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
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CFR Title: 

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ant, but none of its elements is, reviewers should carefully evaluate the licensee’s justification. As an example, consider the case of a system that contains many redundant flowpaths. Single- event importance analysis will tend to dismiss the flowpaths one at a time, effectively dismissing the group as a whole. The focus of the above guidance is that the redundant flowpaths (considered as a subsystem, and recognizing the function they perform), are important and deserve some attention, even though conventional importance measures would not highlight them. However, in the case of redundant systems, the solution need not always be to assign every redundant path to the high-risk contributor category. In this example, especially if the paths are essentially similar, it is arguably necessary to consider CCF. Thus, a program that addresses CCF potential by monitoring component performance may provide the necessary protection against loss of the function, while still allowing a decrease in some level of commitment on the individual members of the group. Verification of Low Safety Significance: In evaluating the qualitative risk-informed categorization, reviewers should consider the integrated decisionmaking process and criteria used by the licensee. In reviews of the licensee’s determination of low safety significance for SSCs or operator actions, reviewers should verify that the licensee appropriately applied risk importance measures and accounted for the results of sensitivity studies. Reviewers should also verify that the licensee considered and compensated for factors such as potential inadequate scope and level of detail of the PRA (see Sections III.2.2.2 and III.2.2.3 of this SRP chapter). Finally, reviewers should verify that, in categorizing an SSC or operator action as low safety significance, the licensee considered the defense-in-depth philosophy and available safety margins. Section III.2.1 of this SRP chapter presents review guidance on these topics. For