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:
s should verify that the safety significance of SSCs not modeled in the PRA (but affected by the proposed application) are appropriately characterized and justified. Addressing Limitations of the Risk Analysis One objective of the integrated decisionmaking process is to overcome certain limitations of the PRA. However, this does not include substituting the analyst's judgment for essential PRA results. One reason for developing PRA models is that the complexity of many facilities makes judgment difficult in many contexts. Generally, if the PRA highlights a plant vulnerability, this should be taken seriously and should not be discounted on the basis of judgment. If the analyst can show that the PRA representation of a vulnerability is invalid, then the PRA should be modified, and the licensee should work with the results of the revised PRA. To address the issue of credit for unmodeled systems that would change a PRA result, the preferred method is to alter the PRA to take the credit. Reviewers should be aware that cases may potentially arise in which credit for an unmodeled system would be seriously complicated by issues of shared support systems, environmental conditions, or other factors such as spatial interaction issues or operator interaction dependencies. To address the issue of making decisions about SSCs that might influence plant response in unmodeled modes or to unmodeled initiators, the acceptable approach is to proceed on the basis of a structured representation of plant response that shows at least qualitatively the initiating events that may pertain, the systems available to respond to each, the functional dependencies of these systems, and the backups available in the event of failure of any particular SSC. While it is possible to accept program reductions for SSCs that are explicitly shown to play no role in SRP 19-B5 unanalyzed modes, it is more difficult to accept reductions for components that do play a role in unanalyzed (e.g., shutdown) modes.