Activities on Safety Improvement of Czech NPPs in Solution of Severe Accident Issues

Transcription

1 Activities on Safety Improvement of Czech NPPs in Solution of Severe Accident Issues Jiří Duspiva ÚJV Řež, a. s. Division of Nuclear Safety and Reliability Dept. of Severe Accidents and Thermomechanics International Conference on Topical Issues in Nuclear Installation Safety IAEA, Vienna, Austria, June 6-9, 2017

2 Outline Background Project on Corium Localization Background of ČEZ Project Requirements to SA Solution ČEZ Supported Activities Conclusions 1

3 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 2

4 Background Activities preceding ČEZ project MELCOR analyses of IVR at modified VVER-1000 Collaboration with Kurchatov Institute Simulations of heat flux distributions using SOCRAT and ASTEC codes First more realistic profiles of heat flux, cooling of RPV as boundary condition Introductory CEZ project (2013) on feasibility of IVR at Temelin NPP Overview of state of the art of IVR Development of first models and analyses for heat flux distribution and RPV cooling Preliminary investigation of technical solution on coolant supply Preliminary testing of cold spray applicability (High Velocity Particle Coating) JRC Coordinated Benchmark on IVR at VVER-1000 ( ) MELCOR, ASTEC, SOCRAT, MAAP, PROCOR, FLUENT, CFX simulations of heat flux distribution Second phase of analytical investigation of IVR (2014) RPV Cooling Evaluation of small scale experiments (BESTH2 facility) with RELAP-3D Cooling of VVER-1000 RPV with RELAP-3D Overview of existing CHF correlations 3

5 Project on Corium Localization (ČEZ) This presentation describes activities for Temelin NPP as example Project initiated in 2015 with duration up to 5 years Scope and approches prepared in collaboration of ČEZ - UJV Six main topics Primary circuit depressurization under SA conditions Corium cooling with water injection into RPV Strategy IVR Feasibility of water supply to cavity Steam release from cavity Feasibility of Deflector Evaluation of consequences of IVR malfunction Verification of IVR efficiency for VVER-1000 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 4

6 Background of ČEZ Project Objective of corium localization To restart heat removal from failed fuel and to stabilize containment conditions with the aim to prevent (or minimize) FP releases Time evolution of activities From IE Prevention of SA From Entry to SA Mitigation of SA in RPV From LHF Mitigation of SA out of RPV Temelín NPP already implemented several additional measures to original project design for SA prevention (applicable also in mitigation phase) Diverse equipment Alternative equipment 5

7 Background of ČEZ Approach SA Mitigation SA Prevention 6 Severe Accidents Alternative Flexible Systems Design Extension Conditions (SBO, LUHS) Additional (Diverse) Systems Design Basis Conditions Safety Systems PAR, In-vessel retention EDU, melt core localization, long term CTMT heat sink Hardened site against extreme weather condition Communication equipment, radiation monitoring Seismic hardened site EDU Diverse equipment (SBO DG; SG, RCS, SFP water supply) Mobile Power & water sources flex equipment & measurement Ventilator Cooling towers EDU Improvement of PAMS EDU

8 Background LHF is assumed as failure of alternative and diverse equipment (cliff-edge effect) Occurence within time range of hours (depends on scenario)? quantification of contribution of new equipment PSA PSA update is ongoing Requirements to new equipment for SA More simple than existing systems Reduced requirements to operating staff and energy supply, etc. All of them, up to now, are focused on coolant injection into RPV/Cntn Open other issues of SAM Long term heat removal from Cntn, if design systems are un-available (sprays, UJ system etc.) Removal of large mass of contaminated water from Cntn processing and storing 7

9 Requirements to SA Solutions Additional measures for SA are reasonable, if fulfil following conditions Effectivity = quantified benefit to safety (prevention of early FP releases and minimize FP releases) + confirmed physical fruitfulness with sufficient margin Reasonable technical feasibility No negative impact to reactor operation (including all procedures/activities during outages) Simplicity applicability under SA condition (limited personal capacity, limited accessibility ) At least partial independency of functionality assurance in comparison with existing emergency systems Consistency of approaches with other utilities operating VVER-1000 (or reactors of similar power) and VVER-1000 designers 8

10 ČEZ Supported Activities Three steps of possible solutions of corium localization Early re-flooding of degraded core (TMI-2 like scenario) (IVR-IN) In case of limited duration of applicability influence of conditions for IVR or ExVC solution to reduced decay power conditions IVR with external RPV cooling (IVR-EX) Recently Deflector for increase of CHF seems to be Necessary for physical feasibility Unacceptable due to too much complicated conditions of installation and removal during each of outage too many risks Success, at least temporary, of previous IVR-IN can provide IVR-EX physically feasible without deflector Corium cooling outside of RPV (ExVC) Various approaches investigated due to mainly very high dose rates in reactor cavity, which would significantly complicate any work there 9

11 IVR-IN Strategy Early re-flooding of degraded core (TMI-2 like scenario) IVR-IN Most effective solution concerning FP releases Already implemented provision for water injection into primary circuit (TB50 system) using external sources of water Analytical evaluation performed using MELCOR code in 2016 Two IE expected (LB LOCA 200 mm, postulated SBO plus primary circuit depressurization) Several cases concerning timing of water injection and alternative systems (TB50, HPI and LPI) Open issues Re-criticality due to injection of non-borated water To be investigated in 2017 Another provisions for extension of time range to start of water injection with any active system 10

12 IVR-ERVC Strategy IVR-EX solution Deflector issues requirements on activities during outages (yearly) Potential risk of incidental event Manipulation with activated parts of deflector structure Water injection to reactor cavity Modifications of venting systems with installation of valves to prevent water leakage 9 valves on three branches in three floors of venting system very high risk of failure not in compliance with requirement on simplicity, but other solution impossible due to Cntn design Early cavity flooding system from pressurized tanks Control of injection rate from water level measurement impossible to test at unit not in compliance with standard requirements to any system at NPP Testing at THS-15 system only possible Additional supporting systems required for water supply Heating of tanks, independent DG, nitrogen pressurized tanks etc. Not in compliance with requirement on simplicity 11

13 IVR-ERVC Strategy IVR-EX solution Steam escaping from reactor cavity Measurement performed Flow area around RPV is sufficient Modifications of biological shielding seem to be necessary Required minimum flow area is ~ 600 cm 2 is not confirmed by measurement Experimental program to assure physical efficiency UJV performed extensive analytical support UJV performed set of small scale test at BESTH2 facility UJV is constructing THS-15 facility ČEZ support this activity and co-finance performing of tests Scope of ČEZ support under negotiation 12

14 Verification of IVR Efficiency for VVER-1000 Combination of analytical and experimental activities Experimental activities Small scale test facility BESTH2 Chemical processes on surface of specimen Corrosion Formation of boric acid crystals on surface Natural convection formation Impact of surface conditions on heat transfer Polished, corroded, coating (High Velocity Particle Coating cold spray technology collaboration with PSU) Impact of surface declination Large scale facility THS-15 Scale 1:1(height and radius) for confirmation of VVER-1000 vessel cooling during applied IVR strategy Segment angle of sector (power capacity) Facility under construction 1. Cavity segment 2. Reduction valve 3. Condenser 4. Cooler 5. Pump 13

15 Summary Applicability of IVR-EX strategy at VVER-1000/320 is very complicated due to existing design solutions High number of new systems, which in higher level of DiD are more complicated, but on the same principles, than those on lower levels All of them must be initiated in very short times after entry to SAMG Within these times still exist possibility to terminate SA using IVR-IN New systems contain set of single points of vulnerability Valves on venting lines, water level measurement with control of injected water mass Complicated feasibility of some parts of solutions Dose rates in reactor cavity Uncertainty in physical feasibility Can be eliminated in experimental program under preparation No reference to any other application at reactor of similar power in operation Decision has be done at the end 2017 No other designing activity at the moment Support of experimental program 14

16 Summary UJV program for support Continuation in experimental program at THS-15 Facility construction has the highest priority Perform of experimental program necessary for EC H2020 IVMR project Additional experimental program on material properties at high temperature Data available up to 350 C, but needed to 1000 C (yield strength, ultimate strength, creep) for structural analysis of RPV wall during IVR application Additional small scale experiments on enhancement of heat removal from RPV surface Additional analytical activities on other corium configurations Three layer configuration Delayed corium formation due to temporary application of IVR-IN Heat flux to RPV wall distribution at lower total decay power 15

17 Conclusion Presentation highlighted some issues related to retrofit of existing NPPs in operation as response to required safety enhancement after Fukushima Daiichi accident Findings from stress tests Requirements from National Action Plans on Strengthening Nuclear Safety of Nuclear Facilities in the Czech Republic There is not possible to define common retrofit activities due to variety of design of units in operation, but Some common recommendation to approached and designing criteria would be useful SA for VVER-1000/320 are not solved in any country operating this type Russian Federation as country of designer does not work on this issue Ukraine no own approaches Bulgaria no activity recently presented (only some analytical work, so designing activities) Czech Republic must define and work independently 16

18 Acknowledgement To partners from ČEZ, a.s. they collaborate on activities mentioned in contribution To colleagues from UJV Řež, a. s. Division on Material Integrity and Engineering Division ENERGOPROJEKT PRAHA Dept on Safety Analysis their outputs were included in this contribution 17

19 Thank You for Your Attention UJV GROUP 18 J. Duspiva