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Previous criticality accidents
While this was Japan's first such accident, similar criticality incidents have occurred, especially in US and Russian military plants and laboratories. All but two of these were prior to the early 1980s. Three (in 1958 and 1964) were very similar to this accident. The last of these was the single previous criticality accident at a commercial fuel plant, in USA, resulting in one death. -
Deaths
Of all the previous accidents, 37 occurred in connection with research reactors or laboratory work for military projects, resulting in ten deaths. Another 22 occurred in fuel cycle facilities, all but one military-related, and resulting in seven deaths. -
1979
In 1979, nuclear power was all the rage. To get in on what was assumed to be a booming business, the huge Sumitomo Metal Mining Company in Japan established a subsidiary called the Japan Nuclear Fuel Conversion Company, later shortened to JCO. It established its main plant next to the city of Tokaimura, in Ibarakin Prefecture, about 60 miles from Tokyo. -
1983
In 1983, JCO received a contract to make fuel for Japan's planned fast-breeder reactor project, which was designed to produce plutonium as it burned its fuel for electric power, and thus make more fuel than it used.This was to be enriched to 20 percent Uranium-235. -
1995
But by 1995, the nuclear industry was in decline, and JCO received fewer and fewer contracts. -
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Tokaimura nuclear accident
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In 1999 three workers
In 1999 three workers received high doses of radiation in a small Japanese plant preparing fuel for an experimental reactor. -
The accident
The accident was classified by the Japanese authorities as Level 4 on the International Atomic Energy Agency International Nuclear Event Scale, indicating an event without significant off-site risk. It was essentially an 'irradiation' accident, not a 'contamination' accident, as it did not result in any significant release of radioactive materials. Japan’s Science & Technology Agency estimated that 2.5 x 1018 fission had occurred, about half in the first few minutes, releasing 81 M.J. -
THE NUCLEAR ACCIDENT
The direct cause of the accident was some workers putting Urania nitrate solution containing about 16.6 kg of uranium, which exceeded the critical mass, into a precipitation tank. The tank was not designed to dissolve this type of solution and was not designed to prevent such accidents to happen. As a result, three workers were exposed to neutron radiation doses in excess of allowable limits. Two of these workers later died. -
uranium
Finally, in 1999, JCO won another contract to convert a batch of 16.8 kilograms of 18.8-percent enriched enriched uranium into a liquid form called uranyl nitrate, for shipping to the experimental fast breeder reactor. -
THE ACCIDENT
Five hours after the start of the critical, evacuation commenced of some 161 people from 39 households within a 350 meter radius from the conversion building. They were allowed home two days later after sandbags and other shielding ensured no hazard from residual gamma radiation. Twelve hours after the start of the incident residents within 10 km were asked to stay indoors as a precautionary measure, and this restriction was lifted the following afternoon.
