The Tokai criticality accident (provisionally rated 5 on the INES scale of 7) occurred in a facility little more than a development-scale plant. The equipment was being used because highly enriched (18.8 per cent U-235) uranium oxide was required to make fuel for the Joyo experimental fast breeder reactor. This is a relatively infrequent requirement.

The process is part of the procedure for converting uranium hexafluoride to uranium oxide. Uranium is dissolved in nitric acid, then precipitated out in a sedimentation tank. The equipment is designed so that the process is carried out in two ‘intrinsically safe’ vessels, with geometry such that it is not possible for criticality to occur. However, this is a slow process, and the procedure was changed so mixing could be done in stainless steel buckets and the mixture poured directly into the sedimentation vessel. On this occasion, it was decided to make a batch containing 16 kg of uranium, although the procedure says the maximum amount should be 2.4 kg. The revised procedures were not submitted to the regulatory body, apparently because the company knew they would not be approved. As the workers were pouring in one last bucketful into the tank, criticality occurred.

To anyone familiar with the nuclear industry, where operating procedures are sacred documents and redundant shutdown systems and emergency equipment are provided, such a situation is beyond belief. In the case of JCO’s plant, there was no way to shut down a critical reaction if it occurred, and no shielding. The workers were not even wearing film badges.

The consequences are that two workers will probably die (one received 17 Gy, the other 10 Gy; the 50 per cent survival dosage is 3-4 Gy), some managers will go on trial (police raided JCO on 6 October to obtain evidence), and the company might close down (its licence was revoked on 6 October, which will cause problems as JCO previously converted 50 per cent of fuel used in Japanese reactors). Regulators will try to enforce the same standards to the peripheral parts of the nuclear industry as they do to nuclear reactor operators. Hopefully, the regulators will also do a little self-examination and find out why they failed to regulate JCO properly.

Recent accidents to Japanese reactors, like the Tsuruga 2 coolant leak and the Monju sodium leak can be attributed to physical failures such as metal fatigue, but the plant systems operated as designed to contain the accident and the operators did what they should. The Tokai accident was entirely a matter of human failure, and no emergency systems even existed.

Sequence of events


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