JCO reactivity accident
Tokai is best known for a nuclear accident that occurred on September 30, 1999, which killed two people. The accident was a criticality accident.
The Tokaimura nuclear accident happened at the JCO reconversion plant.
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 kg of enriched uranium) was poured into the precipitation tank by two technicians, a blue flash of radiation occurred and the two technicians felt severe pain, nausea and had trouble breathing. The radiation alarms went off and the two 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.
Due to the radiation they received, the two technicians died on December 22, 1999, and April 27, 2000, respectively.
I post this as a follow up to the story for September 29th. There was forty two years and a day between the accidents. Only happenstance let this accident cause relatively little harm. It is also very strong evidence that nuclear power is something we humans are just not prepared to handle.