This surprising accident of "criticality" came about in October 1999 in Japan. It did not happen at a nuclear power plant, but in a high tech facility, specialised in enriched fuel, situated in Tokai-Mura, 150 km North-East of Tokyo.
One of the Tokai-Mura unit was dedicated to the production of nuclear fuels for research reactors with an isotope 235 enrichment that coud reach 20%, the yearly being at most 3 tons. The operations consisted in pouring uranium powder (enriched at 18.5% of uranium-235 and previously dissolved in nitric acid) in a container also containing nitric acid. This container normally had an adapted geometry preventing the development of a chain reaction.
To not waste time and finish the work faster, the technicians responsible for the preparation poured in one entire go the enriched uranium in the container. They inserted 16.6kg of enriched uranium, seven times more than the recommended 2.4 kg guaranteeing the absence of any risk of criticality. In doing this, they exceeded the critical mass, triggering an explosion of gamma and neutrons.
In addition, the adapted geometrical equipment had been replaced by a tank of large dimensions, through ignorance or lack of training: The accident would have been avoided if a container in the shape of a flat plate had been used. The investigation showed that the staff from the factory did not respect the safety requirements that were in fact not even indicated.
The exceeding of the critical mass set off the beginning of an atomic explosion. The development of this explosion luckily stopped very quickly. The contamination stayed insignificant and limited to the factory. The authorities evacuated 160 people living in the town to a radius of 350 metres, and 300 000 people received a warning to remain in their houses until the end of the accident.
The accident caused the death of two technicians who made the mixture. One of them, Hishashi Obuchi, had been exposed to a dose 17000 times the standard maximum, set by Japan for one year, and 4 to 5 times the lethal dose. A third technician who was in a neighbouring room, was less exposed and survived .
"Criticality accidents", known since the discovery of nuclear power, are very rare nowadays. Most of them occurred in the 1950s and 60s and did not lead to significant radioactive pollution in the environment.
The Japanese government revoked the exploitation licence given to the “Japan Nuclear Fuels Conversion Company”. The accident of Tokai-Mura coincided with a scandal involving the falsification of data concerning plutonium pellets provided by a British company (Sellafield reprocessing plant). This damaged the image of the nuclear industry of which Japan was counting on reducing its CO2 emissions and the effects of greenhouse gases at the level of 1990.
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