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:
set of minimal cutsets or scenarios contains the significant contributors and their logical combinations for the application in question. Depending on the level of PRA detail (module level, component level, or piece-part level), this may translate into a truncation limit from 10-12 to 10-8 per reactor year. Multiple Component Considerations: As previously discussed, importance measures are typically evaluated on the basis of individual 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 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