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:
quirement could be satisfied by an inventory of event tree success paths, with an indication of the mission success criteria, systems, and SSCs involved in each path. Lacking a full-scope Level 2 PRA, surrogate information should be developed for unanalyzed areas, along the lines described in Section III.2.2.2 of this SRP chapter. This requirement is necessary in order to show the safety functions performed by SSCs (or other plant elements) affected by the application. • Causal models (determination of cause-and-effect relationships) should be developed to support an evaluation (qualitative or quantitative) of the change in risk as a function of the application. This is necessary in order to relate the application to actual risk indices. Documentation of inputs to the decisionmaking panel should be part of the process. Reviewers should verify the scope and depth of the information base, especially information supplied regarding modes and/or classes of initiators unanalyzed in the PRA. Treatment of SSCs Not Modeled in the PRA PRAs do not model all SSCs involved in performance of safety functions for various reasons. However, this should not imply that unmodeled SSCs are not important in terms of contributions to plant risk. For example, SSCs are omitted in some cases because the analysts take credit for programmatic activities that ensure a low failure frequency for that item or a short fault exposure time in the event that it does fail. In such cases, even though the PRA results will not reflect the SSC at all, it would be inappropriate to conclude that the programmatic activity is unimportant. Consequently, one task of the integrated decisionmaking panel is to extrapolate from the PRA and other information sources to draw conclusions about SSCs that are not modeled in the PRA. This does not mean that the panel is to impute to the PRA high-level results that were not generated in the analysis; however, it does mean that if a success path is modeled in the PRA, the