What’s Wrong with the Fault-Tree Linking Approach for Complex PRA Models?

By Steven A Epstein and Donald Wakefield

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Executive Summary

Analysts using complex probabilistic risk assessment (PRA) models at nuclear power plants have two approaches from which to choose: Fault Tree Linking (FTL) or Large Event Tree Linking (ETL) methods. The choice of which method is utilized depends primarily on history (which method was used by the analysts who first developed the specific PRA model) or economics (e.g., which method is used by the majority of PRAs in a utility merger). However, users of either method should be aware of the problems and limitations of their approach. In particular, the current generation of PRA analysts and users need to address problems which a previous generation did not have to face due to the growing complexity of current models and the extensive applications to which models are routinely being applied.

The authors of this article clearly have a bias toward, as well as extensive experience with, the ETL method. A number of tools have been built into the RISKMAN® code which implements the ETL method to solve problems that still remain in FTL. This paper discusses a number of key problems that illustrate, “What’s Wrong with the FTL Approach?” This paper summarizes these problems, provides an estimate of the significance of the problem, and provides references where some of these problems have been examined in greater detail. Note that for some of these problems, the quantitative level of significance can be estimated, while for others, it is unknowable with the current approach.

Since these problems may impact important risk-informed decisions, the PRA analyst is obligated to examine these problems and identify the extent to which they restrict his or her model.