The Tokaimura Accident

Michael E. Ryan
Department of Chemical Engineering
University at Buffalo, State University of New York

Part III: Radiation Exposure

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. Such radiation is most harmful to individuals who are nearby and falls off as the square of the distance. A secondary contribution comes from the volatile fission products, such as isotopes of xenon, krypton, and iodine. At Tokaimura only a very small fraction of the fission decay products are volatile in solution, and concentrations of xenon and krypton are generally thought to have been below the regulatory limits. A third potential source of radiation is the activation nuclei, or nuclei made radioactive by the absorption of neutrons. Studies have found that the decay of activated atmospheric nitrogen can contribute significantly to the total dose if people are not evacuated from the site of the criticality. However, given the neutron absorption cross section of nitrogen, it would appear that at Tokaimura this contribution would have been extremely small.

According to STA, Hisashi Ouchi was exposed to 17 sieverts of radiation and Masato Shinohara was exposed to 10 sieverts of radiation. The other worker in the vicinity at the time the precipitation tank went critical, Yutaka Yokokawa, received a dose of 3 sieverts (the doses were deduced from the measured levels of the relatively long-lived sodium-24 isotope in the victims' bodies).

One sievert (Sv), which equals 100 rems, is a measure of the biological response to the absorbed radiation. A rem refers to "roentgen equivalent in man" and is a measure of dose equivalent. This accounts for the fact that different types of radiation may deliver the same energy per unit mass to the body but do not have the same biological effect. The dose equivalent in rems is found by multiplying the radiation absorbed dose (rads) by a relative biological effectiveness factor. 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.

After the accident the three men were taken by helicopter to the National Institute of Radiological Sciences in Chiba. The two workers who received high radiation doses were transferred to the University of Tokyo Hospital and treated with blood transfusions. 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. Doctors at the University of Tokyo Hospital had used a radical cancer treatment in an attempt to revive his white cell count. It worked temporarily, but Mr. Ouchi was overwhelmed by his other injuries.

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. Their efforts also included the use of a radical cancer treatment. Too weak to cope with conventional blood transfusions, Mr. Shinohara was injected with umbilical cord blood to boost his stem cell count. While this proved successful in restoring his ability to reproduce blood cells, his body was unable to fight infections and other problems, including internal bleeding. Doctors said Mr. Shinohara's condition had declined rapidly after his lung and kidney functions deteriorated.

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. Monitors placed at a number of sites outside the plant had detected the radiation levels. 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.

Most criticality accidents in the past have not involved exposures of private citizens. The JCO facility is located quite close to the surrounding town. Within the 350-meter (1148-foot) radius evacuated immediately after the accident, there were 47 houses and 150 people. Another 310,000 people living within a 10-kilometer (6.2-mile) radius were ordered to remain indoors for 16 hours. Schools were closed, roads cordoned off, and train services were suspended.


  1. Define the relationship between sievert, rem, and rad.

  2. What are the natural sources of radiation to which human beings are exposed?

  3. Demonstrate that 0.50 mSv/hr corresponds to approximately 1000 times higher than the normal background level.

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