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Japan’s Nuclear Criticality Accident


Steven Dolley
Research Director
Nuclear Control Institute

October 4, 1999



When and where did the accident take place?


            An inadvertent nuclear chain reaction, or so-called “criticality accident,” began at 10:35 AM local time on Thursday, September 30 at the JCO Co. Ltd. Conversion Test Building at Tokai-mura, Japan, about 75 miles northeast of Tokyo.  The chain reaction, which gave off intense heat and radiation, could not be stopped until 18 hours later. 


The accident began when workers were converting enriched uranium into oxide powder for use in preparing fuel for the Joyo experimental fast breeder reactor.  This reactor is part of Japan’s plutonium-production program.  The uranium was enriched to 18.8% U-235, far higher than the 3 to 5% enriched uranium used as fuel in Japan’s conventional nuclear power reactors.  Breeder fuel, whether enriched uranium or plutonium, is far more susceptible to criticality accidents than power-reactor fuel.


What happened?


It appears that workers deliberately circumvented safety measures to save time.   A solution of uranyl nitrate was transferred into a large-volume precipitation tank, rather than the smaller, cylindrical container required by regulations.  According to JCO Inc. official Yutaka Tatsuta, one of the injured workers reported that some 16 kilograms of uranium solution had been poured into the precipitation tank, nearly eight times more than its criticality safety limit of 2.4 kilograms.


Workers reported seeing a blue flash and then started to feel ill.  According to one report, “the area was wrapped in a haze of blue smoke.”  Workers told plant staff that “they saw a blue flame rising from the fuel.”  Kenji Sumida, a member of Japanese government’s Nuclear Safety Commission, concluded, “I know this is difficult to believe, but I think that we have no choice but to recognize this accident as having been critical.”  The criticality continued for about 18 hours until the water that was moderating the flow of neutrons and allowing the chain reaction to continue was drained and the tank was flooded with boron, a neutron absorber.


How much radiation was released?


At one point, radiation levels near the plant were 15,000 times above normal background.  A total of at least 49 people were contaminated with radiation, including 39 JCO staff, seven residents, and three firefighters who transported the injured workers.  Two of the workers received such high doses of radiation that they are not expected to survive.


What emergency measures were taken?


Police blocked off a 250-meter radius around the facility.  The local village government requested evacuation within 350 meters of plant; more than 150 people were affected.  Residents beyond this perimeter were requested (several reports said “ordered”) to stay indoors.  Reportedly more than 310,000 people were affected.  School children couldn’t return home and were required to remain inside at their schools.  Late Thursday afternoon, rain began falling in Tokai, and residents were warned to stay out of the rain, and not to ingest rainwater or well water.  An all-clear was issued around 7 AM the next morning (Friday, October 1).


Who is responsible for this accident?


There is plenty of blame to go around.  According to reports, workers in the facility deliberately ignored the criticality limit of 2.4 kilograms, and had poured solution containing between 14 and 16 kilograms of enriched uranium into the container prior to the accident.  This illegal method was standard operating procedure, practiced for over two years and codified in a secret company manual that contradicted Japanese law and was never shown to government safety regulators.  These regulators must accept responsibility for not detecting this major safety violation.


Japan’s Yomiuri newspaper reported that the plant was an experimental facility and the workers thus may have been operating without appropriate safety measures, including protective gear and remote control systems, normally required in larger commercial facilities.  After the accident began, it took JCO Inc. nearly an hour to notify local Tokai authorities, despite a safety agreement requiring immediate notification of any accident, regardless of how serious.


Is this Japan’s first serious nuclear accident?


            The Tokai criticality incident is the latest in a series of  major accidents in Japan’s breeder-reactor and plutonium-fuel program.  In December 1995, the Monju demonstration fast breeder reactor leaked nearly a ton of liquid sodium coolant, which caught fire when it came into contact with air.  Officials of PNC, the company that built and operated Monju, attempted to cover up the extent of the accident by doctoring and destroying evidence.  In 1997, a waste-storage reprocessing plant at the Tokai facility burned and exploded, leading to another cover-up attempt.  PNC has since been reorganized by the government into the Japan Nuclear Cycle Research Institute (JNC) as a result of these accidents and cover-ups.


Has this sort of accident happened elsewhere?


France’s nuclear safety institute DSIN said the Tokai accident was the 60th criticality accident since 1945, 33 of which occurred in the United States and 19 in the former Soviet Union.  Two-thirds of these accidents occurred at research centers.  There had never been a criticality accident previously reported in Japan.


What steps can be taken to prevent such accidents in the future?


    The local government has ordered JCO to halt all uranium-processing activities until safety can be assured, and the national government has undertaken a safety review.  However, past assurances of safety in the Japanese breeder and plutonium program have proven unfounded.  When the Tokai facility was licensed in 1983, its operators assured Japanese government regulators that “critical fission chain reactions could not occur.” 


The Monju breeder’s sodium leak and fire, the Tokai reprocessing plant’s waste explosion and fire, and the Tokai criticality accident all have resulted from Japan’s program to produce and use plutonium fuel and to develop fast breeder reactors.  Not only is plutonium fuel far more expensive than conventional, low-enriched uranium fuel (the fuel of choice in almost all the world’s nuclear power reactors), it is also far more dangerous.  Plutonium and highly enriched uranium are also suitable for use in atomic bombs.  As the latest accident shows, regulatory reforms have failed to improve the safety culture in Japan’s breeder plutonium program.


            The surest way for Japan to prevent such accidents in the future is to shut down its breeder and plutonium fuel programs and to concentrate on safe operation of its nuclear power reactors using low-enriched uranium fuel.  At present, Japan is planning to introduce plutonium into their reactors in the form of so-called “mixed-oxide” (MOX) fuel.  MOX fuel potentially increases both the probability and severity of nuclear power reactor accidents.


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