Document: NUREG-0800
Document ID: ac43f632-9db1-4857-9f56-bfba45965456
Document Type: srp
Title: * These are modifications to a plant’s design, operations, or other activities that require NRC approval. These modifica
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0232/ML023250195.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19
CFR Part: 
CFR Title: 

Content:
ranking results to the important PRA modeling techniques, assumptions, and data. In assessing this information, reviewers should consider the following issues: Different Risk Metrics: Reviewers should ensure that the licensee’s ranking process adequately considered risk in terms of both CDF and LERF. Completeness of Risk Model: Reviewers should ensure that, when determining safety significance contributions using an internal events PRA, the licensee also considered external events, as well as shutdown and low-power initiators, either by PRA modeling or by the integrated decisionmaking process (as detailed in Section C.2 and Appendix B to this SRP chapter). SRP 19-C3 Sensitivity Analysis for Component Data Uncertainties: The licensee should have addressed the sensitivity of component categorizations to uncertainties in the parameter values. Reviewers should be satisfied that SSC categorization is not affected by data uncertainties. Sensitivity Analysis for Common Cause Failures: CCFs are modeled in PRAs to account for dependent failures of redundant components within a system. As discussed in Appendix A to this SRP chapter, CCF probabilities can impact PRA results by enhancing or obscuring the importance of components. This should be addressed by the review. A component may be ranked as a high risk contributor mainly because of its contribution to CCFs, or a component may be ranked as a low risk contributor mainly because it has negligible or no contribution to CCFs. In RIR, removing or relaxing requirements may increase the CCF contribution, thereby changing the risk impact of an SSC. Consideration of Multiple Failure Modes: PRA basic events represent specific failure events and failure modes of SSCs. Reviewers should verify that the licensee performed the categorization by taking into account the combined effects of all associated basic PRA events (such as failure to start and failure to run), including indirect contributions through associated CCF event