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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.
5


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