Selection of risks of 'JOYO' likely to release a large amount of radioactive materials

The Ordinance for standards for the location, structure, and equipment of nuclear reactors used for testing and research (hereinafter referred to as 'Ordinance for installation approval criteria') stipulates the prevention of the spread of accident that could release a large amount of radioactive materials, etc. in its Article 53. As for the accident that could release a large amount of radioactive materials, when the events exceeding the design basis accidents are assumed (in the assumption, troubles caused by external events as common causes, such as natural phenomena, should be considered), it is demanded that to conduct evaluation and measures against the accident that could give excessive radiation exposure to general public near the compound. This report showed selection policy for the 'accidents that could release a large amount of radioactive materials and the like' in 'JOYO,' and selected the accidents. Selected accidents are as follows: (1) accident where reactor shutdown function or radioactive material containment function lowers when reactor output is high, (2) accident where flow-rate reduction rate decreasing function drops when reactor coolant flow-rate is low, (3) accident where decay heat removal function drops when coolant leakage occurs, (4) accident where the function of emergency generators is lost when external power supply is lost, and (5) accident where the water-covering of spent fuel fails due to earthquake as natural phenomenon. (A.O.)