Document: NUREG-0800
Document ID: e32f0820-4e33-476e-aa36-4ca8c2c64af0
Document Type: srp
Title: Use of Probabilistic Risk Assessment in Plant-Specific, Risk-Informed Decisionmaking:
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0119/ML011940192.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
presentation of a vulnerability is invalid, then the PRA should be modified, and the licensee should work wilt 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 -raking 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 unanalyzed modes, it is more difficult to accept reductions for components that do play/ a role in unanalyzed (e.g., shutdown) modes. For such instances, conservative methods viill be considered prudent. SRP 19-B4 To address instances in which a PRA model exists but is considered misleading, caution is indicated. An example would be to down-classify SSCs from a PRA result (i.e., state that a high risk contributor is actually a low contributor), on the basis of panel judgment. It is not acceptable to place on the record both a PRA and a finding that clearly contradicts it. Although the panel is not expected to take the PRA as absolute truth, the test should be whether the record establishes a clear basis for a finding. A technical argument that begins with the misleading PRA result and furnishes supplementary information sufficient to justify