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:
SSCs or human actions. One potential concern that arises from this practice is that single-event importance measures have the potential to dismiss all elements of a system or group, despite the system or group having a high importance when taken as a whole. (Conversely, there may be grounds for screening out groups of SSCs, owing to the unimportance of the systems of which they are elements.) Two potential approaches are used to address the multiple component issue. The first is to define suitable measures of system or group importance. The second is to choose appropriate criteria for categorization based on SRP 19-C4 component-level importance measures. In both cases, it will be necessary for the licensee to demonstrate that the cumulative impact 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 possibility 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 sum 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 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