Database of radiological incidents and related events--Johnston's Archive

Toki-mura criticality accident, 1999

compiled by Wm. Robert Johnston
last modified 11 June 2006

Date: 30 September-1 October 1999

Location: JCO Fuel Fabrication Plant, Toki-mura, Ibarakin, Japan

Type of event: criticality accident at fuel fabrication plant


Three operators were engaged in processes combining uranium oxide with nitric acid to produce a uranium-containing solution for shipment. The uranium involved was 18.8% U-235. The procedure used deviated from that licensed to the facility. In particular the uranium solution was being placed in a precipitation tank for dispensing into shipment containers, not the more narrow vessel (geometrically favorable to minimizing criticality risks) prescribed by license. At about 10:35 AM, while two workers were adding a seventh batch of uranium solution to the tank, a criticality excursion occurred. The two workers, along with a third worker nearby, observed a blue flash and fled the location; simultaneously, gamma-radiation detectors went off in the building and two adjacent buildings, prompting all workers to evacuate to a muster area. Workers were relocated following higher than background radiation readings. The two workers who had been pouring both began vomiting during transport to the hospital. The excursion continued for 20 hours (the facility did not have a procedure for dealing with criticality events) until outside experts were brought in to drain the tank, shortly after midnight. At 3:18 PM an evacuation of residents within 350 meters of the site had been ordered due to 5 rad/hr readings at the facility boundary; at 10:30 PM an advisory was issued to residents within a radius of 10 km to stay indoors. Of the three workers involved in the accident, the one pouring the solution received 600-1,000 rem and died 210 days later; the one holding the funnel received 1,600-2,000 rem and died 82 days later; and the one at a nearby desk received 100-450 rem and was hospitalized for three months. Both workers who died had received transplants of blood stem cells. The highest doses to neighboring residents were between 5 and 25 rem in the case of about 20 residents.

Consequences: 2 fatalities (1,700 and 800 rem), 1 injury (300 rem).


© 2004, 2005, 2006 by Wm. Robert Johnston.
Last modified 11 June 2006.
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