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:
ct of the change has been adequately addressed. While there are no widely accepted definitions of system or group importance measures, it is likely that some licensees will develop new system or group measures. If any are proposed, reviewers should ensure that the measures logically capture the impact of changes to the group. As an example of the issues that arise, consider the following. For front-line systems, one p-ossibility would be to define a Fussell-Vesely type measure of system importance as the sum of the frequencies of sequences involving failure of that system, divided by the SLIm of all sequence 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 systerms. 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. 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). 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 i!3 a concern whether importances are