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:
C power systems. In situations such as these (i.e., when the physical situation has embedded circular dependencies), analysts have to break this circularity to allow for model solution. For changes on systems that are affected by circular logic, reviewers should investigate the manner in which the circularity was broken (usually in the sequencing of functions in the event tree) and should verify that the dependency is still being accounted for in the evaluation of the risk change. Sequence Truncation The staff prefers that licensees calculate the change in risk from the application by requantifying the base PRA model so that the potential effects of originally truncated events can be accounted for should they become important as a result of an application. If the licensee did not requantify SRP 19-A20 the model, or if the application depended on the risk ranking of SSCs from a pre-solved equation, reviewers should use the guidelines provided below. Reviewers should be assured (either by documentation provided in the licensee’s submittal or by an independent staff analysis) that cutset or scenario truncation did not introduce errors into the application results or the logic of the PRA that affects the application. Staff review could also involve performing (or reviewing) sensitivity studies where the truncation limit is lowered for the dominant sequences and event initiators, as well as studying the resultant cutsets or scenarios to see if there are any hidden dependencies or unusual/unexpected event combinations (especially if these involve components affected by the proposed application). Staff review could also include comparing a list of the events affected by the application that is in the final truncated cutset equations to the list of application-specific basic events used in the fault tree and event tree models. This yields a list of events that did not make it pass the truncation process. Documentation should be available to enable reviewers to determine