EXPERIMENTS AT THE ÄSPÖ HARD ROCK LABORATORY

E X P E R I M E N TS ATT H E ÄS P Ö H A R D R O C K L A B O R ATO RY

Backfilland Plug Test

Äspö PillarStability Experiment

Demo Test

CanisterRetrieval Test

TBT

Slurrying Test

Lasgit

LTDE

Horizontal Deposition

RNR Experiment

True Block Scale

Prototype Repository

Lot

RNR Experiment

Two-Phase Flow

Matrix Fluid ChemistryExperiment

Rex Project

Microbe Project

True-1Colloid Project

Zedex

Would you like to know more? Read “Äspö Hard Rock Laboratory – Annual Report 2002” (SKB TR-03-10).

The Äspö HRL consists of a 3,600 metre long spiral tunnel that goes down to a depthof 460 metres. A number of different experiments are being conducted in the tunnel.

Äspö Hard Rock Lab ora to ry

Zedex 2

Äspö Pillar Stability Experiment 4

Demo Test 6

Prototype Repository 8

Horizontal Deposition 10

Backfill and Plug Test 12

Canister Retrieval Test 14

Lot 16

TBT 18

Two-Phase Flow 20

Lasgit 22

Colloid Project 24

RNR Experiment 26

True 28

LTDE 30

Matrix Fluid Chemistry Experiment 32

Rex Project 34

Microbe Project 36

A dress rehearsal is being held in preparation for the construction of a deeprepository for spent nuclear fuel at SKB’s underground Hard Rock Laboratory(HRL) on Äspö, outside Oskarshamn. Here we can test different technicalsolutions on a full scale and in a realistic environment.

The Äspö HRL is also used for field research. We are conducting a numberof experiments here in collaboration with Swedish and international experts.

You can order it or download it as a .pdf file from our website at www.skb.se

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ZedexIn the Zedex experiment we have compared how the rock is affected arounda drill-and-blast tunnel versus a bored tunnel.

When a tunnel is built, the stressesand the water flows in the rock maybe changed. The volume affected bythe tunnel excavation is called theexcavation-disturbed zone, or EDZ.The EDZ consists of two parts. Near-est the tunnel wall is the damagedzone, further out is the stress redistri-bution zone.

The Zedex experiment (Zone of Exca-vation Disturbance Experiment) wasstarted in 1993 in order to find outhow big the EDZ around a drill-and-blast tunnel is compared with a boredtunnel.

The project was concluded in 1998.Besides SKB, our French and Englishcounterparts – Andra and Nirex – alsotook part. Swiss Nagra and GermanBMBF also made important contri-butions to the project.

TWO TU N N E LS

The experiment was conducted intwo parallel tunnels at a depth of 420metres. One was drilled and blastedand the other was bored by means ofa tunnel boring machine.

The blasting method used is knownas careful blasting. Both tunnels havea diameter of about five metres. Thegoal of the project was to understand

how the mechanical and hydraulicproperties of the rock varied depend-ing on how the tunnel was excavated.This is important for long-term safety inthe repository. If many water-bearingfractures are created during excavation,this can open up new transport path-ways for radionuclides. Another goalof the project was to test equipmentand methodology for quantifying theproperties of the EDZ.

Read “ZEDEX – A study of damage and disturbance from tunnel excavation by blasting and tunnelboring” (SKB TR-97-30).

A bored tunnel causes less disturbance of the rock than a drill-and-blast tunnel.

The fractures around the tun-nel were a few centimetresdeep.

Drill-and-blast tunnel Bored tunnel

Fracturescausedby tunnelling

Microfracturescaused bytunnelling

• Disturbance regard-less of tunnellingmethod

• Redistribution ofrock stresses

• No damage tothe rock (no newfractures)

• Small changes inpermeability

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The fractures around a drilled-and-blasted tunnelare much deeper. They were around 30 centi-metres in the walls.

Zedex

You can order it from our website at www.skb.se

M EAS U R E D B E FO R E,D U R I N G AN D AFTE R

In the Zedex experiment we measuredthe properties of the rock before, dur-ing and after tunnelling. A number ofexploratory holes were drilled, bothalong the tunnels and radially. Theproperties of the EDZ were determinedby means of:

• Seismic measurements.

• Measurements of rock movement.

• Ultrasonic measurements.

• Resistivity measurements.

• Observation of natural andinduced fractures.

• Temperature measurements.

• Measurements of acoustic emissionfrom formation of microfractures.

• Measurements of rock stresses.

DAMAG E D Z O N EN EAR TU N N E L WALL

The properties of the rock near thetunnel wall are affected by the excava-tion method used. The changes hereare permanent. This part of the rock istherefore called the damaged zone.

When a tunnel is excavated, new frac-tures are formed. The deepest ones,around 80 centimetres, were found inthe floor of the drill-and-blast tunnel.The fractures in the wall were at themost 30 centimetres deep.

The bored tunnel did not affect therock as far in. There the fracturesextended less than three centimetresinto walls, floor and ceiling. The dam-age was uniform over the whole tun-nel wall.

E D Z R EGAR D LES S O F M ETH O D

The results show that regardless ofwhether the tunnel is drilled-and-blasted or bored, a stress redistribu-tion zone is obtained several metresout from the tunnel. Since a hole hasbeen made, the rock stresses arechanged. These changes are notpurely of a mechanical nature. Otherchanges are small and of a largelytemporary nature.

The deepest fractures, around 80 centimetres, were found in the floor of thedrill-and-blast tunnel.

Zedex

When the deep repository is built, rock pillars will be created between the deposition tunnels andbetween the deposition holes.

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Äspö P i l l a r S tab i l i t y Exper imentIn a new experiment we will investigate how much the rock can take. A narrow pillar betweentwo boreholes will be loaded to the point that the rock’s ultimate strength is exceeded.

A new tunnel was built in the ÄspöHRL during the spring and summer of2003. The tunnel is around 70 metreslong and lies at a depth of 450 metres.This is where the Äspö Pillar StabilityExperiment will take place. Two depo-sition holes will be drilled so closetogether that only a narrow rock pillaris left between them.

U LTI MATESTR E N GTH E XC E E D E D

One borehole will be subjected to aninternal water pressure of 1 MPa.A watertight liner on the inside of thehole prevents water from seepinginto the fractures in the rock.

The rock in the pillar will then beheated. Prior to heating, the stresson the walls of the deposition holesis around 120 MPa. The additionalstresses that arise when the rockexpands cause the rock’s ultimate

strength to be exceeded in a limitedarea. This may cause pieces of therock to spall off from the walls of theholes.

R O C K M EC HAN I CAL M O D E LLI N G

With this experiment we wish todemonstrate our ability to predict rockfailure in a lightly fractured rock mass.We also want to examine how thestrength of the rock is affected by thepressure exerted on the rock by thebackfill in a deep repository. A thirdpurpose is to compare 2D and 3Dcalculation models for predicting thethermal and mechanical stresses on arock block.

In order to achieve these purposes, wefirst simulate the experiment with com-puter models. We have used four dif-ferent programs to get as good resultsas possible. The models predict the

Would you like to know more? Read “Rock stability considerations for siting and constructing a KBS-3 repository” (SKB TR-01-38).

Fractured rock mass in rock pillar.

Rock pillars between deposition tunnels

Rock pillars betweendeposition holes

Rock pillars betweendeposition holes

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temperature distribution, how the rockwill move when it expands due to theheat, and at what temperature the ulti-mate strength will be reached. Bythen performing the experiment andcomparing the results with the com-puter models, we can see how goodthe models are.

MANY D I FFE R E NT I NSTR U M E NTS

Numerous instruments will be installedin the pillar and the rock around theboreholes to measure temperature androck movements. The spalling thatoccurs when the ultimate strength isexceeded will be monitored with specialmicrophones. The microphones capturethe sound that propagates through therock when it reaches its ultimatestrength and energy is released.

Such acoustic measurements will alsobe used to discover if microfractures

are formed in the rock, since thevelocity of the sound wave variesdepending on how fractured the rockmass is.

The practical tunnelling work was pre-ceded by a feasibility study. The pur-pose was to find a suitable place in thelaboratory and an appropriate tunnelconfiguration. It was found that themost favourable conditions are obtainedin a five metre wide tunnel with a roofheight of 7.5 metres. The floor of thetunnel should be semicircular.

P I LLARB ETWE E N H O LES

The deposition holes are bored sothat an approximately one metre thickrock pillar is formed between them.The diameter of the holes should be1.8 metres, just like the depositionholes in the deep repository.

The tunnel was built during the springand summer of 2003. Then we con-ducted a geological characterizationof the rocks in the tunnel.

The deposition holes were bored inOctober 2003. We expect to be fin-ished with this by the end of the year.Heating will start in January 2004 andis planned to take four months. Theexperiment is being conducted incooperation with Posiva, our counter-part in Finland.

Äspö P i l l a r S tab i l i t y Exper iment

The Äspö Pillar Stability Experiment is taking place in a new tunnel in the Äspö HRL.


Äspö Pillar StabilityExperiment

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Demo Tes tIn the Demo Test we are demonstrating emplacement of the copper canistersand the surrounding bentonite in the deposition holes.

Testing machines and deposition tech-nology on a full scale is extremelyimportant in preparation for operationof the deep repository.

The goals of the Demo Test are to:

• Develop and test the methods and theequipment needed for emplacementof canisters of spent nuclear fuel.

• Demonstrate the different steps ofcanister deposition to both special-ists and the public.

• Develop and test criteria and qualitysystems for the deposition technology.

To enable the project to be carriedout, SKB has developed a full-scaleprototype of a deposition machine.It is important that all machines in thedeep repository be functional andpractical, since it is their size thatdetermines how big the tunnels inthe repository will be. The machines’space requirements must also bedetermined accurately, since it is diffi-

cult to enlarge the diameter of thetunnels afterwards.

EASY TO M OVE

The deposition machine is primarilydesigned to be used in drill-and-blastdeposition tunnels. It can be used inbored tunnels, but then some modifi-cations and a temporary driveway inthe tunnel are required.

Blocks and rings of compressed ben-tonite clay are first emplaced in thedeposition hole in a high vertical stackbefore the canister is lowered. The freevolume in a deposition hole must notbe too great. Otherwise there is a riskthat the bentonite will not be denseenough when it is water-saturated.

At present, the space between theinside wall of the bentonite rings andthe canister is only one centimetre.This requires great precision on thepart of the deposition machine. Besidestravelling backwards and forwards,

the machine can also move a fewcentimetres laterally.

CAN I STE R I S T I LTE D

The copper canisters with the spentfuel are brought down to the reposi-tory in special transport casks. Therethey are transferred to a two-pieceradiation protection tube designedfor the deposition machine. The filled

Would you like to know more? Read more about the Demo Test in “Demonstration deposition machine for canisters” (SKB IPR-01-38).

The machine tilts the canister into the depositionhole.

The deposition machine that will be used in the deep repository must be tested under realisticconditions.

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tube is then carried by a truck to thedeposition tunnel, after which it isplaced on a reloading platform andfed into the deposition machine.

Then the top of the radiation protec-tion tube is fixed to the top of themachine by means of a locking deviceand the equipment is driven up to thedeposition hole. The bolts in the bot-tom of the radiation protection tubeare undone, and the lower part of thetube is shifted sideways inside thedeposition machine’s radiation pro-tection box. The machine now slowlytilts the bottom part of the canisterinto the deposition hole and straight-ens it up so that it is vertical and cen-tred in relation to the bentonite stack.Finally, lifting tackle is attached andthe canister is lowered into the hole.

It is difficult to keep the radiationshielding intact on a machine with somany moving parts. We have, forexample, been forced to attach extra

“skirts” to the bottom of the machineto prevent radiation from escaping.

200 CAN ISTE RSPE R YEAR

Today we don’t know how manydeposition machines will be neededin the future deep repository. Thisdepends on the desired pace ofdeposition. In the deep repository, thewhole deposition process will be bothradiation-shielded and remote-con-trolled. At this point we plan todeposit 200 canisters per year.

Despite remote control, radiationshielding is important for safety. If any-thing unforeseen happens during depo-sition, it must be possible for the opera-tors to carry out repairs, for example toreplace something if it doesn’t work. Itmust furthermore always be possible toreturn to a safe state and reverse thedeposition procedure, in other wordsrun it backwards.

During the testing period we willsubject the deposition machine to anumber of malfunctions. These in-clude power failures in various situa-tions. We will also check what theservice requirement is and how muchspace is needed to perform variousservice measures.

Deposition technology has also beendemonstrated in the Canister RetrievalTest and the Prototype Repository.Here, deposition was done using adeposition machine without radiationprotection. One canister was deposit-ed in the Canister Retrieval Test andsix in the Prototype Repository.

Demo Tes t

Inside of deposition machine. The equipment is remote-controlled.

You can order it by e-mail at [email protected]

Demo Test

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Proto type Repos i to ryIn the Prototype Repository we study what long-term changes occur in the barriersunder the conditions prevailing in a deep repository.

To check whether a deep repositoryperforms in keeping with our assump-tions, we have built the PrototypeRepository – a bored deposition tun-nel with six full-scale canisters. Theexperimental area is located at adepth of 450 metres.

It is particularly important to checkheat transport, water saturation,pressure build-up, and how the can-ister, bentonite, backfill and rockwork together.

We will then compare the resultswith the calculation models we useand the other assumptions we makein our safety assessments.

TWO S ECTI O N S

No spent nuclear fuel is being used inthe test. The heat output from the can-isters is instead generated by electricheaters. Instruments in the boreholes,the canisters, the bentonite clay, thetunnel backfill and the surroundingrock measure what is happening.

The Prototype Repository contains sixdeposition holes and is divided into aninner and an outer section. The tunnelswill be backfilled with a mixture of ben-tonite and crushed rock, and the twosections will be separated by a plug.The outer section will be dug out afterbetween four and five years, while theinner section will remain intact for up

to 20 years. All components have thesame dimensions as they are plannedto have in the real deep repository.

The following activities are includedin the project:

• Characterization of the propertiesof the rock.

• Instrumentation.

• Modelling.

• Boring of deposition hole.

• Fabrication of bentonite blocksand rings.

• Canister fabrication.

• Backfilling and plugging.

• Measurement and monitoring.

Would you like to know more? Read “Äspö Hard Rock Laboratory – Test Plan for the Prototype Repository” (SKB HRL-92-24).

All components in the Prototype Repository have the same dimensions as they will have in the real deep repository.

In each of the above activities we aretesting the quality assurance systemwe plan to use in the real repository.

S EVE N C O U NTR I ESAR E PARTI C I PATI N G

The project is being co-funded by theEU for a total of 42 months and hasthe participation of seven countries.Besides SKB and three Swedishexpert groups, the following organiza-tions are participating: Andra (France),GRS and BGR (Germany), Enresa,Aitemin and Cimne (Spain), JNC(Japan), Cardiff University (UK), andPosiva and VTT (Finland).

The electric heaters in the PrototypeRepository suffered the same type ofelectrical malfunctions as the canisterin the Canister Retrieval Test (seepages 14–15). Due to the design ofthe instrument lead-throughs in thecanister lid, residual moisture in thecanister caused an electrolytic mist toform. This in turn gave rise to leakagecurrents to earth, which can at worstcause a short circuit.

I N N E R S ECTI O NS EALE D

The inner section of the PrototypeRepository was backfilled and sealedduring 2001. Measurements are cur-

rently being made. Deposition in theouter section has been delayed dueto the problems with the electricalheaters but is now also finished. Thebackfill material in this part of the tun-nel is in place and the plug is cast.

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Proto type Repos i to ry

Installation of measurement instruments.

Deposition of canister.

You can order it by e-mail at [email protected]

PrototypeRepository

In the Prototype Repository it is particularlyimportant to measure how the repository’sbarriers interact.

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Hor i zon ta l depos i t ionIs it possible to deposit the canisters horizontally without compromising safety?

The deep repository for spent nuclearfuel is based on the KBS-3 method,where KBS stands for Nuclear FuelSafety (in Swedish). The method per-mits canisters to be emplaced eithervertically (KBS-3V) or horizontally(KBS-3H). The canister and bufferare the same in both variants. Thepart of the deep repository locatedabove ground is not affected either.

With horizontal deposition the quan-tity of excavated rock is less. Thismeans that environmental impactduring construction is less as well.

The deposition tunnels are not need-ed in KBS-3H. The deposition holesare instead bored directly from thetransport tunnels. The diameter ofthe deposition holes is about twometres.

E XCAVATE D VO LU M E

This means that the volume of rockthat needs to be excavated is muchless. The excavated volume is reducedby about 900,000 m3 compared withvertical deposition. The number of tripsto haul crushed rock up and down in

the repository is thereby halved. Thegroundwater table is also less affect-ed. The cost of building the repositoryis almost certainly reduced as well,but we don’t know the size of the sav-ing today.

Why isn’t this method SKB’s mainalternative? The answer is that wedon’t yet know if it is technically feasi-ble. Getting many canisters into a200–300 metre long deposition holein a safe and efficient manner is noteasy. It also has to work under thevarying rock conditions that may be


Boring of horizontal deposition holes is done in three stages with the aid of a water hammer.

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encountered. Long-term safety mustbe further investigated as well.

D EVE LO P M E NT N E E D E D

Together with our Finnish counterpart,Posiva, we have started a developmentproject to see whether the methodcan be developed with an undimin-ished level of safety. A feasibility studywas recently concluded, and the pro-ject has now proceeded to design ofthe deposition equipment and variouscomponents in the repository. Hori-zontal deposition differs from vertical

in one important way. Depositiontakes place in parcels. Around thecanister and the bentonite is a steeldeposition cylinder. The cylinder isperforated so that water can get intothe bentonite clay and make it swell.The steel cylinder will eventually cor-rode away.

AI R C US H I O N SLI FT TH E PAR C E L

An advanced remote-controlleddeposition machine that utilizes aircushions or water-driven cushions

to reduce friction is required to movethe nearly 50-tonne parcel with canis-ter and buffer. A spacer block of ben-tonite clay is placed between eachdeposition parcel. When all positionsin a deposition hole have been filled,the hole is sealed with a concrete plug.

Boring of the deposition holes is animportant issue in the project. Theholes must be sufficiently straight andsmooth so that the deposition contain-ers won’t get stuck. A new boringmethod will be used. The equipmentconsists of a water hammer mountedin a steel frame. First a very straightpilot hole is bored with a diameter of25 centimetres. This hole is then usedto guide the boring equipment in asecond stage, when the diameter isincreased to more than 1.4 metres.The final diameter, about two metres,is reached in a third and final boringstage.

F U LL- S CALE TR IALS

Full-scale tests of boring and deposi-tion in the Äspö HRL are planned atthe beginning of 2004. A total of threedeposition holes will be bored. Whilethe holes are being bored, the depo-sition equipment will be designed andmanufactured.

The equipment will be delivered during2005 and the deposition tests will lastuntil the end of 2006. The project willbe concluded by an evaluation in 2007.

Hor i zon ta l depos i t ion


Horizontaldeposition

In horizontal deposition, the 300 metre long holes extend directly from the transport tunnels.

Deposition takes place in parcels. Around the canister and the bentonite is a steel depositioncylinder.

Steel cylinder

End plug(bentonite)

Canister

Bentonite rings

End plug(bentonite)

Steel end

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Backf i l l and P lug Tes t The tunnels in the future deep repository for spent nuclear fuel will be filled with clayand crushed rock and then plugged.

SKB is currently testing the techno-logy for backfilling and plugging depo-sition tunnels in an actual environment.At a depth of 420 metres we havebackfilled a portion of a blasted tunnel,built a plug and installed measurementequipment. We intend to measure thesealing capacity of the backfill and theplug during the next few years. We willalso register how the backfill materialinteracts mechanically with both thebuffer material and the rock.

R E D U C E F LOWS

The main function of the backfill ina deep repository is to prevent thebentonite from swelling up out of thedeposition holes, and to reduce thewater flow along the tunnels. The back-fill material should have such a chemi-cal composition that it does not affectthe buffer or the copper canister.

The work of testing different backfillcompositions commenced with a num-ber of initial field tests in the Äspö HRLin 1995–1996. These studied howmixtures of 10, 20 and 30 percent (byweight) bentonite and crushed rockworked when compacted. SKB alsotried different techniques for compact-ing the material and investigated whatdensities could be obtained in practice.

I N C LI N E D LAYE R S

Preparations for the present-day full-scale tests started in 1998. The innerpart of the 30 metre long test area inthe tunnel has been filled with a mixtureof 30 percent bentonite and 70 percentcrushed rock. The bentonite makes thematerial swell and seals all cavitiesbetween the backfill and the tunnel roof. The backfill has been applied in sec-

tions with an inclination of 35° withthe aid of specially designed machinesdeveloped for the purpose. The inclina-tion makes it easier to handle seepagewater during compaction. Altogetherthere are six sections with the mixtureof bentonite and crushed rock. Thedifferent sections are separated bypermeable drainage mats of plasticfabric from which water is supplied.

C R US H E D R O C K

The outer part of the experimentalarea is built up in the same mannerwith inclined layers, four sectionsand drainage mats. Here the backfillconsists solely of crushed rock. Thespace between the tunnel roof andthe crushed rock is difficult to fill andcompact. With time the material alsosettles, no matter how well packed it


Checking the density of the backfill material.

Construction of plug.

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is from the start. The space at the roofwould therefore serve as a flow pathfor water. In the outer part of the tun-nel, this space has therefore beenfilled with blocks of bentonite andcrushed rock and with bentonite pel-lets. This is not necessary in the innerpart of the tunnel. When the backfillbecomes saturated with water, thebentonite swells and seals the cavities.

There are around 200 different mea-surement instruments in the test area.We measure the water pressure in therock as well as the total pressure, watercontent, local hydraulic conductivity andcompression properties of the backfillmaterial. This collection of measurementvalues started right after installation in1999. In 2001 we increased the waterpressure in the drainage mats to speedup the saturation process. The test area

is now saturated with water. During2003 we measured hydraulic con-ductivity along the tunnel by graduallychanging the water pressure in thedrainage mats. Water flow near theroof and floor and in the centre weremeasured separately.

SPAN ISHCOLLABORATION

The test is being conducted in col-laboration with SKB’s Spanish coun-terpart Enresa, which has undertakento measure and model permeability inthe backfill.The flow tests started during 2003and will continue until the end of 2005.The results will be used to develop themathematical models that we use in oursafety assessments. The concrete plugused to seal the tunnel is 2.4 metres

thick. This is needed so that the tunnelcan withstand the water pressure,which is equivalent to a depth of400 metres. The purpose of the plugis twofold: to cut off the water flowalong the tunnel and to resist the pres-sure exerted by the backfill and thewater.

The plug we have designed is intend-ed to be temporary and only needs tofunction until final closure of the repos-itory. It has not yet been determinedwhat permanent plugs will look likeand what requirements they will haveto meet.

Backf i l l and P lug Tes t


The backfill is emplaced in eleven sections separated by drainage mats.

Backfilland Plug Test

Drainagematerial

Concrete wall

Cable tube

Drainage mats

O-ring ofbentonite

30/70 Bentonite/crushed rock

Blocks of 20/80bentonite/sand and pellets

Concrete plugConcreteblocks

Bentonite blocks and pelletsCrushed rock

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Can is te r Ret r ieva l Tes tIf the deep repository should not perform satisfactorily for some reason,we want to be able to retrieve the spent fuel.

SKB wants to proceed in steps whenbuilding the deep repository. Between200 and 400 canisters (of a total ofabout 4,500) will be deposited in theinitial operating phase. After this anevaluation will be performed. If theresults of the evaluation are positive,the remaining canisters will also bedeposited during regular operation ofthe repository.

If the results of the evaluation are notpositive, it may be decided to free andretrieve the canisters. Like deposition,canister retrieval requires the permis-sion of the regulatory authorities.

T I M I N G I M PO RTANTThe method used for retrieval dependson when it takes place. The longerthe time since deposition, the moredifficult it will be to remove the ben-tonite clay around the canisters. Thelabour and costs incurred by retrievalwill be greatest if the entire reposito-ry has been closed and sealed.

In 1998, SKB started the CanisterRetrieval Test in order to developmethods for freeing and retrievinga canister safely. Most of the practi-cal tests are being done in the ÄspöHRL.

In 2000, a canister with electricheaters was placed in a depositionhole lined with blocks and rings ofbentonite clay. The hole was thenplugged. The canister was to be leftin the deposition hole for betweenthree and five years so that the ben-tonite would become saturated withwater.

Extra water was supplied to speed upthe saturation process, and in themeantime a large number of instru-ments measure pressure, temperatureand several other parameters in thebentonite. The purpose was to obtain

Would you like to know more? Read “Techniques for freeing deposited canisters” (SKB TR-00-15).

Preparations for Canister Retrieval Test. The canister willstand in the deposition hole until the bentonite clay becomessaturated with water.

Bentonite clay can be slurried by flushing with a salt solution.

better models of the water saturationprocess. However, at the end of 2001problems occurred with the electricalheating system. Eventually the heatingsystem stopped working. A great dealof time has been spent on troubleshoot-ing. But we have managed to maintainthe experimental conditions, and so farheating is working as planned.

SLU R RYI NG TH EB E NTO N ITE

When the bentonite has become sat-urated with water, the canister will befreed. We believe that the best meth-

od is to slurry the bentonite using asalt solution containing around 4 per-cent by weight calcium chloride. Thetechnique was tested in a special slur-rying test during the autumn of 2002and the spring of 2003. The testshows that the method works.

A bentonite ring and a bentonite blockwere placed in a tank where slurryingtakes place. A powerful mixer and anannular nozzle with pumps keep thedissolved bentonite suspended in thesalt solution. When the bentonite con-centration in the salt solution is about5 percent by weight, the solution is

pumped to a decanter for dewatering.There the bentonite is separated fromthe salt solution. After dewatering,the dry solids concentration is about50 percent.

The bentonite is placed in a container.The salt solution (the clear liquid) fromthe decanter is reused for the nextslurrying. Fresh salt solution is addedto compensate for the quantity left inthe bentonite after dewatering.

C O NTI N U O US P R O C ES S

Slurrying is supposed to be a continu-ous process. An important part of thetests being conducted is determininghow long it takes to slurry the roughly22 tonnes of bentonite buffer in adeposition hole.

The tests have permitted preliminarychoice and sizing of process equip-ment. The choices were made basedon both the canister to be retrieved inthe Canister Retrieval Test and possi-ble future needs.

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Can is te r Ret r ieva l Tes t


Experimental set-up in slurrying test. Slurrying takes place in the tank in the middle.

Canister Retrieval Test

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LotThe Lot test is intended to show how the bentonite behaves in an environmentsimilar to that in the future deep repository.

The abbreviation Lot stands for LongTerm Test of Buffer Material. Holesbored in the floor of the Äspö tunnel ata depth of 450 metres contain parcelswith four metre long copper tubes sur-rounded by bentonite clay. The tubeshave a diameter of ten centimetres andare equipped with electric heating ele-ments.

A number of different measurementinstruments register heat input, tem-

perature, pressure and moisture con-tent in the bentonite.The experiments were preceded bypilot tests conducted in 1997 and1998. Two test parcels were thenexposed to elevated temperaturesduring a period of just over a year.The parcels were removed and ana-lyzed in the laboratory. The analyses confirm the results ofearlier laboratory experiments.

The current test series includes fiveparcels. The heating elements areused to simulate the decay heat fromspent nuclear fuel. The idea is thatboth KBS-3 conditions (90°C) andmore extreme conditions (130°C) canbe simulated. At the higher of thesetemperatures, the various changeprocesses in the bentonite will beaccelerated.

The project is being conducted incooperation with our Finnish counter-parts, Posiva.

The goals are to:

• Test the models we use to predictthe buffer’s properties and behav-iour after water saturation.

• Test the models we use to predicthow the properties of the bufferchange with time long after watersaturation.

• Study bacterial activity, survival andmobility in the bentonite.

• Study the scope of copper corrosion.

• Determine the bentonite’s capacityto pass gas and at what tempera-ture this occurs.

D O P E D WITH TR AC E R S

Solutes can be transported in stag-nant pore water by diffusion. In thisway the substances move from areasof higher concentration to areas oflower concentration. Diffusion causesthe solutes to be redistributed in thepore water. In the Lot test, the ben-tonite at various locations has beendoped with radioactive tracers (cae-sium-134 and cobalt-60). By deter-mining how far these tracers havemoved by the end of the test period,

Would you like to know more? Read “Long term test of buffer material. Final report on the pilot parcels” (SKB TR-00-22).

Long copper tubes surrounded by bentonite clay have been lowered into boreholes.

17

we can get an idea of the diffusionprocesses in the bentonite.

We are also interested in investigatingwhether bacteria can survive in ben-tonite. If sulphate-reducing bacteria cansurvive and multiply near the canister,this could entail an increased risk ofcanister corrosion. Sulphate-reducingbacteria produce sulphide, which canin turn form copper sulphide if it comesinto contact with the canister. To ensurethat the buffer is not a suitable livingenvironment for bacteria, colonies ofbacteria have been implanted at dif-ferent locations in the bentonite.

Another purpose of the test is to studycopper corrosion. A number of verywell-characterized pieces of copperhave therefore been implanted at dif-ferent locations in the bentonite. Thesepieces will be thoroughly examined inthe laboratory at the end of the testperiod.

FO U R PAR C E LS LE FT

The Lot test started in 1999. The firstparcel was retrieved in 2001 by over-coring. The remaining four test parcelswill remain intact for at least five yearsand will not be retrieved until 2004 atthe earliest.

The retrieved parcel has been ana-lyzed with respect to:

• Physical properties (hydraulic con-ductivity, swelling pressure andplasticity).

• Mineralogical stability (montmoril-lonite, accessory minerals).

• Chemical changes (Eh, pH, corro-sive substances).

• Diffusion of caesium and cobalt.

• Copper corrosion.

The results are currently being com-piled and will be published.

Lot


The overcored test parcel is lifted out of theborehole and analyzed in the laboratory.

Lot

The test parcels are heated and we examine how the bentonite is affected.

18

TBTThe purpose of the TBT test is to determine how the bentonite clay in the bufferis affected by high temperatures.

The TBT test is being conducted bySKB’s French counterpart Andra. TBTstands for Temperature Buffer Test. Inthe test, two canisters of French typehave been placed in the same deposi-tion hole.

The goals of the project are to deter-mine:

• How well the bentonite withstandstemperatures above 100°C.

• The temperature distribution result-ing from deposition with two differ-ent buffer designs.

F IVE P HAS ES

The test is divided into five phases:Design, installation, operation, model-ling, and retrieval.

The project was started in 2002 and isscheduled to be concluded in 2006.

The French canisters are intendedfor reprocessed vitrified waste. Theirdesign therefore differs in severalways from that of the Swedish canis-ters. First and foremost, the materialis not copper but carbon steel. Thewall thickness is 50 millimetres. Thecanisters are much smaller than theSwedish ones. They are three metresin length and 610 millimetres in dia-meter.

The heat output from the French can-isters is also greater than from theSwedish ones. The temperature on thecanister surface will exceed 100°C. Tosimulate these conditions, the canistersare equipped with heating elements.Thermocouples are placed inside thecanisters and on the surface for con-tinuous temperature measurement.There are additional instruments in

the bentonite and the surroundingrock to measure the progress ofwater saturation and the build-up ofswelling pressure in the bentonite.

G O O D M O D E LS

The higher temperature means thatvery good calculation models areneeded to predict the temperaturedistribution in the area around a can-ister. If the models are not reliable, itwill be very difficult to determinewhat dimensions the buffer aroundthe canisters should have.

The calculation models used are cou-pled THM models, which take intoaccount thermal (T), hydraulic (H) andmechanical (M) changes in the buffer.Uncoupled models, which only takeinto account one parameter at a time,will also be used for reference calcula-


Bentonite blocks are lowered into the deposition hole.

19

tions. The buffer used in the TBT testis MX-80, which is the same type ofbentonite used in SKB’s own tests.

To facilitate the supply of water andthe coupled THM calculations, thegap between the wall of the deposi-tion hole and the buffer is filled withsand. Titanium water tubes emptyinto the sand.

When the top canister was emplaced,the space between the canister andthe bentonite was filled with sand. Thespace between the bottom canisterand the bentonite is unfilled.

TBT


The French canisters are of carbon steel and smaller than the Swedish ones.

Test design with two canisters.

One of the canisters is deposited.

TBT

20

Two-Phase F lowTwo-phase flow means that liberated gas in the groundwater flows separately inthe fractures in the rock. This reduces the capacity of the rock to conduct water.

Would you like to know more? Read “Groundwater degassing and two-phase flow in fractured rock” (SKB TR-01-13).

Groundwater at great depths con-tains dissolved gases. The main com-ponents are nitrogen, argon, helium,hydrogen and methane. The propor-tions of the different gases varyregionally as well as with rock typeand depth. The total gas content alsovaries. On Äspö it amounts to aboutthree percent.

D I F F E R E NT O R I G I N S

The gas has several different origins.Some comes from the mantle deepinside the earth, some with water fromthe ground surface. Other sourcesare corrosion processes and micro-bial activity. Gases can also comefrom a defective canister. At depthsas great as 500 metres, all gas is nor-mally dissolved in the water. The pres-

sure inside the fractures in the rockwill be lowered in the vicinity of a tun-nel or a borehole, since they are con-nected to an opening at atmosphericpressure. As a result, the groundwa-ter in the fracture may no longer beable to keep the gas in solution. Thisphenomenon is called degassing.First, gas bubbles form in the water. Ifthe process continues, two-phaseflow occurs in the fracture where thegas and the water flow separately.

D I M I N I S H E D F LOW

At several places in Sweden andabroad, hydrologists have noted thatthe inflow in certain investigatedfractures is lower than expected. Oneexplanation of this phenomenon couldbe that two-phase flow has resulted

from degassing of the groundwater inthe fractures.

Knowledge of how two-phase flowarises and manifests itself near bore-holes and tunnels is necessary inorder to interpret the observations ofhydraulic conditions that are madeduring the site investigations andconstruction of the deep repository.

CAN AF F ECTTH E B U F F E R

Furthermore, this knowledge is need-ed to understand how the buffer andthe backfill behave in the repository,especially during the operating phaseand at closure. A lower inflow ofwater will, for example, mean that ittakes longer for the buffer to becomesaturated.

The experimental station in the Äspö HRL.

Three-dimensional geological model of the experimental area.Geological structures in the experimental area.

21

Two-Phase F low


CALC U LATI O N SS I M U LATE F LOWS

The measurements have also beensupplemented by laboratory experi-ments. The subsequent model calcu-lations included both 3D simulationsof single-phase flow and 2D and 3Dsimulations of two-phase flow. In the3D modelling of two-phase flow, par-ticle transport was also simulated.

Important results are:

• Both model calculations and resultsshow that two-phase flow occurs inthe rock on Äspö.

• Different models provide a consis-tent picture of how the gas plumespreads.

The two-phase flow test was startedin 1994. First a test led by SKB incooperation with USDOE (USA) wasperformed, followed by a bigger testunder the leadership of the Germanorganizations Gesellschaft für Anlagenund Reaktorsicherheit (GRS) andBundesanstalt für Geowissen-schaften und Rohstoffe (BGR).

The tests were conducted in a nicheat a depth of 360 metres andinvolved the following activities:

• Geological mapping of the test area.

• Hydraulic tests.

• Gas flow tests.

• Tracer tests with gas and particles.

• Degassing tests and gas sampling.

• The width of the fractures is toogreat to prevent gas flow. For thisreason, the gas pressure will notexceed the groundwater pressure.

• The unexpectedly low water flowsthat have been observed cannot beexplained solely by two-phase flow.

Vertical fractures

Front

Äspö diorite

Fine-grained graniteHorizontal fractures

Floor

Two-Phase Flow

Vertical fractures

Rock matrix

Horizontalfractures

22

Lasg i tBy pressurizing a canister with helium, we can measure how the gas movesthrough the surrounding buffer.

Inside the copper canister there is aninsert of nodular iron (a kind of castiron) for greater strength. If water wereto enter the canister, the iron in theinsert would corrode and form hydro-gen gas, which collects in the emptyspace in the canister (about one cubicmetre).

S LOW I N C R EAS EO F P R ES S U R E

The pressure inside the canister buildsup very slowly as the insert rusts. Nopressure equalization takes place. Thegas cannot escape from the canister,

since the bentonite in the surroundingbuffer is very impervious. This can inturn lead to a number of problems:

• The pressure in the canister becomesunacceptably high.

• Transport pathways are formed inthe buffer.

• There may also be a risk that thebuffer will dry out. This is not likely,however.

For several years, SKB has conducteda series of laboratory-scale experimentsinvolving gas injection through thebentonite. Experience from these

experiments suggests that gas evolu-tion will probably not be a problem in adeep repository. It is difficult to modelwhat happens when the gas pene-trates through the bentonite. Aboveall, it has been found that the resultsare sensitive to the test conditions.This makes it difficult to apply the lab-oratory results to other scales.

F U LL- S CALE TESTS

To find out more about gas transportthrough the buffer on a full scale, wehave therefore started the Lasgit (LargeScale Gas Injection Test) project in theÄspö HRL.

The purpose of the project is to:

• Carry out and interpret a full-scalegas injection test under the condi-tions that will prevail in a deeprepository.

Would you like to know more? Read “Äspö Hard Rock Laboratory – Annual report 2002” (SKB TR-03-10).

Experimental set-up for the Lasgit test.

23

Lasg i t


• Evaluate the questions surroundingscaling-up and their importance forgas transport and buffer function.

• Learn more about gas transport inbentonite.

• Generate data of such quality thatthey can be used to test and vali-date models.

• Demonstrate that gas evolutioninside a canister does not have anyappreciable negative consequencesfor the deep repository’s barriers.

F I LLE D WITH H E LI U M

The buffer and canister are loweredinto a deposition hole. The hole issealed with a lid, which represents theweight of the backfill. We then pres-surize the canister by pumping in heli-um via a valve in the lid. Then we mea-sure the pressure at which the gas

starts to pass through the bentonite. Itis important that the buffer is complete-ly water-saturated. This normally takesseveral years. But we can shorten thistime to about two years by using ben-tonite blocks fabricated with nearly 100percent water saturation and by fillingthe gap between the bentonite and thewalls of the deposition hole with water.

N O E LECTR I C H EATE R S

No electric heaters will be used in thetest. The simulated situation with cor-rosion and gas evolution is only pos-sible during a late stage when theradioactivity has declined to such lowlevels that the heat output from the fuelis negligible. The experiment is a jointproject between SKB, Posiva (Finland),BGR and GRS (Germany) and Andra(France). The canister was delivered tothe Äspö HRL in July 2003, and we

expect to start the water saturationprocess in January 2004. The firstgas injections will take place in early2006, according to the plans. Thetest will be concluded the followingyear.

Lasgit

All water supply tubes run in channels on the top of the bentonite blocks.

Bufferbentonite

Hydrationtubing Copper

canister

Interfacebetween

individual“donuts” of

bentonite

Hydrationfilter

Wall of emplacementborehole

Gasinjection

filter

Water tube

Borehole wall

Wetting filter

Bentonite buffer

Copper canister

Gas injection filter

Boundary betweenbentonite rings

Tests will be conducted in a deposition hole.

24

Col lo id Pro jec tCan very small particles, known as colloids, transport radionuclidesup to the ground surface?

Colloids are particles that are so smallthat they remain suspended in a solu-tion without sedimenting. Their sizeusually varies between a thousandthand a millionth of a millimetre.

Colloidal particles can be formed as aresult of chemical reactions, for exam-ple oversaturation when two differenttypes of water are mixed. They can alsobe formed by erosion. Colloidal parti-cles in deep groundwaters consist pri-marily of clay, silicon and iron hydroxideparticles. The particles may also con-sist of single large organic moleculesor accumulations of such molecules.

Radionuclides could also form colloids,but only in very concentrated aqueoussolutions. The problem in these solu-tions is the concentration itself, not thecolloid formation.

SKB has investigated colloids for manyyears and carried out a large numberof measurements of the colloid con-centrations at different depths and indifferent types of groundwater.

The results show that the colloid con-centrations near the surface can behigh. At repository depth (about 500metres), however, they are very low,since the salinity is higher. Colloids areonly of importance for long-term safetyin a deep repository because radionu-clides can adhere to colloidal particlesand be transported with them.

CAN B E M EAS U R E DD I R ECTLY

The day the repository is closed, waterwill flow in from all directions and wewill get a mixture of oxygen-poor andoxygen-rich as well as saline and non-saline groundwater, which could leadto colloid formation. The colloids areexpected to be carried away by theflowing groundwater. The time thistakes is determined by the size of thegroundwater flow. It may be a fewyears. In later stages of the repository’shistory, flowing groundwater may erodethe bentonite clay in the buffer so that

colloidal particles are formed. Thesecould then bind any radionuclides thathave escaped from a defect canisterand carry them up to the surface.

So far it has been difficult to study col-loids, since there has not been any sim-ple method for measuring their concen-trations. They are also troublesome tostudy, since they do not, for example,obey the laws of chemical equilibrium.However, modern laser technologymakes it easy to measure colloids inthe field. To find out more, SKB startedthe Colloid Project in 2001 in coopera-tion with our Finnish counterpart Posivaand German BMWA (Bundesministeri-um für Wirtschaft und Arbeit). Accord-ing to the timetable, the project will beconcluded at the end of 2003.

The goals are to:

• Confirm previously measured colloidconcentrations in the groundwater atthe Äspö HRL.

• Determine the stability of the colloids.

Would you like to know more? Read more about colloids in “Diffusion of organic colloids in compacted bentonite” (SKB TR-00-19).

Colloids are particles that are so small that they remain suspended in a solution.

• Investigate to what extent the col-loids contribute towards transport-ing radionuclides.

• Study to what extent the bentonitecontributes to colloid formation.

M EAS U R E M E NTSWITH LAS E R

In 2002, the natural concentration ofcolloids was measured in eight differ-ent boreholes in the Äspö HRL withthe aid of a high-resolution laser. Thesalinity of the groundwater differs in thedifferent boreholes. The measurementsshow that the colloid concentration inthe water declines with increasingsalinity. The particles consist mainly ofaluminium and silicon compounds, forexample clay particles.Further measurements are now beingconsidered in two nearby boreholesthat pass the same fracture with homo-geneous geology. Water with colloidsis pumped into the fracture in one ofthe boreholes, while water is pumped

out of the fracture in the other. In thisway we can measure whether the col-loid concentration declines betweenthe boreholes and determine to whatextent the colloids adhere to the frac-ture-filling minerals. A colloid project isalso under way in the Grimsel RockLaboratory in Switzerland. Laser tech-nology is being used for the measure-ments there too. The groundwater thereis non-saline even at repository depth,and the concentrations that have beenmeasured are much higher than thosein the Äspö HRL.

LAB EXPE R I M E NTS CON FI R M

The field experiments are supplement-ed with experiments in the laboratory.There we have studied how bentoniteclay contributes to colloid formation atdifferent salinities. Bentonite clay wasput in aqueous solutions with differentsalinities. The sedimentation rate of thebentonite particles was then studied.

The laboratory experiments confirmedthe results of the field experiments. Thecolloid concentration was highest at lowsalinities. The chemical composition ofthe water is also of importance. Themore calcium the solution contained,the faster the particles sedimented.

Laboratory experiments are also beingconducted in Switzerland. The conclu-sion from these experiments is that col-loids are not stable in saline water. Thisis true, for example, of the pore waterin compacted bentonite.

The results from the Colloid Project,as well as from the Swiss project, willbe used to verify that the models weuse for colloid transport in our safetyassessments agree with reality.

25

Col lo id Pro jec t


Injection of colloids and measurement of the quantities of injected and naturalcolloids in the draw-off hole.

The colloid concentration decreases with increasing salinity.The measurements were made in different boreholes in theÄspö HRL.

-

300

250

200

150

100

0

0 5000 10000 15000

Col

loid

con

cent

ratio

n (µ

g/l)

KR0012B

SA1229A

HA1330B

HA1330B

KA3110AKA1755A

HA2780A

SA2074A

Chloride ion concentration (µg/l)

50

Natural colloids

Bentonite colloids andtracer

Fracture zone

Packer

5 m

Flow direction

Colloid Project

26

R N R Exper imentAn exchangeable cell in a specially built probe makes it possible to conductexperiments on how radionuclides move.

In the RNR Experiment (RadionuclideRetention Experiment) we are investi-gating how the rock retards and “fil-ters” the radionuclides. Most radionu-clides adhere (sorb) readily on thefracture surfaces in the rock. The radio-nuclides are said to have high Kd val-ues. Kd is the distribution coefficientand is in this case a measure of howreadily a radionuclide is sorbed ontothe rock under given conditions.

Measurements in the laboratory havegiven numerical values of the distribu-tion coefficients. The Kd values canthen be used in the mathematicalmodels that serve as a basis for oursafety assessments.

However, it is difficult to simulate actu-al conditions exactly in the laboratory.The ability of the rock to retain radio-nuclides is dependent on factors suchas the concentrations of colloids anddissolved gases, and the quantity oforganic matter.

No matter how well a laboratory experi-ment is performed, it can never perfect-ly simulate conditions in the rock. Topermit more reliable experiments to beperformed in the rock, we have builtchemical reaction chambers into probesthat can be lowered into a borehole.The probes have been named Chem-lab 1 and Chemlab 2. The experimentscan then be conducted under naturalconditions in terms of pressure, tem-perature and water composition.

S O LU B I LITY I SAF F ECTE D

We are especially interested in howthe oxygen-free conditions that prevailat a depth of 500 metres will influ-ence the solubility of radionuclides,and the retention properties of therock. We also want to find out moreabout to what extent colloids, humicsubstances and organic acids can sorbradionuclides and transport them tothe surface.

The experiments are performed inexchangeable cells inside the Chemlabprobes. The chamber looks differentdepending on what experiment wewant to do. The probes prevent theradionuclides from spreading into thesurrounding rock and groundwater. Inthis way we can work with fragments ofspent nuclear fuel, which would other-wise be too risky.

The design work on the Chemlabprobe began in 1991, and the firstprobe was delivered to the Äspö HRLin 1996. Experiments with diffusion ofradionuclides through bentonite werecommenced in 1997. The investigatednuclides were strontium-85, caesium-134, cobalt-57, iodine-131 and tech-netium-99. Agreement between thelaboratory results and the measuredvalues was good.

Radiolysis experiments with the ele-ment technetium have also been per-formed. Radiolysis is splitting of water

Would you like to know more? Read more in “Chemlab – A probe for in-situ radionuclide experiments” (SKB TR-01-14).

The reaction chamber in the Chemlab probe can be exchanged, depending on whatexperiments we wish to perform.

27

R N R Exper iment


by radiation from the spent nuclearfuel. Different substances are thenformed which can oxidize the tech-netium to forms that are more solubleand mobile than the original form. Anevaluation of the measurement resultsis currently under way. Technetium isnot the only element that oxidizesreadily. Neptunium and plutonium canalso be converted to more solubleand more mobile forms. We are cur-

rently investigating this in cooperationwith Institut für Nukleare Entsorgungin Germany. The experiments arebeing conducted in a natural fracture,which has been overcored.

Later on we will also study:

• To what extent radionuclides thathave been sorbed onto the rock canbe de-sorbed again.

• How colloids and microbes affectsolubility and sorption.

• How radionuclides in spent nuclearfuel are dissolved and sorbed.

The probe makes it possible to conductexperiments directly in the rock fractures.

Illustration of Chemlab 1 and Chemlab 2.

Exp.cell

P

P

DP

P

P

P

T1

V00

V02aV02b

P1 P2

T2 T3

V03aV03b

V04aV04b

V05

V01

Groundwater inlet

Packer

Eh andpH electrodes

Filter

Tracerreservoirs

Experimentalcell

Pressurereduction

Pumps

Pressurereduction

Fractioncollectors

Filter

Groundwaterinlet

Electro valve

Filter

Reservoir

Pump

Internal pres-sure regulator

External pres-sure regulator

RNRExperiment

28

TrueTracer tests are supposed to increase our understanding of how radionuclidesare transported and answer the question whether results obtained on one scaleare also valid on another.

A project has been under way at theÄspö HRL since 1994 to improve ourunderstanding of retention, i.e. how therock retards the transport of differentradionuclides. The project is calledTrue, which stands for Tracer Reten-tion Understanding Experiments.

The experiments and their interpretationare a way to confirm that the calculationmodels we use when we perform oursafety assessments agree with reality.Other participants in parts of the projectare Andra (France), Enresa (Spain),JNC (Japan) and Posiva (Finland).

The basic idea of True is to carry outa series of experiments of graduallyincreasing complexity. Each experimentincludes a series of activities. First, ageological and geometrical character-ization of the test area is done. Thisis followed by hydraulic experimentswhere the permeability of the rock

and the groundwater’s flow paths aredetermined. Then various tracer testsare performed. A number of radionu-clides with different retention propertiesare injected in fractures via boreholes.

By working on different scales, fromtests on drill cores on a decimetrescale in the laboratory to tests in net-works of fractures over distances ofup to 100 metres down in the rock,we learn a great deal about to whatdegree results obtained on one scaleare also valid on another.

The goal is that we should be ableto utilize the gathered information todesign simplified, but reliable, modelson how radionuclide retention workson a large scale.

The results of the first phase of theproject, True-1, were reported in 2000.Here the investigations took place in asingle fracture on a detailed scale

(0.5–10 metres). The purpose of thisstage was primarily to test and adaptmethods and equipment in preparationfor coming experiments.

STR ICT SAFETYR EQU I R E M E NTS

As always in experiments with radioac-tive materials, the safety requirementsare very strict. We must not lose anyradioactive tracers along the way, butmust have full control over how muchwe inject, how much comes out of ourboreholes, and how much is left on thefracture surfaces in the investigatedsystem.

True-1 was preceded by a number oflaboratory experiments on parts of drillcores for the purpose of determiningto what extent radionuclides adhere todifferent rocks and minerals. The trac-ers used were sodium-22, rubidium-

Would you like to know more? Read SKB TR-00-07, SKB TR-02-13, TR-02-14, TR-02-15 and TR-02-16.

•••••••

The True experiments are conducted on different scales.

Detaljskala(~ 5 m)

Laboratorieskala(< 0.5 m)

Blockskala(~ 50 m)

Laboratory scale (< 0.5 m)

Detailed scale (~5 m)

Block scale (~50 m)

29

86, strontium-85, calcium-47, barium-133 and caesium-137. The testsshowed that the substances can beranked in order of their tendency toadhere as follows: sodium < calcium≈ strontium << barium ≈ rubidium <caesium. The tests that were subse-quently done in the Äspö tunnel con-firmed the laboratory results. Of theinvestigated tracers, caesium andstrontium are the ones that are mostrelevant for the deep repository. Asexpected, most of the original quantitywas retained. About 15 months afterthe start of the test, over 60 percentof the original quantity remained onthe fracture surface. We have beenable to show that caesium movesapproximately 100 times slower thanthe groundwater.

The investigated fracture will then befilled with epoxy resin and overcoredfor further examination in the laborato-ry. In this way the geometry and distri-bution of the pore spaces can be deter-mined, as well as the distribution ofthe radionuclides that have adheredto the surface of the rock.

N ETWO R KO F F R ACTU R ES

In True Block Scale we studied howradionuclides move in a rock blockwith its network of small fractures ofvarying size. The scale was 10–100metres. A key question was whetherthe longer time and length scale leadsto greater retention.

The area studied consists of a fracturenetwork defined by five fractures. Thefractures are in turn intersected by fiveboreholes drilled in a fan array throughthe fracture network. Radioactive

tracers were injected at a number ofpoints in the fracture network.

The results of True Block Scale werereported in 2002. Evaluation by meansof mathematical models shows thatthe test scale was presumably of noimportance. At test lengths of around

15 metres, results of roughly the sameorder of magnitude as in True-1 areobtained. The ranking order of the dif-ferent tracers is also the same.

Results from the still-ongoing project,as well as results from new experimen-tal set-ups, will be analyzed in TrueBlock Scale Continuation.

The preliminary indications show thatretention is dependent on the testscale and how complex the flowpaths are. The results will be com-pared with the values obtained frommodelling of new data.

True

You can order or download them as .pdf files from our website at www.skb.se

True-1

True Block ScaleThe experimental site for True-1.

Principle of a tracer test.

•••••••

30

LTD ETo what extent can radionuclides migrate out into micropores in the rock?And how long do they stay there?

By “matrix diffusion” is meant the trans-port of solutes that takes place from afracture into the micropores in the rock.This transport does not occur with theflowing groundwater. Instead it is therandom movements of the particlesthat cause the substance to be trans-ported from areas of high concentra-tions to areas of low concentrations,a process known as diffusion. Solutescan also diffuse into cul-de-sacs andfractures with stagnant water.

SAM P LI N G D I STU R B STH E R O C K

It is difficult to study matrix diffusionin the laboratory. Certain parameters,such as groundwater compositionand pressure, are difficult to re-cre-ate. Nor is the rock any longer undis-turbed. Samples taken for laboratoryexperiments can, for example, bedamaged by the drilling or the stressrelief that takes place when a rocksample is taken out.

In the LTDE experiment we investi-gate to what extent different radionu-clides migrate into the rock matrix.LTDE stands for Long Term DiffusionExperiment.

R ES U LTS C O M PAR E DWe also want to obtain data on thesorption properties of different radionu-clides. Sorption is the process bymeans of which a substance in solu-tion adheres to a solid phase. Sorp-tion can occur by the substance

either being adsorbed by (adhering to)the solid phase or being absorbed(taken up) by it. The data we obtain inthe field experiments for both diffusionand sorption will then be comparedwith the results measured in the labora-tory. These results will then be used toimprove the transport models we use inour analyses of the long-term safety ofthe deep repository. We will also findout whether the results achieved on alaboratory scale are also valid underthe conditions prevailing in the rock.


Installation of equipment for LTDE.

The results from the tests will be used in our safety assessments.

31

I S O LATE D STU B

The experiment is being conducted ina borehole drilled so that the diametergradually diminishes. A “stub” – a smallrock volume around part of a fracture– has been isolated from the surround-ings. The fracture is located 9.8 metresinto the rock from the tunnel wall.

A hole of even smaller diameter hasthen been drilled through the stub.This hole reaches behind the stub,into the undisturbed rock. The stub

itself is 15 cm long. This arrangementhas a twofold purpose: to study howradionuclides diffuse through a frac-ture surface, and to investigate diffu-sion in completely undisturbed rock.

Groundwater is first allowed to circu-late through the packed-off section ofthe borehole in front of the stub. Thisphase began during 2003. When thecomposition of the groundwater hasstabilized, radioactive tracers are in-jected. There they are pumped around

with the circulating groundwater forbetween three and four years. Nowand then samples are taken of thesolution.

The same tracers as in True-1 will beused. In addition, we will investigateseveral other tracers of importance forlong-term safety in a deep repository.

N O P R ES S U R ED I F F E R E N C ES

During the course of the experiment,the tracers will diffuse into the sur-rounding rock. We will make sure thatit is really diffusion that is involved andnot advection (transport with the flow-ing groundwater) by ensuring that thepressure in the packed-off section isthe same as the pressure in the rock.

Substances with low diffusivity are in-jected ahead of substances with highdiffusivity. In this way we can controlthat the radionuclides remain within agiven distance of the borehole.

After approximately four years, thestub will be overcored and taken tothe laboratory for further analyses.The concentrations of tracers in thesurrounding rock will then be deter-mined. The method used in the ana-lyses will depend on which tracer isto be studied.

LTD E


The experiment is being conducted in a part of a borehole that has been isolated fromits surroundings.

LTDE

32

Matr i x F lu id Chemis t ry Exper imentThe water in the rock’s pores can differ in terms of composition and changes fromthe water running in the fractures.

The Baltic Sea has been in turns botha lake and a sea since the most recentice age. This has affected the compo-sition of the groundwater at differentdepths. SKB has taken samples of thegroundwater in the rock’s fractures formany years, giving us plenty of knowl-edge on how water chemistry varieswith depth.

What we haven’t known so muchabout, however, are the properties ofthe water in the pores and microfrac-tures in the rock matrix. This water iscalled matrix fluid.

M O R E R E LIAB LE CALC U LATI O N S

Greater knowledge of how the matrixfluid changes with time gives us betterinput data for the models we use tocalculate how radionuclides are trans-ported in the rock nearest the canisterover very long periods of time.

The composition of the matrix fluid isnot necessarily the same as the com-position of the water in the nearby frac-tures. The matrix fluid is more stagnantand may have entered the pores andmicrofractures during previous geo-logical eras, when the groundwaterhad a different composition.

There may also be an interactionbetween the matrix fluid and the waterin the fractures. In- and out-diffusion ofvarious substances can affect thewater’s composition. By “diffusion” ismeant transport of substances dis-solved in the water (solutes) by ran-dom movements. This takes placefrom areas of high concentration toareas of low concentration. Diffusioncan lead to slow changes in the matrix

fluid. Its salinity, for example, may grad-ually increase or decrease with time.

I N F LU E N C EO F M I N E R ALS

Another factor that influences the com-position of the matrix fluid is the miner-al composition of the rock. Here againthere is an interaction. The chemicalcomposition of the matrix fluid deter-mines which reactions occur with theminerals. The composition of the min-erals determines which substances aredissolved in the water.

Assume, for example, that we have arock matrix consisting of a significantquantity of quartz (granite, for example)

and that the quartz contains fluid inclu-sions. The fluid inclusions, which arelocated centrally in the quartz grains,usually remain intact. Inclusions, whichare located close to the edges of thegrains, can however influence the com-position of the matrix fluid by releasingsalt. Eventually the surrounding ground-water may also be affected. Movementsin the earth’s crust can also cause saltto be released. The quartz minerals maybe fractured by the forces to which therock matrix is subjected, and the matrixfluid may then come out into the sur-rounding groundwater.

Earlier experiments in Canada haveshown that there may be great differ-

Would you like to know more? Read about the Matrix Fluid Chemistry Experiment in SKB TD-02-13, TD-02-18 and TD-03-02.

The composition of the water in the pores ofthe rock matrix influences the ability of the rockto retain radionuclides.

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ences in composition between thematrix fluid and the water running in thesurrounding fractures. We want to knowwhether the same conditions prevail onÄspö and therefore started the MatrixFluid Chemistry Experiment in 1999.The goals of the Matrix Fluid ChemistryExperiment are to:

• Determine the origin and age of thematrix fluid.

• Establish whether present or pastin- or out-diffusion has influencedthe composition of the matrix fluid.

• Derive a range of typical groundwatercompositions that can be used tocalculate what happens in the areaimmediately surrounding the canister.

• Determine how microfractures influ-ence groundwater chemistry.

I N TH E LABAN D I N TH E F I E LD

Methods have been devised for takingsamples of the matrix fluid from prede-termined isolated borehole sections ata depth of 450 metres. The boreholeswere positioned on the basis of para-meters such as rock type, minerals anddepth.

Besides the field experiments, laborato-ry investigations are also included. Thecores from the boreholes were exam-ined to determine rock type and mineralcomposition. We have also measured

porosity and density and performedleaching tests. Diffusion experimentshave also been conducted at theUniversity of Bern.

The results of the measurementsshow that the differences between thecomposition of the matrix fluid andthe water in the surrounding fracturesare very small. This suggests that thematrix fluid is a part of the large-scalegroundwater system on Äspö.

Matr i x F lu id Chemis t ry Exper iment

The reports can be ordered by e-mail at [email protected]

Set-up of Matrix Fluid Chemistry Experiment.

Matrix Fluid Che-mistry Experiment

°°°

Gas regulator

Sampling cylinder

Measuringflask

Packers

Evacuationvalve

Shut-off valve

To measuringflask

Regulating valve

Regulating valve

Regulating valve

Logger

Measuringflask

Measuringflask

Pressure cylinderElectricalconductivitysensor

Regulatingvalve

Evacuationvalve

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Rex Pro jec tApproximately one year after the repository has been closed, all oxygen will havebeen consumed by the minerals and bacteria in the rock. The bacteria in particularare responsible for this consumption.

Oxygen dissolved in the groundwatercan cause the copper canisters to cor-rode. There is not normally any oxygenin the water at a depth of 500 metres.During construction of the repository,the inflowing water will be pumpedaway and the tunnels will fill with air.Some of this oxygen will remain afterclosure and dissolve in the ground-water that refills the repository. Smallquantities of oxygen may also betrapped in air pockets.

M E LTWATE RS E E PS D OWN

Oxygen can also get into the ground-water in other ways. When the landis covered with ice during an ice age,meltwater can seep deep down into

the bedrock. This water is oxygen-rich.Oxygen consumption on the way downis less during warm periods, since allorganic oxygen-consuming matter inthe uppermost soil layers has erodedaway.

In the Rex Project (Redox Experimentin Detailed Scale) in the Äspö HRL,we have studied how oxygen reactswith rock minerals in tunnel walls anddeposition holes.

We have also investigated the capacityof bacteria and minerals to consumeoxygen and how long this takes. Parti-cular interest was devoted to the roleof bacteria in oxygen consumption.

The project started in 1996 and endedin 2001. Besides SKB, Andra (France)and JNC (Japan) also participated.

F I E LD E X P E R I M E NTSO N ÄS PÖ

In the field experiments in the ÄspöHRL, oxygenated water was pumpedaround in a fracture that had been iso-lated from the surroundings. The frac-ture is located at a depth of 380 metresand at a distance of 8.1 metres fromthe tunnel wall.

The total volume of circulating waterwas around one litre. Prior to eachoxygen pulse the fracture was filledwith groundwater from the connectingborehole.The oxygen concentration inthe outgoing water was measuredcontinuously, as were pH and Eh.

The field experiments also includedstudies of the bacteria’s oxygen-con-suming capacity. The investigations

Would you like to know more? Read the report from the Rex Project “O2 depletion in granitic media” (SKB TR-01-05).

The overcored fracture is taken out for analysis in the laboratory.

took place at several sites in the tun-nel. The results show that the bacte-ria consumed oxygen at a rate of bet-ween 10 and 150 micrograms perlitre and day.

The other half of the fracture surfacethat was used in the field circulationexperiments was used in a replicaexperiment, which was done in France.The purpose was to confirm the resultsobtained from the field experiment.

In the replica experiment, the fracturesurface was in contact with about onelitre of water. Different quantities ofoxygen were supplied. The resultsconfirmed the values measured in thefield, i.e. oxygen consumption was ofthe same order of magnitude as in thecirculation experiment.

The field experiments and the replicaexperiment have also been supple-mented by other analyses in the labo-ratory. For example, a mineralogicalstudy has been done of different frac-ture minerals from the Äspö tunnel.

FASTE RTHAN E X P ECTE D

The results show that oxygen is con-sumed faster than previously publisheddata would suggest. Roughly one yearafter closure, all oxygen will be gonefrom the groundwater. The bacteriaare primarily responsible for this oxy-gen consumption. The oxygen is usedto oxidize organic matter in the ground-water, as well as iron and sulphur min-erals in the rock. Methane and hydro-

gen gas that diffuse up through theearth’s crust are also expected to con-tribute considerable reduction capacity.

The conclusions from the Rex Projectwill be utilized in the optimization andconfiguration of the repository. Newcalculations are planned to estimatehow deep an oxygenated glacial watercan seep down and at what depth alloxygen has been consumed.

Rex Pro jec t


Sampling of water.

Rex Project

Fracture

Sampling andinjection of O2

Pump

Measurement of:pH, Eh, O2, pressureand temperature

Packers

Flowmeter

Borehole

Borehole

Reactionchamber

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Experimental set-up for the field experiments. Oxygenated water was circulated in a fracture.

There are life forms under groundwhich, unlike most organisms on thesurface, have no need of either oxy-gen or light to survive. Many of themutilize hydrogen and methane as ener-gy sources and carbon dioxide as acarbon source to build up the organ-ism. The base of the subterraneanecosystem is made up of acetogenicbacteria, which synthesize acetic acidfrom carbon dioxide and hydrogen,and methanogenic archaebacteria,which produce methane from thesame raw materials.

Higher up in the subterranean foodchain live other species that get theirfood from the acetogens and themethanogens. These include thesulphate-reducing bacteria that con-vert sulphate to sulphide, the iron- andmanganese-reducing bacteria thatdissolve iron and manganese oxides,

and the nitrogen-reducing bacteriathat produce nitrogen from nitratecompounds.

TESTS I N TH E F I E LDMicrobiologists from Göteborg Univer-sity have been investigating the pres-ence of bacteria in deep groundwatersfor more than a decade and have con-ducted a number of laboratory experi-ments. Now it is time to take the nextstep and test the hypotheses in thereal world. The Microbe Project start-ed in 2001 at the Äspö HRL. Theexperiment is expected to last untilthe end of 2005.

The copper canisters with spent nuclearfuel have to be protected against corro-sion for at least 100,000 years. Of thesubstances present in the groundwater,it is mainly oxygen and sulphide ionsthat can cause canister corrosion. The

oxygen in the water will probably neverbe a problem for the repository. Oxy-gen is the first substance the bacteriaconsume in their metabolism. One ofthe goals of the Microbe experimentis to measure to what extent microbescan keep the repository oxygen-freeand the groundwater chemically stable.We are currently studying how and atwhat rate naturally occurring bacteriaand archaebacteria consume hydrogenand methane.

Q U ESTI O N S AB O UTLI GAN D S

It is not clear what happens if a cop-per canister is breached and radionu-clides escape into the surroundingenvironment. Bacteria tend to adhereto the rock wall or the rim of the cul-ture dish and can therefore contributeto retention of the radionuclides. But

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Microbe Pro jec tCan subterranean microbes keep a deep repository for spent nuclear fuel oxygen-freeand how do they influence radionuclide transport?

Would you like to know more? Read “Microbial processes in radioactive waste disposal” (SKB TR-00-04).

Deep down in the bedrock there are microorganisms that have no need of either oxygen or light tosurvive.

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some bacteria can also produce lig-ands, which can bind radionuclidesand prevent them from being retainedin the rock. We do not yet know wheth-er ligands can form and be of any im-portance in a groundwater environmentsimilar to that in the future deep repos-itory. We will examine this in detail inthe Microbe experiment.

If radionuclides should reach theground surface, there are several bar-riers. There are iron- and manganese-oxidizing bacteria that flourish wheredeep groundwaters reach the groundsurface. Tests have shown that theseorganisms can act as filters and pre-vent further spreading of the radionu-clides that have escaped from a can-ister and been transported up to theground surface by the groundwater.The work of quantifying the filtrationcapacity of the microbes continues.

TH R E E S ITESThe Microbe experiment is being con-ducted at three different locations inthe Äspö tunnel. Most of the experi-ment is taking place at a depth of450 metres in three cored boreholes.The boreholes intersect water-bearingfractures at different distances from thetunnel wall (12.7 metres, 43.5 metresand 9.3 metres). The equipment hasbeen installed to circulate the ground-water via three identical experimentalcabinets. An analysis instrument formeasuring very low concentrations ofhydrogen and carbon monoxide, as wellas low concentrations of methane andcarbon dioxide, has also been installed.Each experimental cabinet contains fourflow cells with growing microorganisms.

Another part of the project is beingcarried out in a niche at a depth ofabout 200 metres. There a population

of sulphur-oxidizing bacteria has takenup residence in a shallow artificial pond.The bacteria will be used to study thedistribution of stable isotopes of differ-ent elements in sulphate, sulphur andsulphide.

The third experimental site is locatedat a depth of 296 metres. Here weare investigating how iron-oxidizingbacteria can act as a biological filterfor radionuclides and tracers. Ground-water from a borehole flows throughlong shallow aquariums with gravel.Iron-oxidizing bacteria move in andform thick mats of cells, stalks andiron oxides.

Microbe Pro jec t

You can order it or download it as a .pdf file from our website at www.skb.se Microbe Project

In one of the experiments in the Microbe Project, groundwater from a fracturecirculates through a biofilm reactor.

Sulphur bacteria look like strandsof pearls in a light microscope.

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A dress rehearsal is currently beingconducted deep down in the Swedishcrystalline bedrock in the Äspö HardRock Laboratory (HRL). Methods arebeing tested and verified, conditionsanalyzed and assumptions confirmed.When the time comes for final dis-posal of the spent nuclear fuel, wemust be ready with all the necessaryknowledge.

Of course, there is no spent nuclearfuel here. Otherwise the Äspö HRL isvery similar to a future deep repository.Most things are in place: The canis-ters, the machines, the tunnels, andthe boreholes where the canisterswith the spent nuclear fuel will beemplaced.

Box 5864, SE-102 40 Stockholm, SwedenPhone +46 8 459 84 00 Fax +46 8 661 57 19

www.skb.se [email protected]

Documents

EXPERIMENTS AT THE ÄSPÖ HARD ROCK LABORATORY