Chapter 36: Tokaimura Criticality

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on September 16, 2012.

The third nuclear accident, Tokaimura Criticality, occurred in Japan on September 30, 1999, after thirteen years, five months, and four days of the Chernobyl Nuclear accident in the former Ukrainian Republic of the Soviet Union. The accident was classified by the Japanese authorities as Level 4 on the International Atomic Energy Agency (IAEA) and 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, however.  Nevertheless, the accident was caused by bringing together too much uranium enriched to a relatively high level, causing a “criticality” (a limited uncontrolled nuclear chain reaction), which continued intermittently for 20 hours and a total of 119 people received a radiation dose over 1 mSv from the accident, but only the three operators’ doses were above permissible limits. Two of them were hospitalized and died within a period of 7 months.Slide1According to various reports on the subject published by the IAEA, the accident at the nuclear fuel processing facility at Tokaimura seemed to have resulted primarily from human error and serious breaches of safety principles.   The accident did not involve widespread contamination of the environment and that there was little risk off site once the accident was brought under control. The team further concluded that the accident was essentially an “irradiation” accident and not a contamination accident, because it did not result in a significant release of radiological material. Only trace amounts of noble gases and gaseous iodine escaped from the building.  This observation is based on the understanding about nuclear safety that:

  • Safety in the nuclear fuel cycle has always been focused on reactor operations, where a huge amount of energy is released continuously in a small volume of material, and where there are substantial amounts of radioactive materials which would be very hazardous if released to the biosphere;
  • A secondary focus is then the high-level wastes from the reactor, which comprise all the potentially hazardous materials from the reactor core; and
  • Other parts of the nuclear fuel cycle have much less potential for widespread harm to people or the environment. They are correspondingly less regulated in some countries, such as Japan.

As a result of the review conducted by the United States Nuclear Regulatory Commission (NRC), the staff of the Commission agreed with the Government of Japan’s conclusion that the general root causes of the accident were:

1.      Inadequate regulatory oversight:

The regulatory oversight program for the Tokaimura fuel processing facility failed to establish and maintain an adequate safety margin. The licensing review incorrectly concluded that there was “no possibility of criticality accident occurrence due to malfunction and other failures.” Consequently, no criticality accident alarm was required or installed and the facility was not included in the National Plan for the Prevention of Nuclear Disasters. This conclusion relied heavily on the use of administrative controls that were subject to human error;

2.      Lack of an appropriate safety culture:

In the Japanese regulatory framework, as in NRC’s, the licensee is ultimately responsible for the safe operation of nuclear facilities. Deviations from the approved operating procedure began to occur several years before the company developed a second operating procedure for use. That second operating procedure was approved by the manufacturing and quality assurance divisions without the review and approval of the safety management division. This was apparently done to improve production efficiency; and

3.       Inadequate worker training and qualification:

The operators were not given the fundamental safety knowledge the operators would have understood the importance of adhering to the safety limits for the process. The training should have stressed the safety controls for this process to protect against inadvertent criticality.Slide2The philosophy of the regulator was that the system was safe if it was operated in accordance with the approved procedures. In addition, the company did not believe that a criticality accident was a credible event and there were no specific operator training requirements for criticality safety. The operators were also allowed to deviate from the approved procedures to improve production efficiency. Had the operators understood the difference between the safety limits for the 3-5 percent enriched uranium that they usually handled, and the 18.8 percent enriched material involved with this process, they likely would not have taken the shortcuts that resulted in the criticality. The people most vulnerable to the consequences of a failure to implement significant safety controls must be provided with the appropriate safety information.

The staff was able to identify one or more elements in NRC’s regulatory oversight program that would have prevented the identified deficiencies from occurring if they had been in effect at the time of the accident. Based on the review the staff determined that the current NRC oversight program at commercial U.S. nuclear fuel fabrication, conversion and enrichment facilities makes a similar accident unlikely, and no revisions to NRC’s oversight program are needed as a result of the lessons learned.

Unfortunately, the Tokaimura accident is not the only accident related to criticality accidents that occurred in Japan.  These accidents were occurred 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 USA, resulting 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 fuel cycle facilities, all but one military-related, and resulting in seven deaths.Slide3As far as the Tokaimura accident is concerned, it occurred in a very small fuel preparation plant operated by Japan Nuclear Fuel Conversion Co. (JCO), a subsidiary of Sumitomo Metal Mining Co. It was not part of the electricity production fuel cycle, nor was it a routine manufacturing operation where operators might be assumed to know their jobs reasonably well.  The particular JCO plant at Tokai was commissioned in 1988 and processed up to 3 tonnes per year of uranium enriched up to 20 percent U-235, a much higher percentage than for ordinary power reactors. The plant supplied various specialized research and experimental reactors. It uses a wet process.

On 30 September three workers were preparing a small batch of fuel for the JOYO experimental fast breeder reactor, using uranium enriched to 18.8 percent U-235. It was JCO’s first batch of fuel for that reactor in three years, and no proper qualification and training requirements had been established to prepare those workers for the job. They had previously used this procedure many times with much lower-enriched uranium – less than 5 percent, and had no understanding of the criticality implications of 18.8 percent enrichment. At around 10:35, when the volume of solution in the precipitation tank reached about 40 litres, containing about 16 kg U, 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.

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 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.Slide4Five hours after the start of the criticality, evacuation commenced of some 161 people from 39 households within a 350 metre 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 accident did have substantial psychological and economic impacts on the local population. During the accident, about 310,000 people were ordered to remain sheltered and residents living within 350 meters of the facility were evacuated. According to News sources report that JCO expects to pay at least $93 million in compensation to nearby residents and businesses.

The Nuclear Safety Commission (NSC) report, A Summary of the Criticality Accident Investigation Committee, dated December 24, 1999, concluded that there was no significant off site impact on the health of the public nor the environment from radiation or the release of radioactive materials because the amount was so small (releases resulted in exposure rates to several micrograys per hour at the highest point) and the short half-life of the radionuclides released.

Furthermore, the report highlighted that accident response lessons learned emphasized the need to:

  • Provide the capability to detect a continuing criticality;
  • Provide more stringent safety measures for facilities that handle uranium solutions enriched to the 20 percent level; and
  • Disclose and provide information in a timely manner; and transmit information to foreign countries promptly.

The NSC made broad recommendations concerning reassessment of the regulatory system, needed changes to the safety culture of the regulators and industry, and information management. The report called for a reassessment of the nuclear disaster prevention policy, which omitted facilities like JCO. During the initial response, problems were encountered with communications from both the company and regulatory authorities, notification of off-site responders that the accident was a nuclear accident, inadequate disaster-management facilities, and coordination difficulties between the national, local governments, and municipalities.

The report also acknowledged the fact that the accident had a very significant social impact.


  1. NRC Review of the Tokaimura Accident; and
  2. The World Nuclear Association – Tokaimura Accident.