Value Tree Analysis Approach for Integrated Risk-Informed Decision Making: Revision of Allowed Outage Time

Abstract

This study puts forward a methodology of integrated risk-informed decision making (IRIDM) for extension of allowed outage time (AOT) of safety system through Value Tree Analysis (VTA) approach. The purpose of regulatory decision making is to demonstrate compliance with regulatory requirements. Objectivity, transparency, and auditability are the foremost requirements for decisions on nuclear safety. Practical implementation of the IRIDM process may be very difficult because of the problems with the inputs prioritization and the decision options evaluation. An important element of any IRIDM process should be the explicit consideration of all effects because improvements in one area may adversely affect another area and to achieve balance in the overall safety measures. VTA replaces simple ranking of the inputs based on the engineering judgement by a selection of appropriate value functions, as much as possible, basing on the quantitative criteria. In this study, Value Tree for extension of AOT has been developed considering deterministic, probabilistic, and operating experience and economy as inputs. The case study has also been conducted to demonstrate the proposed approach for the extension of AOT of a single inoperable emergency diesel generator from 3 days to 7 days. It is observed that the benefits of the proposed methodology are: transparency, consistency, and reproducibility. This approach significantly makes the IRIDM process well-structured and easier to apply. The VTA complements the existing IRIDM framework proposed by International Atomic Energy Agency. It also increases the accountability and auditability of decisions.