Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decision Making — Main Report (NUREG-1855, Volume 1)

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Publication Information

Manuscript Completed: March 2009
Date Published: March 2009

Prepared by:
M. Drouin (NRC Project Manager)1, G. Parry2
J. Lehner3, G. Martinez-Guridi3
J. LaChance4, T. Wheeler4

3Brookhaven National Laboratory
Upton, NY 11973

4Sandia National Laboratories
Albuquerque, NM 87185

1Office of Nuclear Regulatory Research
2Office of Nuclear Reactor Regulation
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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This document provides guidance on how to treat uncertainties associated with probabilistic risk assessment (PRA) in risk-informed decisionmaking. The objectives of this guidance include fostering an understanding of the uncertainties associated with PRA and their impact on the results of PRA and providing a pragmatic approach to addressing these uncertainties in the context of the decisionmaking.

In implementing risk-informed decisionmaking, the U.S. Nuclear Regulatory Commission expects that appropriate consideration of uncertainty will be given in the analyses used to support the decision and in the interpretation of the findings of those analyses. To meet the objective of this document, it is necessary to understand the role that PRA results play in the context of the decision process. To define this context, this document provides an overview of the risk-informed decisionmaking process itself.

With the context defined, this document describes the characteristics of a risk model and, in particular, a PRA. This description includes recognition that a PRA, being a probabilistic model, characterizes aleatory uncertainty that results from randomness associated with the events of the model. Because the focus of this document is epistemic uncertainty (i.e., uncertainties in the formulation of the PRA model), it provides guidance on identifying and describing the different types of sources of epistemic uncertainty and the different ways that they are treated. The different types of epistemic uncertainty are parameter, model, and completeness uncertainties.

The final part of the guidance addresses the uncertainty in PRA results in the context of riskinformed decisionmaking and, in particular, the interpretation of the results of the uncertainty analysis when comparing PRA results with the acceptance criteria established for a specified application. In addition, guidance is provided for addressing completeness uncertainty in riskinformed decisionmaking. Such consideration includes using a program of monitoring, feedback, and corrective action.

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