The Tokaimura Criticality Accident of 1999

Ryan Dudzinski
March 1, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017


Fig. 1: The Tokaimura nuclear fuel processing plant. (Source: Wikimedia Commons)

The Conversion Test Building, JCO, Ltd, seen in Fig. 1, is located alongside the coast of the Northern Pacific Ocean in the village of Tokaimura, Japan. The facility consists of two units, first plant of which was authorized for power generation in 1966 and decommissioned in 1998, while the second plant built in the 1970's and has remained shut down since March of 2011. It was the Tokaimura criticality accident of 1999, not the dropping of atomic bombs on Hiroshima and Nagasaki that led to more than 200,000 Japanese deaths, that prompted Japan to question its long-lasting relationship with nuclear energy. [1]

Tokaimura Criticality Accident

At approximately 10:35 a.m. on September 30th, 1999, workers at the JCO nuclear fuel processing plant poured a seventh batch of 18.8% enriched uranyl nitrate solution, along with Uranium-235, into a precipitation vessel. The goal of this process is to convert the isotopically enriched uranium hexafluoride into uranium dioxide fuel. However, the addition of the seventh batch of solution caused a self-sustaining chain reaction, otherwise known as a criticality, that persisted for several hours. [2]

Once the nitric acid solution was added to the uranium and the criticality began, neutrons and gamma-rays were emitted, and led to the eventual deaths of the workers Hisashi Ouchi and Masato Shinohara. [2] The reaction was difficult to stop strictly due to the fact that the uranium was already mixed within the solution, providing no way for the workers to prevent more nuclear fission reactions from occurring. It was only until the cool water vessel was drained, which encircled the precipitation vessel and acted as a neuron deflector, that workers were able to cease the criticality. In addition to the death of the two workers, at least 439 people, including plant workers, firemen, and others who responded to the accident, as well as 207 local residents were exposed to elevated levels of radiation. [2]

Yet, the criticality event that changed the course of Japanese nuclear could have been avoided. The criticality's occurrence resulted primarily from a misunderstanding of the chemical process safety procedure and a miscommunication from risk management to the chemical engineers involved. [2] Since the plant had not been operational in the three years prior to the criticality, proper guidelines were not put in place to prevent such an occurrence. At its core, the criticality is attributed to what the Japanese call kaizen. Kaizen is the Japanese word for improvement, carrying the connotation in industry to enhance workplace operations and the environment. [3] So, this drive cultivated by the top management not only led to inadequate risk awareness, but the overall Tokaimura accident that shifted Japanese nuclear in the opposite direction.


It is common understanding that public opinion has played little to no role in Japan's policy formation in relation to other democratic states. Yet, in light of the criticality accident of 1999, the Japanese government produced a variety of new laws and institutional changes to address safety concerns to ensure events like this were prevented in the future. A few of these laws include allowing the Prime Minister to declare a state of emergency and using Self Defense Forces during said emergency situations. [4] As we know, Japan has continued to suffer from the use of nuclear power, such as the tsunami that reeked havoc on the plants in Fukushima, and has adjusted its use of the powerful, but unstable power source accordingly.

© Ryan Dudzinski. 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] T. Holmes, "The Dropping of Atomic Bombs on Japan," Soc. Educ. 69, 209 (2005).

[2] Ryan, Michael E., "The Tokaimura Nuclear Accident, A Tragedy of Human Errors," J. Coll. Sci. Teach. 31, No. 1, 42 (Sept 2001).

[3] A. P. Brunet and S. New, "Kaizen in Japan: an Empirical Study," Int. J. Oper. Prod. Man. 23, 1426 (2003).

[4] M. L. Kotler and I. T. Hillman, "Japanese Nuclear Energy Policy and Public Opinion," James A. Baker III Institute For Public Policy, Rice University, May 2000.