146 Integrating Human Reliability Analysis Approaches in the EPRI HRA Calculator® (PSAM-0430)

The human reliability analysis (HRA) element of a probabilistic risk/safety assessment (PRA/PSA) models the contribution of plant operations and maintenance staff to risk metrics such as core damage and large early release frequency. A well constructed, quality HRA produces both quantitative human error probabilities (HEP) as well as qualitative insights into the factors driving the HEP results. A quality HRA model is constructed by following the process and satisfying the requirements of PRA standards, and in doing so the HRA reflects the “as-operated” plant. Historically, human reliability analyses have contributed to important PRA results and insights.

HRA models and methods for evaluating human error probabilities have evolved over the last 20 years from task-based methods of execution actions (e.g. THERP, NUREG/CR-1278) to newer, “second generation” methods of today (e.g. CREAM). Thus, a variety of HRA methods currently exist, and many can produce significantly different results from the same input. These inconsistencies can affect the ability to develop insights and to make risk-informed decisions as part of probabilistic risk/safety assessment applications. These variations in HRA results can be important as the use of quantitative risk results is incorporated into more and more USNRC programs such as the reactor oversight process and risk-informed performance-based fire methods. In addition to responding to regulatory concerns on a case-by-case or event basis, the use of PRA's to support daily configuration risk management evaluation of maintenance schedules has produced a need to re-evaluate or develop human error probabilities methodically and consistently.

In order to address these needs, the Electric Power Research Institute (EPRI) founded the EPRI HRA / PRA Tools Users Group. The primary objective of the EPRI HRA / PRA Tools Users Group is to develop a tool and guidelines to standardize the application of HRA methods in order to obtain comparable results when evaluating human interactions of similar tasks on plants of similar design, training, procedures, and cues. The secondary objective is for the HRA tool to interface with PRA software such as EPRI Risk & Reliability Workstation products such as WinNUPRA, CAFTA, and Risk Spectrum. The long-term goal for the group is to enable the industry to converge to on a set of common methods. The EPRI / PRA Tools Users Group has now been in operation for several years using a tool called the EPRI HRA Calculator® which embodies a set of HRA guidelines and meets the objectives defined above.

This paper describes the selection and integration of HRA methods embodied in the Calculator®, and summarizes the insights of the first few years of operation of the EPRI HRA / PRA Tools Users Group.