Companion piece · 2026-10-14
At 10:35 a.m. on September 30, 1999, two workers at a nuclear fuel-processing plant in Tokaimura, Japan, were transferring uranium solution into a steel tank.
A radiation alarm sounded. The workers left the room. One soon collapsed.
The contents of the tank had begun sustaining a nuclear chain reaction.
There was no operating nuclear reactor at the centre of this accident. The equipment was part of a chemical processing line, and the operation involved stainless-steel buckets, a funnel, and a tank intended for another purpose.
The reaction continued for approximately twenty hours. Three workers received severe radiation exposures. Two eventually died.
THE FACILITY AND ITS ASSIGNMENT
Tokaimura, in Ibaraki Prefecture, was an established centre of Japan's nuclear industry, with research, fuel-processing, and power facilities.
The accident occurred at JCO, a subsidiary of Sumitomo Metal Mining. Established in 1979 and renamed JCO in 1998, the company converted uranium into forms suitable for nuclear-fuel manufacture.
The particular order involved uranium enriched to 18.8 percent uranium-235, intended for the experimental fast reactor Jōyō.
The plant's approved procedures imposed a limit of 2.4 kilograms of uranium per processing batch. Equipment dimensions and batch limits were intended to prevent an accidental chain reaction, a hazard known as criticality.
These safeguards mattered even though the facility was not operating a nuclear reactor.
PROCEDURES THAT CHANGED
Producing a uniform shipment required mixing material from multiple batches.
Beginning in 1986, JCO used a complicated blending procedure that was not part of its approved process. In 1995, the company adopted a simpler approach, combining solution in a narrow storage tank. That change also lacked regulatory approval.
On September 29, 1999, workers made another adjustment. They began using a wider precipitation tank to combine the batches, transferring solution by hand.
The precipitation tank was unsuitable for holding this quantity of enriched uranium. Unlike the narrow storage vessel, its geometry did not provide an equivalent safeguard against criticality.
Investigators later found that the changes had developed through routine production decisions, inadequate training, and insufficient oversight. The final procedure was not an isolated improvisation.
SEPTEMBER 30: THE ACCIDENT
Three JCO employees were assigned to the operation: Hisashi Ouchi, 35; Masato Shinohara, 40; and their supervisor, Yutaka Yokokawa.
Ouchi and Shinohara worked beside the precipitation tank. Yokokawa was in an adjoining area.
Four batches had been transferred on September 29. The men resumed the remaining work the following morning.
During transfer of the seventh batch, at 10:35 a.m., the contents of the tank reached criticality.
The total uranium mass had risen to approximately 16.6 kilograms, almost seven times the approved batch limit.
An initial burst of neutron and gamma radiation struck the workers. Accounts described an unusual flash of light, although the precise observation was not consistently documented.
The men evacuated. Ouchi began vomiting and lost consciousness. Medical personnel were summoned.
The reaction did not stop when the workers left.
Following the initial burst, fission continued at a lower, fluctuating rate. Water surrounding the precipitation tank helped reflect neutrons back into the solution, sustaining the reaction.
THE EMERGENCY RESPONSE
JCO reported the accident to national and local authorities more than forty minutes after the initial alarm. Early information was incomplete, and emergency personnel were not immediately given a clear account of the radiation hazard.
Local officials eventually ordered 161 people living within approximately 350 metres to evacuate. Around 310,000 people within ten kilometres were advised to remain indoors.
By evening, specialists had established that criticality was continuing.
Ending it required changing the conditions inside the tank while limiting additional radiation exposure to response personnel.
During the early hours of October 1, teams of JCO workers approached external piping in carefully timed operations. They drained water from the cooling jacket, removing an important neutron reflector.
At 6:14 a.m., neutron readings fell sharply. A boric-acid solution was subsequently introduced to prevent renewed criticality.
The chain reaction was over.
THREE DIFFERENT MEDICAL OUTCOMES
The three men had received very different radiation doses. Later medical estimates placed Ouchi's exposure at approximately 16–25 gray-equivalent, Shinohara's at 6–9, and Yokokawa's at 2–3.
They were transferred for specialist treatment. Both Ouchi and Shinohara suffered severe damage to blood-forming tissues and other organs. Their care included intensive supportive treatment and stem-cell transplantation.
Ouchi died on December 21, 1999, 83 days after the accident.
Shinohara died on April 27, 2000.
Yokokawa survived and was discharged from hospital.
Dose assessments were also conducted for hundreds of workers, responders, and nearby residents. Investigators concluded that, apart from the three primary victims, radiation doses were generally too low to produce immediate radiation illness.
The principal public exposure resulted from radiation emitted during the continuing reaction, rather than extensive environmental contamination.
ACCOUNTABILITY AND CHANGES
JCO's operating authorization was revoked. In March 2003, the Mito District Court convicted the company and six employees or managers in connection with the accident.
The sentences imposed on the individuals were suspended. JCO was fined one million yen. Former plant manager Kenzo Koshijima received a suspended three-year custodial sentence and an additional fine.
Japanese authorities also strengthened inspection requirements, operating procedures, and emergency coordination. A new law on nuclear emergency preparedness was enacted in December 1999.
The official investigation identified a failure extending beyond the workers at the tank: unauthorized process changes, poor criticality awareness, and inadequate management and regulatory oversight.
The uranium was being processed for an authorized purpose.
The method used to process it was not authorized.
The criticality accident demonstrated that nuclear safety requirements applied to chemical processing equipment just as they did to reactors.
The equipment had been modified in practice. The physical conditions required for a self-sustaining chain reaction had not changed.
This has been Nuclear Shadows. I'm Morgan Miller.
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