120 - 12/99 - Radiation Incident - Japan - What can go wrong - Clearance of Radioactive Materials


The recent incident at the JCO Company plant on the Tokaimura Nuclear Site in Japan highlights the need for strict adherence to regulations when dealing with radioactive materials. There is a clear similarity between the incident in Japan and the potential problem referred to in Bulletin 80 - 2/99 "Clearance of Radioactive Cargoes - A Cautionary Note".  The potential problem being warned about in Bulletin 80 was exactly what occurred in the Japanese incident - a "Criticality Excursion".

The Japanese incident happened in a research facility on the Tokaimura site. The site contains a number of nuclear research and production facilities as well as civil nuclear reactors (Tokaimura No 1, which is still in operation). The incident occurred in a small research building belonging to the JCO Company which is owned by the Sumitomo Corporation. The Tokaimura No 2 nuclear reactor unit was not involved in the incident in any way.

The operation being carried out in the building was the conversion of Uranyl Nitrate into Uranium Fuel for the Joyo experimental fast breeder reactor

(both these materials are regularly transported internationally by land and sea)

. The type of incident was a Criticality Excursion due to the bringing together of more than the permitted amount of enriched uranium in a solution of nitric acid. At the point in the process where the accident occurred, the amount of uranium should have been limited to 2.4 kgs but the workers added 16 kgs. This, in combination with the presence of water in the nitric acid solution caused a self sustaining chain reaction.

A Criticality Excursion results in high radiation energy fields being given off in all directions. In total 36 workers, three firemen and seven members of the public were thought to have received abnormal doses of radiation. The hazard at the time of the incident was potentially life threatening to persons within 200 metres of the incident, falling off to a very low hazard at 500 metres.

It is stressed that this was a radiation beam hazard and not radioactive contamination. A radiation beam is similar to the heat you would feel standing next to a fire whereas radioactive contamination would occur when radioactive ash was deposited on your clothes and body. No significant radioactive contamination would be expected to be found outside the immediate vicinity of the building in this type of incident and that seems to have been confirmed by subsequent monitoring.

The hazard in a Criticality Excursion only lasts as long as the chain reaction is self sustaining. In the case of this accident the chain reaction continued on and off for 17 hours from 1033 hrs on 30 September.

The reason for describing the details of the incident in Japan is to bring home to Members the need for clearance of any proposed radioactive consignments.  In all cases members should consult the Club for their nuclear consultant's confirmation.

Source of Information :

Roger Cheshire

RPC Transport Consultants

Tel +44 (0) 1829 732059

Fax +44 (0) 1829 732541




Staff Author