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Tokai JCO Criticality Accident

Report [32]2/8/2000

Rushed Interim Report Plays Down the Accident

The Interim Report

On November 4th, the Science and Technology Agency (STA) released data on the estimated scale of the September 30th accident at the JCO Tokai plant. The STA also announced the estimated radiation dose received by the local residents, based on this data. The following day an interim report with urgent suggestions reflecting those results was presented at the fifth meeting of the Uranium Conversion Facility Plant Criticality Accident Investigation Committee of the Nuclear Safety Commission (NSC).

The STA has asserted that about 1mg of uranium fissioned in the criticality accident. This amount is extremely close to the amount provisionally calculated by CNIC on the basis of data gathered before November 3rd by the Japan Atomic Energy Research Institute from analysis of the uranium sample collected from the precipitation basin by JCO employees on October 20th. However, when the sample was collected, the equipment set up to stir the contents of the precipitation tank failed to operate, and the sample was taken without the contents of the solution being stirred uniformly. Thus the sample represents only the top layer of the solution. CNIC is critical of the way this sample was collected, and there is a possibility that a higher amount of uranium underwent fission.

The STA also released results of tentative "theoretical" estimation on the exposure level of the area around the site of the accident. These results revealed that by the time criticality stopped at 6:15 a.m on October 1st, at a point 80 meters away from the JCO plant, where the closest public street lies, radiation would have been 160 times higher than the annual exposure dose limit of 1mSv for the general public. The radiation would have been 13 times higher than the annual exposure limit 200 meters away from the site, and over two times higher 350 meters away where evacuation was advised. It must be pointed out that this comparison is based on the annual dose limit, and residents and employees were exposed to amounts between two and 160 times the annual limit in a matter of hours. We assume that a considerable number of residents were exposed to radiation levels close to these figures by remaining too long around the site, or failing to evacuate at an earlier stage.

STA's Claims on Possibility of Cancer

The STA asserts that exposure under 200mSv will not result in cancer; however, nothing can be further from the truth. This assertion was based on exposure data from Hiroshima and Nagasaki A-bomb victims, yet in 1999 the International Commission on Radiological Protection made it clear that this data was not fit for evaluating exposure to a relatively low radiation dose with long-term effects. When exposed to radiation, varying health effects are seen according to the exposure dose. The possibility of contracting cancer increases in accordance with the exposure dose. However, even with a relatively low dose, an exposure to radiation will increase the possibility of contracting cancer no matter how small the amount is. There is a possibility that there will be adverse health effects for the local residents in the future, and these people must have a guarantee of sufficient aftercare over an extensive period.

The Accident Investigation Committee appealed for the revision of safety inspections and the establishment of safety at nuclear-related work sites. Upon receiving this request, the NSC began on November 11th the revision of the Safety Review Standards regarding nuclear fuel facilities. There is a need for an independent critical analysis of this accident. Thus CNIC is preparing to set up a Committee for Comprehensive Social Impact Assessment of the JCO Criticality Accident. Various aspects of the accident will be evaluated and a final report will be put out.

(By Chihiro Kamisawa)



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