Tokaimura nuclear accidents
The Tokaimura nuclear accidents refer to two nuclear related incidents near the village of Tōkai, Ibaraki Prefecture, Japan. The first accident occurred on 11 March 1997, producing an explosion after an experimental batch of solidified nuclear waste caught fire at the Power Reactor and Nuclear Fuel Development Corporation radioactive waste bituminisation facility. Over twenty people were exposed to radiation.
The second was a criticality accident at a separate fuel reprocessing facility belonging to Japan Nuclear Fuel Conversion Co. on 30 September 1999 due to improper handling of liquid uranium fuel for an experimental reactor. The incident spanned approximately 20 hours and resulted in the deaths of two workers; of 436 people whose doses of radiation exposure were evaluated, none of them exceeded annual regulatory dosage limits.
It was determined that the accidents were due to inadequate regulatory oversight, lack of appropriate safety culture and inadequate worker training and qualification. After these two accidents, a series of lawsuits were filed and new safety measures were put into effect.
By March 2000, Japan's atomic and nuclear commissions began regular investigations of facilities, expansive education regarding proper procedures and safety culture regarding handling nuclear chemicals and waste. JCO's credentials were removed, the first Japanese plant operator to be punished by law for mishandling nuclear radiation. This was followed by the company president's resignation and six officials being charged with professional negligence.
Background
was an important energy alternative for natural-resource-poor Japan to limit dependence on imported energy, providing about 30% of Japan's electricity up until the Fukushima nuclear disaster of 2011, after which nuclear electricity production fell into sharp decline.Tōkai's location and available land space made it ideal for nuclear power production, so a series of experimental nuclear reactors and then the Tōkai Nuclear Power Plant – the country's first commercial nuclear power station – were built there. Over time, dozens of companies and government institutes were established nearby to provide nuclear research, experimentation, manufacturing, and fuel fabrication, enrichment and disposal facilities. Nearly one-third of Tōkai's population relied upon nuclear industry-related employment.
1997 nuclear waste accident
On 11 March 1997, Tōkai's first serious nuclear incident occurred at PNC's bituminization facility. It is sometimes called the Dōnen accident, 'Dōnen' being an abbreviation of PNC's full Japanese name Dōryokuro Kakunenryō Kaihatsu Jigyōdan. The site encased and solidified low-level liquid waste in molten asphalt for storage, and that day was trialing a new asphalt-waste mix, using 20% less asphalt than normal. A gradual chemical reaction inside one fresh barrel ignited the already-hot contents at 10:00 a.m. and quickly spread to several others nearby. Workers failed to properly extinguish the fire, and smoke and radiation alarms forced all personnel to evacuate the building. At 8 p.m., just as people were preparing to reenter the building, built up flammable gases ignited and exploded, breaking windows and doors, which allowed smoke and radiation to escape into the surrounding area.The incident exposed 37 nearby personnel to trace amounts of radiation in what the government's Science and Technology Agency declared the country's worst-yet nuclear accident, which was rated a 3 on the International Nuclear Event Scale. A week after the event, meteorological officials detected unusually high levels of caesium 40 km southwest of the plant. Aerial views over the nuclear processing plant building showed a damaged roof from the fire and explosion allowing continued external radiation exposure.
PNC management mandated two workers to falsely report the chronological events leading to the facility evacuation in order to cover-up lack of proper supervision. Dōnen leadership failed to immediately report the fire to the Science and Technology Agency. This delay was due to their own internal investigation of the fire causing hampered immediate emergency response teams and prolonged radioactivity exposure. Dōnen facility officials initially reported a 20% increase of radiation levels in the area surrounding the reprocessing plant, but later revealed the true percent was ten times higher than initially published. Tōkai residents demanded criminal prosecution of PNC officials, reorganization of company leadership and closure of the plant itself. Following public outcry, the facility closed until reopening in November 2000 when it was reinstated as a nuclear fuel reprocessing plant.
Later, Prime Minister Ryutaro Hashimoto criticized the delay that allowed radiation to continue to impact local areas.
1999 accident
The second, more serious Tōkai nuclear accident occurred about four miles away from the PNC facility on 30 September 1999, at a fuel enrichment plant operated by JCO, a subsidiary of Sumitomo Metal Mining Company. It was the worst civilian nuclear radiation accident in Japan prior to Fukushima. The incident exposed the surrounding population to hazardous radiation after the uranium mixture reached criticality. Two of the three technicians mixing fuel were killed. The incident was caused by lack of regulatory supervision, inadequate safety culture and improper technician training and education.The first issue contributing to the accident was the lack of regulatory oversight. There was no criticality accident alarm and the site was not included in the National Plan for the Prevention of Nuclear Disasters. This led to delays in informing the nearby public of the accident. In addition, the regulator did not conduct routine inspections to determine that the production process was not being carried out correctly.
The second contributing issue was the use of an unapproved procedure. The accident occurred while manufacturing fuel at a different enrichment which was made infrequently. Because of a combination of reduced revenue, layoffs and inexperience, the company felt pressured to make this fuel despite not having the procedure approved. They stated that they did not submit a procedure for making this type of fuel to the regulator because they knew it would not get approved, and they would not be able to make it at all.
The JCO facility converted uranium hexafluoride into enriched uranium dioxide fuel. This served as the first step in producing fuel rods for Japan's power plants and research reactors. Enriching nuclear fuel requires precision and has the potential to impose extreme risks to technicians. If done improperly, the process of combining nuclear products can produce a fission reaction which, in turn, produces radiation. In order to enrich the uranium fuel, a specific chemical purification procedure is required. The steps included feeding small batches of uranium oxide powder into a designated dissolving tank in order to produce uranyl nitrate using nitric acid. Next, the mixture is carefully transported to a specially crafted buffer tank. The buffer tank containing the combined ingredients is specially designed to prevent fission activity from reaching criticality. In a precipitation tank, ammonia is added forming a solid product. This tank is meant to capture any remaining nuclear waste contaminants. In the final process, uranium oxide is placed in the dissolving tanks until purified, without enriching the isotopes, in a wet-process technology specialized by Japan.
Pressure placed upon JCO to increase efficiency led the company to employ an illegal procedure where they skipped several key steps in the enrichment procedure. The technicians poured the product by hand in stainless-steel buckets directly into a precipitation tank. This process inadvertently contributed to a critical mass level incident triggering uncontrolled nuclear chain reactions over the next several hours.
Victim report
Two of the workers were working on the tank at the time of the accident; the third was in a nearby room. All three immediately reported seeing blue-white flashes. They evacuated immediately upon hearing the gamma alarms sound. After evacuating, one of the workers that was at the tank began experiencing symptoms of radiation exposure. The worker passed out, then regained consciousness 70 minutes later. The three workers were then transferred to the hospital, which confirmed that they were exposed to high doses of gamma, neutron, and other radiation.In addition to these three workers who immediately felt symptoms, 56 people at the JCO plant were reported to have been exposed to the gamma, neutron, and other irradiation. In addition to the workers at the site, construction workers who were working on a job site nearby, were also reported to have been exposed.
Nuclear criticality event chronology
JCO facility technicians Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa were speeding up the last few steps of the fuel/conversion process to meet shipping requirements. It was JCO's first batch of fuel for the Jōyō experimental fast breeder reactor in three years; no proper qualification and training requirements were established to prepare for the process. To save processing time, and for convenience, the team mixed the chemicals in stainless-steel buckets. The workers followed JCO operating manual guidance in this process but were unaware it was not approved by the STA. Under correct operating procedure, uranyl nitrate would be stored inside a buffer tank and gradually pumped into the precipitation tank in increments.At around 10:35, the precipitation tank reached critical mass with its fill level at about of uranium. The hazardous level was reached after the technicians added a seventh bucket containing aqueous uranyl nitrate, enriched to 18.8% U, to the tank. The solution added to the tank was almost seven times the legal mass limit specified by the STA.
The nuclear fuel conversion standards specified in the 1996 JCO Operating Manual dictated the proper procedures regarding dissolution of uranium oxide powder in a designated dissolution tank. The buffer tank's tall, narrow geometry was designed to hold the solution safely and to prevent criticality. In contrast, the precipitation tank had not been designed to hold unlimited quantities of this type of solution. The designed wide cylindrical shape made it favorable to criticality. The workers bypassed the buffer tanks entirely, opting to pour the uranyl nitrate directly into the precipitation tank. Uncontrolled nuclear fission began immediately, emitting intense gamma and neutron radiation. At the time of the event, Ouchi had his body draped over the tank while Shinohara stood on a platform to assist in pouring the solution. Yokokawa was sitting at a desk four metres away. All three technicians observed a blue flash and gamma radiation alarms sounded. Over the next several hours the fission reaction produced continuous chain reactions.
Ouchi and Shinohara immediately experienced pain, nausea, and difficulty breathing; both workers went to the decontamination room where Ouchi vomited. Ouchi received the largest radiation exposure, resulting in rapid difficulties with mobility, coherence, and loss of consciousness. Upon the point of critical mass, large amounts of high-level gamma radiation set off alarms in the building, prompting the three technicians to evacuate. All three of the workers were unaware of the impact of the accident or reporting criteria. A worker in the next building became aware of the injured employees and contacted emergency medical assistance; an ambulance escorted them to the nearest hospital. The fission products contaminated the fuel reprocessing building and immediately outside the nuclear facility. Emergency service workers arrived and escorted other plant workers outside of the facility's muster zones.
The next morning, workers ended the chain reaction by draining water from the surrounding cooling jacket installed on the precipitation tank. The water served as a neutron reflector. A boric acid solution was added to the precipitation tank to reduce all contents to sub-critical levels; boron was selected for its neutron absorption properties.