On September 30, 1999, a severe accident happened at the JCO Company Ltd. nuclear fuel factory 130 kilometres north of Tokyo in the village of Tokaimura. The nuclear fuel plant was producing nuclear fuel rods for the commercial nuclear industry through a chemical process.

A nuclear chain reaction was not considered a possibility at a nuclear fuel factory and therefore the factory did not have an emergency plan. Several violations of basic security standards in the nuclear industry increased the extent of the accident.

The accident happened when three workers mixed uranium oxide with nitric acid using a stainless steel container instead of a mixing apparatus. This procedure was recommended in an illegal manual drafted by the JCO Company, probably with the intention of saving time and money.

The manual had never been approved by the supervising ministry as by law required and furthermore the procedure was known to be very risky. In spite of this, the method had been in use at the factory for seven years.

By circumventing the mixing apparatus, the nuclear fuel could be inserted all at once and that led to a chain reaction. The workers were not wearing protective clothes and were exposed to high levels of radiation. Two of them later died from the radiation.

The JCO factory is located very near to the village Tokaimura and at least 200 villagers were exposed to radiation. An estimated 600 people, including firemen and plant workers, were exposed.

The problems were magnified by the fact that the factory did not have a mechanical system to interrupt the chain reaction and so the ventilation system kept spreading radioactive material from inside the building to the surrounding area. One consequence of the accident was that the plant owner lost his operating licence.

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Links and references:

A case study of the accident:
http://ublib.buffalo.edu/libraries/projects/cases/...

Chronology of the accident:
http://archive.greenpeace.org/nuclear/reactor/toka...