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
ectio
n (H
igh
Like
lihoo
d)M
itiga
tion
(Low
Lik
elih
ood)
Floo
ding
Wat
er E
leva
tion
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