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Tokai JCO Criticality Accident

Report [2] 10/2/99 21:00

(1) Conduct a thorough research and release complete data on the findings of the released radioactivity!

There has been a very limited amount of data given regarding the radioactivity released. However, CNIC has done a provisional trial calculation on the amount of uranium that reacted, and the amount of radioactivity released.

Of the uranium in the precipitation basin that caused the criticality accident, we assume that roughly 1-10grams of uranium 235 underwent fission, and 1016 to 1017 Bq (becquerel) of fission products were produced. (When assumed that the radioactive materials released at Chernobyl was roughly 1019 Bq, this estimated amount is 1/1000 to a couple hundredth of one. This amount is based on very limited information, thus we apologize for the possibility of the numbers entailing some inaccuracy. )

When it is assumed that 1/100 of the produced 1016 to 1017 Bq fission materials (radioactive materials) were released, based on the assumption that all the radioactive gas was released and some of the other radioactive materials were released as well, the amount of radioactive materials released was 1014 to 1015 Bq, i.e. one hundred to one thousand  teraBq. According to IAEA International Nuclear Event Scale, an accident involving a release of hundreds to thousands teraBq radioactivity is categorized as level 5. STA announced that this accident was level 4, however, in reality it is a level 5 accident.

We assume that an amount of radioactive materials corresponding to the amount produced in the facility were released to the outside of the facility, and that the radioactive materials with short halflives have already attenuated. However, it is certain that no small amount of radioactive materials are still left inside the precipitation basin.

It has been reported that the gamma ray emitted from the remaining radioactive materials are kept inside because of a shelter consisting of layers of aluminum was put around the facility. However, this shelter is an unreliable tentative treatment, and there are serious doubts for its sufficiency.

There is a possibility that some of the remaining radioactive materials are still being released at the moment, and the possibility of such materials being released in the future cannot be denied as well.

At 10/2/99 6:30 p.m. a "Declaration of Safety" was issued by the government saying that the area 350meters radius of the site has no contamination and thus they would call off the evacuation request. However, judging from the conditions supposed above, we do not believe that the government has sufficient enough facts to release such a declaration.

In addition, the exposure check done for the area residents who request such an examination is only checking if the surface of their body is contaminated by radioactive materials, and is not a check for previous exposure (internal and other exposure by neutron rays, gamma rays, and radioactive materials ) from the accident. Thus this examination cannot determine whether a person has been exposed or not.

(2) The realities of the exposure is not known yet.

It is said that the 49 workers and area residents have been exposed. However, this is not a number based on examinations for various types of exposure. The examination was conducted to detect exposure by neutrons, and thus relates only to the number of people who had sodium24, that is produced by neutron exposure, in their body.

Thus there has not been an investigation on exposure by gamma-ray and internal exposure by inhalation of radioactive materials. There are good reasons to believe that a much higher number of people have been exposed. There must be an all inclusive investigation and a complete release of the findings.

In the early morning of 10/1/99, workers went to the space underneath the floor of the accident site to extract coolant water in order to contain the criticality reaction. Due to that operation, 18 people were exposed to an amount of 20mSv to 103mSv. It is regulated by law that the annual dose limit for workers are 50mSv/y, and 100mSv for one operation during emergencies. However, many workers were exposed to an amount beyond the regulated annual dose limit, and one of them was even exposed to an amount beyond the emergency limit. There should be a system that does not involve such exposure to the workers. JCA and the Science Technology Agency (STA) are greatly responsible for bring on such exposure to the workers. There must be a detailed data for exposure.

(3) An absent of criticality control measures brought about this accident.

According to "Basic Guidelines for the Licensing of Nuclear Facilities" put to effect in 2/7/1980, it is stated that for all facilities dealing with nuclear fuel materials, "there must be a counter measure set up to prevent criticality reaction under all possible scientifically conceivable conditions." If this policy was pursued, all installations of the facilities must be designed to prevent criticality even in a case of human error (i.e. the facilities must be designed with criticality prevention installment). Criticality control by the amount and density of nuclear materials can cause criticality by human errors. However, there were no installation for criticality control in the precipitation basin where the reaction took place.

In addition, the guideline requires that "there must be a proper counter measure set up for in case of a criticality accident." However, no sufficient measures could be taken to stop this criticality, and it was left alone. Furthermore, there was no neutron detection system in the plant to detect criticality. These facts are clearly in violation of the guideline. The company that designed such an illegal facility, and the STA that licensed the facility hold great responsibility. The cause of this accident cannot be explained away as human error.

There are many facilities with units that do not have criticality prevention installment. This accident has made it clear that such facilities must be fit with criticality prevention installment. All facilities including JCO must be reassessed under renewed licensing review for the safety and the location of the sites to prevent any type of damage to the area residents, even in a case of a worst criticality accident.


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