The Japan Atomic Energy Agency along with other organizations currently operate a number of nuclear technology research facilities in the town.
The accident was caused when seven times the allowable limit of 18.8% enriched uranium dioxide was mixed with nitric acid in order to form uranyl nitrate, and was put in a precipitation tank to homogenize. At 10:35 am, when the seventh bucket (making a total of 16.6 kg of 18.8% enriched uranium) was poured into the precipitation tank by three technicians, a blue flash of radiation occurred and the three technicians felt severe pain, nausea and had trouble breathing. The radiation alarms went off and the three technicians and their supervising technicians immediately left the building.
The criticality was stopped 20 hours later by draining the cooling water jacket around the precipitation tank and filling it with argon, and purging the tank with boric acid. Through a radiochemical analysis of the uranium solution from within the tank (N. Shinohara et al., Radiochimica Acta, 2001, 89, 135-138) the amount of neptunium and plutonium formed during the event was estimated.