Fire PRA Uncertainty

Fire PRA Quantitative Uncertainty and Sensitivity Analyses to Support Fire PRAs for US Nuclear Utilities

Fire PRAs are complex and can consist of in the order of 1,000 fire damage states (similar to an internal events initiating event). The development and quantification of these fire damage states has considerable uncertainty, which has not been fully addressed in recently conducted fire PRAs. Furthermore the PRA Standard includes requirements to address uncertainties in a fire PRA and applications of a fire PRA are expected to address uncertainties. Whilst US industry and NRC documents (EPRI 1016737 and NUREG 1855) provide guidance for addressing uncertainties, neither guidance document explicitly addresses fire or other hazards with the same level of detail provided for internal events and internal flooding at power.

Consequently, Jacobsen Analytics have developed and implemented an approach for identifying and addressing fire PRA parameter uncertainty during the course of two fire PRA projects being performed in conjunction with its US associate company, Scientech on behalf of US utilities. The method has two significant advantages in addition to the prediction of overall uncertainty ranges; namely the ability to assess the importance of specific input parameters to the overall variance of the risk outcome and the ability to predict a less conservative mean estimate of the risk than would otherwise be possible. The approach has recently undergone two industry peer reviews. In both cases the approach and implementation has been highly commended by the review teams including recognition as a 'best practice'. An overview of this approach is provided in a white_paper.

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The method is now under consideration by EPRI for further refinement and extension to more fully characterise the uncertainties based on data provided in NUREG/CR 6850, NUREG/CR 1934 as well as other published sources and expert judgment.