Accident in Tokaimura Nuclear Fuel Treatment Plant
The uranium fuel treatment facility is located in Tokaimura (Japan), 120km northeast of Tokyo, in the Ibaraki Prefecture. It is currently owned by the company JCO.
The nuclear accident of the installation took place on September 30, 1999, in the conversion building of the nuclear plant.
The installation consists of three auxiliary uranium conversion buildings:
- One with an annual capacity of 220 tons of uranium per year for low enrichment (approximately 5%).
- Another with an annual capacity of 495 tons of uranium per year for low enrichment (less than 5%).
- Another, the one that had the accident, with an annual capacity slightly higher than 3 tons of uranium per year for high enrichment (not exceeding 20%).
In this third building, concentrated uranium oxide powder is produced from the transformation of uranium hexafluoride. It used to not work continuously. The installation was used only for very specific orders of immediate production. Practically it was only in operation 2 months a year.
Causes of the accident
To understand what happened first we have to briefly explain the uranium enrichment process at the Tokaimura plant.
The uranium enrichment process is done by previously converting the uranium into a compound, uranium hexafluoride, which is gaseous under normal conditions. The next step is the conversion of enriched uranium in the form of uranium hexafluoride to uranium oxide, which is achieved in a tank with an aqueous solution of uranyl nitrate.
The compound is converted by precipitation and sedimentation, and later by calcination, into ceramic fuel pellets, which will constitute the fuel elements of some nuclear reactors.
According to the established internal operating procedure, the uranium oxide solution (U3O8) should be in a tank prepared for this purpose, then transferred to a solution of pure uranyl nitrate and homogenized with a nitrogen gas purge.
Subsequently, the mixture was poured into the precipitation tank cooled by water to evacuate the residual heat generated by the exothermic reaction that occurs.
To prevent the occurrence of a criticality (a self-sustaining fission chain reaction), the procedure established limits for the amount of uranium to be transferred to the precipitation tank, a maximum amount of 2.4 Kilograms of uranium.
The work procedure was modified in November 1996, without permission from the competent regulatory authorities, allowing the treatment of the dissolution of uranium oxide in stainless steel buckets, which did not comply with the appropriate measures. This new method of work had been carried out several times before the accident occurred.
Thus, when preparing the JOYO reactor fuel in September 1999, the workers dissolved the U3O8 powder in nitric acid in the stainless steel buckets and poured the solution directly into the precipitation tank.
The solution used of 16 liters of uranium oxide, enriched to 18.8% of uranium-235, was distributed in four stainless steel buckets to be poured into the precipitation tank.
On the morning of September 30, when the volume reached 40 liters, equivalent to 16 kilograms of uranium, much higher than the initially limited amount, the critical mass needed to initiate a nuclear fission reaction in auto chain was reached. maintained, accompanied by the emission of neutrons and gamma radiation.
Development of the nuclear accident
The worker, who added the seventh bucket of uranium nitrate to the sump, saw a blue flash of Cherenkov radiation. He and another worker who was near the sink immediately experienced pain, nausea, shortness of breath and other symptoms; A few minutes later, already in the decontamination room, he vomited and lost consciousness.
There was no explosion, but the result of the nuclear reaction was an intense gamma, and neutron radiation from the sedimentation tank, which triggered the alarm, and then the actions to locate the nuclear accident began. In particular, 161 people were evacuated from 39 residential buildings within a 350-meter radius of the company (they were allowed to return to their homes after two days). 11 hours after the start of the nuclear accident, a gamma radiation level of 0.5 millisievert per hour was recorded in one of the sites outside the nuclear plant.
The nuclear chain fission reaction continued intermittently for approximately 20 hours, after which it stopped due to the fact that water was added to the cooling jacket surrounding the settling tank. Water played the role of a neutron reflector, and boric acid was added to the settler (boron is a good neutron absorber); This operation involved 27 workers, who also received a certain dose of radiation. The ruptures in the nuclear chain reaction were caused by the fact that the liquid boiled, the amount of water became insufficient to reach criticality and the chain reaction was attenuated. After cooling and condensing the water, the reaction was resumed.
The neutron radiation stopped along with the chain reaction, but for some time the dangerous level of residual gamma radiation from the fission products remained in the sink. For this reason, it was necessary to install temporary protection against bags of sand and other materials. Most volatile radioactive nuclear fission products remained inside the building due to the fact that they maintained a lower pressure than outside and were then collected using high efficiency air filters. However, some of the noble radioactive gases and iodine 131 entered the atmosphere.
Consequences of the accident
The accident directly affected the three workers who prepared the sample, who had to be hospitalized, two of them in critical conditions, and who died one at 12 weeks and another, after 7 months.
In addition, 56 more workers in the plant were exposed to radiation, of which at least 21 people received important doses and had to be under medical evaluation.
In a radius of 200 meters around the facility, access was restricted, and additionally, the Japanese authorities established evacuation measures for 161 people, from the areas located at a distance of 350 meters from the plant.
As a preventive measure, the 310,000 people who lived 10 km away were warned not to leave their homes, until the situation was under control, lasting their confinement for 18 hours.
Once the criticality ended, adding boric acid to the solution of the precipitation tank, and thanks to the containment systems of the site, always in depression with respect to the outside, the levels of radiation in the exteriors returned to normal.
According to the IAEA, the radiation levels of the areas near the plant, in mid-October 1999, had recovered the natural background levels. The measurement of iodine-131 in soils and vegetation outside the facility determined that the food had not been affected.
The accident was classified as level 4 according to the INES Scale ("accident without significant risk outside the site"), since the amounts of radiation released to the outside were very small, and within the established limits, but within the site, the damages produced in the equipment and biological barriers were significant, in addition to the fatal exposure of workers.
From the accident, to which all the indications point as a human failure, the fuel manufacturing plants in Japan were completely automated, to ensure that a criticality accident did not occur again, equipping the systems with neutron control equipment, and using dry conversion methods, intrinsically safer.
Last review: December 13, 2018