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:
o mean values. However, this does not imply that a detailed propagation of uncertainties is always necessary; in many cases, it is possible to show that a point estimate is an acceptable approximation of the mean value using qualitative arguments about the risk contributors. For example, if a formal propagation has not been performed, it is necessary for the licensee to demonstrate that the result is not affected by the so-called state of knowledge correlation (specifically, that there are no significant contributing cutsets or scenarios that involve multiple events for which the probabilities are determined using the same parameter, particularly if the parameter value is very uncertain). It is not uncommon for licensees to use point estimate values without defining probability distributions on the values. In such instances, it is not possible to characterize the point estimate as a mean value. However, for the more significant parameters, some characterization of uncertainty is essential to demonstrate that the point estimate is not an optimistic value. Model uncertainty: Reviewers should determine if the results are strongly impacted by the specific models or assumptions adopted for the assessment of important elements of the FPRA, 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