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Tokaimura nuclear accident

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The Tokaimura nuclear accident was a serious nuclear radiation accident in Japan. It took place at a uranium-reprocessing facility in Tokaimura, northeast of Tokyo, Japan, on 30 September, 1999. The accident occurred in a very small fuel preparation plant operated by JCO.[1]

The direct cause of the accident was some workers putting uranyl 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.[1]

Many emergency workers and nearby residents were hospitalized and hundreds of thousands of others were forced to remain indoors for 24 hours.[2]


  1. 1.0 1.1 Tokaimura Criticality Accident: Nuclear Issues Briefing Paper #52, June 2000
  2. In The Wake of Tokaimura, Japan Rethinks its Nuclear Picture