Tokaimura Criticality Accident
A criticality accident at a uranium fuel processing plant caused by workers improperly mixing fuel, directly killing two workers.
How It Happened
Workers at the JCO fuel conversion plant were preparing enriched uranium solution. Following an unofficial shortcut procedure to speed up work, they poured far too much uranium solution (about 16 kg instead of the permitted 2.4 kg) into a precipitation tank not designed for this purpose. The mass exceeded critical threshold, initiating a self-sustaining nuclear chain reaction.
The Criticality
The blue flash of Cherenkov radiation and a wave of heat announced the criticality. Workers Hisashi Ouchi and Masato Shinohara received lethal radiation doses. A third worker received a serious but survivable dose. The chain reaction continued intermittently for approximately 20 hours before being stopped.
Medical Treatment
Ouchi received an estimated 17 Sv of radiation dose - far beyond the 6-7 Sv typically considered lethal. His chromosomes were essentially destroyed. He survived for 83 agonizing days, undergoing experimental treatment including stem cell transplants. His case became a landmark in radiation medicine.
Causes and Lessons
The accident resulted from complete failure of safety culture at JCO. Workers had been using an illegal shortcut procedure for years with management knowledge. The company had prioritized production speed over safety. The accident led to major reforms in Japan's nuclear regulatory framework.
📅 Timeline
Criticality begins when excess uranium added to precipitation tank
Workers exposed to intense neutron radiation
Emergency services notified
350m evacuation ordered; 310,000 residents told to stay indoors
Criticality stopped by draining water from jacket around tank
Hisashi Ouchi dies after 83 days of intensive treatment
Masato Shinohara dies after 211 days