"The Tokaimura Accident"

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


This case was developed for use in a chemical engineering laboratory, unit operations, or chemical plant design course in order to introduce the subject of hazard identification and process safety. Students in these courses would have completed courses in general chemistry, calculus-based physics, and an introductory chemical engineering course involving material and energy balances. A rudimentary knowledge of nuclear chemistry or nuclear physics would be helpful.

The case is based on actual events. This particular accident was chosen because of the associated dramatic aftermath. Although the chemical and nuclear industries are indeed very safe, the potential for serious consequences as a result of an accident is especially significant.


  • To describe the nature of accidents and provide an accurate perspective of chemical process safety.

  • To illustrate the importance of chemical process safety including hazard evaluation and loss prevention.

  • To introduce risk assessment and risk management as related to the chemical process industries.

  • To discuss industrial hygiene and chemical toxicology.

  • To introduce emergency planning and response.

  • To examine flowsheet review and incident investigation.

  • To describe the professional and ethical responsibilities of the chemical engineer.


Nuclear fuel produces a significant portion of electrical power in most developed countries. Nuclear fuel is derived from uranium as well as plutonium. Uranium serves as the source for most commercial power plants. Although 235U occurs naturally with the common isotope 238U, 235U fissions spontaneously while 238U does not. 235U decays by alpha emission and must be concentrated or enriched so that a critical mass capable of initiating a fission chain reaction can be achieved in the reactor. Military applications of uranium fission require an enrichment on the order of 30% or above, distinctly different from commercial fuel.

The fuel is regulated by nuclear licensing agencies around the world, such as the Nuclear Regulatory Commission (NRC) in the United States. Regulation and control of uranium isotopic enrichment processes is essential because of the potential for the unauthorized procurement and construction of nuclear weapons. Safety regulations concerning the amount of uranium, enrichment technology, equipment configuration, and associated materials have been promulgated to ensure that a critical mass is not accidentally generated. A criticality accident implies that a fission reaction will be sustained by liberated neutrons.

Part I - Nuclear Power and Nuclear Fuel Reprocessing in Japan

In Part I, students explore the social, political, technical, and economic issues relating to the energy needs and energy policies of industrially developed countries. In this section, some factual details are presented on nuclear fuel reprocessing in Japan as well as basic concepts of nuclear chemistry. Questions are posed on alternative sources of energy along with their associated advantages and disadvantages. Students are asked to construct a block flow diagram of the uranium oxide purification process.

Question I.1: List several alternative sources of energy for industrial and consumer needs along with their associated advantages and disadvantages.

Answer to Question I.1: Alternative sources of energy include fossil fuels (coal, petroleum, natural gas), hydroelectric, geothermal, solar, wind, tidal, ocean (wave energy conversion), biomass, nuclear, etc. A discussion of possible advantages and disadvantages would include considerations relating to economics, environmental impact, the renewable nature of the source of energy, availability, technical feasibility for the intended use, etc.

Question I.2: What are the major sources of energy in the United States?

Answer to Question I.2: The table below summarizes the approximate energy consumption by source in the United States.

Energy SourceU.S. Consumption - 1999(quadrillion Btu)
Natural Gas22.10
* firewood, crop waste

As the table shows, approximately 8% of all energy consumed in the United States is derived from nuclear energy. It is important to realize that a substantial portion of energy is derived from fossil fuels that are extensively used for heating and transportation applications.

It is interesting to note that North America represents 7% of the world's population but consumes 30% of the world's energy. In 1999 the United States accounted for 25.5% of the world consumption of coal and 26.8% of the world consumption of natural gas. The per capita consumption of gasoline for several countries is indicated in the table below.

CountryPer Capita Consumption of Gasoline (1997) (gallons)
United States459

* Source: National Geographic, March 2001

A summary of the number of nuclear plants and the percentage of electrical power derived from nuclear energy for various countries is indicated in the table below. For the United States, nuclear energy represents a fairly significant proportion of electrical power generation. For many other countries, nuclear energy represents the predominant source of power generation. It is interesting to note that the United States operates one-fourth and Japan one-eighth of all nuclear plants worldwide.

Country Number of Plants Capacity(MW) Output(billion kWh) Percentage of Power from Nuclear
Argentina 2 935 6.6 9.0
Armenia 1 376 2.1 36.4
Belgium 7 5,712 46.6 57.7
Brazil 1 626 3.9 1.3
Bulgaria 6 3,538 14.5 47.1
Canada 14 9,998 70.4 12.7
China 3 2,167 14.1 1.2
Czech Republic 4 1,648 13.4 20.8
Finland 4 2,656 22.0 33.0
France 59 63,103 375.0 75.0
Germany 20 22,282 160.4 31.2
Hungary 4 1,729 14.1 38.3
India 11 1,897 11.5 2.7
Japan 53 43,691 306.9 36.0
South Korea 16 12,990 97.8 42.8
Lithuania 2 2,370 9.9 73.1
Mexico 2 1,308 9.6 4.9
Netherlands 1 449 3.4 4.0
Pakistan 1 125 0.7 1.2
Romania 1 650 4.8 10.7
Russia 29 19,843 110.9 14.4
Slovak Republic 6 2,408 13.1 47.0
Slovenia 1 632 4.5 36.3
South Africa 2 1,842 13.5 7.4
Spain 9 7,470 56.5 31.0
Sweden 11 9,432 70.1 46.8
Switzerland 5 3,079 23.5 36.0
Taiwan 6 4,884 36.9 25.3
Ukraine 16 13,765 67.4 43.8
United Kingdom 35 12,968 91.2 28.9
United States 103 97,145 719.4 19.5
TOTAL 435 350,653 2,394.6 17.0

Source: J. Johnson, "New Life for Nuclear Power?" Chemical & Engineering News, 78, 40, 39 (2000).

Question I.3: Sketch a block flow diagram for the uranium oxide purification process.

Answer to Question I.3:

Block flow diagram for the purification process at the JCO Reprocessing Plant

Part II: Accident Chronology

Part II gives a detailed account of the accident chronology. In addressing the questions, instructors can incorporate concepts of hazard evaluation and flowsheet review. Students can also consider whether modifications to the design could have prevented the accident from occurring, personnel protective equipment, operating procedures, and management policies.

Question II.1: What mistakes did the technicians make that resulted in this accident?

Answer to Question II.1: Although the mixing of uranium oxide with nitric acid had not been approved by STA, the technicians were not at fault in following this procedure since it had been incorporated into the operating manual. The technicians erred in adding the bucket contents directly to the precipitation tank rather than to the buffer tank. In doing so, they added approximately seven times more enriched uranium than permitted under the company's operating license.

Question II.2: Were there any flaws in the equipment or design of the process that contributed to the occurrence of this accident?

Answer to Question II.2: One potential design flaw was the ease by which the buffer tank could be bypassed. A different equipment layout of the buffer and precipitation tank, warning signs, and stricter adherence to proper operating protocol could have greatly reduced the potential for this type of accident.

Question II.3: What steps could JCO management have taken to eliminate or reduce the possibility of this type of accident?

Answer to Question II.3: JCO management should have enforced stricter protocols and reviews of operating procedures. Greater emphasis on safety and potential hazards should have been included in regular training programs for operators and supervisors.

Part III: Radiation Exposure

Part III elaborates on the dramatic and tragic consequences of the accident. This is done to emphasize the potentially serious nature of industrial accidents along with the associated personal consequences to the individuals directly involved and their families. This part of the case lends itself to an in-class discussion of the possible widespread impact of an industrial accident to the public or surrounding community. The three questions associated with this section relate to basic concepts of radiation measures and radiation exposure.

Question III.1: Define the relationship between sievert, rem, and rad.

Answer to Question III.1: A rad is an acronym for radiation absorbed dose and is a measure of the radiation dose (energy) actually absorbed by an object. An absorbed dose of one rad is equivalent to the delivery of 10 mJ/kg of ionizing radiation. A rem is an acronym for roentgen equivalent in man and is a measure of dose equivalent. A roentgen is a measure of exposure. One roentgen is defined as the ability to deliver 8.78 mJ of energy to 1 kg of dry air at standard conditions. The rem takes account of the fact that different types of radiation may deliver the same amount of energy per unit mass to the human body but the biological effects may not be the same. The dose equivalent in rems is obtained by multiplying the absorbed dose (in rads) by a relative biological effectiveness factor. For x-rays and electrons this effectiveness factor is approximately equal to 1, whereas for slow neutrons the effectiveness factor has a value of approximately 5. Personnel-monitoring radiation devices such as film badges are designed to register the dose equivalent in rems. One sievert is equal to 100 rems.

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

Answer to Question III.2: Cosmic rays and radioactive elements in the earth's crust constitute natural sources of radiation exposure. Diagnostic and therapeutic x-rays used in medicine constitute radiation sources of human origin. The absorbed radiation dose from sources of natural and human origin is approximately 0.2 rad per year.

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

Answer to Question III.3:

0.50 mSv/hr = 0.05 rems/hr = 438 rems/year

This is between 450 and 2000 times higher than a background level of 0.2 rad/year depending on the value used for the relative biological effectiveness factor (1-5).

Part IV: Immediate Aftermath

After a brief discussion in Part IV of the reaction of company management, government leaders, the public, and the media in the immediate aftermath of the accident, students work in teams of three to prepare a recommended response by company officials and the government. The students also consider how much responsibility or blame is borne by various individuals. Discussion points might include the following:

  • The workers should have been provided with more stringent training and certification. The company should have had an appropriate emergency response plan. The company should also have designated an authoritative spokesperson to provide accurate information and address questions by the public and the media. It is important for company officials to participate fully and cooperatively with emergency response personnel and government agencies in order to resolve the crisis.

  • Some government officials may have overreacted to the situation by suggesting that persons within a 10 km radius be checked for radiation exposure. Although well intended, these screening examinations were costly to conduct with essentially little benefit. The program may also have exacerbated the anxiety of persons in the community rather than ameliorate their concerns.

  • It is difficult to judge how we might react when directly confronted with a serious and emotional incident such as this. In all likelihood we would probably not react very differently if we and our family members were involved in a similar situation.

Part V: Update of Situation

Part V reports on what has happened since the accident. At this point, student groups compare their recommended responses in Part IV to the actual events and actions that have unfolded to date. Some discussion points might include the appropriateness of the compensation paid by the company to local businesses and residents, criminal charges brought against company officials, level of fines, and jail sentences.

Part VI: Lessons Learned

Part VI discusses some of the causes and other extenuating factors surrounding the accident. The questions psoed ask the students to critically review the equipment design, process topology, operating procedures, and company policies and identify oversights or modifications that could have ensured a greater level of safety.

Question VI.1: In view of this accident, what equipment or design changes, if any, would you recommend for this type of process?

Answer to Question VI.1: One possible design modification would be to install equipment that would prevent bypassing the dissolving tank.

Question VI.2: In view of this accident, what changes in operating procedures, if any, would you recommend for this type of process?

Answer to Question VI.2: The operating procedures could include a better information management system in order to follow government and company restrictions. Analysis of 235U, other uranium isotopes, impurities, etc., could be made at various stages in the process to ensure compliance with process and criticality safety specifications. Regular safety training and certification programs should be required of all operators and laboratory personnel.

Question VI.3: In view of this accident, what policy options would you propose when an accident of this type occurs?

Answer to Question VI.3: This question relates to basic government policy and the debate over the role of nuclear energy. The discussion of these options connects to the discussion points raised in Part I.

Part VII: Historical Perspective

Part VII provides a short historical chronology of some other nuclear accidents. Students research two well-known nuclear accidents, namely, those that occurred at Three Mile Island and at Chernobyl. Later, during their research of fast-breeder reactor technology, students study the cost-benefit analysis of this technology for Japan.

Question VII.1: Research the accident that occurred at Three Mile Island. What were the factors that led to this accident?

Answer to Question VII.1: There are many sources of information concerning the incident at Three Mile Island. Two references are:

    Houts, P.S., P.D. Cleary, and T-W. Hu. The Three Mile Island Crisis. University Park, PA: Pennsylvania State University Press, 1988.

    Ford, D.F. Three Mile Island: Thirty Minutes to Meltdown. New York: Viking Press, 1982.

Question VII.2: Research the accident that occurred at Chernobyl. What were the factors that led to this accident?

Answer to Question VII.2: There are many sources of information concerning the Chernobyl disaster. Two references are:

    Mould, R.F. Chernobyl: The Real Story. Oxford: Pergamon, 1988.

    Haynes, V. and M. Bojcun. The Chernobyl Disaster. London: Hogarth, 1988.

Question VII.3: Research fast-breeder reactor technology. What are the benefits that could be derived from this technology?

Answer to Question VII.3: Fast-breeder reactor technology is fuel efficient since it generates more fuel than it consumes by converting nonfissile uranium into fissile plutonium. Fast-breeder reactors are capable of extracting one hundred times more energy from the same amount of uranium. Breeder reactors require fuel reprocessing. A readable reference on energy from nuclear power is:

    Hafele, W. Energy from Nuclear Power. Scientific American 263(3):136. 1990.


This is a discussion case that can be effectively covered in two class periods. For a course comprising 50-minute periods, Parts I-IV are handed out at the beginning of class. Students receive no additional materials or references concerning the case. The students have five minutes to read Part I, then spend 10 minutes addressing and discussing each of the questions at the end of Part I. A similar format is followed for Part II. As homework, the students are asked to read Parts III and IV and to prepare a short written response to each of the questions posed in Parts III, which they hand in at teh next class period. The students are then assigned to a group to complete the group assignment in Part IV, handing in their written statements the following class period. During the second class period, approximately 25 minutes are devoted to a discussion of Parts IV and V. The remainder of the time is spent reading Parts VI and VII and discussing the questions posed in those sections.

For a course comprising 80-minute periods, you could follow a similar format as outlined above, spending additional time on discussion. Alternatively, during the additional time available in the second class period, students could debate the material and their group assignment. This approach has the advantage of a more lively exchange among the students, with fewer papers for the instructor to grade. The discussion or debate could conclude with a structured summary of the case and the lessons learned.


The following websites provide useful information and statistics relating to energy usage and production in the United States.


    Anonymous. 1999. Science and technology: All over in a flash. Economist 353(8140):102, October 9, 1999.

    Ball, K. 2000. Japan's Criticality Accident. Industrial Hygiene News 23(4):48.

    Johnson, J. 2000. New Life for Nuclear Power? Chemical & Engineering News 78(40):39.

    Kumagai, J. 1999. In the wake of Tokaimura, Japan rethinks its nuclear future. Physics Today 52(12):51.

    Levi, B.G.1999. What happened at Tokaimura? Physics Today 52(12):52.

    Sumitomo Metal Mining Company Ltd., Annual Report, JCO Criticality Accident:

    Wilson, E. 1999. Processing errors led to nuclear accident. Chemical & Engineering News 77(41):19.

Acknowledgements: The author would like to acknowledge the many helpful and constructive comments and suggestions provided by Professor Graham Peaslee, Chemistry Department, Hope College. This case study was developed with support from The Pew Charitable Trusts and the National Science Foundation as part of the Case Studies in Science Workshop held at the University at Buffalo on June 12-16, 2000.

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