A Comparison of Japanese and U.S. Nuclear Power Plants in Reactor Operating and Incident Rates (Japanese)

Nuclear power is a very important source of energy in Japan, with 53 reactors currently in operation and together accounting for about 25% of total power supply in the country. In recent years, however, the average operating rates of those reactors have been lowered to the 60% level due to a series of factors including force majeure such as earthquakes, facility and/or operational problems, and related regulatory requirements. In contrast, the average operating rates of nuclear power reactors in the United States have been stable at the 90% level since around 2000. Thus, there exists a significant gap between Japan and the U.S. in the operating rates of nuclear power reactors.In this paper, I have classified Japanese and U.S. nuclear power reactors by type and degree of aging; computed and aggregated the operating rates and number of incidents for each group over the past 10 years; statistically analyzed changes in the operating rates and the frequency of incident occurrence as well as the relationship between the two variables; and performed a quantitative comparison and analysis of factors causing differences in the operating rates between Japan and the U.S. as well as of the effects of regulatory systems.It has been found that the differences in the operating rates between Japan and the U.S. are largely accounted for by the fact that the average operating rates of Japanese boiling water reactors (BWRs) are about 30 percentage points lower than that of BWRs in the U.S. Meanwhile, the incidence rates of preventable incidents have been and still are lower in Japan than in the U.S. when limited to cases requiring a reactor shutdown. However, when compared in terms of the incidence rate of overall incidents (covering all cases regardless of whether a reactor shutdown was involved or not), such differences have become almost negligible in recent years. This is because the overall incidence rate has significantly decreased over the years in the U.S. whereas Japan has witnessed a rise in the incidence rate for BWRs.Regarding the relationship between the operating rate and the incidence rate of preventable incidents, the impact of incidents - measured in terms of the percentage points by which the operating rate is decreased per unit of incidence rate - is greater for Japan and there is an observable tendency for Japanese reactors to take a prolonged period of time before resuming operations once they are shut down. However, the longer shutdown period as a direct result of an incident is not the primary cause of the differences in operating rates between Japan and the U.S. It has been found that, in the case of BWRs, operating rate differences are primarily attributable to differences in the length of time reactors need to be shut down for incident prevention maintenance and work necessitated by past incidents as well as for the purpose of regular inspections. Meanwhile, in the case of pressurized water reactors (PWRs), the incident rate is extremely low both in Japan and the U.S. and differences in the operating rates between the two countries are mostly accounted for by differences in the length of time required for regular inspections.Accordingly, it is believed that measures to prevent incidents are important in the case of BWRs toward increasing the overall operating rates of nuclear power reactors and decreasing incidents in the future. Likewise, measures to extend and optimize the interval periods between regular inspections are necessary in the case of PWRs.