Uncertainty and Sensitivity Analyses (Task 15)

Task Overview

Background

This task describes an approach for identifying and treating uncertainties throughout the Fire PRA process. The treatment may vary from quantitative estimation and propagation of uncertainties where possible (e.g., in fire frequency and non-suppression probability) to identification of sources without quantitative estimation, where knowledge of a quantitative treatment of uncertainties is beyond the state-of-the-art. The treatment may also include one-at-a-time variation of individual parameter values to determine the effect on the overall fire risk (sensitivity analysis).

Purpose

This procedure describes the approach for identifying and treating uncertainties throughout the Fire PRA process and identifying sensitivity analysis cases. It also prescribes a review for the identified uncertainties among the Fire PRA analysts to establish an integrated approach of addressing the effects of these uncertainties on the results of the analysis. At this time, the procedure provides a general approach to be followed and does not provide a comprehensive list of specific uncertainties to be addressed. As pilot Fire PRAs and other studies are completed, this procedure may be revised accordingly.

Scope

This procedure covers the identification and treatment of uncertainties throughout the Fire PRA. As such, it provides: (1) background on the subject of uncertainty found in Appendix U, (2) classification of types of uncertainty, and (3) a general approach with regard to practical implementation of treating expected uncertainties in the Fire PRA, as described in Appendix V.

Related Element of ASME/ANS PRA Standard

Uncertainty and Sensitivity Analysis (UNC)

Related EPRI 1011989 NUREG/CR-6850 Appendices

Appendix U, Appendix for Chapter 15, Uncertainty Analysis-Technical Bases

Appendix V, Appendix for Chapter 15, Discussion of Individual Task Uncertainties

Supplemental Guidance

Risk-informed applications are expected to address uncertainties according to guidance such as:

• EPRI 1026511, Practical Guidance on the Use of Probabilistic Risk Assessment in Risk-Informed Applications with a Focus on the Treatment of Uncertainty provides supplemental guidance beyond EPRI 1016737 and NUREG-1855
  • Appendix B provides an assessment of potential sources of model uncertainty associated with the internal fire hazard. This appendix identifies the minimum set of generic sources of model uncertainty that need to be addressed for meeting the related PRA standard supporting requirements associated with the internal fire hazard.
• EPRI 1016737, Treatment of Parameter and Modeling Uncertainty for Probabilistic Risk Assessments provides a generic list of model uncertainties and examples of how to characterize those sources.
• NUREG-1855 Revision 1, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk Informed Decisionmaking provides guidance on how to treat uncertainties associated with PRAs to support a risk-informed application to the U.S. NRC. This process includes identifying and characterizing the uncertainties associated with the PRA, performing an uncertainty analysis to understand the impact of the uncertainties on the results of the PRA, and factoring the results of the uncertainty analyses into the decisionmaking.