ML21145A166
| ML21145A166 | |
| Person / Time | |
|---|---|
| Site: | Duane Arnold  |
| Issue date: | 05/27/2021 |
| From: | Joel Wiebe Plant Licensing Branch III |
| To: | NextEra Energy Duane Arnold |
| Wiebe J | |
| References | |
| EPID L-2020-PPM-0015 | |
| Download: ML21145A166 (23) | |
Text
Public Meeting to Share Risk Insights Duane Arnold Energy Center (DAEC) Derecho May 27, 2021
Objectives
- Consistent with our Principles of Good Regulation of openness, and clarity, we are sharing risk insights from our regulatory response to the August 10th Derecho.
- Openness Transparent and publicly available information on our evaluations and analysis.
- Clarity Evaluation considered a sampling of sites with different design characteristics to understand generic implications to the fleet.
Characteristics chosen to get a broad understanding to support analysis and gather risk insights that could benefit overall fleet.
Risk insights can significantly vary based on site, plant design, and plant operating characteristics.
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Agenda Time Topic Speaker 1:00pm Meeting Kick-Off J. Wiebe - 5 mins Welcome Remarks M. Franovich - 5 Mins Meeting Objectives A. Zoulis - 10 mins Event Summary NextERA - 15 mins NRC Activities following Event S. Weerakkody - 10 mins Risk Insights from ASP Analysis C. Hunter - 15 mins Risk Insights from LIC-504 Analysis M. Leech - 20 mins Owners Group Discussion PWROG/BWROG - 20 mins 2:40pm Break 10 mins 2:50pm Panel Discussion Industry/NRC - 40 mins 3:30pm Public Comments NRC/Public - 30 mins 3 Public Meeting on Risk Insights Associated with DAEC Derecho
NRC Activities Following Event Sunil Weerakkody Senior Level Advisor US NRC Division of Risk Assessment Office of Nuclear Reactor Regulation 4 Public Meeting on Risk Insights Associated with DAEC Derecho
NRC Responses to DAEC Derecho
- Different Entities of NRC coordinated different activities to fulfill NRC mission.
- Region III Performed Management Directive (MD) 8.3 evaluation to determine level of follow-up inspection needed.
- Office of Nuclear Regulatory Research (RES)
Performed the Accident Sequence Precursor (ASP) analysis to determine the overall risk-significance of the event.
- Office of Nuclear Reactor Regulation (NRR)
Used NRR Office Instruction LIC-504, Integrated Risk-Informed Decision Making for Emergent Issues.
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NRC Region III Actions
- MD 8.3 Evaluation - Decision Documentation for Reactive Inspections
- Resident on-site within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> of the storm.
- Conducted inspections to assess immediate actions, plant stability, verify safety barriers, and evaluate radiological impacts.
- ADAMS Accession No. ML21022A415, August 11, 2020.
- DAEC Integrated Inspection Report
- No findings or violations of more than minor significance were identified during this inspection.
- ADAMS Accession No. ML20314A150, November 6, 2020.
- ADAMS Accession No. ML21022A418, January 22, 2021.
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Office of Nuclear Reactor Research Actions
- Accident Sequence Precursor (ASP) Analysis
- Mean conditional core damage probability (CCDP) was 8x10-4.
- High (comparative) risk-significance although emergency systems remained functional after the event and there were no violations/findings.
- ADAMS Accession No. ML21056A382, March 4, 2021.
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Office of Nuclear Reactor Regulation Actions
- LIC-504 - Determination of the Need for Prompt Regulatory Actions in Response to Insights Gleaned from Duane Arnold Nuclear Power Plant.
- ADAMS Accession No. ML20315A117, November 25, 2020.
- Duane Arnold Energy Center LIC-504 Team Recommendations.
- Issue an information notice to licensees about the risk insights gained through the NRCs analysis of the DAEC derecho event.
- Share risk insights obtained from the LIC-504 analysis with the NRCs regional staff.
- Identify opportunities to engage with external stakeholders (e.g., PRA practitioners, Owners groups) about the insights gained during this evaluation.
- Update two SPAR models during fiscal year 2021 and 2022 as part of the normal update process to further enhance staffs understanding of risk insights gained from LIC-504.
- ADAMS Accession No. ML21078A127, March 30, 2021.
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ASP Analysis Risk Insights Chris Hunter Senior Reliability and Risk Analyst US NRC Division of Risk Assessment Office of Regulatory Research 9 Public Meeting on Risk Insights Associated with DAEC Derecho
ASP Program History 1978 1982 Initial ASP Report NUREG/CR-2497 Lewis Committee
- NRC established the ASP Program as part of the Office of Analysis
- An independent evaluation of WASH- and Evaluation of Operational Data.
1400 recommended increased use of operational data to assess the risk from commercial NPPs.
Rogovin Report
- After the TMI accident, a special inquiry concluded the following: The systematic evaluation of operating experience must be RIS 2006-24 undertaken on an industry-wide basis, both by the utility industry, which has the greatest direct *Allowed the ASP Program to leverage SDP risk stake in safe operations, and by the NRC. evaluations to prevent duplication of effort.
1980 2006 10 Public Meeting on Risk Insights Associated with DAEC Derecho
What is Precursor?
- An accident sequence precursor is an initiating event and/or degraded condition that increases the likelihood for core damage.
- The precursor threshold is a conditional core damage probability (CCDP) or increase in core damage (CDP) of greater than or equal to 10-6.
- Threshold for initiating events is plant-specific and can be higher.
- Analyses consider concurrent degraded conditions or unavailabilities due to maintenance.
- Maximum exposure time of 1 year is used for degraded conditions.
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ASP Process 12 Public Meeting on Risk Insights Associated with DAEC Derecho
Duane Arnold ASP Analysis
- Risk dominated by Station Blackout (SBO) scenarios.
- Offsite power recovery credit not provided within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
- Identified several key analysis uncertainties.
- Performed sensitivities to evaluate the impact of these uncertainties.
- Sent the preliminary ASP analysis to the licensee for a 60-day review per Regulatory Issue Summary 2006-24.
- NRR and Region 3 given opportunity to provide additional comments.
- The licensee and PWROG provided comments.
- Final ASP analysis was modified based on offsite power recovery could be credited within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, resulting in final mean CCDP of 8x10-4.
- ADAMS Accession No. ML21042A079, February 9, 2021.
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Uncertainties/Sensitivities Key Uncertainty Mean CCDP % Change No FLEX Credit 7.8x10-3 ~+900%
No FLEX Credit for Successful High Pressure CoolanI 1.7x10-3 +120%
Scenarios x2 increase 7.7x10-4 -3%
FLEX Hardware Reliability Multiplier x5 increase 9.3x10-4 +18%
x10 increase 1.2x10-3 +57%
x5 increase 1.9x10-3 +135%
FLEX Human Error Probabilities x5 decrease 5.8x10-4 -27%
72-Hour AC Power Recovery Requirement 1.1x10-3 +34%
Potential Effects of Bypassing 1.5x10-3 +89%
Emergency Service Water Strainer 14 Public Meeting on Risk Insights Associated with DAEC Derecho
LIC-504 Analysis Risk Insights Matt Leech Reliability and Risk Analyst US NRC Division of Risk Assessment Office of Nuclear Reactor Regulation 15 Public Meeting on Risk Insights Associated with DAEC Derecho
The LIC-504 Analysis
- The Accident Sequence Precursor Analysis conducted by the Office of Research focuses on examining the risk from the specific event for the Duane Arnold plant.
- The LIC-504 conducted by the office of Nuclear Reactor Regulation (NRR) examined several plants by running a similar scenario to the one Duane Arnold experienced during the derecho and examining the risk results.
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First Step of the LIC-504
- The first step of the LIC-504 was to determine if immediate shutdown orders or other prompt actions were needed.
- Initially we took a group of 7 plants that had similar characteristics to Duane Arnold.
These were single unit sites, without Station Blackout Diesels, and Ultimate Heat Sink systems that could potentially experience storm debris from a derecho.
- They were of mixed design: Pressurized Water Reactors including designs by Westinghouse, and Combustion Engineering; Boiling Water Reactors (BWR) including BWR-4 design with Mark 1 containment, and BWR-6 with a Mark 3 containment.
- Performed a conservative upper bound analysis and concluded that there is no need to issue orders to shut down or impose prompt compensatory measures. This conclusion was documented in ADAMS Accession Number ML20315A117 in November of 2020.
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Next Step of the LIC-504
- To support the next step of LIC-504, we refined the analysis and reduced conservatism. The objective is to generate risk insights and provide recommendations.
- The risk analysis in this step reduced conservatism in a variety of areas:
- Provided credit for Diverse and Flexible Mitigation Capability, or FLEX, equipment and strategies
- Refined the models on a case-by-case basis using readily available information to improve accuracy
- Credited operator's ability to bypass the strainers at plants that had bypass capability.
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DAEC LIC-504 Analysis Process For each plant we were analyzing the following:
We were solving the model for a weather-related Loss Of Offsite Power (LOOP) with offsite power recovery not possible for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
We also changed the probability for an Emergency Service Water (ESW) strainer to fail at various failure rates And examined the results 19 19
Analysis Uncertainties
- LIC-504 had some uncertainties that could significantly influence the results.
- What failure probability to assign to the Emergency Service Water strainers to fail?
Very influential to the results.
- What initiating event frequency to use for the event?
The industry average Weather-Related Loss of Offsite Power frequency was used.
Likely conservative because some Weather-Related Loss of Offsite Power events would not be as severe as a derecho that could impact the Emergency Service Water.
- All sites do not have the same level of risk to a derecho, especially how it will impact their Ultimate Heat Sink.
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Derecho Frequency 21 21
Risk Insights
- The following design attributes reduce the risk significantly for this type of derecho event.
- The ability for operators to bypass a clogged strainer.
- Availability of an alternate source of cooling water to Emergency Diesel Generators (e.g.,
fire protection water).
- The availability of additional diesel generators (not including FLEX diesels) that are not dependent on Emergency Service Water for cooling.
- Service water intake traveling screens on a safety related power supply.
- FLEX provided a significant reduction in risk for this type of event
- Risk reductions varied between a factor of 1.5 to 11.5.
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LIC-504 Recommendations
- The LIC-504 final closure memo was finished and made publicly available April 2021
- It had three main recommendations
- Issue an Information Notice of the risk insights to share with industry
- Share insights with our regional inspectors
- Share insights with external stakeholders about the risk insights gained from the evaluation.
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