Overview of Post-Fukushima Severe Accident R&D Support to Canadian Nuclear Power Plants

Transcription

1 Overview of Post-Fukushima Severe Accident R&D Support to Canadian Nuclear Power Plants V.S. (Krish) Krishnan Project Manager, Nuclear Safety & Environmental Affairs IAEA Technical Meeting on Post-Fukushima R&D Strategies and Priorities 2015 December 15-18, Vienna

2 Acknowledgements This presentation is based on the results of work conducted by COG in the Joint Project & Services and R&D program areas, specifically: Joint Project 4426: Severe Accident Support to Industry - Post Fukushima. Project Manager Jeff Weed Safety & Licensing Program. Program Manager Holly Anderson 2

3 Presentation Outline Introduction COG Joint Project: Severe Accident Support to Industry Post Fukushima Recommendations for further R&D R&D in progress and planned Summary 3

4 Nuclear Power Stations in Canada Nuclear Power Stations in Canada are regulated by the Canadian Nuclear Safety Commission(CNSC)under the authority of the Nuclear Safety and Control Act 4

5 CANDU Owners Group COG Members Supplier Participants: Is a Federally Incorporated Not-for-Profit Corporation President is accountable to a Board of Directors Board is accountable to the Shareholders Voting AECL, Canada Bruce Power, Canada NB Power, Canada OPG, Canada SNN, Romania AMEC-FW (NSS) Areva B&W-XT Canada Cameco CPUS Hatch Non Voting CNNO, China KHNP, Korea NASA, Argentina NPCIL, India PAEC, Pakistan Kinectrics-Candesco RCM Technologies SNC Lavalin-Candu Stern Labs Worley Parsons 5

6 Background CANDU Severe Accident related R&D and support NPPs have been in place for a number of years in Canada at COG and AECL/CNL 2006: COG Joint Project (JP) 4056 produced the first CANDU Generic SAMGs and Technical Basis Documents Similar to Westinghouse Owners Group approach Covered single unit, at-power reactor events Utility-specific SAMGs developed subsequently 2011 March: Fukushima event External hazard larger than accounted for in original design basis Multiple units affected on site Irradiated Fuel Bays impacted Some units in outage configurations Challenges to containment and fission product retention 6

7 Canadian Response to Fukushima 2011 April: CANDU Industry Integration Team formed Address common issues (WANO SOERs, CNSC actions) Ensure alignment, consistency and share information 2011 October: CNSC Task Force Report (INFO 0824) established general principles for: Strengthening defence-in-depth Enhancing emergency response Improving regulatory framework 2011 December: CNSC Staff Action Plan (INFO 0828) Specific Fukushima Action Items (FAIs) to address task force report recommendations 2012 April: COG JP 4426 (Severe Accident Support to Industry Post Fukushima) established 7

8 COG JP 4426: Severe Accident Support to Industry Post Fukushima Participants OPG, BP, NBP, CNL and SNN Objectives Provide guidance to utilities to ensure a systematic and consistent approach for assessment activities to close CNSC Fukushima Action Items Update SAMG Technical Basis Document and Generic SAMG documents to reflect OPEX from initial development of SAMG OPEX and Lessons Learned from Fukushima event Specific considerations related to multi-unit stations Specific issues related to units shutdown or at low power and IFB events Recommendations for further R&D 8

9 JP 4426 Topical Reports and Main Outputs 1. Instrumentation & Equipment Survivability Assessment Methodology and Guidelines 9. CANDUIn-Vessel Retention (IVR) Summary of Supporting Technical Basis 2. Plant Habitability Methodology 10. CANDU IVR Update of Technical Basis Documents (TBD) and Severe Accident Management Guidelines 3. Multi-Unit Events 11. OPEX Review 4. Shut Down and Low Power Events 12. Fukushima Lessons Learned 5. Irradiated FuelBay (IFB) Methodology 13. Topical Report on Emergency Water Makeup 6. IFB TBD Update 14. Topical Report on Aerosols 7. IFB Update for SAMG 15. Topical Report on Hydrogen Management 8. CANDU IVR Summary of Experiments on Corium Behaviour and Interaction with Calandria Vessel 16.Topical Report on Containment Integrity Methodology & Assessment 9 9

10 JP 4426 General Recommendations Incorporate feedback from implementation of station-specific SAMG. Useful for: IFB methodology and assessment Habitability methodology Feedback from implementation of revised Computational Aid CA-4 for hydrogen flammability in containment and new CA-7 for accident progression timing evaluation Review further Fukushima feedback and lessons learned 10

11 JP 4426 Specific Recommendations In-Vessel Retention (IVR) strategy Aerosol behaviour Containment integrity MAAP4_CANDU severe accident analyses 11

12 Areas for Improvement of CANDU IVR Knowledge Three Areas: IVR by Internal Calandria Vessel Cooling IVR by External Calandria Vessel Cooling CANDU-Specific Conditions During Severe Core Damage 12

13 In-Vessel Retention (IVR) CANDU6 CalandriaVessel and CalandriaVault / Shield Tank Section View Following core collapse and boil-off of moderator, dry Terminal Debris Bed (TDB) located in bottom of CV Retaining TDB in CV requires: 1. Supply of cooling water to calandria vault and end shields 2. CV shell must withstand thermal loads and thermochemical interactions 3. CV shell must withstand relevant mechanical loads 13

14 CANDU Core Retention Strategy 14

15 IVR by Internal Calandria Vessel Flooding Pertains to timely deployment of Emergency Mitigating Equipment such that formation of coherent, molten pool is prevented (IVR states 3-6). Internal CV Cooling in the Absence of External Cooling Evaluate plausible core debris configurations in contact with the calandria shell shields for the thermal response of the calandria shell with no external cooling (PNGS A) Internal CV Cooling with External Calandria Shell Cooling Apply methodology from above to evaluate calandria shell temperature 15

16 IVR by External Calandria Vessel Cooling Pertains to long-term IVR of CANDU core debris, which is cooled externally (IVR states 7a-7b) Corium Properties Establish range of plausible corium compositions and obtain key properties (k, v, ρ) Corium Behaviour Assess CV survivability using CFD models benchmarked against LWR data. End Shield Characteristics and Behaviour Thermalhydraulics within porous media of shielding balls should be investigated. 16

17 IVR by External CalandriaVessel Cooling (contd.) Chemical Interactions of Corium with Retention Boundaries Need tests to determine onset of significant interaction between Zr and 304L stainless steel CV welded joints may have different compositions and thus different interaction with Zr. Review welded joint records. Methodology for Calandria Vessel Survivability Assessment Perform deterministic CV structural survivability assessment during severe core damage 17

18 CANDU-Specific Conditions during Severe Core Damage Improved understanding and modelling of CANDU core behaviour under SCD progression Core Damage Progression and Symptoms Experimental and analytical work recommended on CANDU core break-up Assessment of different CANDU core damage progressions and configurations to understand how they could be interpreted as observable plant symptoms and transient loads on CV boundaries Capability to model solid debris motions in a CANDU CV and flooding of a degraded core Behaviour of Breached CV Compile and consolidate information on discharge characteristics of plausible, low elevation openings in CV boundaries 18

19 Aerosols Behaviour Accumulation of aerosol on filters or in vent lines is more likely to result in partial blockage rather than total plugging, hence lower flow rate and increased pressure drop, which can lead to failure of the filter medium if conventional membrane type used in EFADS/FADS. Filters designed specifically for severe accident conditions unlikely to fail in this way but the overall flow characteristic may be adversely affected. Occurrence of conditions likely to give rise to large masses of aerosol in the containment atmosphere is found to occur late in accident progression and not at all if IVR is successful. Potential impact of inert aerosol generation on the effectiveness of the EFADS/FADS and emergency filtered venting in CANDU-6 plants should be evaluated. 19

20 Selected ongoing and planned R&D Programs Hydrogen IVR Core Collapse FP Behaviour 20

21 Hydrogen Program: PAR Behaviour Issue: PAR effectiveness for Beyond Design Basis Accidents Identified relevant compounds that may be released during a BDBA that have potential to affect PAR catalyst performance I 2, CH 3 I, Cl 2, HCl, CO, hydrazine, CsI and nitrate Identified conditions expected 100 C, 3 vol% H 2, dry and 50% RH Constructed the Catalyst Activity Bench Scale (CABS) rig to test these conditions Tests to date show hydrazine, CsI and Iodine do not affect catalyst performance 21

22 Hydrogen Program: PAR Behaviour Issue: Hydrogen Isotope Effects on PAR Performance Deuterium may be released in some specific accident scenarios in CANDU reactors instead of light hydrogen Assess the results from small-scale PAR catalyst experiments performed in 2014/2015 & determine if further large-testing is necessary Perform large-scale PAR experiments with deuterium in the CTF (Combustion Test Facility) to quantify the difference between deuterium and light hydrogen (if necessary) Study PAR-induced ignition Study combustion in stratified H 2 layers 22

23 Severe Accidents Program In Vessel Retention In-vessel corium retention Critical heat flux measurements Core Collapse and FP release Calandria vessel heat stress response during in-vessel retention phase of a postulated severe accident Critical heat flux at the step between main and subshells at the bottom of the calandria vessel Critical heat flux in the end shield in the presence of obstructions representing lattice tubes Assessment of calandria failure modes in MAAP-CANDU 23

24 In-Vessel Retention 24

25 Calandria Vessel Heat Stress Response During IVR Issue:During IVR, water levels may drop below the top of the shield tank / calandria vault if water is not added. Temperature near top of CV cylindrical shell would climb once uncovered, potentially leading to plastic deformation The situation will be studied experimentally with a scaled apparatus Technical objective: provide quantitative and qualitative data demonstrating CV integrity during IVR Planned timeline: Work Plan in 2015 Experiments performed in 2018 Report on experimental results in

26 Critical Heat Flux at the Step between Main and Sub Shells at the Bottom of the Calandria Vessel Issue:Efficient heat transport from the CV cylindrical shell to the water of the calandria vault / shield tank necessary for maintenance of IVR, Impact on CHF of the step between the main shell and sub shell of the CV is unknown The impact of CV step on CHF to be studied experimentally with a large-scale apparatus Technical objective: provide quantitative data determining the CHF in the vicinity of the CV step Planned timeline: Experiments performed in 2016 Report on experimental results in

27 Critical Heat Flux in the End Shield in the Presence of Obstructions Representing Lattice Tubes Issue:Efficient heat transport from the calandria tubesheet to the water of the end shields is necessary for maintenance of IVR - Presence of lattice tubes in end shield may reduce CHF The impact of lattice tubes on CHF in end shields to be studied experimentally with a large-scale apparatus Technical objective: provide quantitative data determining the impact of lattice tubes on CHF in the end shield geometry Planned timeline: Work Plan in 2016 Experiments performed in 2016 Report on experimental results in

28 End Shields CHF Experiments Assembled Apparatus Inner Tank Side View During Testing 28

29 End shield CHF Test Results Initial series of tests measured CHF in end shields completed and documented Measured values of CHF support feasibility of IVR Predicted heat flux delivered to calandria tubesheets (< 200 kw/m 2 ) well below measured values of CHF in end shield geometry (> 375 kw/m 2 ) Qualitative observations of post-dryout heat transfer suggest mitigating effect of shielding balls on consequences of dryout (i.e. shielding balls appear to enhance post dryout heat transfer) Further experiments planned to determine the impact of a larger heated surface and the presence of lattice tubes 29

30 CV CHF during IVR Following core collapse and boiloff of moderator, dry Terminal Debris Bed located in bottom of CV Efficient heat removal through calandria cylindrical shell necessary to maintain wall integrity and IVR CV main shell is 7.6 m in diameter Radius of curvature approximately double that of LWR lower heads Depth of TDB in main shell ~1 m Much shallower than TDBs in LWRs Minimum value of CHF in LWR studies found to be at bottom (φ=0) GAS FLOW PATTERNS SUSPENDED DEBRIS properties BROKEN & mobility CHANNEL dissassembly DEBRIS-WATER INTERACTIONS INTACT CHANNELS failure TERMINAL DEBRIS properties & coolability 30

31 CV CHF Experiments Heating Plate with Cartridge Heaters and Thermocouples Heating plate sealed to a steel chimney and insulated on top and sides Heat flux measured by embedded thermocouples and power supplied to cartridge heaters Overall design guided by avoidance of edge effects and asymmetry Apparatus Overall View 31

32 Summary of CV CHF test results Two test series measured CHF at the bottom of the CV cylindrical shell a total of 44 times using a large scale apparatus Measured values of CHF support feasibility of IVR Further experiments planned to determine the impact of obstructions representing reactor structures, and CHF in the vicinity of the step between the main shell and sub shell 32

33 FP Behaviour: International Collaboration Phébus Issue: Severe Accident Fuel and Fission Product Behavior COG was a participant of this important project, which started in the early 90s A database of the important documents and data was created A summary of FP behavior during all 5 Phébus tests written X-ray images show damage to the fuel pins during the test 33

34 Other Human and Organizational Factors Under Severe Conditions Response to Fukushima Daiichi and Daiini and emphasized the importance of human and organizational factors under high stress and hazardous working conditions involving the interaction of individuals, organizations and technology. Individual decision-making under stressful and uncertain conditions Technology and equipment impacting effective management of an emergency situation Organization and management structure for command and control Research required (for example, how to model human resilience in PSA?) 34

35 Concluding Remarks Canadian nuclear industry: Has funded significant R&D activities in the past and is making additional R&D investments to resolve remaining phenomena considered to be of high significance, especially those related to understanding and modeling of severe accident phenomena Has recently completed post-fukushima assessment of Canadian NPPs Is following up on recommendations for long-term R&D, and working on collaborative testing with international research organizations 35

36 CANDU Excellence through Collaboration 36