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:
t is important that reviewers become familiar with their structure and with the assumptions embedded in them. In particular, it is important to identify assumptions or approximations that might impact the application. Such assumptions and approximations are not always explicitly documented. The guidance provided below discusses approaches that reviewers can adopt to assess the appropriateness of the modeling of the LB change in the event trees. Reviewers should familiarize themselves with the structure of the event trees and the associated assumptions that are used in the construction of the event trees. Specific issues to consider should include the conditions created by the initiator and the chronological requirements for systems operation and/or operator responses for the different event tree branches. Reviewers should be satisfied that, if simplifications or assumptions were made in the structure and logic of the event trees, these would remain justifiable in light of the LB change. Reviewers should also study the functional and physical dependencies for each phase of the sequence and, at the same time, the interaction between operators and systems as the sequence unfolds. The timing of the events and time dependencies should also be understood. A review of the general structure and philosophy underlying the pertinent plant emergency and abnormal operating procedures will provide valuable insight on the validity of the event tree structure and logic. Specifically, reviewers should ensure that the following factors are addressed in the evaluation of the LB change: • The event trees reflect changes (if any) to the initiating event groupings. • The models and analyses are consistent with the as-built and as-operated plant, i.e., the functions necessary for safe shutdown are included, relevant systems are credited for each function, and plant emergency operating procedures (EOPs) and abnormal operating procedures (AOPs) are correctly represented. In addition,