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:
ially increased by having CCF contributions that are too large so that the impact of the change is obscured. These cases will impact applications involving risk categorization by lowering the relative contribution (and importances) of the affected SSCs. An understanding of these effects can be obtained from sensitivity analyses performed by removing the pertinent CCFs or by using more realistic values for the CCFs. • A common modeling approximation is to include CCF contributions only from that combination of SSCs that fails the function of the system. For example, if system success is defined as success of one out of four components, usually only a single term representing a CCF of all four components is included. If the success criterion were two out of four, the corresponding CCF term would represent failure of any three or all four 19.2-54 June 2007 SSCs in the group. While probabilistically this usually corresponds to the dominant contributions, care has to be taken when the application relies on assessing the impact on risk of having one train unavailable. In this case, the effective success criterion of the remaining part of the system changes, so that in the case of the one-out-of-four system, a CCF of three SSCs becomes a possible contributor. The impact of not modeling the lower-order CCF contributors should be investigated. Note that this can impact applications for which the justification of the change relies on risk categorization, as well as those that require an evaluation of changes to risk. c. Evaluation Findings Reviewers should verify that information provided and review activities conducted support the following conclusions: • Common cause failure has been suitably addressed, and the licensee has systematically identified component groups sharing attributes that correlate with CCF potential and that affect the application. • Where applicable, the licensee's performance monitoring program addresses a phased implementation approach to reduce