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:
sults are strongly impacted by the specific models or assumptions adopted for the assessment of important elements of the PRA and whether the sensitivity analyses that have been performed (if any) are sufficient to address the most significant uncertainties with respect to these elements. In some cases, particularly for small changes in risk or for relatively minor changes, there may be relatively few issues related to model uncertainties. In other cases, where the baseline risk values are to be estimated, the modeling issues should include all those that have a significant impact on the evaluation of the baseline risk values. Model uncertainties arise when there are several alternative approaches to the analysis of certain elements of the PRA model. They are typically addressed by adopting a specific model or making a specific assumption. In such cases, the licensee should document why the particular model or assumption used is appropriate both for the base case risk evaluation and for the analysis of the impact of the change. In certain cases, it may be necessary to perform sensitivity analyses using alternative reasonable models or assumptions to demonstrate the robustness of the conclusions. In deciding what are reasonable alternatives, reviewers should consider whether the alternatives have some precedent and whether they have a reasonable engineering basis. 19.2-21 June 2007 Reviewers should pay particular attention when the characterization of a model uncertainty is such that the results fall into a bimodal or multi-modal distribution and one or more of the modes exceed the acceptance guidelines. The results should then be reviewed on the basis of an evaluation of the significance of the hypotheses associated with the modes that exceed the guidelines. • Completeness Uncertainty: Reviewers should determine whether the licensee has adequately addressed the limitations in the PRA scope and other completeness issues, either by limiting the scope of the