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:
uence frequencies. Such a measure would need to be carefully interpreted if the numerator included contributions from failures of that system as a result of support systems. Similarly, a Birnbaum-like measure could be defined by quantifying sequences involving the system, conditional on its failure, and summing up those quantities. This would provide a measure of how often the system is critical. However, the support systems again make the situation more complex. To take a two-division plant as an example, front-line failures can occur as a result of failure of support division A in conjunction with failure of front-line division B. Working with a figure of merit determined by the "total failure of support system" would miss contributions of this type. 19.2-77 June 2007 In the absence of appropriately defined group level importance measures, reliance should be made on the integrated decisionmaking process to make the appropriate determination (see Section C.2 of this appendix). Relationship of Importance Measures to Risk Changes: Importance measures do not directly relate to changes in risk associated with implementation of a set of changes proposed in an application. Instead, the risk impact is indirectly reflected in the choice of the value of the measure used to determine whether an SSC should be classified as being of high or low safety significance. This is a concern whether importances are evaluated at the component or group level. Therefore, the criteria for categorization into low and high significance should be related to the acceptance guidelines for changes in CDF and LERF. This implies that the criteria should be a function of the base case CDF and LERF, rather than being fixed for all plants. Thus, reviewers should determine how the choice of criteria relates to, and conforms with, the acceptance guidelines described in Regulatory Guide 1.174. If component level criteria are used, they should be established taking into account the fact that the