The excessive maintenance of the nuclear power plants (NPPs) may cause the early (infant) failure in Japan. An easy analysis; the Weibull analysis was applied to the evaluation of the failure mode. The Weibull analysis needs the hazard data. The maintenance information of the equipment which caused plant shutdown was required for the hazard calculation. However, maintenance information of the equipment was not open. Therefore, all equipment was assumed to be maintained during every shutdown. This assumption was based on renewal process. However, a repair after unplanned shutdown of NPP is generally a restoration of only failed function without system overhaul. The system must be considered to age continuously. The system was not renewed. The operation data must be regarded as one continuous data before and after unplanned shutdown. An improvement of the Weibull analysis was required for NPPs. The model of the Weibull analysis was investigated. The competitive model in which shutdown caused by other than focused equipment/cause may be supposed to be continuous data could not be applied for a comprehensive analysis. Furthermore, the calculation method of the Weibull analysis was investigated. The calculation method of the hazard was viewed. A denominator of the hazard is the number of data which is cut for every continuous data by renewal process. However, multiple considerations of operation periods before unplanned shutdowns might cause underestimation of the failure rate in case of restoration process. Therefore, a dominator of the hazard was not supposed to be the number of data but the number of survived equipments (plants) at each time according to the definition of the hazard. This improved method is for the restoration process. The performance of Japanese NPPs was evaluated by improved method. The failure modes of Japanese NPPs were early failure modes. Moreover, performances of U.S. NPPs was tried to be evaluated by improved method. Operation data was collected from “NRC Power Reactor Status Reports”. However, many “maintenance outage”s which are the shutdowns of unknown origin were found. Therefore, DOE information was supplemented to investigate the “maintenance outage”. Failure modes of U.S. NPPs were the early failure modes, and failure rates were larger than Japanese NPPs.

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