In 1999 the Japanese nuclear industry suffered a significant nuclear accident in Tokai-mura in Ibaraki, Japan. It happened in a nuclear fuel processing facility.
To understand Ibaraki's criticality accident, we first briefly explain the uranium enrichment process at the Tokaimura plant.
What Is the Uranium Enrichment Process?
The uranium enrichment process is carried out by previously converting uranium into a compound called uranium hexafluoride. Uranium hexafluoride is gaseous under normal conditions.
The next step is converting the enriched uranium in the form of uranium hexafluoride into uranium oxide, which is achieved in a tank with an aqueous solution of uranyl nitrate.
The compound is converted into ceramic fuel pellets. These pellets are the nuclear fuel used in a nuclear power plant.
The uranium oxide solution (U3O8) had to be in a tank arranged for this purpose. Afterward, it had to be transferred to a pure uranyl nitrate solution and homogenized with a nitrogen gas purge.
Subsequently, the mixture had to be poured into the water-cooled precipitation tank to evacuate the residual heat generated by the exothermic reaction.
The procedure had limits for the amount of uranium that had to be transferred to the precipitation tank to prevent a self-sustaining nuclear chain reaction. The maximum must be 2.4 kg of uranium.
Ibaraki Nuclear Disaster Causes
The procedure was modified in November 1996 without permission from the competent regulatory authorities.
Thus, when preparing the JOYO power reactor fuel in September 1999, the workers dissolved the U 3 O 8 powder in nitric acid in the stainless steel buckets and poured the solution directly into the precipitation tank.
The used solution of 16 liters of uranium oxide highly enriched was distributed into four stainless steel buckets to be poured into the tank.
On the morning of September 30, when the volume reached 40 liters and reached the critical mass necessary for initiating a nuclear fission chain reaction, it started emitting neutrons and gamma radiation.
Immediate Effects of Ibaraki Nuclear Accident
The worker who added the seventh cube of uranium nitrate to the sink - Hisashi Ouchi-, saw a blue flash of Cherenkov radiation. He and another worker near the sink immediately experienced pain, nausea, shortness of breath, and other symptoms.
A few minutes after the accident occurred, he vomited and lost consciousness.
There was no explosion, but the result of the nuclear reaction was intense gamma and neutron radiation from the sedimentation tank, which triggered the alarm. Then actions began to locate the nuclear accident.
Eleven hours after the start of the nuclear accident, the gamma radiation level was about 0.5 millisievert per hour at one of the sites outside the nuclear plant.
The nuclear fission chain reaction continued for about 20 hours. After this time, the reaction stopped because workers added cooling water surrounding the tank.
The cooling water played a neutron reflector, and boric acid was added to the settler (boron is a good neutron absorber).
The breaks in the nuclear energy chain reaction were caused by the fact that the liquid boiled, the amount of water became insufficient to reach criticality, and the chain reaction went down. After cooling and condensing the water, the reaction was resumed.
The neutron radiation ceased, but the dangerous level of residual gamma radiation from fission products remained in the sink for some time.
Most of the volatile radioactive nuclear fission products remained inside the building. However, some of the radioactive noble gases and iodine 131 entered the atmosphere.
Consequences of the Tokaimura's Accident
The Tokaimura nuclear accident directly affected the three workers preparing the sample, who had to be hospitalized. the three men were Yutaka Yokokawa, Masato Shinohara and Hisashi Ouchi.
Two of them were in critical condition. Hisashi Ouchi died at 12 weeks and the other after seven months. It is estimated that one of the dead workers received radiation between 1 and 20 sieverts.
Fifty-six more workers at the plant received radiation—at least 21 people who received significant doses and had to be under medical evaluation.
In a radius of 200 meters around the facility, access was restricted.
Japanese authorities evacuated 161 people from areas 350 meters from the plant.
Three hundred ten thousand people who lived 10 km away were warned not to leave their homes until the situation was under control, their confinement lasting 18 hours.
Once the chain reaction ended, the radiation levels outside returned to normal.
According to the International Atomic Energy Agency IAEA, the radiation levels near the plant, in mid-October 1999, had recovered their natural levels. Levels of iodine-131 in soils and vegetation outside the facility showed that the food had not been affected.
The accident reached level 4 in the INES Scale (“accident without significant risk outside the site”).
Since the accident, which all indications point to as a human failure, Japan's atomic energy authorities decided that the fuel processing plants in Japan must be fully automated.