The Tokaimura Incident

Anthony Brown
February 24, 2015

Submitted as coursework for PH241, Stanford University, Winter 2015


Fig. 1: Location of the Tokaimura Accident in Japan. (Source: Wikimedia Commons)

Since the early 1970's, Japan has focused on decreasing the country's reliance on foreign fuel imports. With very few natural resources, Japan has embraced nuclear energy and currently receives over 35% of its electricity from nuclear power. Nuclear power plants now reside throughout the country and Japan has focused on reprocessing spent nuclear fuel rather than disposing of it as waste.The Tokaimura nuclear fuel processing plant is operated by JCO Company Ltd. and is located approximately 135 kilometers northeast of Tokyo. [1] This plant employs about 172 people and is one of 15 nuclear facilities in Tokaimura. The main function of this plant is to convert isotopically enriched uranium hexafluoride into uranium dioxide fuel.

Accident Details

On September 30, 1999, an accident occurred at a uranium fuel conversion test facility of JCO Co.,Ltd. Tokai-mura, Japan. JCO has operated mainly conversion facilities to produce uranium oxide powder or uranyl nitrate solution from low enriched uranium hexafluoride. The accident was triggered by pouring a sufficient amount of the 18.8% enriched uranium solution into a precipitation vessel, causing a high level reaction, and was terminated after approximately 19 hours by draining the cooling water around the vessel, which acted as a neutron reflector to maintain the condition. As a result, this accident gave serious radiation doses to 3 employees and fatal doses to 2 of them; further, neutrons and gamma-rays emitted by the accident caused doses to many residents, JCO employees, and emergency personnel who attempted to terminate the condition and to rescue the 3 employees. The dominant dose for the residents and the JCO employees was caused by neutrons and gamma-rays produced in the precipitation vessel. The individual dose was estimated for 234 residents, 169 JCO employees and 260 emergency personnel, respectively. [1] The Tokaimura accident of 1999 is the third most serious accident in the history of nuclear power, after the 1986 Chernobyl accident and the 1979 Three Mile Island accident but unlike the other cases, the Tokaimura accident did not involve a nuclear power station but a nuclear fuel factory where no nuclear chain reaction should ever happen. [2] The accident happened when workers preparing nuclear fuels mixed uranium oxide with nitric acid using a stainless steel container instead of a mixing apparatus. The shortcut was described in an illegal operating manual drafted by the company. The manual had never been approved by the supervising ministry, as was legally required. The procedure violated some of the most basic safety requirements that were well known in the nuclear industry since the early 1940's. [2]

Public Opinion on Nuclear Activities

The Tokaimura accident was the worst nuclear accident in Japan and it shattered the trust of the Japanese people in the industry's management capabilities for nuclear power generation and affected the future of Japan's nuclear power industry. The 1999 Tokaimura accident has contributed greatly to negative public confidence in government and corporate nuclear oversight. The share of Japanese people feeling uneasy about nuclear power grew from 21 percent before the accident to 52 percent afterwards. [3] In an October 1999 Japan Public Opinion Company survey, only 11 percent supported government plans to increase the share of nuclear power, 51 percent favored maintenance of current plans, while another 33 percent wanted to see a reduction in, or end to, nuclear power. [3] The belief in a high degree of safety and trust in the Japanese nuclear power industry may have evaporated with the Tokaimura accident. In 2001, the Japanese government planned to increase the number of nuclear power plants from 52 to between 62 and 65 by 2010, but by 2010 only two new power plants have been in operation. [3] Public perception in Japan towards nuclear power is finally improving over a decade after the Tokaimura accident and public surveys show that Japanese people are now supporting maintaining or expanding existing nuclear capacity.

© Anthony Brown. 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] S. Tanaka, "Summary of the JCO Criticality Accident in Tokai-Mura and a Dose Assessment," J. Radiat. Res. 42, Suppl., S1 (2001).

[2] W. D. Hoover, Historical Dictionary of Postwar Japan (Scarecrow Press, 2011).

[3] V. Vivoda, Energy Security in Japan: Challenges after Fukushima (Ashgate Publishing, 2014).