The Tokaimura nuclear accidents were two nuclear incidents which occurred 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 (PNC) 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. (JCO) 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 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
Nuclear power 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 (about 70 miles from Tokyo) 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, 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 (動燃事故, Dōnen jiko), '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 (bitumen), and placed the mixture in metal drums for storage. That day, the site was trialing a new asphalt-waste mix ratio, 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. Temperature controls in the building, designed to prevent the asphalt from reaching its ignition point, were malfunctioning at the time. 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 (25 miles) 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 chronology of events that led to the facility's evacuation, in order to cover up the lack of proper supervision. Dōnen leadership failed to immediately report the fire to the Science and Technology Agency (STA). This delay was due to their own internal investigation of the fire, which hampered the deployment of emergency response teams and prolonged the radiation 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
On 30 September 1999, another, more serious event known as the Tōkai nuclear accident (Japanese: 東海村JCO臨界事故, romanized: Tōkai-mura JCO-rinkai-jiko) occurred about four miles away from the PNC facility, 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 the Fukushima Daiichi nuclear disaster in 2011. The incident exposed the surrounding population to hazardous radiation after a uranium mixture reached criticality, triggering uncontrolled nuclear chain reactions over the next several hours. The three technicians mixing fuel were exposed to a large amount of radiation, and two of those three died from complications of radiation poisoning in the months following. Official investigations concluded that the released radiation caused no harm to the surrounding public or environment, but the event nevertheless impacted the local economy and affected public perceptions.
Background
Regulatory deficiencies
There was no criticality accident alarm because the licensing board incorrectly concluded that a criticality accident could not happen at the site. The site was also not included in the National Plan for the Prevention of Nuclear Disasters; the lack of a disaster plan led to delays in informing the nearby public of the accident.
The regulator also did not conduct routine inspections to determine whether production processes were being carried out correctly.
Company errors
Because of a combination of reduced revenue, layoffs and inexperience, JCO felt pressured to manufacture fuel at a different enrichment which was made infrequently, using a procedure that had not been approved by regulatory bodies. The unapproved process in which they made the fuel skipped several crucial safety steps. JCO stated that they did not submit this procedure to the regulator because they knew it would not get approved, in which case they would not have been 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 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 fuel, a specific chemical purification procedure is followed. The steps included feeding small batches of uranium oxide powder into a designated dissolving tank to produce uranyl nitrate using nitric acid. Next, the mixture is carefully transported to a buffer tank, specially designed to prevent fission activity from reaching criticality. In a precipitation tank, ammonia is then added, forming a solid product and capturing any remaining nuclear waste contaminants. Lastly, uranium oxide is placed in the dissolving tanks until purified, without enriching the isotopes, in a wet-process technology specialized by Japan.
The fuel being made on the day of the event was JCO's first batch for the Jōyō experimental fast breeder reactor in three years; no proper qualification and training requirements were established to prepare for the process.Pressure placed upon JCO to increase efficiency led the company to skip several key steps in the enrichment procedure. Under correct operating procedure, aqueous uranyl nitrate would be stored inside a buffer tank and gradually pumped into the precipitation tank in 2.4 kg (5.3 lb) increments. Instead of pumping the uranyl nitrate, the technicians used buckets to pour the product by hand directly into the precipitation tank; the workers followed JCO operating manual guidance in doing this, but were unaware it was not approved by the STA.
Chronology of events
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. To save processing time, and for convenience, the team mixed the chemicals in stainless-steel buckets. At around 10:35, the precipitation tank reached critical mass with its fill level at about 16 kg (35 lb) of uranium. The hazardous level was reached after the technicians added a seventh bucket containing uranyl nitrate, enriched to 18.8% 235U, 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. The workers bypassed the buffer tanks entirely, opting to pour the uranyl nitrate directly into the precipitation tank. Unlike the buffer tank, the precipitation tank had not been designed to hold unlimited quantities of this type of solution; its wide cylindrical shape made it favorable to criticality. Uncontrolled nuclear fission (a self-sustaining chain reaction) 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 (possibly Cherenkov radiation) 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, one of which was experiencing symptoms of radiation exposure and had passed out and regained consciousness 70 minutes later, and contacted emergency medical assistance; an ambulance escorted them to the nearest hospital. The fission products contaminated the fuel reprocessing building and immediate surroundings of the nuclear facility. Emergency service workers arrived and escorted other plant workers outside of the facility's muster zones.
56 other people at the JCO plant were reported to have been exposed to the radiation. Construction workers at a job site nearby were also reported as exposed.
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.
Evacuation
By mid-afternoon, the plant workers and surrounding residents were asked to evacuate. Five hours after the start of criticality, evacuation began of some 161 people from 39 households in Tōkaimura within a 350-metre radius from the conversion building. Twelve hours after the incident, 300,000 residents surrounding the nuclear facility were told to stay indoors and cease all agricultural production. This restriction was lifted the next afternoon. Almost 15 days later, the facility instituted protection methods with sandbags and other shielding to protect from residual gamma radiation.