to 100 meters of the tunnel length and the boulder diameter should be at least 600 mm. The regions where the tunnel and shaft entrance reach the original ground surface will be landscaped with natural boulders that have a diameter of at least 600 mm. The depth of the rock backfill should be sufficient to provide a very robust mechanical barrier to any human intrusion into the tunnel or shaft excavations. Shaft backfill The shafts are long, vertical structures, where the self-weight of backfill as well as lithostatic, hydrostatic and other stresses are quite large by the time the base (below disposal level) is reached. As a result of this high stress state, some deformation of the backfill is inevitable. These deformations, which are mainly expressed as settlement, will be dominated by the elastic and plastic deformation (including creep) of the backfill. The magnitude of the settlement will depend on the mechanical properties of the backfill material installed as well as the behaviour of a number of mechanical (concrete) plugs along the length of the shafts. For conservative design purposes, the EDZ is assumed to exist along the entire length of the shafts and this will need to be accommodated in the design of the backfill and plugs installed (through keying to cut off the EDZ). The self-weight of the backfill will mean that deformation will be vertically downwards and so it should be flexible (plastic) enough to tolerate some deformation without endangering its functionality. Shaft backfill in near-surface regions The space in the permafrost layer will be backfilled following the same principles as for the access tunnels using granular materials and intrusion obstructing plugs close to the surface and so almost all its consolidation–related deformation can be expected to occur quite soon after its installation (first few years). This could result in formation of a gap in the uppermost section of the backfill, between it and any overlying rigid plugs. This

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could be accommodated through installation of a zone of backfill immediately below the rigid plugs that is capable of swelling to fill any voids that might otherwise form due to granular material settlement. Similarly, settlement of underlying materials could result in the mechanical plugs installed in the shaft having to support a substantial load from the overlying mass of granular fill materials. Shaft backfill below maximum depth of permafrost penetration Below the maximum depth of permafrost penetration, the backfill in the shafts (and access tunnel) has a hydraulic barrier function. This entails use of lower permeability filler material in the tunnel and hence a much higher plasticity than the near-surface granular fill. This higher plasticity and lower hydraulic conductivity means that these materials will experience settlement deformation that can span the period from construction out to several hundred years after its installation (depending on hydraulic, hydro-geochemistry and stress conditions). Such long-term deformation could compromise the backfill performance unless it is engineered to allow for some swelling capacity, which would make up for localised settlement deformations. At depths approaching the disposal level (below HZ20) the shaft backfill will have more stringent hydraulic requirements (lower hydraulic conductivities). In order to achieve the required hydraulic properties, the backfill in this region could be placed as either precompacted blocks and pellets or as an in situ compacted mass. It should be noted that in situ horizontal compaction of bentonite-aggregate materials can still be accomplished to substantial densities and so this is also a viable means of achieving the density requirements of the backfill, as demonstrated in the Enhanced Sealing Project (Dixon et al. 2009; Martino et al. 2011) and in field trials reported on by Korkiala-Tanttu et al. (2007). Locations where the shafts connect with other excavations will need to be backfilled such that these features do not become pathways that allow for preferential groundwater movement. This can be accomplished through installation of high swelling capacity materials (e.g. precompacted bentonite blocks). Nold (2006) presented a concept of how the lower part of a lift shaft could be backfilled with compacted bentonite (Figure 5-5). The length of the bentonite backfill plug shown in that concept is quite long (40 m) and it is separated from the coarser granular shaft backfill using several filter layers. The purpose of the filter layers is to ensure the long-term stability of the structure e.g., to ensure that there will be as little as possible mixing of the materials during water flow through the materials. Although this concept was developed for a sedimentary rock environment, the application of it could be readily adapted to a shaft in crystalline rock (Olkiluoto).

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Figure 5-5. Generic concept for sealing connection between a shaft and tunnels in a repository (Nold 2006). Shaft and tunnel intersection with hydraulic features An important consideration in designing the closure system for the disposal facility is the placement of material in locations where the underground openings intersect with water conductive structures or fracture zones. This is essentially construction of a miniature compartment intended to isolate a small but potentially disruptive feature. These intersected features can be isolated either through use of a low permeability fill installed in the affected region or else use of low permeability materials below (and above) the intersection with the hydraulic feature. The option of backfilling the intersection area with a low permeability material, like blocks of swelling clay (or other material having hydraulic performance at least as low as the surrounding rock, i.e., crushed rock with a suitable grain distribution) is intended to have the result that this location does not provide a preferred transport pathway from the tunnel/shaft into the fracture zone. This is the approach that was adopted in the joint Posiva, NWMO, SKB and Andra Enhanced Sealing Project where a full-scale shaft was plugged (Dixon et al. 2009, Martino et al. 2011, Dixon et al. 2012). A relevance affecting backfill material choice is the transportation of fine-grained material into hydraulic features, and thus the granular framework is to be such that supporting frame remains even though smaller particles are washed away.

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5.2.2 Plugs at Olkiluoto site The three primary environmental zones at the Olkiluoto site are: 1) surface to the HZ19 area 2) from 100 to 300 meters including HZ19 and HZ20 stuctures and 3) below HZ20 structure. These will drive the design and composition of the concrete structures within the closure system. The principles for these plugs are use of standard, frost-resistant, sulphate-resistant, high strength, high pH concrete in regions above HZ20. Below HZ20 a low permeability, low pH concrete is needed to provide isolation as needed and to minimize pH effects on the environment at the disposal level. The design requirements of plugs that are anticipated to be needed in a deep geological disposal facility at the Olkiluoto site were presented in Table 3-2. A general layout of the types and locations of plugs and their locations has been developed. Figure 5-2 shows the general locations where plugs may be necessary in the underground disposal facility. Figure 5-6 shows the general locations where the central tunnel plugs would be needed. Figure 5-7 shows the plugs that may be needed to isolate the technical rooms and shaft access locations.

Figure 5-6. General illustration of plugs in central tunnels.

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Figure 5-7.Conceptual example of the different types of plugs that can be used to isolate technical rooms.

As described previously in Section 4.1, the homogeneous backfilling concept considered by SKB for closure does not utilise backfills that are tailored to the various regions of the disposal facility and instead relies on a single (high-quality) backfill for all regions. The homogeneous backfilling concept will also require installation of plugs within the disposal facility in order to provide sealing and restraint to the backfilled deposition tunnels, central tunnels as well as to isolate hydraulically-significant features that may be encountered at the disposal facility site. The plugs described for the Olkiluoto site are therefore generically applicable to both closure concepts (for the one suggested for Olkiluoto and the other considered by SKB), although their actual locations within the disposal facility will differ as the results of repository layout and site-specific conditions. 5.2.3 Composition and number of plugs required for Olkiluoto site Plug composition The location and function of the cement-based closure components (plugs) identified as being needed in Posiva’s closure concept for a disposal facility at Olkiluoto is described in Section 5.2. Basically, the three primary environmental zones at the Olkiluoto site will drive the design and composition of the concrete structures within the closure. The principles for these plugs are use of standard, frost-resistant, sulphate-resistant, high strength, high pH concrete in regions above HZ20 (~ -200 m). Below HZ20 a low permeability, low pH concrete is needed to provide isolation as needed. Final concrete mixes will be defined closer to the start of the closure operations when the entire closure design will be finalised. As part of the process of evaluating the volumes of non-backfill materials left in the disposal facility and to allow for assessment of the potential effect of concrete materials on the pH evolution of the region surrounding it, it is necessary to produce some initial estimations of how many plugs will be needed.

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Preliminary estimation of the number of plugs Figures 5-2 and 5-7 show the general layout of the proposed KBS-3V disposal facility at the Olkiluoto site, plugs needed for the access tunnel and shafts and the technical rooms, respectively. From this proposed layout the number of the needed mechanical and hydraulic plugs can be estimated. Table 5-3 lists the types and general locations for these plugs. The information in Table 5-3 also provides guidance regarding the sequencing of the backfill and plugging of the disposal facility as well as for planning and costing of the closure process. The information regarding plug types and locations will need to be re-evaluated once the disposal facility is fully excavated, actual conditions are determined and a final layout for the repository is settled on. Closure sequencing will also have an effect on the number and location of the plugs, and any changes in the size of the disposal facility would also require re-evaluation of the number and type of plugs needed.

The size of the plugs and amount of constituents depend on the location, depth and size of the tunnel and purpose for which the plug is planned to be used. Excluding intrusion obstructing plugs and deposition tunnel plugs, the amount of cement in the plugs can be estimated to be in maximum half of the cement in deposition tunnel plugs. This will mean the presence of a substantial quantity of cementitious material. Additionally there will be a significant amount of steel and other stray materials left in the disposal facility as components of the mechanical plugs. All of these materials must be taken into consideration when examining the influence of engineering on the post-closure evolution of the repository. Karvonen (2011) included estimations of plug materials, according to then unpublished preliminary designs, into estimations on foreign material quantities that will remain in the disposal facility after its closure. Those quantities are similar to designs presented here, except for the copper that is at present not considered as a plug component. Table 5-3. The estimated numbers and types of plugs needed for Olkiluoto site.

Area Mechanical

Plugs* Hydraulic

Plugs Intrusion Obstructing

plugs

Central tunnels 21 6

Technical rooms and miscellaneous spaces

7

Access tunnel 3 5 1

Shafts 5 20 5

Total plugs 36 31 6

*number of plugs is for generic estimation purposes only

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Shotcrete materials installed during disposal facility operations could perhaps be left in areas not associated with tunnel plugs. Where necessary, shotcrete can be removed using several techniques, including mechanical hammering, water jet sawing, a combination of a mechanical hammer and a water jet saw, a mechanical cutter and a mechanical cutter installed to an excavator. A mechanical cutter installed to an excavator seems to be the most cost effective alternative. The efficiency of shotcrete removal and the suitability of different support methods will need to be more closely evaluated (Saanio et al. 2006). 5.3 Discussion about knowledge with uncertainties for the closure of the

disposal facility designed to be constructed at Olkiluoto Chapter 4 of this document described materials, technologies and examples of backfill and sealing demonstrations of relevance to Posiva’s disposal facility method and its application at the Olkiluoto site. A general layout for a disposal facility at the Olkiluoto site has been generated, together with preliminary closure backfill and plug locations. Beyond the knowledge already developed with respect to the ability to backfill and close the regions other than the deposition tunnels, there are still a number of items that could be further investigated or demonstrated at field-scale. These are associated with both the short (pre-closure and immediately post-closure) and the longer-term evolution of the disposal facility site. Part of the data for the information gaps will be gathered within studies and tests for deposition tunnel backfill and plugs. Over the short-term there are still questions associated with: The ability of the backfill to remain physically stable enough under conditions of

water inflow similar or higher than in the deposition tunnels (mechanical erosion), especially during the period preceding installation of plugs. This is the subject of ongoing work by Posiva.

Defining the consistency with which backfill materials can be placed within the openings, particularly with respect to ensuring that adequate density is maintained.

Full definition of materials and natural analogues that support their use.

Longer-term (post-closure) issues have been discussed in Performance Assessment report and in other TURVA-2012 portfolio reports, including the uncertainties listed below: Determining the vulnerability of the backfill to erosion as the result of mechanical

processes (groundwater flow in intersecting hydraulic features, or along the backfilled tunnels),

Establishing what chemical reaction processes (interaction with nearby structures e.g., concrete, grout, iron or other stray materials present in the disposal facility) will affect system performance.

Evaluating what potential exists for chemical erosion of closure materials to substantially alter system performance (e.g. dilute water intrusion as a result of glaciation).

Assessing the long-term durability of the closure structures (concrete-based) under the anticipated groundwater conditions, following interaction with adjacent closure

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materials. This will include evaluation of the effects of their potential loss of function with time on overall system performance.

Assessing the mechanical performance of the near surface structures during glacial cycle.

Beyond the outstanding questions related to closure system performance there is a continuing need to develop means of simulating (and evaluating) the overall site performance with time using numerical models. Initial numerical simulations (Hartley et al. 2013) provide encouraging results (good system performance). In support of model development and qualification process there is also the need to undertake large system simulations and demonstrations, with gradually more complex (and site relevant) physical simulations of the backfilling and closure system. These will provide data for use in modelling as well as confirmation of the practicality of the performance specifications currently established for the components of the closure system.

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6 SUMMARY The final goal of disposal facility closure is to ensure the safe, long-term isolation of the spent nuclear fuel. This will in part to be accomplished by so far as possible restoring the original host rock conditions in the vicinity of the repository once operations have concluded. In order to accomplish this Posiva has developed or identified materials and methods that can be used to achieve effective closure of the underground disposal facility and connections to the surface or nearby hydraulically-significant features. The use of plugs within the disposal facility is important to both the short- and long-term performance of the closure system. These are primarily expected to consist of cement-based materials (concrete) and have hydraulic or mechanical functions that are associated with their location and the site-specific conditions. Selection of the location for each of the plugs will be vital in assuring their performance. Materials and methods for use in closure of the openings beyond the deposition tunnels have been described and a range of potentially viable compositions and installation methods have been tested and evaluated. Information currently available indicates that backfill materials can be installed to sufficient density using either in situ compaction or as an assembly of precompacted swelling-clay blocks and then pellets placed between the blocks and the surrounding rock. The swelling and hydraulic behaviour of a range of potential backfill materials has been summarised and provides a means of tailoring fill materials to the surrounding conditions. For regions requiring stringent material performance, backfill of adequate characteristics can be identified and specified. For regions requiring less stringent performance, appropriate materials can also be selected based on such information. The effects of groundwater conditions and rate of inflow to the excavations have been identified as potential issues with regards to system robustness and means to address them discussed. The closure concept for closure of a disposal facility provides some features that are attractive for application at the Olkiluoto site. In particular, the closure concept allows for a flexible approach to be taken when installing both sealing materials (closure backfill) and structures (plugs) within the wide range of opening sizes that will exist in Posiva’s disposal facility. This allows for the effects of geological structure and hydrological features to be included in the process of defining backfill and sealing structures. Detailed design and location of the backfills and plugs within the disposal facility will only be possible once a repository layout is defined and site-specific characteristics clearly established. Further field work and numerical simulations support to confirm the viability of the closure concept for closure of spent nuclear fuel disposal facility and to begin the process of detailed defining what materials and structures are required in what locations.

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REFERENCES Safety Case portfolio main reports Complementary Considerations Safety case for the disposal of spent nuclear fuel at Olkiluoto – Complementary Considerations 2012. Eurajoki, Finland: Posiva Oy. POSIVA 2012-11. ISBN 978-951- 652-192-6. Design Basis Safety case for the disposal of spent nuclear fuel at Olkiluoto 2012 - Design Basis. Eurajoki, Finland: Posiva Oy. POSIVA 2012-03. ISBN 978-951-652-184-1. Features, Events and Processes Safety case for the disposal of spent nuclear fuel at Olkiluoto 2012 - Features, Events and Processes. Eurajoki, Finland: Posiva Oy. POSIVA 2012-07. ISBN 978-951-652-188-9. Performance Assessment Safety case for the disposal of spent nuclear fuel at Olkiluoto 2012 - Performance Assessment. Eurajoki, Finland: Posiva Oy. POSIVA 2012-04. ISBN 978-951-652-185-8. Synthesis Safety case for the disposal of spent nuclear fuel at Olkiluoto 2012 - Synthesis. Eurajoki, Finland: Posiva Oy. POSIVA 2012-12. ISBN 978-951-652-193-3. Safety case portfolio supporting reports Backfill Production Line report Backfill Production Line 2012 - Design, production and initial state of the deposition tunnel backfill and plug. Eurajoki, Finland: Posiva Oy. POSIVA 2012-18. ISBN 978-951-652-199-5. Site Description Olkiluoto Site Description 2011. Eurajoki, Finland: Posiva Oy. POSIVA 2011-02. ISBN 978-951-652-179-7. Other references Andersson, J., Ahokas, H., Hudson, J., Koskinen, L., Luukkonen, A., Löfman, J., Keto, V., Pitkänen, P., Mattila, J., Ikonen, A. & Ylä-Mella, M. 2007. Olkiluoto site description 2006. Olkiluoto, Finland: Posiva Oy. Posiva Report 2007-03. ISBN 978-951-652-151-3. Arenius, M., Hansen, J., Juhola, P., Karttunen, P., Koskinen, K., Lehtinen, A., Lyytinen, T., Mattila, J., Partamies, S., Pitkänen, P., Raivio, P., Sievänen, U., Vuorinen, U. & Vuorio, M. 2008. R20 summary report: The groundwater inflow management in

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ONKALO - the future strategy. Olkiluoto, Finland: Posiva Oy. Working Report 2008-44. Autio, J., Johansson, E., Hagros, A., Anttila, P., Rönnqvist, P., Börgesson, L., Sandén, T., Eriksson, M., Halvarsson, B., Berghäll, J., Kotola, R. & Parkkinen, I. 2008. KBS-3H design description 2007. Stockholm, Sweden: Swedish Nuclear Fuel and Waste Management Co. (SKB). R-08-44. Autio, J., Hassan, Md. M., Pintado, X., Keto, P. & Karttunen, P. 2012. Backfill Design 2012. Eurajoki, Finland: Posiva Oy. POSIVA 2012-15. ISBN 978-951-652-196-4. Börgesson, L., Dixon, D., Gunnarsson, D., Hansen, J., Jonsson, E. & Keto, P. 2009. Assessment of backfill design for KBS-3V repository. Olkiluoto, Finland: Posiva Oy. Working Report 2009-115. Chandler, N.A., Cornut, A., Dixon, D., Fairhurst, C., Hansen, F., Gray, M., Hara, K., Ishijima, Y., Kozak, E., Martino, J., Masumoto, K., McCrank, G., Sugita, Y., Thompson, P., Tillerson, J. & Vignal, B. 2002. The five year report on the tunnel sealing experiment: An international project of AECL, JNC, ANDRA and WIPP. Chalk River: Atomic Energy of Canada Limited (AECL). AECL-12127. Dahlström, L.-O. 2009. Experiences from the design and construction of plug II in the Prototype Repository. Stockholm, Sweden: Swedish Nuclear Fuel and Waste Management Co. (SKB). R-09-49. Dahlström, L.-O., Magnusson, J., Gueorguiev, G. & Johansson, M. 2009. Feasibility study of a concrete plug made of low pH concrete. Stockholm, Sweden: Swedish Nuclear Fuel and Waste Management Co. (SKB). R-09-34. Dixon, D. 2000. Porewater salinity and the development of swelling pressure in bentonite-based buffer and backfill materials. Olkiluoto, Finland: Posiva Oy. Posiva Report 2000-04. ISBN 951-652-090-1. Dixon, D. & Keto, P. 2008. Backfilling techniques and materials in underground excavations: Potential alternative backfill materials for use in Posiva’s spent fuel repository concept. Olkiluoto, Finland: Posiva Oy. Working Report 2008-56. Dixon, D.A., Stroes-Gascoyne, S., Kjartanson, B. & Baumgartner, P. 2005. Development of sealing materials for application as engineered barriers in a deep geologic repository for used fuel disposal: Filling the gaps. Ottawa: Canadian Nuclear Society, Waste Management, Decommissioning and Environmental Restoration for Canada’s Nuclear Activities: Current and Future Needs, May 8-11, 2005. Dixon, D., Lundin, C., Örtendahl, E., Hedin, M. & Ramqvist, G. 2008a. Deep repository - Engineered barrier systems. Half scale tests to examine water uptake by bentonite pellets in a block-pellet backfill system. Stockholm, Sweden: Swedish Nuclear Fuel and Waste Management Co. (SKB). R-08-132.

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