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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
First Studies on Corium Spreading and Cooling
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
First Studies on Corium Spreading and Cooling
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
First Studies on Corium Spreading and Cooling
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
Temelin NPP ExVC Analytical Activities
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
Temelin NPP ExVC Analytical Activities
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
No cooling of corium
Homogeneous corium pool
One cavity approximation
Duration – 66 hours (progression till LHF is not modelled)
11
Temelin NPP ExVC Analytical Activities
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
Homogeneous corium pool
One cavity approximation
Initial and boundary conditions from integral calculation of SBO Scenario
Duration – 66 hours (progression till LHF is not modelled)
4. Stand alone analysis of SBO scenario with MELCOR code
Aim – comparison of homogeneous and stratified corium modeling
Initial and boundary conditions from integral calculation
Top cooling of corium
Stratified corium pool
One cavity approximation
Duration – 66 hours (progression till LHF is not modelled)
12
Temelin NPP ExVC Analytical Activities
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
Top cooling of corium
Homogeneous corium pool
Two cavity approximation
Initial and boundary conditions from integral calculation of SBO Scenario
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
Initial and boundary conditions from integral calculation
No cooling of corium
Homogeneous corium pool
Duration – 66 hours (progression till LHF is not modelled)
13
Temelin NPP ExVC Analytical Activities
7. Stand alone analysis of SBO scenario with MELCOR Aim – assessment of limestone concrete as sacrificial material
Top cooling of corium
Homogeneous corium pool
Two cavity approximation
Initial and boundary conditions from integral calculation of SBO Scenario
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
Initial and boundary conditions from integral calculation
Top cooling of corium
Homogeneous corium pool
Duration – 66 hours (progression till LHF is not modelled)
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
Temelin NPP ExVC Analytical Activities
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
Temelin NPP ExVC Analytical Activities
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