Tokaimura nuclear accident

nuclear accident

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 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]

After the accident


The three workers who worked at the uranium-reprocessing facility were Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa. Two of them died of radiation poisoning. Hisashi Ouchi, aged 35, died 12 weeks after the accident. He had lost most of his skin, and was kept alive for 83 days, according to his parents and wife will. Ouchi was closest to the tank when the accident occurred. He ended up as the first victim of this nuclear accident.[3] Seven months after the accident, Masato Shinohara died, aged 40.

Ouchi was reported to have received 17 sieverts (sv) of radiation, Shinohara 10 sv and Yokokawa 3 sv; 8 sieverts is considered a fatal dose, and 50 milli sieverts is the maximum limit of annual dose allowed for Japanese nuclear workers.[4]

Another 83 workers were subjected to higher radiation than normal.


  1. 1.0 1.1 "Tokaimura Criticality Accident: Nuclear Issues Briefing Paper #52, June 2000". Archived from the original on 2008-03-03. Retrieved 2008-05-18.
  2. "In The Wake of Tokaimura, Japan Rethinks its Nuclear Picture". Archived from the original on 2018-07-22. Retrieved 2008-05-18.
  3. Lamar, Joe (1999-10-09). "Japan's worst nuclear accident leaves two fighting for life". BMJ : British Medical Journal. 319 (7215): 937. doi:10.1136/bmj.319.7215.937a. ISSN 0959-8138. PMC 1116790. PMID 10514143.
  4. Conachy, James. "Worker's death exposes the dirty secrets of Japan's nuclear industry". Retrieved 2017-10-17.