Seismic and External Flood Hazard Risk Insights

Milton Valentín, P.E.NRC Risk and

Reliability Analyst

Timeline of NRC Efforts

1990 2000 2010 2020

1991: NUREG-1407, Submittal and Guidance for IPEEE

1995: GL 88-20 Supplement 5 (IPEEE Modified Seismic Requirements)

2002: NUREG-1742, Perspectives Gained from IPEEE

2005: GI-199Seismic Hazard Estimates

2009: External Event PRA Standard Endorsed in RG 1.200

2010: GI-204 Upstream Dam Failure

2011: PRE-11 Downstream Dam Failure

2012: 10 CFR 50.54(f) Request for Information on Fukushima Reevaluations

1992: NUREG/CR-4839, External Event Screening

1987-89: NUREG/CR-5042,Evaluation of External Hazards

2017: POANHI

2015: PFHA Workshops

Post-Fukushima ActionsHow did we do it?• Applied a safety-first

mentality

• Used latest methods and information

• Supported improvement of the state of knowledge

• Endorsed industry guidance

What did we do with it?• Confirmed safety for

licensing basis events

• Enhanced protection and mitigation for beyond-design-basis events

• Improved the state of knowledge

Reevaluated Flooding Hazards

Annual Exceedance Probability (AEP):

AEP ≥ 1x10-3 + Margin or 1x10-4

High Likelihood—Protection

AEP < 1x10-3 + Margin or 1x10-4

Low Likelihood—Mitigation

NEI 16-05, Revision 1, “External Flooding Assessment Guidelines,” June 2016

Prot

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Like

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(Low

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Floo

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Seismic Risk Can Be Nontrivial

Frequent Seismic Risk Contributors

Estimated Risk Reduction from SPRA-Based Plant Modifications

Safety Enhancements ΔSCDF ΔSLERFOperator actions and training to prevent relay chatter

17% 6%

Alternate letdown isolation ~ 92%Alternate power to hydrogen ignition system

~ 50%

Emergency supply transformer anchorage

16.5% ~

Diverse and robust auxiliary feedwater supply

40% 60%

Conclusion• U.S. plants are better prepared against external hazards as

the understanding of the hazards and the tools to quantify risk have evolved.

• U.S. plants have improved their mitigation and protection capabilities commensurate with the improved understanding of the risk.

• The NRC staff continues to enhance the state of knowledge:– POAHNI– PFHA workshops– PRA standard/guidance/tools development and updates

Acronyms10 CFR – Title 10 of the Code of Federal Regulations10 CFR 50.54(f) – Regulation stating that licensees shall, upon the Commission’s request, submit written statements to enable the Commission to determine whether a license should be modified, suspended, or revoked.AC and DC – Alternate Current and Direct CurrentAEP – Annual Exceedance ProbabilityCDF – Core Damage FrequencyFLEX – Diverse and Flexible Coping Mitigation Strategies GI – Generic IssueGL – Generic LetterIPEEE – Independent Plant Examination for External EventsLIP – Local Intense Precipitation

LOCA – Loss-of-Coolant AccidentLOSP – Loss of Offsite PowerNEI – Nuclear Energy InstituteNUREG – NRC ReportsNRC – U.S. Nuclear Regulatory CommissionPFHA – Probabilistic Flooding Hazard AnalysisPOAHNI – Process for Ongoing Assessment of Natural Hazard InformationPRA – Probabilistic Risk AssessmentPRE – Pre-Generic Issue SummaryRG – Regulatory GuideSCDF – Seismic Core Damage FrequencySLERF – Seismic Large Early Release FrequencySPRA – Seismic Probabilistic Risk AssessmentΔSCDF – Delta Seismic Core Damage FrequencyΔSLERF – Delta Large Early Release Frequency