Summary and Causes of Criticality Accident at JCO

JCO 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.

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.