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:
tial 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 SRP 19-A18 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 SRP chapter 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. • For some complex groups of operator actions (e.g., the response to an anticipated transient without scram (ATWS) in a BWR, or the choice to go to recirculation rather than residual heat removal (RHR) in response to a small LOCA in a PWR), the PRA analysts may have chosen to adopt a bounding approach to the accident scenarios that precludes having to address subsequent actions. This could mean that the equipment associated with those actions might be overlooked in the change process. c. Evaluation Findings The staff safety evaluation report should include language equivalent in