To maintain the fuel debris retrieval (FDR) operations at the Fukushima Daiichi Nuclear Power Station, the design of a monitoring system is important. However, there is a scarcity of research that evaluates the specific physical quantities (monitoring items) to be monitored on-site and their significance from both working and safety functions. In this study, we propose a risk analysis method combining objective tree (OT) and failure mode and effect analysis (FMEA), which includes the steps of determining the monitoring items and calculating the criticality ranking tailored to the characteristics of the FDR operations. The proposed method is applied to the partial-submersion method, and the results of the risk analysis are evaluated by nuclear facility design experts. As a result, it was confirmed that the proposed method can be utilized in the design of the monitoring system.

A nuclear power plant consists of tens of thousands of equipment and facilities. To maintain and manage these equipment and facilities, it is necessary to classify their importance and conduct appropriate maintenance and resource allocation for each facility. In the case of operating plants, this concept has been developed on the basis of the existing importance classification. On the other hand, decommissioning plants have importance levels different from those of operating plants because the requirements for safety functions change from the operation phase and there is no requirement to install criticality management facilities, but the concept has not been developed yet. In this study, on the basis of the data obtained from plant operation and maintenance records, we examined one path to determining the importance level of decommissioning plants, focusing on the relationship risk assessment and maintenance of facilities.

An accident tolerant fuel (ATF) cladding, which is more resistant to accidents than the conventional Zircaloy cladding, is under development. One of these claddings is a chromium (Cr)-coated cladding with an outer surface coated with Cr, which is expected to improve the resistance to high-temperature steam oxidation. In this study, electrochemical plating was applied to coat a Cr layer on the cladding outer surface, and its properties under accidental conditions were evaluated. In a loss-of-coolant accident (LOCA), the cladding will burst and both the outer and inner surfaces of the cladding will be oxidized. Thus, as-received Zircaloy-4 and Cr-coated claddings were tested for oxidation in high-temperature steam to investigate differences in oxidation behavior, hydrogen absorption behavior, and mechanical properties after oxidation. Oxidation tests were conducted using a thermobalance. The amount of oxidation of coated samples decreased by half compared with that of uncoated samples, indicating that the coating was effective in inhibiting oxidation. However, the hydrogen absorption of coated samples was found to be higher than that of uncoated samples. In this paper, we discuss the mechanism behind this difference in hydrogen absorption and its effect on mechanical properties.