The 1999 Tokaimura Incident: Lessons Learned and Safety Measures for the Future

On September 30, 1999, three Japanese workers at a small nuclear fuel preparation plant received high doses of radiation, with two of them dying from full-body radiation doses. Due to human error and serious breaches of safety principles, over 100 people received individual doses of over 1 mSv.

This incident became one of the “most thought-provoking experiences in a nuclear energy related facility”, teaching lessons on management systems and emergency preparedness and response.

JCO Plant at Tokai

Image from Wikimedia Commons

Japan Nuclear Fuel Conversion Co. (JCO), a subsidiary of Sumitomo Metal Mining Co., operated this particular fuel preparation plant at Tokai. It was commissioned to process up to 3 tonnes of uranium per year for various specialised research and experimental reactors. 

The plant processed enriched uranium hexafluoride (UF6) gas through various stages to convert it into uranium dioxide (UO2) powder. This powder was then shaped into pellets and used as fuel in nuclear reactors. 

The Criticality Accident

Prior to the incident, the JCO plant made a number of changes to their initial process. 

The plant was operating in batch mode, with the maximum batch mass limit of 2.4 kg U for enrichment of 16 wt.% to 20 wt.%. However, JCO processed more than six times as much—more than 15 kg U.

In 1995, four years before the accident, they changed procedure again without the approval of The Science and Technology Agency (STA). Instead of “cross-blending” with product delivery bottles, a whole delivery lot was homogenised in a pure UNH storage tank at one time. 

On the day before the accident, they made another modification. Instead of the pure UNH storage tank, they used a precipitation tank which had “unsafe configuration from a view point of prevention of criticality.”

Image from Clés CEA

Around 10AM of September 30, JCO facility technicians Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa were trying to speed up the last few steps of the fuel/conversion process and meet shipping requirements. One of the three workers was pouring the solution to the precipitation tank when they felt a strong shock and heard a strange sound. An unconfirmed report states that there was a blue light flashing. They immediately went outside of the room. A small quantity of the solution was left in a stainless steel beaker. 

The gamma ray monitor installed at the ceiling of the room triggered an alarm for the very high dose detected.


Although the workers managed to escape to the decontamination room, they were still severely exposed. Ouchi and Shinohara suffered from pain, nausea and difficulty in breathing and passed away months later. Yokokawa, the supervisor, had minor radiation sickness. 

Residents within about 350m of the facility were evacuated as requested by the mayor of Tokaimura, while those within a 10 km radius were advised to stay indoors.

Image from IAEA

The accident was classified as Level 4 on the International Atomic Energy Agency International Nuclear Event Scale, as it was not a contamination accident and did not cause significant offsite risk. 

However, rumours caused social and economical impacts—returned goods, boycotts of products, cancellation of hotel reservations, and more. 

Following the criticality accident, safety protocols at nuclear facilities in Japan were reevaluated. Operational staff were well trained especially in recognising and responding to facility and equipment errors. Stricter regulations and safety standards were implemented to prevent similar incidents in the future. Radiation monitoring was strengthened, even in facilities where low exposure is expected.

Real Time Radiation Monitoring in Nuclear Power Plants

Real-time monitoring can help identify potential issues at a nuclear power plant and allow for preventive repairs and maintenance. In the event of a radiation release or other emergency, radiation monitoring can help emergency responders quickly assess the situation and take appropriate action to protect public health and safety.

If you’re looking for area radiation monitors or personal radiation monitoring devices, you can count on SensaWeb. With our monitors, you can easily detect and interdict radioactive materials. 

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