| Case Name |
Accident caused due to a critical phenomenon of nuclear fission at the JCO uranium-processing plant |
| Pictograph |
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| Date |
September 30, 1999 |
| Place |
Tokai, Ibaragi, Japan |
| Location |
Nuclear-fuel processing factory |
| Overview |
On September 30th, 1999, there was an accident due to a critical phenomenon at a factory that processed high-concentration uranium fuel in a region with nuclear facilities in Ibaragi Prefect. Three workers were exposed to radiation severely, and two of the three died. Nearby inhabitants were also exposed to radiation in great numbers. The operation manual prepared at a national level was repeatedly changed, including on the day prior to the accident, without permission. Therefore, it became easy to use a container with a shape and structure in which a critical state could arise. Critical phenomena occurred with a large amount of U235 in a small nuclear reactor, and there was a large amount of the emission of neutrons. The critical state was ended after 20 hours by suicide squads of JCO who drew off jacket water at the judgment of the government field countermeasures headquarters. |
| Incident |
In September, 1999, a criticality accident occurred at a uranium fuel processing factory in a region with atomic energy facilities in Ibaragi Prefecture. The accident occurred at JCO Tokai factory, which usually produced low-concentration uranium fuel (uranium 235 concentration of 3-5%). At the time of the accident, the factory produced highly concentrated uranium fuel (uranium 235 concentration 18.8%), which was used in a fast breeder experimental reactor JOYO. The work was entrusted a few times a year. The facilities, which were usually used for low-concentrated fuel, diverted operations to highly concentrated uranium fuel temporarily. Processing was carried out batch-wise, and the accident occurred at the final stage, which is a homogenization process with mixing of some batches of a nitric acid solution of uranyl nitrate. A critical state occurred suddenly when some batches were put into a vessel to homogenize the solution, and a powerful neutron beam was irradiated. Three operators were heavily exposed to radiation, and two of them died later.
The critical state was not resolved quickly, and inhabitants within a 350 m radius were forced to take refuge, and those up to a 10 km radius were recommended to stay indoors. The critical state was ended by drawing off jacket water from the homogenizer vessel by destroying the drain valve. As the work was executed under exposure of powerful radiation, some suicide squads were organized by JCO employees. Each suicide squad worked for a very short time, and after many shifts the critical state was finally ended. In addition, a boric acid solution was put into the vessel for the homogenization operation. The critical state continued for 20 hours. |
| Processing |
Manufacture |
| Individual Process |
Filling and subdivision |
| Process Flow |
Fig2.Process flow
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| Substance |
Uranyl nitrate, Fig3 |
| Type of Accident |
Environmental pollution, health hazard |
| Sequence |
From mid-September to September 28th, 1999, highly concentrated uranium fuel was refined with many batches.
On September 29th, the refined fuel was dissolved in nitric acid again by batch processing, and a nitric acid solution of uranyl nitrate was prepared. Then, some of the batch was mixed for homogenization.
The uranyl nitrate solution of four batches was put into the vessel. (2.4 kg of uranium in a batch of 6.5 liters)
On September 30th, three batches were also added to the vessel.
About 10:35, a criticality state occurred, and three workers were exposed to radiation.
10:43, fire fighters received an emergency request from JCO. The fire fighters were aware of a radiation accident.
After communications with Tokai village, Ibaragi Prefect., and Science and Technology Agency urgent countermeasures were taken. Countermeasures headquarters at each level were set up. Government countermeasures headquarters, led by the prime minister, was also set up.
15:00, the Tokai village headman directed inhabitants within a radius of 350 m to take refuge.
22:30, the Ibaragi Prefecture governor made a recommendation to stay indoors to inhabitants within a radius of 10 km.
About 22:30, there were no indications that the critical state had been resolved, and the government field countermeasures headquarters decided to draw off water from the jacket of the vessel.
00:25 on October 1st, two person, the first suicide squads, who were selected from JCO employees, started draw-off work.
06:04, the 10th suicide squad succeeded in drawing off the water, and the critical state stopped.
08:39, injection of a boric acid solution into the vessel was completed, and the critical state ceased. Recommendations to inhabitants were reversed. |
| Cause |
In short, "the cause was only one that a uranyl nitrate solution including uranium above the critical mass was injected into the settling vessel, which had not been designed for criticality safety, and was originally intended for a different purpose." The settling vessel used for remelting and homogenization was the vessel in which the accident occurred, and use of the vessel was different from the original application. Homogenization work should be done in a container called a dissolution tower, but JCO thought the work was difficult, and decided to change the vessel without obtaining permission. On the day the accident occurred, the vessel was changed again to an unlicensed vessel.
The whole process was submitted to Science and Technology Agency at that time, and all of the work had to be done using the facilities and methods approved. The following descriptions are shown in Fig3: Change of the mode of operation for remelting and homogenization.
In the approved manual, remelting and homogenization are done using a dissolution tower, as shown in Fig3. However, homogenization work was transferred to a storage tower after dissolving in a stainless bucket, because using the dissolution tower was difficult for subdivision work in the final stage. The changed method still required hard work on distributing to the final subdivision container and too much time to mix for homogenization. The work was changed again.
As using the storage tower was inconvenient, the settling vessel having a short and thick shape with a water jacket was used. As the surface area was small, neutrons had difficulty in leaving the vessel. Besides, the neutrons were reflected by the water in the jacket. The critical state, which did not occur in the tall storage tower, occurred in the settling vessel.
In homogenization of the final process, one lot which included seven batches from upstream was handled, and the quantity of uranium in one batch was 2.4 kg. |
| Response |
Three workers were exposed to radiation and carried to NIRS, which was located in Chiba city via an emergency hospital in Ibaragi Prefecture. The following were done. Nearby inhabitants were recommended to take refuge and those in a wider area to stay indoors. To stop the critical state, jacket water was drawn off by suicide squads, and a boric acid solution was injected into the vessel to completely stop the process. |
| Countermeasures |
In the case of this accident, accident prevention countermeasures returned to the basis of ensuring safety, and adhering to defined procedures simply and honestly. (Measures and rumored damage to inhabitants are omitted.) |
| Knowledge Comment |
1. Thoughtlessly changing of the work method brings about hazards. Why was it repeated many times?
2. It is very important to consider countermeasures assuming a worst situation and the thorough and rapid report when an emergency occurs. It is the minimum safety management and crisis management not only for the atomic energy industry but also every industry. In particular, it is important to report to the highest ranking person when a serious situation is anticipated, without worrying about a false alarm. |
| Background |
The real cause was a selfish corporate culture of JCO that changed the operation manual from the nationally approved one at their discretion, and operated according to a hidden manual. Moreover, they changed it again in an expedient way on the day prior to the accident. The company's organization allowed orally the change which was proposed on the previous day without confirming the operation. The person who made a decision misunderstood the concentration of the solution. This change was fatal. |
| Sequel |
Some consumers did not want to buy foods produced in Ibaragi Prefecture as they might have been contaminated with radioactivity (damage from rumors) from the viewpoint of food safety. |
| Reason for Adding to DB |
First accident in Japan caused due to a critical phenomenon of nuclear fission |
| Scenario |
| Primary Scenario
|
Poor Value Perception, Poor Organizational Culture, All the Company Illeagal, Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Organizational Problems, Poor Management, The Priority is always the Opinion of the Site, Planning and Design, Poor Planning, Insufficient Operation Design, Non-Regular Action, Change, Change of Operation Procedure, Bad Event, Chemical Phenomenon, Critical State, Secondary Damage, External Damage, Bodily Harm, Death, 2 person died, Bodily Harm, Harm to Physical Well-being, Radioactivity Exposure, Damage to Society, Social Systems Failure, Forced Refuge ,etc., Loss to Organization, Social Loss, Trust Damage
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| Sources |
Eiji Oshima. Blind spot in accident prevention countermeasures. Research on disasters 31. pp.43-49 (2000)
Yoichi Uehara. JCO accident safety technique personnel observed. Research on disasters 32. pp.5-13 (2001)
Mito district court. Defendant contravened Law for the Regulation of Nuclear Source Materials and Nuclear Fuel Material and Reactors No.865, 2000. Lower court main ruling. Mito district court home page. (2003)
Yomiuri Shimbun editorial office. The blue flash. Document Tokai criticality accident. (2000).
|
| Number of Deaths |
2 |
| Number of Injuries |
664 |
| Consequences |
Ibaragi Prefecture and Tokai Village recommended inhabitants within a 350 m radius range to take refuge and those within a 10 km radius range to stay indoors. Interruption of railway. Roads were closed. The source of drinking water supply was switched from Kuji River to Naka River. Drinking of well water and rainwater were forbidden. Schools were closed within a radius of 10 km. Hospital visits were controlled.
Rumors of pollution of crops damaged farmers.
Based on the results of monitoring on the surrounding area by the emergency technology recommendation organization of Nuclear Safety Commission, the government countermeasures headquarters made recommendations. As a result, in the afternoon of October 1st, the recommendation to stay indoors within a 10 km range was canceled by Ibaragi Prefecture, etc., and the recommendation to take refuge within a 350 m range was also canceled next day, on October 2nd. |
| Multimedia Files |
Fig3.Chemical formula
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Fig4.Settling tank in which the critical accident was caused
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| Field |
Chemicals and Plants
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| Author |
KOBAYASHI, Mitsuo (Office K)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
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