Update on CANDU Safety Issues, COG Large LOCA and Severe Accident Projects

Transcription

1 Excellence through Collaboration Update on CANDU Safety Issues, COG Large LOCA and Severe Accident Projects Krish Krishnan and Jeff Weed IAEA Workshop on Good Practices in HWR Operation Buenos Aires, Argentina, 2013 November COG Official Use Only

2 Outline Focus of Presentation CANDU Safety Issues Large LOCA Analytical Solution Severe Accident Support to Industry Post Fukushima Summary 2

3 Focus of Presentation Mission of COG is to improve the performance of CANDU stations worldwide through Member collaboration COG Nuclear Safety & Environmental Affairs Program facilitates Peer Groups, Committees, Working Groups and Task Teams sponsored by Members An example is the CANDU Safety Issues Task Team (CSI TT) sponsored by the Nuclear Safety Peer Group and the Chief Nuclear Engineers Forum COG Joint Projects & Services Program identifies Member issues that are common and develops cost effective solutions to these issues through pooling of Member expertise and resources Large LOCA Analytical Solution and Severe Accident Support to Industry Post Fukushima projects are two examples This presentation will focus on the above 3 examples 3

4 1. CANDU Safety Issues 4

5 CANDU Safety Issues Background In 2007, the Canadian Nuclear Safety Commission (CNSC) assessed the status of outstanding safety issues associated with the design and analysis of Canadian CANDU reactors The initial list of issues was developed using the IAEA TECDOC-1554*, information from currently operating reactors, life extension assessments, and pre-licensing reviews of new CANDU reactors The original listing consisted in excess of 70 issues covering 20 subject areas (next slide) * Generic Safety Issues for Nuclear Power Plants with Pressurized Heavy Water Reactors and Measures for their Resolution 5

6 CANDU Safety Issues Subject Areas AA CI CS EH EP ES FH GL IC IH MA OP PC PF PSA RC RP SM SS TR Accident Analysis Component Integrity Containment External Hazards Emergency Preparedness Electrical and Other Support Systems Fuel Handling General Instrumentation and Control Internal Hazards Management Operations Primary Circuit and Associated Systems Physics and Fuel Probabilistic Safety Assessment Reactor Core Radiation Protection Surveillance and Maintenance Safety Systems Training 6

7 CANDU Safety Issues RIDM Process CNSC and Canadian Industry subsequently established a joint Working Group to further review the issues identified as applicable to the reactors in Canada with an aim of clearly defining and achieving common understanding of them. CNSC, with input from Industry, in the meantime established a Risk Informed Decision Making (RIDM) process, which was used to assess the risk significance levels for the issues. Key elements of the RIDM process are: Issue definition Risk estimate and evaluation Risk Significance Level Risk Control Measures (RCM) Monitoring of RCM Implementation 7

8 CANDU Safety Issues Categorization Result: 16 Category III CANDU Safety Issues (CSI) with the balance classified as Category I and Category II. I. Is satisfactorily addressed in Canada. II. Is a concern in Canada appropriate measures are in place to maintain safety margins. III. Is a concern in Canada measures are in place to maintain safety margins, but the adequacy of these measures needs to be confirmed. Category III issues should not be viewed as questioning the safety of operating reactors, which have attained a very high operational safety record, but rather as areas where: uncertainty exists, safety assessment has been based on conservative assumptions, and where regulatory decisions are needed or will need to be confirmed. Three of the 16 issues were reclassified to Category II. Four of the remaining 13 were specifically identified with Large LOCA. 8

9 CANDU Safety Issues Category III (2011 Status) Issue # Generic Safety Issue Category Category III CANDU Safety Issue LOCA Category 1 Accident Analysis AA 3 - Computer Code and Plant Model Validation Non-LLOCA 2 Component Integrity CI 1 - Fuel Channel Integrity and Effect on Core Internals Non-LLOCA 3 General GL 3 - Aging of Equipment and Structures Non-LLOCA 4 Internal Hazards IH 6 - Need for systematic assessment of high energy line break effects. Non-LLOCA 5 Physics and Fuel PF 18 - Fuel bundle/element behaviour under post-dryout conditions Non-LLOCA 6 Physics and Fuel PF 19 - Impact of ageing on safe plant operation Non-LLOCA 7 Physics and Fuel PF 20 - Analysis methodology for NOP / ROP Non-LLOCA 8 Probabilistic Safety Assessment PSA 3 - Open Design of the Balance of Plant Steam Protection Non-LLOCA 9 Safety Systems SS 5 - Hydrogen control measures during accidents Non-LLOCA 10 Accident Analysis AA 9 - Analysis for void reactivity coefficient LLOCA 11 Physics and Fuel PF 9 - Fuel behaviour in high temperature transients LLOCA 12 Physics and Fuel PF 10 - Fuel behaviour in power pulse transients LLOCA 13 Physics and Fuel PF 12 - Channel voiding during a Large LOCA LLOCA 9

10 CANDU Safety Issues COG Task Team In 2011, COG Nuclear Safety Peer Group established the CSI Task Team Mandate of CSI TT: To coordinate industry effort to address Category III CSIs and to promote their reclassification to Category II Member representation: OPG, BP, HQ, NBP, AECL/CE Facilitator: COG Nuclear Safety & Environmental Affairs Periodic Industry-only meetings to coordinate effort, monitor work progress, and reclassification requests Periodic Industry/CNSC status update meetings 10

11 Category III Issues Status Summary Non-LLOCA 18 (covering 5 stations) reclassified (40%) 14 (covering 5 stations) requests for reclassification submitted (30%) Utilities plan to submit requests for reclassification of the remainder by 2014 LLOCA PF12 (GAI 00G01) reclassified to Category II in utilities have requested reclassification of the remaining 3 issues following completion of the LLOCA JP (Part 2 of this presentation) 11

12 2. Large LOCA Analytical Solution 12

13 Large LOCA Project Background and Objective An instantaneous LBLOCA up to a guillotine pipe failure has been the traditional basis for design and safety analysis Known not to be realistic Used in the absence of historical experience and a better understanding of pipe fracture mechanics Safety analysis margins for LBLOCA have eroded over the years Issue closely linked to positive void-reactivity coefficient Discovery issues from analysis and R&D More sophisticated codes Increased stringency of safety analysis assumptions Sensitivity of results to changes in input data / assumptions (driven by power pulse) Pushes resources to the LBLOCA R&D program but tests cannot encompass the full range of parameters for stylized bounding events CNSC concern with LBLOCA overall and associated Category 3 CANDU Safety Issues PSAs and limited experience show LBLOCA not the major risk contributor Need risk-informed tools Hence Composite Analytical Approach 15

14 Large LOCA Project Participants and Schedule Funding Participants Bruce Power New Brunswick Power Ontario Power Generation Candu Energy Inc. (Originally AECL) Hydro Quebec Project and Technical Management COG Joint Projects & Services Completion Date Final Report Issued: June

15 Large LOCA Project Composite Analytical Approach CAA is a Risk Control Measure that would give a more realistic assessment of LBLOCA, and support the prediction of larger margins Provides both frequency and consequence information to assist in resolving the LBLOCA issues: Three methodologies + two core inputs Methodologies are technologically independent Pipe break opening characteristics Large pipe break frequency Best Estimate Analysis and Uncertainty (BEAU) Core inputs strengthen existing practice Validated physics tools Defined and quantified physical barriers

16 Large LOCA Project Composite Analytical Approach (CAA) Overall Logic 16

17 Large LOCA Project CAA-Technical Areas Complementary reinforcing analytical activities add confidence and alleviate reliance on any single element Risk Control Measure: Composite Analytical Approach Tech Area 4 Failure Probability (reclassify breaks) Break Opening Characteristics COG R&D Tech Area 3 Integrated Application of BEAU+ Threshold Break Size + Realistic Break Opening to LLOCA Improved LBLOCA Margins Quantify Coolant Void Reactivity & Uncertainties Define Safety Limits & Confirm Adequacy of Safety Margins Tech Area 1 Common Core Activities Tech Area 2 Multi-layered approach provides defense in depth and mitigates uncertainty 17

18 Large LOCA Project Key Outcomes: Technical Area 4 Break Size Reclassification and Break Opening Break size reclassification involves the application of probabilistic fracture mechanics (PRAISE-CANDU PFM code was developed for this purpose) use PFM to demonstrate that the predicted frequency of breaks of a size larger than the threshold break size will be less than 10-5 per reactor year, with high confidence. A detailed state of the art review of large pipe failure research programs supports a more realistic, though still conservative, break opening model for large CANDU HTS piping. Two-step model of break dynamics: Instantaneous (within 5 ms) opening to 10% of the pipe cross-sectional area; Slower opening (within 5 s) up to 100% of the pipe cross-sectional area 20

19 Large LOCA Project Key Outcomes: Technical Area 3 Pilot Analysis for Representative CANDU DBA-Significant Reduction in Power Pulse Traditional Safety Analysis Maximum Powered Bundle (FP) Maximum size DBA (TBS) at NPS 10 is equivalent to % RIH for CANDU designs Composite Analytical Approach (with LOE) 21

20 Large LOCA Project Improvement in Safety Margin Significant Reduction in Peak Fuel Enthalpy and Peak Reactivity Design Change 22

21 Large LOCA Project Overall Results Probability of a Large LOCA is extremely low and supports reclassifying largest breaks to the Beyond Design Basis Accident (BDBA) category of RD-310. Large LOCA safety margins are substantially larger than those afforded by traditional safety analysis Multiple methods of analysis and sensitivity analysis provide robustness and diversity Reinforces the basis for break size reclassification and for demonstration of adequate safety margins for Large LOCA. The overall adequacy of the analytical basis for the Composite Analytical Approach has been demonstrated Resolution plans for confirmatory work to address residual R&D or code validation gaps have been established. Scope of any residual work is significantly reduced by the more moderate consequences made possible by implementation of the new methodology. Supports re-categorization of the LLOCA related CSIs 23

22 Large LOCA Project Status Project complete. Final Report issued in June 2013 Technical team and management oversight involved >60 industry staff Main Body + 10 appendices (1686 pp. total) 24

23 3. Severe Accident Support to Industry - Post Fukushima 25

24 Severe Accident Support to Industry Member Goals COG Joint Project proposed by CANDU Industry Integration Team (CIIT) in early 2012 to support utility responses to regulatory action items (including Fukushima Action Items (FAIs) issued by Canadian Nuclear Safety Commission) All COG Members were invited to join. Current Members: BP, NBP, OPG, AECL and SNN. Buy-In proposal to join sent to NASA Goal is to update the current Technical Basis Document (TBD) and Generic Severe Accident Management Guidelines (SAMGs) 26

25 Severe Accident Support to Industry Project Scope and Deliverables Seven areas of work: Shutdown and Low Power Events Multi-Unit Events Containment Integrity In-Vessel Retention Habitability Update of TBD and SAMG Instrumentation and Equipment Survivability 20 major topical reports deliverable in the 7 Project areas Topical reports will then be combined in overall updates of TBD/SAMGs 27

26 Severe Accident Support to Industry Schedule Project Steering Committee, utility representatives and vendors meet frequently to maintain a focus on deliverables and reviews Original schedule drawn up for Joint Project (JP) was through end of 2014 and into 2015 Needs of utilities to use JP outputs and perform work prior to responding to regulators has accelerated schedule Most current work is planned to be completed in 2013, with issue of final updated TBD and Generic SAMGs in first quarter of

27 Severe Accident Support to Industry Deliverables Instrument Survivability No current standardized methodology for use in CANDU Project will provide a Tool Kit Review requirements, evolving international standards, guidelines and OPEX to determine the current industry best practices in I&ES Develop a common instrumentation and equipment survivability assessment methodology for CANDU in alignment with international practices Provide ready made templates that utilities can use to support the station specific I&ES 29

28 Severe Accident Support to Industry Deliverables Habitability Current guidelines do not address Habitability of control areas subsequent to a severe accident Like I&ES, need a Tool kit for utilities use: Essential for implementing SAMGs and deployment of EME (Emergency Mitigating Equipment). Generic report on Methodology for CANDU Habitability can be used by utilities as basis for their detailed assessment of each plant. Use of emergency dose levels during severe accidents is a topic of much discussion with CNSC. The Habitability Report is one input to help reach agreement. 30

29 Severe Accident Support to Industry Deliverables - Shutdown and Low Power How to plan and manage Severe Accidents initiated from these states: SD/LP plant conditions not covered in the previous SAMG documentation Include outage configurations that are in the PRAs and in the approved outage heat sink manuals, including states where SAMG equipment may not be available Include insights from severe accident progression analysis Define SAMG entry conditions and exit criteria to allow utilities to develop station-specific guidance 31

30 Severe Accident Support to Industry Deliverables Multi-Unit These lead to the need for special considerations: Multi-unit events for Ontario-specific CANDU reactors are unique in terms of common containment envelope Multi-units with common site also present special challenges (similar to Fukushima experience) Report develops technical basis, and understanding of multi-unit events Review systems and components that are available to be used for different numbers of units at different stages of an accident Considers potential impact on nuclear emergency plans, SAMG management and resource allocation 32

31 Severe Accident Support to Industry Deliverables Reactor Integrity Demonstration of retention of Corium within the Calandria Vessel through In-Vessel Retention (IVR) as a viable scenario is a key deliverable of the Joint Project. Working with current knowledge base and experimental data, the most complete integrated data and results set ever assembled will be produced This will be a prime component in support of the IVR argument 33

32 Severe Accident Support to Industry Deliverables Containment Integrity Maintaining Containment Integrity is key to managing releases Management of hydrogen, aerosols, non flammable gases Use of recombiners (PARs), igniters, filtered venting Maintaining containment pressure integrity to minimize uncontrolled releases 34

33 Severe Accident Support to Industry Status - 1 The project has completed the preparation of most of the topical reports. The following are published: Instrumentation and Equipment Survivability Shut down and low power events Multi unit events IFB (2 of 3 reports) Fukushima Insights The following are in final review and will be published shortly: Reactor Integrity / In Vessel Retention (IVR) Containment Integrity Habitability 35

34 Severe Accident Support to Industry Status - 2 The topical report outputs and recommendations are in the process of being integrated into the existing Technical Basis Documents (TBD) and Generic Severe Accident Management Guidelines (SAMGs) that currently exist for CANDU. The full integration process will take approximately 4 months, with the revised document being available end of March Once the generic SAMGs are issued, each utility has the option of taking and developing further to meet station specific needs, as has been done for the current set of SAMGs. 36

35 Summary COG, as part of its mission, is actively involved in facilitating common programs that help utilities in enhancing safe operation Three examples were provided in this presentation 37

36 Acknowledgements CANDU Safety Issues Task Team Chair Evan Davidge, OPG Large LOCA Project Steering Committee Chair Peter Purdy, Bruce Power Severe Accident Support to Industry Project Steering Committee Chair Mark Knutson, OPG 38

37 CANDU Excellence through Collaboration 39