Development of importance measures reflecting the risk triplet in dynamic probabilistic risk assessment; A Case study using MELCOR and RAPID

動的確率論的リスク評価におけるリスクトリプレットを反映した重要度指標の開発; MELCORとRAPIDを用いたケーススタディー

Zheng, X. ; 玉置 等史 ; 柴本 泰照 ; 丸山 結 ; 高田 毅士  ; 成川 隆文*; 高田 孝*

Zheng, X.; Tamaki, Hitoshi; Shibamoto, Yasuteru; Maruyama, Yu; Takada, Tsuyoshi; Narukawa, Takafumi*; Takata, Takashi*

While traditional risk importance measures (RIMs) in probabilistic risk assessment (PRA) are effective for ranking safety-significant components, they often overlook critical aspects such as the timing of accident progression and consequences. Dynamic PRA offers a framework to quantify such risk information, but standardized approaches for estimating RIMs remain underdeveloped. This study addresses this gap by: (1) reviewing traditional RIMs and their regulatory applications, highlighting their limitations, while introducing newly proposed risk-triplet-based RIMs, consisting of timing-based worth (TBW), frequency-based worth (FBW), and consequence-based worth (CBW); (2) conducting a case study of Level 2 dynamic PRA using the JAEA'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 TBW, FBW, and CBW provide differentiated insights into risk significance, enabling multidimensional prioritization of systems and mitigation strategies, for example, TBW quantifies the delay effect of mitigation systems and CBW evaluates consequence-mitigating potential. The study underscores the potential of dynamic PRA and risk-triplet-based RIMs to support risk-informed and performance-based regulatory decision-making, particularly in contexts where the timing and severity of accident consequences are critical.