How Hisashi Ouchi became the world’s most radioactive man (2024)

• In 1999, human error derived from a lack of training and supervision caused a criticality accident at theJapan Nuclear Fuel Conversion Company’s nuclear plant in Tōkai, Japan.
• Technician Hisashi Ouchi became “the world’s most radioactive man” and died after receiving 17 sieverts of radiation, much more than a human body can take.
• The accident led to the company’s credentials for the operation being canceled and the allocation of million-dollar sums in compensation to other radiation victims.

The 1999 Tokaimura nuclear accident was the worst nuclear accident in Japan after the natural disaster-induced f*ckushima nuclear accident of 2011.

On the morning of September 30, 1999, human error resulted in a criticality accident at the Japan Nuclear Fuel Conversion Co. (JCO) uranium processing plant near Tokai, 70 miles (~112 kilometers) northwest of Tokyo.

Due to improper handling of uranium, an uncontrolled nuclear fission chain reaction exposed two technicians and their supervisor to high levels of radiation. It prompted an evacuation of the plant and the surrounding areas.

One of the technicians, Hisashi Ouchi, would later become known as “the world’s most radioactive man,” having absorbed 17 sieverts of radiation. To put this into perspective, the dose of radiation that is estimated to be lethal for 50 percent of the population within 30 days is generally around 4 to 5 sieverts.

Ouchi received a dose more than three times this lethal threshold, and he suffered from the devastating health effects of radiation poisoning for 83 days before dying.

Here’s the full story.

1999 Tokaimura nuclear accident

The village of Tokai, in Ibaraki, has been the center of Japan’s nuclear industry since the Japan Atomic Energy Research Institute and several nuclear installations, such as the Japan Atomic Energy Agency, were established nearby.

In fact, the 1999 Tokaimura nuclear accident was not the first nuclear accident that took place in Tokai. Two years earlier, on March 11, 1997, there was an explosion at the Power Reactor and Nuclear Fuel Development Corporation’s (PNC) radioactive waste bituminization facility after solidified low-level liquid waste caught fire, exposing workers and Tokai residents to ionizing radiation.

Although the event was severe enough to result in the plant’s closure, no director casualties were associated with it.

However, the 1999 Tokaimura nuclear accident was different.

At the time, the JCO plant’s main activity was turning uranium hexafluoride into enriched uranium dioxide fuel for research and experimental reactors.

To do this, it usually employed a “wet process” consisting of dissolving uranium oxide power with nitric acid to create pure uranyl nitrate; the uranyl nitrate was then moved to a buffer tank designed to prevent fission, and then to a precipitation tank, in 5.3 pounds (2.4 kilograms) increments.

The solution was mixed with ammonia in the precipitation tank to form a solid product and eliminate residual nuclear waste contaminants. This tank was equipped with water cooling jackets to dissipate excess heat generated during the exothermic chemical reaction, hence the term “wet process.”

However, after the accident, it was revealed that the JCO workers were assigned to mix the uranium oxide in stainless steel buckets and then pour the solution directly into the precipitation tank, bypassing criticality controls.

While the workers had previously performed this task with lower levels of enriched uranium (less than 5 percent), they were not adequately trained to handle higher levels of enrichment (more than 18 percent), and they inadvertently put too much uranium in the precipitation tank (35 pounds, or 16 kilograms).

This was seven times the mass limit the Science and Technology Agency (STA) specified as safe.

The tank quickly reached critical mass, which is the smallest amount of fissile material required for a self-sustained nuclear chain reaction. As a result, the material began releasing gamma and neutron radiation. There was no explosion, but the workers saw a blue flash, and gamma radiation alarms were set off.

Because they were using a wet process, the water in the solution provided neutron moderation, speeding the reaction. (Most fuel preparation plants use dry processes.)

Hisashi Ouchi (35) had his body draped over the tank at the time, which is why he received such a high dose of radiation. Masato Shinohara (39), a technician assisting Ouchi, received 10 sieverts. Supervisor Yutaka Yokokawa (54), sitting at a desk 13 feet (4 meters) away from them, received 3 sieverts.

Another 24 JCO workers received around 48 millisieverts (0.048 sieverts) of radiation.

Radiation effects on the human body

Acute Radiation Syndrome (ARS) is a serious medical condition caused by exposure to a high dose of radiation in a short period. The severity and specific symptoms vary depending on the dose, exposure duration, and radiation type.

Generally, people exposed to radiation may experience skin reddening or radiation burns, headache, nausea, vomiting, diarrhea, fatigue, and fever about 2 hours after exposure. But radiation damages living tissues at a cellular level, and further developments include harm to the bone marrow, including depletion of white blood cells (leukopenia) and red blood cells (anemia).

Gastrointestinal symptoms, such as bleeding, dehydration, and electrolyte imbalances, may manifest due to damage to the cells lining the gastrointestinal tract. If the radiation damages the neural tissues, the central nervous system can also be affected, resulting in confusion, seizures, and loss of consciousness.

Acute Radiation Syndrome (ARS) doesn’t have a cure, and treatment, at that time, was based on supportive care. There are now a few treatment options for internal decontamination, but untreatable ARS occurs at a dose of eight to 10 sieverts.

Having received a dose of 17 sieverts, Hisashi Ouchi fainted and then started vomiting a few moments after the accident. By the time he was taken to the National Institute of Radiological Sciences in Chiba, he was suffering from severe radiation burns, diarrhea, and dehydration. He had a white cell count close to zero, which meant he effectively no longer had an immune system.

He was transferred to a radiation ward at the University of Tokyo Hospital, where he received a blood stem cell transplantation, with his sister as a donor. He was also treated with blood transfusions, broad-spectrum antibiotics, painkillers, and granulocyte colony-stimulating factor (a glycoprotein that encourages the bone marrow to produce stem cells).

However, doctors had seen that chromosomes in his bone marrow were shattered into pieces. None of this would work.

Hisashi’s skin started to fall off, and he required multiple cultured skin grafts as it wouldn’t regenerate on its own. He couldn’t eat, so he was fed with an IV. Soon, he began to develop breathing problems and had several cardiac arrests, up to three in one hour. He had not signed a Do Not Attempt Resuscitationorder, and at first, his family wanted doctors to continue reviving him. Still, they changed their minds after one cardiac arrest left him unresponsive and with possible brain damage.

Hisashi Ouchi finally died on December 21, 1999, from a last cardiac arrest triggered by multiple organ failure.

Masato Shinohara, Ouchi’s coworker, survived until April 2000. He had been treated with skin grafts and blood stem cell transfusions from the umbilical cord of a newborn. He also went through cancer treatment.

However, radiation had not only damaged his bone marrow but also his lungs and liver, and like Ouchi, he died from multiple organ failure seven months after the accident.

Their supervisor, Yukata Yokokawa, suffered from minor radiation sickness and was discharged from the hospital three months after the accident.

The aftermath

About 161 people from 39 families in a radius of 0.2 miles (350 meters) from the nuclear facility were evacuated in the five hours after the accident. Another 300,000 people in Tokai were told to stop spending time outdoors and refrain from harvesting crops or drinking well water.

Radiation tests and medical checkups were performed on everyone in a radius of 6 miles (10 kilometers) from the nuclear facility, and found that at least 667 people had been exposed to radiation. Some had to be hospitalized.

Radiation tests on Tokai’s water, sea, and dairy products all came out negative. A few samples of vegetation contained low levels of radiation.

The nuclear chain reaction lasted 20 hours in total.

To interrupt it, emergency service workers had to drain water from the cooling jacket that surrounded the precipitation tank. This is because water acted as a neutron reflector, reflecting some of the neutrons into the reaction, making it more challenging to halt.

Simultaneously, a boric acid solution was added to the precipitation tank. Boron can absorb neutrons and reduce their availability for sustaining the nuclear chain reaction.

The combined effect of these actions brought the contents of the precipitation tank to sub-critical levels.

Investigation into the accident revealed that workers at the JCO plant had followed operating manual guidance not approved by the Science and Technology Agency (STA). Technicians and engineers lacked qualification, communication, and supervision. The International Atomic Energy Agency (IAEA) concluded that the accident had been caused by “human error and serious breaches of safety principles.”

As a result, the STA removed JCO’s authorization to operate in the nuclear industry. The company also had to pay $121 million in compensation to 6,875 claims from people affected by the radiation.

Six JCO officials faced charges of negligence resulting in death. Ouchi and Shinohara’s supervisor, Yokokawa, was one of them. They all pleaded guilty in April 2001.

The 1999 Tokaimura nuclear accident was classified as Level 4 on the International Atomic Energy Agency’s (IAEA) International Nuclear Event Scale (INES), meaning that consequences were localized and had no significant impact beyond the immediate vicinity.

However, it led to JCO becoming the first Japanese plant operator to face legal consequences for mishandling nuclear radiation. It encouraged the introduction of new legislation regarding operational safety and inspection requirements in Japan’s nuclear industry.