The accident occurred on September 30, 1999, when Hisashi Ouchi and two of his colleagues added a seventh bucket of aqueous uranyl nitrate solution to a precipitation tank. Upon adding, the tank reached critical stage and went into a self-sustaining nuclear fission chain reaction releasing intense gamma and neutron radiation.
During the accident, Ouchi was exposed to 17 sieverts of radiation with 8 sieverts being normally considered fatal and 50 milli sieverts being the maximum limit of annual dose allowed for Japanese nuclear workers.
Ouchi’s exposure to the radiation was so severe that his chromosomes were destroyed and his white blood cell count plummeted to near-zero. Most of his body had severe burns and his internal organs received severe damage.
Ouchi is considered the first fatality of his kind in Japan, perhaps the only person to ever receive such a huge amount of radiation in such a short amount of time. The amount of radioactive energy that he was exposed to is thought to be equivalent to that at the hypocenter of Hiroshima atomic bombing. The immensity of radiation completely destroyed his body, including his DNA and immune system. According to the book A Slow Death: 83 Days of Radiation Sickness, “[N]one of Ouchi’s chromosomes could be identified or arranged in order.”