In order to develop rationalized maintenance plans of nuclear power plants, it is necessary to consider characteristics of each plant. For sodium-cooled fast reactor (SFR) plants, there are constraints on inspections due to the specialty that sodium equipment needs to be handled, which is one of the important points when considering rationalization of maintenance. Therefore, we previously proposed a maintenance optimization scheme based on the System Based Code (SBC) concept. The concept of the SBC intends to give a theoretical procedure to optimize the reliability of the system by administrating every related engineering requirement throughout the life of the system from design to decommissioning. “ASME Boiler and Pressure Vessel Code, Code Case, N-875 Alternative Inservice Inspection Requirements for Liquid-Metal Reactor Passive Components” was already developed based on the SBC concept. The basic procedure of the code case has been also implemented in “ASME Boiler and Pressure Vessel Code, Section XI, Division 2” which provides requirements for Reliability and Integrity Management (RIM) programs for nuclear power plants including advanced reactors. One of proposed scheme goals is to develop detailed procedures of preparing a maintenance plan based on the process in the code case. In this study, the procedures to determine inspections for potential degradation and additional inspections in terms of defense-in-depth have been further clarified. The modified maintenance optimization scheme consists of the following processes. At first, potential active degradation mechanisms are identified. If there are any degradation mechanisms that are not addressed in the design codes, structural reliability evaluations are conducted to confirm whether the component of interest has sufficient reliability for the degradation mechanisms of concern. In the case the evaluated reliability does not meet a predetermined reliability target, inspections for the potential degradation are required. Next, the detectability of hypothetical defects before they grow to an unacceptable size in terms of plant safety is assessed. A method which has sufficient detectability, either in direct way such as volumetric examinations or in indirect way such as monitoring, is required as an additional inspection. If any methods are not applicable, reliability evaluation with a penalty is required. The penalty is determined conservative hypothesis relating safety function of the component of interest. If the evaluated reliability meets to the reliability target, additional inspections can be exempted. In this paper, the modified maintenance optimization scheme is also illustrated by a quantitative trial evaluation of the core support structure of the next SFR under development in Japan.

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