Document: NUREG-0800
Document ID: e32f0820-4e33-476e-aa36-4ca8c2c64af0
Document Type: srp
Title: Use of Probabilistic Risk Assessment in Plant-Specific, Risk-Informed Decisionmaking:
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0119/ML011940192.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
uidance 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 i:tat 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 shou d 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 samrE! 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 ,cperating 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 nece-3sary for safe shutdown are included, relevant systems are credited for each function, and plant emergency operating procedures (EOPs) and abnormal operating orocedures (AOPs) are correctly represented. In addition, where the proposed change affects any of these elements, the change is properly modeled. Changes to the plant's design or operations could affect the dependencies (functional, phenonnenological, and operational) among the top events in event trees. Section A.4 of this SRP chapter presents additional