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Criticality Accident at the Uranium Conversion Facility, JCO Co.Ltd.


October 19, 1999
The Federation of Electric Power Companies

The accident that occurred on September 30, 1999 in JCO Co. Ltd. was Japan's first criticality accident, exposing workers to high radiation levels and causing exposure to some members of the general public adjacent to the plant. Emergency advisories were issued for residents close to the plant to evacuate their homes and for others in the general vicinity to remain indoors. The cause of this accident appears to have been the use of illegal procedures. This accident has a grave ramifications for nuclear safety in Japan.

We have no direct contractual relationship with JCO. But, as an industry that also works with nuclear energy, we recognize the severity of the JCO accident. At the same time we believe that if we are to secure a stable supply of energy, sustain economic growth and provide protection for the global environment, we need to continue with our steady development of nuclear power generation and the nuclear fuel cycle in Japan.

The Federation of Electric Power Companies of Japan (FEPC) has set up a JCO Accident Special Response Committee, through which we will again ensure the safety-first principle in the electricity companies and seek to secure additional safety measures. In our efforts to ensure the sharing of the safety culture and practices throughout the nuclear industry, it is our intention to establish a safety center, based on the mutual checks and information exchange regarding operation management issues that the world's nuclear operators have carried out since the Chernobyl accident. We will invite all businesses in Japan dealing with nuclear fuel, radioactive waste management and transportation to take part in this new safety center.

1. JCO Conversion Plant

The Nuclear Fuel CycleJCO is a company that handles conversion of enriched uranium hexafluoride to uranium dioxide in the process of making fuel for nuclear power plants.

The uranium dioxide becomes fuel assembly at the fuel fabrication plant for loading into nuclear reactors.

At the time of the criticality accident, highly enriched fuel (18.8% enrichment) was being processed for use in the experimental fast reactor Joyo.

Normal processing of fuel (5% or less enrichment) for nuclear power stations is carried out on a different production line.

2. Cause of the JCO Accident (Assumption based on information released to date)

JCO failed to adhere to government-approved procedures
and poured more than the restricted amount of uranium
into the precipitation tank.

The correct procedure, employing a solution tank and dedicated pump, only allows uranium solution to be introduced into the precipitation tank in amounts determined to prevent continuing nuclear fission leading to criticality.

In this case, however, a stainless steel bucket was used to manually pour uranium solution directly into the precipitation tank. Since this solution was about 7 times the allowed uranium limits, criticality was soon reached.

Authorized Procedure

3. No contingency plans had been made for criticality at the JCO facility.

Since both the conversion plant where the accident occurred handles uranium in solution or compound and additional restrictions had been applied to container size & type, as well as to amounts of uranium handled, the plant was operated on the premise of non-critical conditions preventing self-sustaining nuclear fission. For this reason, criticality had not been considered a possible contingency.

At nuclear power stations, on the other hand, where uranium is handled as processed fuel and nuclear fission is used to generate electricity, nuclear reactors are designed on the premise of criticality. Radioactive materials are contained and multiple safety measures are in place to prevent any effects on residents in the vicinity.

Safety Measures in Nuclear Power Stations

  1. Design to prevent equipment failure and operating mistakes.
  2. System to allow for immediate reactor shutdown in case of abnormal conditions.
  3. System to cool reactor and contain radioactive materials in the event of accident.

Fivefold Walls shield Radioacrivity

4. Practically no radioactive materials were detected outside the JCO plant.

The increase in radiation levels in the plant vicinity during the accident was largely due to the release of neutron and gamma radiation from the precipitation tank in which criticality had been reached. A post-accident investigation of soil, river and other environmental samples, as well as schools and other public facilities detected practically no radioactive material. The Japanese government has also declared all agricultural and marine products from the region to be safe.

The JCO accident did cause an increase in radiation levels in the vicinity following the accident but was not the same as the Chernobyl accident which dispersed large amounts of radioactive materials, causing widespread ground contamination.

JCO accident / Major accident causing dispersion of radioactive material

The Nuclear Safety Commission and the Science & Technology Agency are currently investigating the cause of the accident.