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:
ated or quantified, reviewers should establish that performance monitoring is capable of detecting CCF before multiple failures are likely to occur subsequent to an actual SRP 19-A14 system challenge. In addition, to reduce fault exposure times for potential common cause failures, phased or incremental implementation should be considered as part of the effort to protect against CCF. Reviewers should ensure that the impact of the change is not inappropriately made insignificant by the choice of CCF probabilities for SSCs unaffected by the change. This can occur in two ways. First, the cutsets or scenarios containing events which represent failures of SSCs affected by the change may include CCF contributions from other SSCs which are too small. Second, the contribution of cutsets or scenarios which do not contain affected SSCs may be artificially 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 which 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 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