Document: NRC Regulatory Guide
Document ID: 22ccfd5e-c5d8-4615-a02c-32369aa9f533
Document Type: regulatory_guide
Title: Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1930/ML19308B636.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.200
CFR Part: 
CFR Title: 

Content:
d out. The analysis of how the flood pathways and water levels cause the failure of SSCs following ingress into the plant structures is similar to the analysis in the internal flood PRA. The technical elements for an external flood PRA are similar to those for an internal flood PRA and seismic PRA. The types of external flood phenomena that should be considered in the analysis are dependent on the site. Both natural phenomena, such as river or lake flooding, ocean flooding from high tides or storm surges, unusually high precipitation, tsunamis, seiches, etc., as well as human-caused events such as failures of dams, levees, and dikes, are considered. It is assumed that the external flood PRA is based on modifications made to an existing, up-to-date internal events, at-power PRA. Because of its dependence on the internal events model, the external flood PRA incorporates the elements of regulatory positions C.1.2.1 and C.1.2.2, as necessary. External Flood Hazard Analysis estimates the frequency of external flood at the site using a site-specific probabilistic hazard analysis that incorporates the available recent site-specific information and uses up-to-date databases. Uncertainties in the models and parameter values are properly accounted for and fully propagated to allow the derivation of a mean hazard curve from the family of hazard curves obtained. External Flood Fragility Analysis is an evaluation that is performed to estimate plant-specific, realistic flood fragilities for those structures, or systems, or components (or their combination) whose failure contributes to core damage or large early release. External Flood Plant Response Analysis uses an external flood PRA model that includes all significant flood-caused initiating events and other failures that can lead to core damage or large early release. The model is adapted from the internal events, at-power PRA model to incorporate unique flood- analysis aspects that are different from the at-power, internal