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Tokai JCO Criticality Accident

Report [3] 10/7/99 12:20
revised 9/10/99 20:20

Worst Ever Nuclear Accident in Japanese History Was an Accident Waiting to Happen

1. Description of the accident

On September 30, 1999 at 10:30 a.m., there was an accident with radioactive release at a JCO Ltd. owned plant where uranium is reconverted as part of a process to fabricate fuel for nuclear reactors. Initially, the plant workers were not aware of the nature of the accident. However, gradually it became clear that it was one of the worst type of nuclear accidents, a criticality accident. The plant workers were slow in realizing the situation as a criticality incident because, the plant operators had never entertained the possibility of a criticality occurring. Furthermore, there were no plans or systems to contain criticality. Thus the accident progressed further into a frightening disaster.

This criticality accident at the plant was triggered when workers transferred enriched uranium oxide, dissolved into solution by adding nitric acid, into a precipitation basin. An excessive amount of uranium enriched to an extremely high percentage of 18.8%, was poured into the precipitation basin against the regulated standard limit. Thus the precipitation basin functioned in the same way as a nuclear reactor and the fission chain-reaction began. It was at 3 p.m. the same day that researchers of the Japan Atomic Research Institute pointed out to the plant operators that the accident might possibly be a criticality accident and the operators began to take the situation seriously. However, the site of the accident could not be entered because of the strong neutron and gamma radiations, and the fission reaction continued until 6:30 a.m. the next morning.

During that time, altogether forty-nine workers, residents, and firefighters were highly exposed, and the three workers that were at the accident site were exposed to a lethal dose of radiation. One of those three seriously exposed workers was exposed to 8 to 17 Sv of radiation. Although their conditions have not taken a turn to the worst, this worker and one other worker were together exposed to a lethal dose of radiation, and are still in critical condition. In addition, in the early morning of October 1, eighteen workers carried out the extraction of coolant water from the precipitation basin to stop the criticality condition. All of them were exposed and one of the exposed workers had a radiation count of up to about 100mSv.

Criticality incidents occurred in the early history of nuclear energy in America and in Russia at military facilities and some research institutes, however, there has been no such incidents recently. Furthermore, there has never been an incident like this one in which the criticality condition continued for so long. This is the worst accident ever in the history of nuclear power development in Japan. Based in the International Nuclear Event Scale, the Japanese government categorized this accident as a level 4, however CNIC is certain that this accident belongs to category 5. This is because it can be supposed with well-grounded scientific evidence that there were 10E+16 to 10E+17 Bq of rare gas released. Criticality was stopped at 6:30 a.m. on Oct. 1 by extracting cooling water from around the precipitation basin, however, there is still a large amount of fission products in the precipitation basin and the accident site cannot be accessed. (as of 10/4)

2. The cause of the accident.

The direct cause of the accident is that a worker had filled 16kg of (highly enriched ) uranium into a precipitation basin that was suppose to be filled with no more than 2.4kg of uranium of this enrichment concentration. This caused the precipitation basin to reach criticality. The media is putting heavy emphasis on the fact that the workers transferred the solution in a stainless steel pail with their bare hands to save time, and that it is this human error that is the main cause of the accident.

It is shocking that uranium solution was transferred with bare hands. This itself is a shameful violation of safety regulations, however, it is not the direct cause which triggered criticality. Criticality was triggered because the workers and the overseers were not well enough aware that they were dealing with uranium enriched to as high as 18.8% enrichment, and an excessive amount of uranium-235 was filled into a tank as if the solution was low enriched uranium of 5% or less concentration.

This JCO owned plant usually deals with low enriched uranium for light water reactors in Japan. However, at the time of the accident, they were handling highly enriched uranium to be used for the fuel for Joyo Fast Breeder Test Reactor (MARK 3). Since there is inherent danger of criticality to such facilities, the form, shape, and size of the container must be designed with limitations to counter criticality even in the case of human error. This is called "geometrical control" to counter criticality, and the most significant error was that there was no geometrical control to this facility.

3. Has the accident come to an end?

The criticality condition ceased at 6:30 a.m. Oct.1. At 4:30 p.m. the same day, the request to the residents within a 10km radius of the plant to stay indoors was lifted. At 6:30 p.m. on Oct. 2, the evacuation advice to the residents within a 350m radius of the plant was lifted and essentially saying that the government had made its "Safety Declaration". However, that did not mean that the situation became free of danger. As already mentioned, there is a precipitation basin full of radiation in the plant and it has been left untouched. Radiation is still being released from this basin, and there is still a possibility of a massive radiation release. As high as 54Bq per kg of Iodine 131 was detected by researchers of the Research Reactor Institute, Kyoto University from the leaves of a mugwort plant from the vicinity of the site. Radioactive materials were produced by activation, and sodium 24 was detected from the soil up to an area 3km away from the plant. According to our assumption, a large amount of activated products were released into the area within the 350m radius of the plant, and there is a possibility that area residents returning to their houses could take radiation into their bodies by eating salt and other foods that was in their houses at the time of the accident. Thus it is still too early to issue a "safety declaration." Furthermore, our calculation together with the measurements by the company strongly suggests that residents living within 600-700m radius had been exposed to neutron doses beyond legal annual limits.4. Parties responsible for the accident.It is clear from the above mentioned facts that the company as an organization (entity) holds responsibility for this accident. However, the Science and Technology Agency and the Nuclear Safety Commission are also deeply responsible in that they allowed this plant to operate despite the fact that it was not designed to counter criticality. In addition, this plant was built in the middle of a residential area, and the possibility of criticality accident was not included when the site location review was done.Both parties as overseeing agencies are deeply responsible for this fact as well.5. Lessons learned from this accident.Recently, serious accidents have occurred frequently in Japan, and the fabrication and falsification of inspection data and product data have been made public as well. With each incident and scandal, the STA has set up investigation committees and put out suggested measures for improvement, and in some instances, the responsible parties were fined. However, instead of making improvements with these measures, the situation has become worse, and now it has resulted into a level 5 accident. Nothing can be expected from the "Accident Investigation Committee" set up by STA. There must be a thorough investigation into the accident, and a review of the nuclear industry itself by a third party group that includes citizens. Furthermore, there must be a review of the entire energy policy of Japan that up until now has been heavily reliant on nuclear energy. This is an urgent task. Without this fundamental review, a disaster even worse than this accident is inevitable.In particular, there are a significant number of nuclear fuel fabrication facilities similar to the JCO plant that likewise lack installations to counter criticality. Considering the seriousness of this accident, all such facilities should immediately stop operating, and go through a full safety inspection conducted by a third party. In addition, since the irresponsibility of the Japanese nuclear industry has been exposed to this extent, and has given deep fear to Japanese citizens, the "plu-thermal plan," i.e. the plan to utilize MOX fuel in light water reactors, must be frozen since it has never been attempted in a large scale in Japan and has yet to be proven safe.


Citizens' Nuclear Information Center
Akebonobashi Co-op 2F-B, 8-5 Sumiyoshi-cho,
Shinjuku-ku, Tokyo, 162-0065, Japan