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:
e changed significantly so that the HEP should be modified. Reviewers should also be aware that the impact of the change can be obscured if the accident sequences that do not contain affected SSCs are artificially increased in value by HEPs that are too large. These cases will impact applications involving risk categorization by lowering the relative contribution of the affected SSCs. An understanding of these effects can be obtained from sensitivity analyses performed by removing the pertinent HEPs or by using more realistic values for the HEPs. Another consideration associated with the potential masking of important SSCs is that the SSCs might not be included in the model used to perform the evaluation of risk. This can happen in several ways: • Cutsets or scenarios containing the SSCs may be truncated because HEPs in the same cutset or scenario are too low. Such truncation should only be a concern if the logic model was not re-solved to determine the change in risk (for example, in applications that depend on SSC risk ranking using a pre-solved equation). The preferred resolution to this would be a request for re-solution with the appropriate changes made to all affected SSCs. Section A.9 of this appendix discusses this in more detail. • SSCs may not be included in the logic model structure because HEPs are so high that they are assumed to dominate the unavailability of a function, and therefore the associated hardware is not modeled. However, the hardware could still be a contributor to the calculation of risk importance. For example, the hardware (as a group) will have the same risk importance (in terms of Risk Achievement Worth) as the associated HFE. This suggests that the licensees should identify the important operator actions for applications in RIR, as well as the equipment required to perform the specific function associated with the action. The equipment should then be dispositioned in accordance with its importance in achieving that function. •