Tokaimura nuclear accident
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Japan's worst nuclear radiation accident 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 criticality accident was 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 eventual criticality. Three workers were exposed to (neutron) radiation doses in excess of allowable limits, and two of these workers died; a further 119 received lesser doses of 1 mSv or greater.[1]
Dozens of emergency workers and nearby residents were hospitalised and hundreds of thousands of others were forced to remain indoors for 24 hours.[2]