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:
c. Similarly, if changes involve modification to procedures to cross-tie electrical buses, the review might focus on the loss of offsite power event trees. b. Review Guidance and Procedures Event tree sequence models are used to model the responses of plant systems and operations personnel to initiating events. When the LB change request requires the review of event trees, it 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 19.2-44 June 2007 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