In Japan, the policy statement of risk informed nuclear safety regulation has been issued by Nuclear Safety Commission (NSC) and the implementation plan of utilization of risk information to nuclear safety regulation has been established by Nuclear and Industrial Safety Agency (NISA). Additionally, Japanese industrial code of maintenance of nuclear power plant (JEAC 4209-2003) suggests the importance to perform maintenance effectively and efficiently by utilizing risk information. Consequently, it is necessary to study the way to utilize risk information into the operation of nuclear power plant.

Utilization of risk significance has capability to make various activities, such as maintenance safely and more efficiently in nuclear power plant. On the other hand, studies on risk informed categorization of structures, systems and components (SSCs) are not yet performed sufficiently and there are a few studies on risk impact after changing the maintenance program in Japan.

In this study, the authors performed preliminary risk analysis assuming that the reliability of SSCs or on-line maintenance activity would be varied due to changing the maintenance program by utilizing risk significance. SSCs were categorized into four regions by using the numerical thresholds for Fussell-Vesely (FV) and risk achievement worth (RAW). Core damage frequencies (CDFs) were evaluated with the assumption that failure probability of all SSCs categorized into a region would be varied at same rate considering the maintenance program change applied to those SSCs. And incremental conditional core damage probabilities (ICCDPs) from maintaining SSCs on-line for some duration were evaluated. Furthermore, sensitivity analyses of the risk significance thresholds were performed.

Sensitivity analyses show that changing the maintenance program by applying different FV threshold value in categorizing SSCs impacts the increase of CDF, and changing the maintenance program by applying different RAW threshold value in categorizing SSCs impacts the increase of total ICCDP. It is necessary to use appropriate FV and RAW threshold value when changing the maintenance program by utilizing risk significance with ensuring the nuclear safety.