ML22299A091
| ML22299A091 | |
| Person / Time | |
|---|---|
| Issue date: | 10/26/2022 |
| From: | Sunil Weerakkody NRC/NRR/DRA |
| To: | |
| References | |
| Download: ML22299A091 (15) | |
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Using Integrated Risk-Informed Decisionmaking Process to Address HEAF Events at U.S. Nuclear Power Plants Sunil Weerakkody, Ph.D.
Senior Level Advisor in Probabilistic Risk Assessment Division of Risk Assessment Office of the Nuclear Reactor Regulations U.S. Nuclear Regulatory Commission October 2022 1
LIC-504 Effort JS Hyslop Charles Moulton Ching Ng Sunil Weerakkody Nick Melly Kevin Coyne Reinaldo Rodriquez Gabe Taylor Siva Lingam Sandia National Laboratory Two US Reference Plants 2
3 Refresher - Integrated Decision-Making Process for Emergent Issues (LIC-504)
Developed as a lessons learned from a 2001 Davis Besse reactor vessel head degradation Event Provides a structured process and expectations to document decisions for issues that may warrant safety orders Provides guidance to apply integrated decisionmaking including risk, defense-in-depth, and safety margins considerations Has been used frequently for a range of emergent plant-specific and generic issues Most recent update (Revision 5, NRC ADAMS Accession No. ML19253D401) has incorporated significant changes (e.g., consideration of Enterprise Risk) based on lessons learned from the use of previous versions.
Recent LIC-504 assessments have incorporated some elements of NRCs Be RiskSMART framework (NUREG/KM-0016) to develop recommendations.
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1 Use performance measurement strategies to monitor the change Proposed increases in risk are small and are consistent with the Commissions Safety Goal Policy Statement Maintain sufficient safety margins Change is consistent with defense-in-depth (DiD) philosophy Change meets current regulations unless it is explicitly related to a requested exemption or rule change Integrated Decision-making (RG 1.174)
Radiation exposure to plant workers Enterprise Risk Be RiskSMART framework Factors Considered in LIC-504 Implementation -
Refresher on LIC-504 Process 4
10-6 10-3 10-5 10-4 LIC-504 Risk-Informed Regulatory Actions (examples)
Change in CDF Regulatory Intensity 10-7 Weigh a Spectrum of Regulatory Options Immediate regulatory action -
compensatory measures Formal backfit analysis ( 10-4 )
50.54(f) letters Bulletin Information Notice/Outreach Smart inspection samples -
within baseline program No Actions Use RIDM -
not numbers alone; Facility DID and SM are also considered 5
Applying the Be Risk Smart Framework Two coordinated tracks
- Developed and documented risk-informed options to disposition HEAF issues for management consideration.
- Updated HEAF PRA Methodology, which was the primary input to examine the change in estimated risk significance.
- Research Acvies
- In coordination with EPRI, developed and validated a HEAF PRA methodology that characterizes the thermal heat transfer from an arc.
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Some Features of the Updated HEAF PRA Method (Enhanced realism)
- Credits the ability of electric raceway fire barriers to prevent potential damages to cables;
- Substantial changes to zones of influence of HEAFs for non-isophase bus ducts and for low and medium voltage switchgears;
- Uses updated HEAF frequencies; and
- Enables modelling of more realistic HEAF damage potential that considers factors such as arc duration 7
HEAF LIC-504 - Scope Copper and aluminum HEAF zones of influenceshould be treatedthe samebased on the current state of knowledge.
The LIC-504 assessment was then focused on examining the change in estimated HEAF risks associated with the use of the new HEAF PRA methodology.
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HEAF LIC-504 STEP 1 - NO IMMEDIATE SAFETY CONCERN (NRC ADAMS ACCESSION NO. ML21272A262).
STEP 2 - DETAILED EVALUATION USING DRAFT OF UPDATED HEAF PRA METHODOLOGY (NRC ADAMS ACCESSION NO. ML22158A071) 9
HEAF LIC-504 - Work Completed Visited one BWR and one PWR; Use of best available information provided by each reference plant was instrumental in staffs ability to enhance realism of results.
Reviewed accident sequence precursors associated with HEAFs in US plants and the Maanshan Event (NRC. ADAMS Accession No. ML02120364). and EPRI Report No.
3002015459, HEAF scenarios of reference plant PRAs to develop risk-informed insight.
Reviewed EPRI Report No. 3002015459, HEAF scenarios of reference plant PRAs, and NEA/CSNI/R(2013)6 to develop risk-informed insights.
The team issued Publicly available memo on July 22, 2022 (NRC ADAMS Accession Nos.
ML22201A000-A0003) documenting teams analyses and recommendations.
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HEAF LIC-504 - Staff Insights (Based on analyses best available information from reference plant using the draft updated HEAF PRA methodology THE RISK OF HEAF COULD BE HIGHER OR LOWER THAN CALCULATED UNDER THE PREVIOUS METHODOLOGY RISK VARIES SIGNIFICANTLY BASED ON PLANT CONFIGURATION FOR CERTAIN CONFIGURATIONS, THE ESTIMATED RISK FROM BUS DUCTS CAN BE NOTABLY HIGHER THAN PREVIOUSLY MODELED CONCLUDED THERE IS NO SIGNIFICANT INCREASE IN TOTAL HEAF RISK, NOT WARRANTING THE NEED FOR ANY ADDITIONAL REGULATORY REQUIREMENTS 11
HEAF LIC-504 - Some risk-informed Insights (Based on review of ASP events, Maanshan HEAF event and EPRI report 3002015459).
HEAFS LEADING TO STATION BALCKOUT SCENARIOS CONSTITUTE THE HIGHEST HEAF RELATED RISKS.
THEREFORE, PLANT DESIGN AND OPERATIONAL CHANGES THAT HAVE BEEN ADOPTED TO ENHANCE THE MITIGATION OF BEYOND DESIGN BASIS ACCIDENTS NRC ORDER [EA-12-049] ARE LIKELY TO REDUCE HEAF RELATED RISKS.
HEAFS THAT CAN LEAD TO SBOS ARE LIKELY TO INITIATE AT BUSES OR SWITCHGEAR THAT ARE ESSENTIAL TO SUPPLY AC POWER FROM BOTH OFFSITE POWER AND EMERGENCY DIESELS (OR ANOTHER EMERGENCY SUPPLY). RESOURCES FOCUSED TO MINIMIZE THE LIKELIHOOD OF HEAF OCCURRENCE AT THOSE SWITCHGEAR AND BUSES (E.G., IMPROVED PREVENTIVE AND PREDICTIVE ELECTRICAL MAINTENANCE) CAN REDUCE HEAF RELATED RISKS. MEASURES TAKEN TO MINIMIZE THE POSSIBILITY OF A HEAF AT ONE EMERGENCY BUS, CAUSING FAILURE OF THE REDUNDANT ELECTRICAL TRAIN DUE TO CONSEQUENTIAL FAILURES (E.G., DUE TO SMOKE, OR DESIGN DEFICIENCIES), WILL ALSO MINIMIZE THE SBO RELATED HEAF RISKS.TION WITH RESPECT TO CIRCUIT BREAKERS, MAINTENANCE OF THE UNIT AUXILIARY TRANSFORMER (UAT) BREAKER IS PARTICULARLY IMPORTANT BECAUSE ITS FAILURE CAN LEAD TO AN EX-TENDED DURATION GENERATOR-FED FAULT AT THE FIRST SWITCHGEAR BUS. OPERATING EXPERIENCE HAS SHOWN THIS BREAKER TO FAIL DURING AUTOMATIC BUS TRANSFERS.
WITH RESPECT TO SWITCHGEAR, CRITICAL SWITCHGEAR, SUCH AS FEEDER CIRCUIT BREAKERS THAT CARRY HIGHER CURRENTS AND SWITCHGEAR THAT IS PART OF A BUS TRANSFER SCHEME, PROPER MAINTENANCE OF CONNECTIONS ON BOTH THE BUS DUCT SIDE AND THE CIRCUIT BREAKER SIDE IS ESPECIALLY IMPORTANT.
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Recommendations Information Notice Issue Risk insights into NRRs ongoing PRA configuration control initiative.
Incorporate Integrating risk insights into NRRs inspection program in accordance with ROPs change control processes.
Consider Risk insights with internal and external stakeholders.
Communicate 13
HEAF LIC-504 - Next Steps Webinar Insights with internal and external stakeholders.
(Fall 2022)
Info Notice Information Notice.
(Winter 2022)
Complete Supporting technical documents (RES).
(December 2022) 14
CONCLUDING REMARKS The NRC has successfully developed a process to address safety issues that emerge as a result of world-wide nuclear power plant operating experiences in an efficient and effective manner. NRR developed an Office Instruction entitled, LIC-504, Integrated Risk-Informed Decisionmaking for Emergent Issues that describes this process, which enables NRC staff to use best available information to assess risk (quantitative or qualitative), defense-in-depth, and safety margins. This process allows for the NRC to disposition issues in a timely manner, consistent with risk-informed decision-making principles.
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