Tokaimura criticality accident, Japan

This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).


caption Emergency response personnel at the site of the Tokaimura criticality accident, Japan.

On September 30, 1999 an accident happened at a small fuel preparation plant operated by Japan Nuclear Fuel Conversion Co. (JCO), a subsidiary of Sumitomo Metals and Mining, in the village of Tokaimura, 130 km northeast of Tokyo. Two workers died as a result of radiation exposure.

The particular JCO plant at Tokai was commissioned in 1988 and processes up to 3 tonnes per year of uranium enriched up to 20% uranium-235 (235U), much more than in ordinary nuclear power reactors. The plant supplied various specialized research and experimental reactors. It used a wet process.

The approved nuclear fuel conversion procedure involved the dissolution of uranium oxide (U3O8) powder in a dissolution tank, then its transfer as pure uranyl nitrate solution to a buffer column for mixing, followed by transfer to a precipitation tank. This tank is surrounded by a water cooling jacket to remove excess heat generated by the exothermic chemical reaction. The prevention of criticality was based upon the general licensing requirements for mass and volume limitation, as well as upon the design of the process. A key part of the design was a column with a criticality-safe geometry as a buffer to control the amount of material transferred to the precipitation tank.

However, the work procedure was modified three years ago, without permission from the regulatory authorities, to allow the dissolution of uranium oxide to be performed in stainless steel buckets. The mixing designed to occur in the buffer column was undertaken by mechanical stirring in the precipitation tank, thus bypassing the criticality controls. The shape of the hundred-liter precipitation tank (450mm diameter and 660mm high) enhanced the likelihood of criticality.

The accident

caption Plan of site from STA, data quoted below from monitoring point A, that from B is slightly lower.

On 30 September, three workers were preparing a small batch of fuel for the JOYO experimental fast breeder reactor (FBR), using uranium enriched to 18.8% uranium-235 (235U). It was JCO's first batch of fuel for that reactor in three years, and no proper qualification and training requirements appear to have been established to prepare those workers for the job. At around 10:35, when the volume of solution in the precipitation tank reached about 40 liters, containing about 16 kg uranium, a critical mass was reached.

At the point of criticality, the nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation, triggering alarms. There was no explosion, though fission products were progressively released inside the building. The significance of it being a wet process was that the water in the solution provided neutron moderation, expediting the reaction. Most fuel preparation plants use dry processes.

The criticality continued intermittently for about 20 hours. It appears that as the solution boiled vigorously, voids formed and criticality ceased, but as it cooled and voids disappeared, the reaction resumed. The reaction was stopped when cooling water surrounding the precipitation tank was drained away, since this water provided a neutron reflector. Boric acid solution (neutron absorber) was finally was added to the tank to ensure that the contents remained subcritical. These operations exposed 27 workers to some radioactivity. The next task was to install shielding to protect people outside the building from gamma radiation from the fission products in the tank. Neutron radiation had ceased.

The radiation (neutron and gamma) emanated almost entirely from the tank, not from any dispersed materials. Buildings housing nuclear processing facilities such as this are normally maintained at a lower pressure than atmosphere so that air leakage is inward, and any contamination is removed by air filters connected to an exhaust stack. In this case, particulate radionuclides generated within the conversion building were collected by the high-efficiency particulate air filters, though noble gases passed through the filters. A smoke test on 5 October confirmed that the negative pressure had been maintained (i.e., the structural integrity of the building was satisfactory) and that the ventilation system was working. However, owing to the detection of low levels of iodine-131 being released to the environment through the exhaust, it was later decided to stop ventilation and to rely on the passive confinement provided by the building.

Five hours after the start of the criticality, evacuation commenced of some 161 people from 39 households within a 350 meter radius from the conversion building. They were allowed home two days later after sandbags and other shielding ensured no hazard from residual gamma radiation. Twelve hours after the start of the incident, residents within 10 km were asked to stay indoors as a precautionary measure, and this restriction was lifted the following afternoon.

The effects, and analysis

The accident was classified by the Japanese authorities as Level 4 on the International Atomic Energy Agency (IAEA) International Nuclear Event Scale (INES), indicating an event without significant off-site risk. It was essentially an 'irradiation' accident, not a 'contamination' accident, as it did not result in any significant release of radioactive materials.

The International Nuclear Event Scale (INES)
Level, Descriptor Off-Site Impact On-Site Impact Defence-in-Depth Degradation Examples
7 - Major Accident Major Release: Widespread health and environmental effects   Chernobyl, Ukraine, 1986 (fuel meltdown and fire)
6 - Serious Accident Significant Release: Full implementation of local emergency plans   Mayak at Ozersk, Russia, 1957 (reprocessing plant criticality)
5 - Accident with Off-Site Risks Limited Release: Partial implementation of local emergency plans Severe damage to reactor core or to radiological barriers  Windscale, UK, 1957 (military). Three Mile Island, USA, 1979 (fuel melting).
4 - Accident Mainly in Installation Minor Release: Public exposure of the order of prescribed limits Significant damage to reactor core or to radiological barriers, worker fatality  Saint-Laurent, France, 1980 (fuel rupture in reactor). Tokai-mura, Japan, 1999 (criticality in fuel plant for an experimental reactor).
3 - Serious Incident Very Small Release: Public exposure at a fraction of prescribed limits Major contamination, acute health effects to a worker Near Accident. No safety layers remaining Vandellos, Spain, 1989 (turbine fire, no radioactive contamination). Davis-Besse, USA, 2002 (severe corosion). Paks, Hungary, 2003 (fuel damage)
2 - Incident nil Significant spread of contamination, overexposure of worker Incidents with significant failures in safety provisions  
1 - Anomaly nil nil Anomaly beyond the authorized operating regime  
0 nil nil No safety significance  
Below Scale nil nil No safety relevance  
Source: International Atomic Energy Agency

The three workers concerned were hospitalized, two in a critical condition. One died 12 weeks later, another 7 months later. The three had apparently received full-body radiation doses of 10-20,000, 6-10,000 and 1-5000 millisieverts (about 8000 mSv is normally a fatal dose). Doses for a further 436 people were evaluated, 140 based on measurement and 296 on estimated values. None exceeded 50 mSv (the maximum allowable annual dose), though 56 plant workers exposed accidentally ranged up to 23 mSv and a further 21 workers received elevated doses when draining the precipitation tank. Seven workers immediately outside the plant received doses estimated at 6 - 15 mSv (combined neutron and gamma effects).

The peak radiation level 90 meters away just outside the nearest site boundary was 0.84 mSv/hr of gamma radiation, but no neutron levels were measured at that stage. The gamma reading then dropped to about half that level after nine hours at which stage 4.5 mSv/hr of neutron radiation was measured there, falling to about 3 mSv/hr after a further two hours, and then both readings falling to zero (or background for gamma) at 20 hours from the start of the criticality.

Neutron dose rates within one kilometer are assumed to be up to ten times the measured gamma rates. Based on activation products in coins from houses near the plant boundary and about 100 m from the reaction, it was estimated that some 100 mSv of neutron radiation would have been received by any occupants over the full period of the criticality. However, the evacuation of everyone within 350 meters of the plant had been ordered 5 hours after the start of the accident. The final report on the accident said that the maximum measured dose to the general public (including local residents) was 16 mSv, and the maximum estimated dose 21 mSv.

While 160 TBq of noble gases and 2 TBq of gaseous iodine were apparently released, little escaped from the building itself. After the criticality had been terminated and shielding was emplaced, radiation levels beyond the JCO site returned to normal.

Only trace levels of radionuclides were detected in the area soon after the accident, and these were short-lived ones. Products from the area would have been as normal, and entirely safe throughout. Radiation levels measured by the IAEA team in residential areas in mid-October were at the normal background levels. Measurement of iodine-131 in soils and vegetation outside the plant showed them to be well under levels of concern for food.

According to the IAEA, the accident "seems to have resulted primarily from human error and serious breaches of safety principles, which together led to a criticality event". The company conceded that it violated both normal safety standards and legal requirements, and criminal charges are being laid. The fact that the plant is a boutique operation outside the mainstream nuclear fuel cycle evidently reduces the level of scrutiny it attracts.

Japan's atomic energy insurance pool said would make a payment to JCO in respect to the accident, its first such payment ever. However, this would be limited to one billion yen, with further liability (the total estimated at 13 billion yen or A$200 million), being met by JCO or its parent company. The plant's operating licence was revoked early in 2000.

Mainstream fuel fabrication plants in Japan are fully automated, engineered to ensure that criticality does not occur, equipped with neutron monitoring systems and fully prepared for any possible criticality accident. Most plants use a dry process in any case, which is intrinsically safer. No major civil reactor uses uranium enriched beyond 5% uranium-235.

The events of 30 September placed Japan’s entire nuclear enterprise under scrutiny. Critics of the IAEA assessment contend that the accident also resulted from lax government oversight of the nuclear industry. Critics were particularly concerned by what they perceived as a slow and inadequate initial repsonse to the accident, a poor emergency plan, and poor enforcement of occupational health and safety procedures on the part of the government. On paper, the government-appointed Nuclear Safety Commission acts as the industry watchdog. But some question its independence--they say it is dominated by pro-nuclear scientists with official ties. The commission even works out of the offices of the Science and Technology Agency, the bureaucracy that regulates the industry. These analysts called for an independent nuclear regulatory safety committee in Japan.

Previous criticality accidents

While this was Japan's first such accident, similar criticality incidents have occurred, especially in US and Russian military plants and laboratories. All but two of these were prior to the early 1980s. Three (in 1958 and 1964) were very similar to this accident. The last of these was the single previous criticality accident at a commercial fuel plant in the US, which resulted in one death.

Of all the previous accidents, 37 occurred in connection with research reactors or laboratory work for military projects, resulting in ten deaths. Another 22 occurred in nuclear fuel cycle facilities, all but one military-related, and resulting in seven deaths. The energy released in each of these accidents ranged from about 0.03 MJ to 3 GJ (on basis of IPSN report quoting fissions ranging from 1015 to 1.2 x 1020, and each fission yielding 3 x 10-11 Joules. Petrol @ 34 MJ/litre). The energy released in the similar US accident was about 3 MJ, though due to the prolonged criticality here, some 80 MJ was released, equivalent to the combustion of just over two liters of petrol/gasoline.

The fuel preparation accidents were all in wet processes, due to putting too much uranium-bearing solution in one tank. Mostly, these then erupt rather like a saucepan of milk boiling over, and the fission reaction ceases as the material is ejected and dispersed in the immediate vicinity. None of the previous accidents resulted in significant release of radioactivity outside the plants. Practically all were in Russian or US plants, and in reviewing these accidents recently the need for a high level of staff training was emphasized.

Further Reading



Hore-Lacy, I., & Association, W. (2009). Tokaimura criticality accident, Japan. Retrieved from


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