Tokaimura Nuclear Accident

Gracia Mahoney
March 22, 2018

Submitted as coursework for PH241, Stanford University, Winter 2018


Fig. 1: The location of the Tokaimura nuclear accident. (Source: Wikipedia Commons)

In September 1999, the Tokaimura nuclear accident occurred at the JCO uranium processing plant. Local officials were unprepared for the accident due to their belated ability to remove local people from the area and the lack of information outputted to alarm and inform people regarding the accident. [1] The Tokaimura processing facility was approximately 120 miles northeast of Tokyo (see Fig. 1). [2] One of the workers exposed by the levels of radiation emitted in the Accident died 83 days after exposure, ultimately due to multiple organ failure - which illustrates the lethal effects of radioactive emissions. [2] The surface levels of I-129/I-127 in soil was 100 times higher than the level of I-129/I-=127 in soil 20-25 cm deep. [3] Additionally, in terms of reporting on the accident, mistranslation between Japanese and English lead to inaccurate reporting of the account. [1]

Aftermath of Accident

The main role of the JCO plant was to convert isotopically enriched uranium hexafluoride into uranium dioxide fuel. [4] The plant poured a seventh batch of 18.8% enriched uranyl nitrate solution with U-235 into a precipitation vessel leading to a chain reaction of fissions. [4] The greatest source of radiation exposure that resulted from the accident were the flux of neutrons and gamma rays. [4] This type of radiation is very harmful to individuals who are nearby. [4] One of the three workers who were exposed to the high nuclear levels, Hisashi Ouchi, was exposed to 17 Sieverts of radiation and ultimately passed away. [4] At least 439 people, including first responders and 207 local residents, were exposed to radiation from the accident. [4] As a result of the accident, authorities warned locals to not drink tap water or harvest their crops. During a 10 day period following the 10,000 people sought medical check-ups. This was extremely costly to the state of Japan. [4]


An event as tragic that occurred in Japan could happen elsewhere in the world if dangerous fissile fuels are carelessly handled. Nuclear plants must adopt extremely strict safety procedure to minimize human error to prevent a tragedy like the Tokaimura nuclear accident. [5]

© Gracia Mahoney. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] C. Barnard, "The Tokaimura Nuclear Accident in Japanese Newsweek: Translation or Censorship?" Japanese Studies 20, 281 (2000).

[2] T. Ishii et al., "Brief Note and Evaluation of Acute-Radiation Syndrome and Treatment of a Tokai-mura Criticality Accident Patient," J. Radiat. Res. 42, 5167 (2001).

[3] Y. Muramatsu and Y Ohmomo. "Iodine-129 and Iodine-127 in Environmental Samples Collected From Tokaimura/Ibaraki, Japan," Sci. Total Environ. 48, 33 (1986).

[4] M. E. Ryan, "The Tokaimura Nuclear Accident: A Tragedy of Human Errors," J. Coll. Sci. Teach. 31, 42 (2001).

[5] D. Bellis et al., "Airborne Emission of Enriched Uranium at Tokai-Mura, Japan," Sci. Totel Environ. 264, 283 (2001).