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:
ncluded in the original risk analysis based on screening analyses, the licensee should have determined whether initiating events previously screened out because of low frequency might now be above the screening threshold as a result of a proposed application. Plant changes could increase the frequency of initiating events that were relatively infrequent to begin with, or these changes could affect structures, systems, and components (SSCs) or operator actions that were credited with the satisfactory mitigation of initiating events. If initiating events increased in frequency as a result of an application to the point where it became important (i.e., could no longer be screened out), reviewers should verify that the licensee has modified the scope of the analysis to reflect this change. Low frequency of an event, by itself, is not usually sufficient as a criterion for screening purposes. The consequences of non-mitigation of the events also play a big part in this process. For example, interfacing system loss-of-coolant accidents (ISLOCAs) are often assessed as low-frequency events. However, because of their impact on public health and safety, these ISLOCAs can be important. Therefore, for potentially high-consequence events, even if the event frequency is below a screening criterion, the features that lead to the frequency being low (for example, surveillance test practices, startup procedures) should be taken into account in reviews of PRA applications. The licensee should also have evaluated proposed plant changes to determine whether the changes could result in initiators that were not previously analyzed in the PRA. For example, changes might enhance the potential for spurious operation of components that might, in turn, cause initiating events, or changes might increase the likelihood for operator errors of commission that could result in plant trips. If the licensee identified mechanisms for producing new initiators, reviewers should ensure that the licensee