Case study

Case Background

  • Copyright Information: Held by the National Center for Case Study Teaching in Science, University at Buffalo.

  • Publication Date: Originally published June 25, 2001.

  • Author: Michael E. Ryan, Department of Chemical Engineering, University at Buffalo.

Nuclear Energy in Japan

  • Energy Policy: Post-1970s oil crises, Japan focused on reducing foreign fuel dependence.

  • Nuclear Dependency: Provides approximately 36% of Japan’s electricity.

  • JCO Company: Operates the Tokaimura nuclear fuel processing plant, pivotal in Japan’s energy self-sufficiency.

  • Location: About 120 km northeast of Tokyo, employs 140 in a 40-acre site.

JCO Plant Function

  • Main Function: Convert isotopically enriched uranium hexafluoride into uranium dioxide fuel.

  • Types of Uranium: Uses 235U (fissile isotope) and 238U (inert isotope).

  • Fission Process: Uranium nuclei split, converting mass into approximately 200 MeV energy.

Purification Process

  • Purification for Joyo Reactor: Involves converting uranium oxide into uranyl nitrate.

  • Regulations: Limited to processing 2.4 kg of 18.8% enriched uranium.

  • Chemical Steps:

    • Dissolving Tank: Uranium oxide mixed with nitric acid.

    • Buffer Tank: Stabilizes mixture, prevents criticality.

    • Precipitation Tank: Ammonium salt solution added for ammonium diuranate formation.

Accident Chronology (Part II)

  • Incident Date: Mixing began on September 28, 1999, with an accident occurring on September 30.

  • Faulty Practices: Technicians mixed high-purity enriched uranium oxide with nitric acid outside the approved process.

  • Criticality Event: Adding the seventh bucket caused a blue flash (Cherenkov radiation) indicating a self-sustaining chain reaction.

Consequences of the Accident

  • Immediate Effects: Pain, nausea, and mobility issues for technicians; gamma radiation alarms signaled the criticality.

  • Management Response: Initial confusion; workers not adequately trained to handle the emergency.

  • Critical Mass Dynamics: Water presence in the tank reduced the critical mass needed for a chain reaction.

Radiation Exposure (Part III)

  • Types of Radiation: Neutron and gamma rays are chief exposure sources in a criticality accident.

  • Radiation Measurements:

    • Hisashi Ouchi: 17 sieverts, died December 21, 1999.

    • Masato Shinohara: 10 sieverts, died April 27, 2000.

    • Yutaka Yokokawa: 3 sieverts, discharged by December 20, 1999.

Severity of Radiation Exposure

  • SV to Rem Relationship: 1 sievert (Sv) = 100 rems, with higher doses correlating to increased risk.

  • Emergency Evacuations: Over 310,000 ordered to remain indoors; local residents faced confusion and panic.

Immediate Aftermath (Part IV)

  • Lack of Preparedness: No emergency plan; misinformation about contamination of food and water.

  • Public Health Response: 10,000 residents sought check-ups, yielding few benefits.

Company and Government Response

  • Company Actions: Ineffective communication and response; management did not adhere to emergency protocols.

  • Government Actions: Evacuations and emergency protocols initiated; however, communication was poor.

Update of Situation (Part V)

  • Plant Shutdown: JCO's operational license canceled; compensation claims handled poorly.

  • Criminal Charges: Six company officials arrested for professional negligence; significant financial compensation to affected residents.

  • Public Sentiment: Rise in anti-nuclear sentiment following the accident, with increased public opposition to nuclear power.

Lessons Learned (Part VI)

  • Operator Training: Highlighted failures in operator understanding of critical mass and handling procedures.

  • Communication Failures: Need for improvement in training and regulation enforcement.

Historical Perspective (Part VII)

  • Previous Criticality Accidents: Similar incidents at Oak Ridge, Mayak, and Chernobyl.

  • Long-term Impacts: Public distrust in nuclear power, influencing future regulatory frameworks and plant operations.

Study Questions

  1. Analyze the root causes of the accident and suggest improvements in handling criticality processes.

  2. Examine the regulatory failures contributing to the accident.

  3. Assess the effectiveness of communication following the incident.