機器の状態遷移および経時的変化の影響を考慮したPRA手法の提案

摘要

One of the most important issues of the current PRA methodology is the precise modeling of dynamic changes such as state transitions among several states including fault(s) or maintenance of the nuclear facility, safety-related systems or components by fault-tree analysis and event-tree analysis. Moreover, though safety-related systems are usually in the stand-by state during normal operating conditions of a nuclear power plant, modeling of the dynamic changes in safety functions, along with changes in component failure rates due to the aging effect in the stand-by state or continuous/intermittent effects originating from external hazards, is also carried out under the same situation. On the basis of the background described above, the authors proposed a reliability analysis methodology of using the Markov state transition model applied to the digital reactor protection system of an ABWR plant, and demonstrated the applicability of the developed methodology using the component failure modes discussed in DIGREL, the task group of WGRisk belonging to OECD/NEA/CSNI. These studies showed that the PRA methodology including the state transition model can consider state transitions of components and time-dependent changes in component failure rates, and the relationship between this methodology and minimal cut sets for calculating the core damage frequency was also clarified.%既存の原子力発電所の安全審査においては，決定論的な設計基準が設定され，これに沿った設計を行うことで安全性を確保してきた。一方，設計基準を超える過酷事故が起こる可能性を残余のリスクと呼び，原子力発電所の安全性を向上させるためには，適切な残余のリスクの評価と評価結果に基づく対応が急務とされてきた。