Determination of in-service inspection requirements for fast reactor components using System Based Code concept

Abstract In our previous study (Takaya et al., 2015a), we proposed a new process for determining in-service inspection (ISI) requirements using the System Based Code concept. The proposed process consists of two complementary evaluations, one focusing on structural integrity and the other on plant safety. First, the structural reliability of a specified component is evaluated considering potentially active degradation mechanisms, including those that are not explicitly addressed in the design codes. If the structural reliability meets the requirement, the second evaluation can be conducted, which assesses the detectability of defects before they can grow to an unacceptable size, taking plant safety into account. If there is any feasible way to detect defects, it is adopted as an ISI requirement. Otherwise, a structural integrity evaluation would be required under a sufficiently conservative hypothesis. In other words, if the additional requirements are met, detectability is not an obligation. In this study, the ISI requirements for a reactor guard vessel (RGV) and core support structure (CSS) of a prototype sodium-cooled fast breeder reactor in Japan (Monju) were investigated using the proposed process. Creep-fatigue and fatigue were chosen as the potentially degradation mechanisms for the RGV and CSS, respectively. The Stage I evaluations using the Monte-Carlo method showed that both components had sufficient reliability if these degradation mechanisms were considered. At Stage II, the reliability levels of the components were evaluated assuming initial fully circumferential cracks with a depth equal to 10% of the thickness as additional requirements because there was no available inspection method for the components. It was shown that both components had sufficient reliability even with the additional requirement based on conservative hypothesis. The failure occurrences of these components were practically eliminated. Hence, it was concluded that no ISI requirements were needed for these components. The proposed process is expected to contribute to the realization of effective and rational ISI by properly taking into account plant-specific features.