More than 30 workers at the Tokaimura plant are thought to have been exposed to radiation. Two are in a critical condition and are expected to be given bone marrow transplants. Officials said workers had caused the accident at the plant by pouring too much uranium solution into a tank. He also held an emergency meeting of the cabinet which set up a special task force - the first time it has taken such a measure after a nuclear accident. Washington has meanwhile announced that a joint American and Russian team is being sent to Japan.
Criticality is the point at which a nuclear chain reaction becomes self-sustaining.
The French nuclear institute said the incident was the 60th in the world since 1945, following 33 such accidents in the United States and 19 in the former Soviet Union. Workers normally use up to 2.3kg of uranium in each procedure to prevent a criticality accident, officials said. US help US President Bill Clinton pledged US assistance to Japan in the wake of the accident. The leak is the latest in a series of accidents at Japanese nuclear facilities, including a previous one at Tokaimura in which 35 workers were exposed to radiation. Japan has 51 commercial nuclear power reactors, providing one third of the country's electricity.