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Three workers were preparing a small batch of fuel, uranium enriched to 18.8% U-235. None of them had proper training or qualification. At 10:35 a.m., the uranium reached a critical mass. 119 people, including 27 workers, were exposed to some radioactivity. The criticality continued for about 20 hours.
http://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/tokaimura-criticality-accident.aspx -
JCO worker succumbs after 83 days. (n.d.). Retrieved from https://www.japantimes.co.jp/news/1999/12/22/national/jco-worker-succumbs-after-83-days/#.WigIbEuGPrc
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Most of the residents near the Tokaimura plant were unaware of the accident and were unwittingly exposed to nuclear radiation. The day after the tragedy, over 49 people were being treated for radiation exposure.
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The reaction started at 10:35 a.m., on September 30, 1999. It continued for the next 20 hours, finally coming to a stop at 6:30 a.m. on October 1, 1999.
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The residents of Tokaimura were allowed back to their houses on October 2, 1999 with sandbags and other shields to guard themselves from residual gamma radiation. Those further from the site had bunkered down in their houses and were allowed to leave the afternoon following the accident.
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The Nuclear Safety Commission places the blame on the Science and Technology Agency on November 5, 1999. This, paired with public outcry, may have lead to multiple laws restricting nuclear energy in Japan.
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After 83 days of being kept alive against his will, Hisashi Ouchi died of multiple organ failure. Despite multiple blood transfusions, skin transplants, and body fluids being administered, his body could not withstand the fatal doses of radiation he was exposed to- over 17,000 times the legal limit and possibly the most ever received by a human in such a short time.
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Residents of nearby towns stated that they faced "anxiety over delayed effects from radiation" and "fears that there might be another nuclear-related accident." Many people experienced sleeplessness, headaches, weakness, and tiredness.
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The JCO was the company that owned the Tokaimura plant. Their license was revoked in March of 2000.
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Masato Shinohara passed away of multiple organ failure on April 27, 2000. His body and mind slowly deteriorated over the seven months he was kept alive. Once they lost the ability to speak, both Shinohara and Hisashi Ouchi, the other fatality of the accident, had to resort to writing to communicate with the doctors. Shinohara's last statement was, "Mommy, please."
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In March 2011, the second building of the Tokaimura plant was shut down, ending the plant as a whole.
