New details on Japan nuclear accident

December 02, 1999

In September, a nuclear accident in Japan made international headlines, as workers at a fuel processing plant in the village of Tokai (also known as Tokaimura) inadvertently started a nuclear chain reaction when adding enriched uranium to a tank at the plant.

While it will be some time before the official report on the incident is available, Physics Today magazine reveals some newly uncovered details on the Tokai accident in the December issue.

The Physics Today account provides new information on why the workers added about 7 times as much uranium to a given container as was allowed. The article identifies three errors that were made, the first by the plant's operating company in modifying the government-approved procedure without authorization, the second and third by the workers themselves, possibly with concurrence of their immediate management.

Most accounts have provided the impression that the workers were adding fuel to a tank they were supposed to use but that they were doing so with buckets rather than through pipes or funnels. But as the Physics Today article reveals, they were actually using a container not intended for the purpose. Its shape was not right and it had a water jacket around it for cooling. Both the shape of the tank and the presence of the water jacket (which reflected neutrons back into the tank) contributed to the onset of a nuclear chain reaction.

The Physics Today report includes several estimates by nuclear experts on the magnitude of the accident, based on different methods. These estimates help put the Tokai accident into perspective vis a vis other nuclear accidents.

The Physics Today article is freely available online
Contact: Barbara Goss Levi (Sr. Editor, Physics Today), (805) 965-3483,

American Institute of Physics

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