On the 30 September 1999, at the Japan Nuclear Fuel Conversion Co. fuel preparation plant at Tokai, three workers began preparing a batch of enriched uranium for use in an experimental fast breeder reactor. Operational procedures were violated through the use of manual preparation, water cooling and improper configuration of precipitation tanks. A critical mass of uranium-235 was reached in the precipitation tank and a nuclear fission reaction became self-sustaining. Intense gamma and neutron radiation were emitted from the tank and the vigorous boiling of the reactant released radioactive material throughout the building. Criticality lasted intermittently for twenty hours before being poisoned with the addition of a neutron absorber, and residences within 350m of the facilities were evacuated for two days. Two of the operators received fatal doses of radiation from this accident.
This accident shows that uncontrolled fission reactions can happen like in nuclear bombs.
When criticality was achieved, the fission reaction became self-sustaining, but the conditions were not right for a rate of reaction required for a nuclear explosion to occur. The intense heat from the fission resulted in the mixture boiling vigorously. When this occurred, voids were created, which reduced the mixture to subcritical. The resultant cooling then removed the voids and criticality resumed. The mixture was in essence oscillating either side of criticality. This accident actually shows that even in the most uncontrolled of accidents, nuclear explosion cannot happen. Nuclear bombs require effort, not negligence.
Clearly, enrichment facilities are negligent.
It would be crass to judge the entire fuel fabrication industry based on the misconduct of one small facility. This facility received less scrutiny than most and was more cavalier because it was working on experimental technology. Mainstream commercial facilities follow regulations more appropriately.
This shows how fast breeder technology is dangerous.
Fast breeder technology at this stage cannot be judged, since making it practical has indeed proven difficult. If it was considered that fast breeders were fit for commercial use, they would be used. At this stage, it is experimental.
We should stop experimental nuclear technology to prevent this from happening again.
Experimental technology should be treated with care. But in this case, it was not. That is why a criminal case was pursued against JCO.
The grave effects of this accident show how we cannot take the chance that operators will not follow the rules.
Despite the obvious tragedy of the deaths of two workmen, the dose equivalent to the general public over the course of the criticality was no more than 21mSv. This is clearly well above usual levels, but still well below any threatening levels. No long term increase in the radiation environment has been recorded.
What are you trying to say?
This was clearly one of the more serious mistakes made by the civil nuclear industry. The regulations laid down are not in dispute since they would have prevented this accident. The concern is for the low level of scrutiny that allowed JCO to violate them in such a cavalier way. This was brought about because the facility in question was working on experimental technology and this accident is no reflection on the mainstream industry. Potential indictment is directed at the experimental cycles but because rules were flouted, it is difficult to generalise this. When regulations are violated, a lot of damage can happen in many industries.
The radiological effects were noticeable, but apart from the three workers who received doses above the legal limit, including two fatal doses, there has yet to be any discernible long term effects. It serves as a reminder of the dangers of taking a cavalier attitude to hazardous substances, dangers present in a wide range of industries. It does not highlight the nuclear industry as being any more dangerous than many other heavy, particularly energy industries.