Document: NUREG-0800
Document ID: 7916b088-fb90-4163-84fe-027bd315bcc5
Document Type: srp
Title: REVIEW OF RISK INFORMATION USED TO SUPPORT PERMANENT PLANT-
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0717/ML071700658.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.2
CFR Part: 
CFR Title: 

Content:
ents the general guidance in Appendix B to this SRP section. b. Review Guidance and Procedures Identification of Functions, Systems, and Components Important to Safety: The PRA can provide significant qualitative insights that emerge simply from considering whether and how systems are invoked in particular scenarios. If a front-line system is credited in success paths, 19.2-78 June 2007 it is "important" in some sense, and at least some of its SSCs must also be important in some sense, even if a given single-event importance measure does not reflect this. However, the real importance of a system is a function of whether alternative, diverse systems that could fulfill the same function. Systems that are the only means of providing the function would be considered more important than those for which there are viable alternatives. A system that supports an important front-line system could also be considered important. This does not mean that all such systems cannot be candidates for relaxing current requirements; however, it does mean that components in system trains credited in the PRA should be explicitly considered during the integrated decisionmaking process. Either by evaluating the licensee's documentation or by conducting an independent verification, reviewers should complete the following steps: • Identify all systems that are relied upon in plant response to an initiating event, whether explicitly modeled in the PRA or not (e.g., room cooling systems, and instrumentation and control systems associated with indications rather than control may not be modeled), and identify the function(s) they perform or support. • Determine whether failure of components screened out on the basis that they are elements of "unimportant" systems could affect a system that is relied upon in the plant's response to an initiating event. Reviewers should then verify that at least some elements of each of the important systems identified above are considered "safety significant."