01 Overview
On September 30, 1999, three workers at the JCO uranium fuel conversion facility in Tลkai triggered an uncontrolled nuclear chain reaction while mixing uranium solution in buckets - completely bypassing approved procedures to speed up production.
02 Cause
Workers bypassed approved procedures and mixed uranium nitrate solution directly into a precipitation tank using buckets. They added a total of 16 kg of uranium - seven times the legal limit - causing a self-sustaining chain reaction. The facility had no interlocks to prevent this.
03 Impact
Two workers received lethal doses and died within months. A third worker received a high dose but survived. 439 people near the facility were exposed to radiation. Approximately 310,000 residents within 10 km were asked to shelter indoors for 18 hours. The chain reaction continued for about 20 hours.
04 Response
Workers initially tried to stop the reaction manually. The chain reaction was halted by draining water beneath the tank to remove the neutron moderator. The plant was evacuated; nearby residents were sheltered indoors.
05 Legacy
Tokaimura was the world's first criticality accident to cause deaths since SL-1 in 1961. It led to sweeping reforms in Japanese nuclear regulation and criminal convictions of JCO executives - a harbinger of the systemic failures later revealed by Fukushima.