Journal Article

Development of Importance Measures Reflecting the Risk Triplet in Dynamic Probabilistic Risk Assessment: A Case Study Using MELCOR and RAPID

Tamaki, Hitoshi, Takata, Takashi, Narukawa, Takafumi, Sibamoto, Yasuteru, Maruyama, Yu, Zheng, Xiaoyu, Takada, Tsuyoshi

2025

Overview

While traditional risk importance measures in probabilistic risk assessment are effective for ranking safety-significant components, they often overlook critical aspects such as the timing of accident progression and consequences. Dynamic probabilistic risk assessment offers a framework to quantify such risk information, but standardized approaches for estimating risk importance measures remain underdeveloped. This study addresses this gap by: (1) reviewing traditional risk importance measures and their regulatory applications, highlighting their limitations, and introducing newly proposed risk-triplet-based risk importance measures, consisting of timing-based worth, frequency-based worth, and consequence-based worth; (2) conducting a case study of Level 2 dynamic probabilistic risk assessment using the Japan Atomic Energy Agency’s RAPID tool coupled with the severe accident code of MELCOR 2.2 to simulate a station blackout scenario in a boiling water reactor, generating probabilistically sampled sequences with quantified timing, frequency, and consequence of source term release; (3) demonstrating that the new risk importance measures provide differentiated insights into risk significance, enabling multidimensional prioritization of systems and mitigation strategies; for example, the timing-based worth quantifies the delay effect of mitigation systems, and the consequence-based worth evaluates consequence-mitigating potential. This study underscores the potential of dynamic probabilistic risk assessment and risk-triplet-based risk importance measures to support risk-informed and performance-based regulatory decision-making, particularly in contexts where the timing and severity of accident consequences are critical.

• Publisher: MDPI AG
• Subject: Accidents / Boiling water reactors / Case studies / Decision making / dynamic probabilistic risk assessment / Failure / MELCOR / Nuclear energy / Preventive maintenance / Probabilistic risk assessment / Regulation / Risk assessment / risk importance measure / risk triplet / risk-informed decision making / severe accident