Two people died, one was seriously injured, over 600 were exposed to radiation, and 320,000 people were evacuated. Behind each of these “dry” figures lies a human destiny, turned upside down after an accident at an experimental nuclear fuel reprocessing plant operated by JCO in Tokai Mura, Ibaraki Prefecture, Japan. The incident occurred on September 30, 1999. It was classified as level 4 on the International Nuclear Event Scale. It is considered the most serious nuclear accident in Japan before Fukushima (2011).
One of the victims, Hisashi Ouchi, 35, was exposed to a dose of radiation considered “super-fatal” – 16-17 Sv. He suffered severe burns and almost complete destruction of his bone marrow. Despite this, doctors fought for his life for 83 days. His organs began to fail. He underwent several stem cell transplants. He was kept alive artificially with equipment. He died three days before Christmas Eve. His case became a symbol of the suffering caused by radiation exposure.
His colleague Masato Shinohara, 39, received half the radiation dose. Nevertheless, he suffered massive damage to his bone marrow and immune system. He was treated for months, including with transplants and sterile conditions. He died on April 27, 2000, after more than seven months of struggle.
Causes of the accident
When preparing fuel for the reactor, instead of using the prescribed automated procedure, technicians manually poured uranium solution into a container that was not designed for such quantities. The critical mass was exceeded and an uncontrollable chain reaction occurred.
Consequences
The incident caused a huge public scandal. The Japanese government tightened control over the nuclear industry. JCO lost its license, and managers and employees were brought to trial for negligence.
Chronology of events
Around 10:30 a.m.
Three technicians from JCO prepare a uranium solution for an experimental reactor. Instead of using the prescribed automatic system, they pour the uranium solution by hand into a container (settling tank) that has no protection against criticality. The tank contains about 16.6 kg of uranium (20% enrichment) – significantly above the safety limit.
11:35 a.m.
Critical mass is reached in the tank. A continuous chain reaction occurs – strong neutron and gamma radiation begins to be emitted. The three technicians are exposed to enormous doses:
Hisashi Ohuchi – about 17 Sv (fatal dose).
Masato Shinohara – about 10 Sv.
Yutaka Yokokawa – about 3 Sv.
At noon
The staff does not immediately realize the scale of the incident. High radiation levels are measured in the plant. About 150 staff members are evacuated.
2:30–3:00 p.m.
5:00 p.m.
The National Agency for Nuclear and Industrial Safety confirms that there is a “serious nuclear criticality.” Teams are mobilized in the area to measure and contain radiation.
By 9:30 p.m.
The reaction continues for almost 20 hours with interruptions. Only then do the operators manage to stop the criticality by draining the water serving as a moderator and adding boric acid (which absorbs neutrons).
After the accident
The two most heavily irradiated workers are taken to a hospital in Tokyo.
In the following days, the authorities begin mass testing of the population – over 600 people are registered as irradiated.
The incident went down in history as the most serious nuclear accident in Japan before Fukushima (2011).
What the technicians did
Bypassing the procedure
The three technicians decided to speed up the process because the task was considered “routine” and the slow automatic system was perceived as inconvenient.
They used metal buckets (like ordinary containers) to pour the uranium solution.
Pouring into the wrong container
Instead of the special device, they poured the solution into a settling tank, which is round and contains water (acting as a neutron moderator).
This was a critical mistake—the geometry of the tank allowed critical mass to accumulate.
Exceeding the limit
According to the instructions, they were supposed to use up to 2.4 kg of uranium per stage.
In reality, they poured a total of 16.6 kg of uranium, more than six times the permissible amount.
Why did they break the rules?
Habit: it turns out that this bucket method had been used unofficially at JCO for years to “save time.”
Insufficient control: neither management nor regulatory authorities actually monitored compliance with procedures.
Underestimation of risk: technicians and supervisors believed that uranium in this form “could not become critical.”
This combination of arbitrary changes, systematic practice, and lack of oversight led to the first and most serious accident of its kind in the Japanese nuclear industry.
