Tokaimura Criticality Accident

On September 20, 1999, a nuclear reaction occurred at the Tokaimara Uranium Processing plant, about 90 miles northwest of Tokyo. While processing uranium with nitric acid to make nuclear fuel, workers bypassed several safety steps, and mixed too much uranium and set off the accidental uncontrolled reaction. It is believed they put 35 pounds of uranium into the tank - well over the 4.8-pound limit.

Fifty-five people, mainly plant workers and emergency personnel who responded to the accident, were exposed to radiation. Three workers were exposed to about the same amount of radiation as was received in 1986 by workers at the Chernobyl plant in the Ukraine. The two workers who caused the accident later died from the radiation exposure. Authorities ordered 161 people evacuated from their homes, and another 310,000 were advised to stay indoors for 18 hours as a precaution.

The government classified the incident as "level four" on a 1-7 scale of nuclear accidents, making it Japan’s worst. A level four accident is one where there is a leak of a small amount of radioactive material outside a nuclear facility. The accident at Chernobyl was a level 7, while the accident at Three Mile island was rated at 5.

The plant reopened in November 2000. A similar accident has occurred only once in the United States, and that was in 1964 at a fuel processing plant in Wood River Junction, Rhode Island.