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Tokai JCO Criticality Accident

Report [19]11/18/99 20:15

Accident Investigation Committee Puts Out an Interim Report

Concerning the September 30th criticality accident at JCO Tokai plant, the Science and Technology Agency (STA) on November 4th released data on the estimated scale of the accident and, based on that, an inferential radiation dose received by the local residents. 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 at the criticality accident. This amount is extremely close to the amount provisionally calculated by CNIC based on the data gathered by the Japan Atomic Energy Research Institute by November 3rd from analyzing 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 basin failed to operate, and the sample was taken without the contents of the solution being evenly distributed. 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.

Furthermore, the STA’s inferential results on the exposure dose of the area around the site of the accident was based on uncertain figures and thus is not a reliable data. Their results revealed that by the time the criticality stopped at 6 a.m on October 1st , at a point 80 meters away from the JCO plant where the closest public street lies, there would have been radiation 160 times higher than the annual exposure dose limits of 1mSv for the general public. The radiation would have been 13 times higher than the annual exposure limits 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 fair number of residents were exposed to radiation level close to these figures by remaining too long around the site, or failing to evacuate at an earlier stage.

The STA asserts that exposure under 200mSv will not result in cancer; however, nothing can be further away from the truth. The exposure data from Hiroshima and Nagasaki A-bomb victims, which the STA is using as the base for the above assertion, has been clearly rendered by the International Commission on Radiological Protection in 1999 as not fit for evaluating exposure to 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 however small the amount is. There is the possibility that there will be adverse health effects to the local residents in the future, and there is a need to guarantee sufficient aftercare that will be carried out over an extensive period.

The Accident Investigation Committee appealed for the revision of safety inspections and the establishment of safety at the 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.



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