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Small fuel preparation plant operated by JCO (formerly Japan Nuclear Fuel Conversion Co.)
-The accident occurred in a small fuel preparation plant operated by JCO
-The plant was commissioned in 1988, carried out research and built experimental reactors.
-They processed up to 3 tonnes of uranium enriched up to 20% U-235 to make nuclear fuel.
-They did this using a wet process, which expedited the nuclear fission chain reaction. -
Operators Began to Modify Procedure (Pt.1)
-Nuclear fuel preparation involved mixing uranium oxide (U3O8) powder in nitric acid in a dissolution tank.
-They then transferred it to a storage column, and then to a precipitation tank
-The storage column was built to prevent criticality and to control the amount of mixture transferred to the precipitation tank. -
Operators Began to Modify Procedure (Pt.2)
-Several steps bypassed: they modified the process to allow the uranium oxide to be mixed in stainless steel buckets, rather than the dissolution tank. This was not regulated.
-They then tipped the solution into the precipitation tank.
-To speed up the process, operators skipped mixing process in the storage column and instead did that part in the precipitation tank. There was no proper control of the amount tipped into the tank. -
Day of the Accident
-Hisashi Ouchi and Masato Shinohara, under supervisor Yutaka Yokokawa, prepared fuel with uranium enriched to 18.8% U-235 for a reactor.
-They had only previously worked with less than 5% enriched uranium, and they had no idea how lethal this level of enrichment could be.
-10:35am: critical mass reached in the precipitation tank, with 40 L of the solution containing 16 kg of U.
-Nuclear fission chain reaction began, emitting gamma and neutron radiation.
-Workers saw a flash of blue light. -
After the Reaction
-The nuclear fission reaction continued for 20 hours. Boric acid solution, a neutron absorber, was added to the tank to control the radiation.
-27 workers were exposed to radioactivity
-5 hours after the reaction took place, about 161 people who lived within a 350 meter radius from the building were asked to evacuate. They were allowed to return two days later, and were given sandbags and shielding to protect from gamma radiation. -
Dose Received by Workers
-The incident was classified as Level 4 on the International Atomic Energy Agency (IAEA) International Nuclear Event Scale
-All three workers were hospitalized and in critical condition. They received full body radiation doses of 16,000-20,000, 6000-10,000 and 1000-5000 milligrays (8000-10,000 mGy being a fatal dose)
-24 other workers received about 48 mSv (annual allowable dose is 50 mSv). Other workers and some members of the public received between 5-24 mSv. -
Immediate Effects on Workers
-Ouchi received one of the largest doses of radiation known
-Tests on Ouchi and Shinohara showed that their lymphatic blood counts were at almost zero.
-Symptoms included nausea, vomiting, and dehydration
-6 days after the criticality, Ouchi’s chromosomes in his bone marrow cells were examined and showed that they were broken into pieces.
-The next day, with his sister as a donor, he received a peripheral bone stem cell transplant. -
Death of Hisashi Ouchi
-Ouchi's body continued to break down. When medical tape was removed from his skin, his skin came off with it due to the inability of his cells to repair and regenerate his skin.
-He experienced extreme pain, breathing problems, and after two weeks, he was unable to eat.
-Ouchi’s heart stopped two months after the incident, but he was revived. He died of multiple organ failure on December 21st, 11:21pm. -
Masato Shinohara and Yutaka Yokokawa
-Shinohara showed signs of improvement, but the radiation damage to his respiratory system lead to pneumonia and caused difficulty breathing.
-Shinohara also died of multiple organ failure in April 27, 2000.
-Yokokawa was released three months later with minor radiation sickness. -
Plant Faces Criminal Charges
-JCO faced criminal charges for violating safety standards and legal requirements
-JCO's operating license revoked in early 2000
-After the incident, there were more strict safety standards, regulations, and monitoring of radiation
-6 former executives and employees of JCO were arrested in October 2000
-Yokokawa was among those arrested and faced negligence charges
-The incident lead Japan to reduce number of reactors planned for the next 11 years, to 13 reactors instead of the planned 16-20.