Tokai JCO Criticality Accident
Report  15 July 2000
Inquiry into the Responsibility of the Former PNC and STA
Nuke Info Tokyo 78
By Hideyuki Ban
The JCO Criticality Accident Assessment Committee, organized by CNIC together with the Japan Congress Against A- and H- Bombs and established in Dec. 1999, recently compiled and released an interim report which criticizes the government's safety review of the JCO facility and emphasizes the responsibility of those who commissioned that review. Following is a brief summary of the report.
The amount of uranium nitrate put into the precipitation tank was said to be 16.6 kg, but examination of the contract between JCO and Japan Nuclear Cycle Development Institute (JNC) for the order of the uranium solution, and other documents, indicates that only about 15 kg was supposed to have been manufactured. Clarifying this point is of the utmost importance in relation to the amount that led to criticality, but the most basic of facts such as this are not examined by the Nuclear Safety Commission (NSC)'s investigation at all. The commission finished its accident inquiry in December and has already disbanded, despite not having tracked down the accident's cause.
The main cause of worker's exposure was neutron emissions. On two occasions the government investigation reviewed the workers' exposure assessments, both times lowering their exposure dose. The government's assessment of exposure dose from the accident is grossly under-estimated and will have to be reviewed because the recommendations of the International Commission on Radiological Protection Publication 60, which will be legally adopted in Japan from 2001, assess the impact of neutrons on the human body (quality factor) at double than the quality factor that was used for the current government estimation.
In addition, it has become clear from our survey of local residents that there were many who experienced during the accident, or who are still experiencing, various physical symptoms and illnesses. There needs to be a thorough investigation into the relationship between the accident and these symptoms. An important part of such research would be an investigation into the effects of internal and external exposure from radioactive iodine and rare gases, which have short half-lives.
It was pointed out during the government's safety review of the JCO plant's license application that the precipitation tank was not designed with geometrical control. However, the Science and Technology Agency (STA) avoided dealing with this problem by double-checking the mass control of the tank. Their conclusion was that since the workers would never violate the mass control, criticality was an "impossibility." This clear fault in the reviewing process was never brought up in the NSC's Investigation Committee, but it is obvious that the STA and the NSC, which are in charge of safety reviews, carry grave responsibility for letting the matter slide.
In addition, the NSC's Investigation Committee deliberately avoided pursuing the responsibility of JNC, which placed the order for the particular uranium solution. JNC (formerly PNC) made an order for uranium solution which had a very high concentration of 370 g per liter and demanded procedural specifications for the homogenization process which were difficult for JCO to carry out at its plant. It is written in the contract between JNC and JCO that JCO must provide JNC with the conversion process manual and the conversion process summary (outline) before preparation of the particular uranium solution. Obviously JNC was aware of the illegal procedures that were adopted at the JCO plant. And of course the company was aware from the very beginning that the conversion building of the JCO plant was not installed with sufficient equipment to prepare uranium nitrate solution and was thus unsuitable for preparing high-enriched, high-concentrated uranium solution. The responsibility of JNC is actually the heart of the cause of this accident. Our investigation has gone into depth in this matter. We plan to release the final report of our investigation this fall.