Lessons Learned 5 Criticality Accident at Fuel Fabrication Plant

Radiological Safety Training for Plutonium Facilities

DOEHDBK11452001

Handouts

Lessons Learned 5

Criticality Accident at Fuel Fabrication Plant

The criticality accident that occurred in September 1999 at the Fuel Conversion Test

Building in Tokai-mura, Japan, was the first in which measurable exposures occurred to off-

site members of the public. Two operators lost their lives and another received a significant

dose. The excursion continued for nearly twenty hours before it was terminated by workers

directed by government officials.

There were two deviations from the license-authorized procedures that caused the

accident. First, the company procedures specified that the dissolution step was to be

conducted in open, 10-liter stainless steel buckets instead of the dissolution vessel. The

much more serious procedural departure was the transfer of the nitrate solution into the

unfavorable geometry precipitation vessel instead of the prescribed geometry columns.

Factors contributing to the accident include a weak understanding by personnel at all

levels of the factors that influence criticality, and specifically, a lack of realization that the 45

liters of solution, while far subcritical in the intended storage tanks, could be supercritical in

the unfavorable geometry precipitation vessel. Secondly, there was a mind-set at the plant

and the regulatory authority that a criticality accident was not a credible event.