- In 1999, human error derived from a lack of training and supervision caused a criticality accident at the Japan Nuclear Fuel Conversion Company’s nuclear plant in Tōkai, Japan.
- Technician Hisashi Ouchi became “the world’s most radioactive man” and died after receiving 17 sieverts of radiation, much more than a human body can take.
- The accident led to the company’s credentials for the operation being canceled and the allocation of million-dollar sums in compensation to other radiation victims.
The 1999 Tokaimura nuclear accident was the worst nuclear accident in Japan after the natural disaster-induced Fukushima nuclear accident of 2011.
On the morning of September 30, 1999, human error resulted in a criticality accident at the Japan Nuclear Fuel Conversion Co. (JCO) uranium processing plant near Tokai, 70 miles (~112 kilometers) northwest of Tokyo.
Due to improper handling of uranium, an uncontrolled nuclear fission chain reaction exposed two technicians and their supervisor to high levels of radiation. It prompted an evacuation of the plant and the surrounding areas.
One of the technicians, Hisashi Ouchi, would later become known as “the world’s most radioactive man,” having absorbed 17 sieverts of radiation. To put this into perspective, the dose of radiation that is estimated to be lethal for 50 percent of the population within 30 days is generally around 4 to 5 sieverts.
