Before the Fukushima-Daiichi Accident, Japanese nuclear power plants (NPPs) were utilized with a capacity factor of around 70％, 20％ lower than the US capacity factor of around 90％, which is a consequence of Japanese NPPs being operated with a shorter fuel cycle and longer outage period. One reason for this situation is that Japanese decision making is strongly focused on equipment reliability. In a typical pressurized-water reactor, however, core damage frequency (CDF) during refueling outage is higher than that during operation, that is, a short fuel cycle could possibly increase the total CDF of NPPs. In this paper, a decision-making rule using an index representing CDF par power generation is firstly proposed. Secondly, using this rule, a decision process is simulated to optimize the fuel cycle and refueling outage period while showing the effects on CDF reduction in each plant. Thirdly, by applying this decision process to all Japanese NPPs, the total CDF reduction in Japan is indicated. This simulation shows that the change of decision-making rule will bring about an 18％ CDF reduction or 16％ increase in power generation in total in Japan. At the same time, each NPP gains strong incentive to improve its own safety because this new rule permits a higher capacity factor operation only for the NPPs that are safer than the average.
Not just whether nuclear power stations should be restarted, but how local agreement over the restart should be achieved has been controversial issues in Japan since the Fukushima Nuclear Disaster. In this paper, public attitudes towards local agreement in the case of Hamaoka Nuclear Power Plants are explored with a postal questionnaire survey in Shizuoka prefecture. Through descriptive statistics and factor analysis, the study shows that extending the “local” scale and judgement by ordinary citizens is given more support than the conventional local agreement process. Factor analysis reveals three factors behind respondents’ attitudes towards local agreement: “conventional decision-makers”, “narrow localism” and “national interests”. The analysis of the factor scores reveals that attitudes towards local agreement differ depending on attitudes towards the restart of the plant and the prefectural referendum, as well as generation, while no significant difference is found among genders and residential areas excluding the second factor. By clarifying the public attitudes towards how local agreement should be made, this study makes a significant step toward the design of a socially more agreeable local agreement.
The Japanese government is beginning to consider radiation protection in the “specific reconstruction reproduction base area” of the Fukushima nuclear power plant, the evacuation order of which will be lifted by 2023. It is essential to grasp the present situation of radiation contamination and evaluate exposure dose in the area to realize the lifting of this evacuation order zone. Many surveys on the evaluation of the distributions of air dose rate have been carried out, and exposure dose has been estimated using the results since the Fukushima Daiichi Nuclear Power Plant accident. Nevertheless, more detailed information on exposure is needed for the area because the radiation level is relatively high. This will also be helpful in preparing a prudent evaluation plan. This study is aimed at evaluating the detailed contamination situation in the area and estimating exposure dose with consideration of areal circumstances. Work was carried out for (1) an airborne survey of the air dose rate using an unmanned helicopter and ground-based measurement (walk-survey), (2) the evaluation of airborne radiocesium and (3) the estimation of external/internal effective doses for the typical life patterns assumed. Our study resulted in a detailed map of the air dose rate and clarified the distribution pattern in the area. Moreover, the exposure dose of residents was evaluated by considering some life patterns based on this map.
The purpose of this paper is to clarify the true cause of the Fukushima meltdown accident from the viewpoint of reactor design. There are only two factors. One is that a total loss of battery power was not considered in the basic design, and the other is that the fail-close design was selected in long-term core cooling systems, as the confine function was preferred over the cool function in the three safety functions of stop （nuclear reaction）, cool （decay heat） and confine （radioactivity） in the basic criteria. After the loss of all power （battery and station power） in Unit 1, the IC system failed because of the fail-close design. After the loss of all power in Unit 2, the core-cooling function of the RCIC system failed because of the instability of two-phase flow in the turbine steam line, since the core decay heat was confined in the reactor and containment vessel owing to the fail-close design of the vent valves. As loss of station power occurred but battery power was available in Unit 3, operators were able to activate the RCIC system, which was tripped by the trip signal after one day. Then, the HPCI system was started automatically but stopped after about 10 hours because of the loss of battery power. After the loss of core cooling, core meltdown occurred in Units 1 to 3. The lessons learned from this accident are that the core-cooling system should be designed to withstand the loss of all power in its basic design and that the cool function should be preferred over the confine function.