The Tokaimura Accident (28 September 1999)

"The Tokaimura nuclear fuel processing plant is operated by JCO Company Ltd. and is located approximately 120 kilometers northeast of Tokyo....  The main function of the JCO plant is to convert isotopically enriched uranium hexafluoride into uranium dioxide fuel.... The uranium used in the process has been enriched to contain up to 5% of the fissile isotope, ²³⁵U.

[A standard process at the plant was to treat] high-purity enriched uranium oxide with nitric acid to form uranyl nitrate for shipping. On the morning of September 30, 1999, three technicians, Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa, were running fuel through the last steps of the conversion process. To speed up the process, they mixed the oxide and nitric acid in 10-liter stainless steel buckets rather than in the dissolving tank. For convenience, they added the bucket contents directly to the ... precipitation tank rather than to the buffer tank. That was a crucial error because the tall, narrow geometry of the buffer tank was designed to preclude the onset of criticality. In filling the precipitation tank, the crew added ... about 16 kg of enriched uranium....

Masato Shinohara stood on a platform and was pouring the uranyl nitrate solution into the precipitation tank while Hisashi Ouchi held a glass funnel in an inlet at the top of the tank. [Yutaka Yokokawa was seated at a desk]. At approximately 10:35 a.m. the technicians added the seventh bucket and saw a blue flash. The two technicians near the vessel began to experience pain, waves of nausea, some difficulty in breathing, and problems with mobility and coherence. The gamma radiation alarms activated immediately. The blue flash that they had seen was a result of the Cherenkov radiation that is emitted when nuclear fission takes place and ionizes air. The addition of the seventh bucket had caused a self-sustaining chain reaction. The mixture, in other words, had gone critical. Mixing in the precipitation tank caused the fissile uranium species to disperse so that the reaction fizzled out. However, the critical mass later reassembled, initiating another chain reaction that released more neutrons and gamma radiation. This cycle was repeated several times over many hours.

None of the three technicians realized what had happened. Mr. Ouchi had been draped over the top of the tank and was experiencing the greatest difficulty. The other two workers helped him out of the building in response to the gamma radiation alarms. A worker in an adjoining building noticed the injured and confused technicians and called for medical assistance. An ambulance arrived quickly and removed the affected workers....

The exact critical mass for the 18.8% uranium mixture in the JCO precipitation tank is not known.... [T]he critical mass is greatly reduced when the fuel is in solution because water acts as a "moderator." Light atoms such as hydrogen slow the neutrons released by decaying ²³⁵U nuclei between fissions, making it more likely that they will be absorbed and trigger another nucleus to decay. The critical mass was further reduced at Tokaimura because a water jacket surrounding the precipitation tank reflected neutrons back into the tank.... Judging from the levels of gamma and neutron radiation measured near the plant perimeter, the criticality excursion seems to have lasted about 20 hours. After that time, the radiation levels dropped below detection limits.

The greatest source of radiation exposure in a criticality accident is the flux of neutrons and gamma rays that emanates directly from the fissioning nuclei and rapidly decaying fission products.... According to STA, Hisashi Ouchi was exposed to 17 sieverts of radiation ... Masato Shinohara was exposed to 10 sieverts of radiation [and] Yutaka Yokokawa received a dose of 3 sieverts.... One sievert (Sv), which equals 100 rems, is a measure of the biological response to the absorbed radiation.  Normally people receive an average of 0.003 Sv annually from natural causes. Half of all individuals exposed instantaneously to 4 Sv die within 30 days. Doses of 10 and 17 Sv are above the levels normally considered fatal.

Yutaka Yokokawa did not require transfusions and was discharged from the hospital on December 20, 1999. Hisashi Ouchi, 35, died of multiple organ failure on December 21, 1999. He suffered serious burns to most of his body, severe damage to his internal organs, and had a near-zero white blood cell count. Masato Shinohara, 40, died on April 27, 2000, of multiple organ failure despite a seven-month fight by doctors after his exposure to the large dose of radiation.

At least 439 people, including plant workers, firemen, and others who responded to the accident, and 207 local residents were exposed to elevated levels of radiation. In October 2000 the total number of people who received some radiation exposure from the accident was revised upward to 667. At one of the closest monitoring sites, STA reported dose rates of 4.5 mSv/hr for neutrons and 0.50 mSv/hr for gamma rays about 11 hours after the onset of criticality. That gamma dose rate was about 1000 times higher than the normal background level."