In 1999 three workers received high doses of radiation in a small Japanese plant preparing fuel for an experimental reactor.
The accident was caused by bringing together too much uranium enriched to a relatively high level, causing a 'criticality' (a limited uncontrolled nuclear chain reaction), which continued intermittently for 20 hours.
A total of 119 people received a radiation dose over 1 mSv from the accident, but only the three operators' doses were above permissible limits. Two of the doses proved fatal.
The cause of the accident was "human error and serious breaches of safety principles," according to the International Atomic Energy Agency.