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:
these results have been factored into the PRA or into the current application, • The plant risk profile to ensure that the accumulation of changes has not created dominant risk contributors. If the licensee's submittal includes past changes made to the plant (but not submitted to the NRC) that reduced the plant risk, especially changes related to the current application, reviewers should consider such changes in the integrated decisionmaking process. Benefits from the implementation of the Maintenance Rule can also be credited for the applicable SSCs. Beyond cumulative effects, synergistic effects are also possible, and some of these might not emerge from a quantification of the PRA. For example, if conventional importance ranking approaches are employed to determine the importance of SSCs, it would be possible that multiple requirements could be relaxed on certain "low" significant components under multiple applications. If the QA (potentially affecting the failure rate) and the test interval (potentially affecting fault exposure time) were to be relaxed for the same component, the component unavailability could increase more than expected (since failure rate and fault exposure time combine multiplicatively in the calculation of unavailability). If the effects of QA on failure rate could be quantified convincingly, this would be addressed explicitly, but this cannot presently be ensured. As a result, potential exists that different applications might lead to unintended and unquantified synergistic effects on the unavailability of a given component. Synergistic effects on a given element can be addressed by showing that the basic event model adequately reflects the effects of programmatic activities, and that the calculated unavailability, propagated through the PRA, is consistent with the needed performance with regard to the risk indices and the defense-in-depth concept. However, it is more straightforward simply not to allow for the relaxation of multiple