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R&D Activities to Resolve ExVC Strategy for

VVER-1000 Reactor

Jiří Duspiva

IAEA I3-TM-52206

on Phenomenology and Technologies Relevant to In-

Vessel Melt Retention and Ex-Vessel Corium Cooling

Shanghai, China, October 17-21, 2016

1

Outline

Background

First Studies on Corium Spreading and Cooling

Temelin NPP ExVC Analytical Activities

Project on Corium Localization

Strategy ExVC

Conclusions, Suggestions

2

Background

ÚJV Řež provides complex services in severe accident management to Czech NPPs owned and operated by ČEZ a.s.

Accident progression Evaluation of source term Identification of severe accident management strategies

Supporting analyses for optimization

Validation of existing SAMGs Supporting analyses for

Control room habitability

Development of layout of hydrogen mitigation system

Fukushima Dai-ichi event accelerated interest of utility (ČEZ) to enhance SAM

Implementation of H2 removal system designed to SA H2 source (2015) Modifications for primary circuit depressurization Selection of corium localization strategy

VVER-440/213 Dukovany NPP – IVR implemented

VVER-1000/320 Temelin NPP – not yet decided, R&D program initiated 2015 with

parts to both strategies IVR and ExVC

3

Background

Activities preceding ČEZ project for ExVC strategy

First idea on corium spreading and cooling with top flooding from 90’

Analyses confirmed positive effect on reduction of concrete ablation

Open issue – is corium after initiation of MCCI fully coolable?

OECD MCCI and MCCI2 projects + CCI7 test

Identification of potential for MCCI termination due to top cooling, but

Much higher for common sand/limestone than siliceous concrete

Extensive validation on CCI tests

CORQUENCH and ASTEC/MEDICIS against experimental values

MELCOR/CORCON - code to code comparison approach

Temelin plant applications (siliceous concrete)

Some analyses identified potential of MCCI termination, but

General conclusion - MCCI is not possible terminate, if concrete ablation already initiated

Recent approach – application of refractory material to prevent MCCI for time needed to cool down corium after spreading

Further research mainly on optimization of refractory material is foreseen as well as on spreading of corium (dry, under water)

4

Background

Analysis of plant vulnerability (early 90’) identified serious

issues in the phase after LHF

Fast progress of MCCI resulting in early melt-through to channel of

ionization measurement loss of containment integrity and potential

release of corium to non-hermetic lower rooms

Corium ablation in axial direction is fast enough to melt-through

containment basemat

MELCOR analysis of TLCD scenario

(Report UJV Z-143-T, 1996)

Penetration depth 2.75 m in axial and 0.9 m

in radial direction at the end of the 1st day of SA

Ratio of axial to radial ablation incorrect for siliceous

concrete as well as LCS

Idea to spread corium and cooldown from top

with water

Based on ideas of repairing of LPI or HPI ECCs

5


Analytical evaluation of coolable thickness of corium

Assumptions

Adiabatic condition at border corium-concrete

Constant temperature at border corium-water

Criteria of coolability of solidified layer

Temperature of corium layer is not increasing, i.e. generated heat is removed to water

Temperature at border corium-concrete is below concrete ablation temperature

Conservative corium conductivity assumed (3 W/m-K)

Coolable layer of corium ≤ 0.15 m

Ratio of ablation products in corium will strongly determine real conductivity of corium

Separation of layers will influence heat transfer

Coolability of molten corium pool more complicated

Enhanced heat conductivity in corium

Formation of top crust

6


Analytical evaluation of spread corium cooling

Spreading to area ~ 100 m2 (cavity + spreading space)

Reduction of corium layer thickness significant slowdown of axial

ablation and termination of radial ablation

Cooling of spread corium

Reduction of axial ablation at the end

of the 1st day to 1.0 m from

2.75 m non-cooled and non-spread, and

1.4 m non-cooled, but spread

7


Conclusion from introductory studies

Potential of at least principal reduction of ablation rate (up to

termination of ablation) confirmed

Further R&D needed

Experimental data on corium coolability during MCCI

Possibility of joining of programs

EC 5th FWP project LPP (1998-2000)

EC 6th FWP project SARNET (2004-2008)

EC 7th FWP project SARNET2 (2009-2013)

OECD/NEA MCCI (2002-2005) and MCCI2 (2009-2012)

Extension of analytical tool portfolio (MELCOR/CORCON)

ASTEC/MEDICIS and CORQUENCH

Modeling of new phenomena of Melt Eruptions and Water Ingression

8

Application of MCCI and MCCI2 Outcomes

Extensive validation activities with

MEDICIS/ASTEC

Direct validation on CCI tests (CCI-1, CCI-2, CCI-4, CCI-5, and CCI-7)

CORQUENCH

Validation on CCI-7 test

CORCON/MELCOR

Direct validation impossible due to geometry

assumption of cavity

Indirect validation performed via. benchmarking

with MEDICIS and CORQUENCH

on VVER-1000/320 data

Main conclusion on siliceous concrete

Already initiated MCCI can’t terminated

Idea for solution with delayed MCCI

9


Goals

Corium cooling effect on major MCCI characteristics for siliceous concrete

Assessment of SAM strategies effectiveness

Proposal for alternative solutions (if SAM unsuccessful)

SA scenarios calculated

SBO

LB LOCA

Analytical approaches

Integral (whole scenario and whole unit)

Stand-alone (only MCCI phenomena plus initial and boundary conditions)

Code used for calculations

MELCOR 1.8.6 YV_3481

MEDICIS from ASTEC V2.0R1P2

CORQUENCH 3.03

10


1. Integral analysis of SBO scenario with MELCOR code

Aim – set up initial and boundary conditions for stand alone calculations

No cooling of corium

Homogeneous corium pool

One cavity approximation

Duration – 3 days (72 hours)

2. Stand alone analysis of SBO scenario with MELCOR code

Aim – confirmation of correctness of initial and boundary conditions in stand alone case via. comparison of integral and stand alone predictions

Initial and boundary conditions from integral calculation




Duration – 66 hours (progression till LHF is not modelled)

11


3. Stand alone analysis of SBO scenario

Aim – assessment of water cooling effectiveness, code to code comparison

MELCOR/CORCON, ASTEC/MEDICIS, and CORQUENCH codes

Top cooling of corium



Initial and boundary conditions from integral calculation of SBO Scenario


4. Stand alone analysis of SBO scenario with MELCOR code

Aim – comparison of homogeneous and stratified corium modeling



Stratified corium pool



12


5. Stand alone analysis of SBO scenario Aim – assessment of more detailed cavity description, code to code comparison

MELCOR/CORCON, ASTEC/MEDICIS, and CORQUENCH codes



Two cavity approximation


Duration – 66 to 152 hours (progression till LHF is not modelled)

6. Stand alone analysis of SBO scenario – sensitivity study Aim – corium conductivity sensitivity study

MELCOR/CORCON, and CORQUENCH codes





13


7. Stand alone analysis of SBO scenario with MELCOR Aim – assessment of limestone concrete as sacrificial material



Two cavity approximation


Duration – 66 (progression till LHF is not modelled)

8. Stand alone analysis of SBO scenario – study of refractory lining Aim – assessment of refractory lining, code to code comparison

MELCOR/CORCON, and CORQUENCH codes





Steps 1, 2, 4, 5 and 8 performed also for LB LOCA scenario Only with MELCOR code

Additional study complementary to step 8 with delayed failure of refractory liner and initiation of MCCI

14


Conclusions from analysis

MCCI is complex and complicated process with many uncertainty and indefiniteness in modeling

insufficient knowledge of MCCI phenomena

limitation of calculation models describing MCCI

uncertainty of model parameter values

inaccuracy of material property data

uncertainty of initial and boundary conditions

inaccuracy and instabilities of numerical methods used for solution of large system of equations

Possible retrofit plant solution can fulfilled only limited objection

To extend as much as possible time of Cntn basemat melt-through

Termination of MCCI with siliceous concrete would be benefit which can’t be fully confirmed based on SOAR

Refractory liner is potential solution for extension of time to failure

Analytical results identified time range of 8 to 12 hours for cooldown and solidification

15


Conclusions from analysis

Refractory liner is potential

solution for extension of time

to failure

Analytical results identified time

range of 8 to 12 hours for

cooldown and solidification

(Tsolid ~ 1750 K based on

composition and prediction in

MELCOR)

Complementary analysis of liner

and concrete heat up

Refractory liner (100 mm)

Steel liner (8 mm)

Concrete

800 K

16

Project on Corium Localization

Project initiated in 2015 with duration up to 5 years

Six main topics

Primary circuit depressurization under SA conditions

Corium cooling with water injection into RPV

Strategy IVR

Strategy ExVC

Corium spreading and localization in GA302

Modifications in cavity and spreading spaces

Overview of coolant supply

Containment response to SA

and long term issues

SA initiated in SFP

17

Corium Spreading and Localization in GA302

Solution to enable easy spreading from cavity to GA302

Shielding and hermetic doors

Definition of requirements on “opening”

Design of modifications of doors

Requirements on barriers to protect spreading to forbidden places

Transport corridor

Floor penetrations

Impact to outages

Removable barriers to enable transport of loads

Spreading evaluated with 1D correlation

in dry 11 to 18 m

under water 9.5 to 15.5 m

18

Modifications in Cavity and Spreading Spaces

Installation of refractory liner

To prevent any impact to reactor operation

Venting system in cavity

Thermal insulation and biological shielding in cavity

To prevent any impact to activities during outage

Control of RPV in period 6 years – specific equipment using rails

Supporting legs of rails in GA302 can be lined

Rails in cavity has to be substituted and wheels of equipment changed

Change of activation samples in cavity in period 1 year – specific equipment using rails

Identical solution as for previous

Geometrical requirements

Material requirements

Solution in cavity strongly influenced with high radiation

Possibility of alternative solution if the liner installation impossible

First ideas, but not yet technological proposal

19

Modifications in Cavity and Spreading Spaces

Installation of refractory liner - Material requirements

Materials used in analytical studies come from experiments

ZrO2 and MgO or other? – selection of candidates is on-going in

collaboration with division on Chemistry of Fuel and Waste

Management and University of Chemistry and Technology in

Prague

Selection criteria – high melting temperature, low potential for dissolution

in reaction with corium and optionally low heat conductivity with high

thermal capacity

Their functionality has to be confirmed experimentally -

objective

Dissolution kinetics of candidate material in reaction with prototypic

corium

20

Overview of Coolant Supply

Evaluation of time ranges for water supply to containment

Spreading in reflooded GA302 expected

Idea for solution with dry spreading was in conflict with requirement on no impact to activities during outages (permanent tall barrier with water locks to be opened with melt-through holding wires)

Sufficient water layer during spreading to prevent stratified steam explosion

Reflooding Cntn to 1 m level in spreading area during 4 hours from entry to SAMG (core exit temperature > 650°C)

Based on MELCOR analyses of several scenario progression and

Requirements for doors melt-through

Identification of possible water sources in auxiliary building

Evaluation of SAMG assumptions (SAG-4 Inject into Containment)

Potential extensions or proposals for additional water sources

Alternative (mobile) equipment already installed at Temelin NPP and can also be used

21

Conclusions

Corium localization is key measure in SAM

Application of IVR or ExVC strategy to existing reactors

requires extensive efforts on

Evaluation of efficiency of proposed strategy

Evaluation of feasibility within design of existing reactor

UJV performs extensively R&D activities to investigate

applicability of ExVC strategy to VVER-1000/320 Temelin NPP

Analytical program

Preparation of experimental program on refractory liner material

Identification of needs for new systems or components

Decision on application of any strategy at Temelin NPP has

to be done at the end 2017

22

Conclusions

Comparison of applicability of IVR – ExVC at Temelin NPP Both solutions has advantages and disadvantages

Both solutions are influenced with many uncertainties

IVR solution at Temelin NPP Uncertainty in heat flux distribution – configuration with 2 or 3 layers and fluxes from layers, between layers and so on

Water supply short time window for reflooding of cavity, if missed strategy is lost

water supplied into cavity via venting system – complicated solution with many active parts, long term reflooding is fully active

Cntn is untouched if successful, risk of steam explosion if IVR fails

ExVC solution at Temelin NPP Refractory liner material unknown yet

Implementation in cavity influenced by high radiation

Water supply – initially high amount (~1000 m3), but to any location in Cntn

Solution on Cntn basemat, potential of loss of integrity in very late phase

Process of decision - selection from IVR or ExVC will be very complicated

23

Acknoledgement

To colleagues from UJV Řež, a. s.

Division 25000 ENERGOPROJEKT PRAHA

their outputs were included in this contribution

24 •J. Duspiva

UJV GROUP

Thank You for Your Attention

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R&D Activities to Resolve ExVC Strategy for VVER-1000 … · R&D Activities to Resolve ExVC Strategy for VVER-1000 Reactor ... application of refractory material to ... Initial and