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Tokaimura Criticality Accident: What Happened To One Of The Most Irradiated Humans In History?


Rachael Funnell


Rachael Funnell

Writer & Senior Digital Producer

Rachael is a writer and digital content producer at IFLScience with a Zoology degree from the University of Southampton, UK, and a nose for novelty animal stories.

Writer & Senior Digital Producer

hisashi ouchi nuclear accident

Tokai Nuclear Plant, Japan's first nuclear power station. Image credit: ENERGY.GOV - HD.15.058, Public Domain

A terrible incident unfolded in Tokaimura, Japan, on 30 September 1999 when an uncontrolled chain reaction involving radioactive material was triggered. This would become the country’s worst nuclear accident. Over the next 20 hours, 49 people within the plant were exposed to dangerous levels of radiation – though it’s expected the total reach may have exceeded this.

Among them was Hisashi Ouchi, then aged 35, who was exposed to 17 sieverts (Sv) of radiation – 10 Sv more than the lethal dose, which is considered to be around seven. Attempts were made to save the technician’s life, but the devastating effect the radiation had on his body eventually killed him 83 days later.


The Tokaimura Criticality Accident

The accident occurred in a small fuel preparation plant in the Ibaraki Prefecture which supplied specialized research and experimental reactors, operated by JCO (formerly Japan Nuclear Fuel Conversion Co.), reports the World Nuclear Association. Those involved were preparing fuel for a reactor by mixing 2.4 kilograms (5.3 pounds) of enriched uranium with nitric acid.

The dangerous procedure was supposed to take place in a dissolution tank, directed by the approved nuclear fuel preparation procedure – but the workers had been instructed to follow a different procedure that hadn’t been approved. This saw them manually mixing 16 kilograms (35.3 pounds) of the fissile material in a stainless steel bucket.

“The uranium reached a critical mass at 10:35 am and set off an uncontrolled chain reaction that emitted radiation for almost 20 hours,” the BMJ reported.

“The three workers who carried out the operation reported seeing a blue flash – the Cerenkov radiation that is emitted during a critical reaction – before collapsing with nausea. They were rescued by colleagues and taken to a local hospital by emergency services.”

What happened to Hisashi Ouchi

Following the deadly exposure, Ouchi was taken to the National Institute of Radiological Sciences in Chiba. He had severe radiation burns to most of his body, as well as significant injury to his internal organs. Ouchi, alongside the two other workers, experienced profuse sweating and vomiting which put them at risk of dehydration.

Blood analysis also revealed the radiation had caused Ouchi’s lymphocyte count to plummet to almost zero. When lymphocytes – a type of white blood cell, involved in the immune system – get too low it’s termed lymphopenia (or lymphocytopenia), leaving the body vulnerable to infections. It can be brought on by blood disorders like Hodgkin’s lymphoma and leukemia, as well as AIDS – and, evidently, exposure to radiation.

In a bid to save Ouchi’s life, doctors at the University of Tokyo Hospital tried carrying out a peripheral stem cell transplant from his brother. While the cells engrafted and began making blood cells, he continued to have issues with bone marrow suppression, requiring rigorous infection control.

Skin loss and gastrointestinal injuries meant he was losing a lot of blood and bodily fluids and he developed respiratory failure due to fluid on the lungs. He suffered a cardiac arrest on day 58 due to hypoxia, but was revived.


His condition continued to worsen as his kidneys and liver shut down, he continued to battle with respiratory failure and developed hemophagocytic syndrome, a life-threatening condition characterized by an overactive and abnormal immune response.

After 83 excruciating days, Ouchi finally succumbed to his injuries suffering a fatal cardiac arrest due to multiple organ failure. His colleague, Masato Shinohara, survived for seven months with the aid of skin grafts, cancer treatments, and an umbilical cord blood stem cell transfusion, but eventually died after 211 days, also due to multiple organ failure.

Lessons Learned From The Tokaimura Criticality Accident

The fate of these men was a rare demonstration of the devastating effects of acute radiation sickness involving exposure to neutrons which were detailed in a report titled “Lessons Learned From The JCO Nuclear Criticality Accident In Japan In 1999”.

The failings of JCO regarding the serious breaches of safety principles led to criminal charges, and the plant’s operating license was eventually revoked by the year 2000.


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