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Tokai JCO Criticality Accident

Report [13]11/2/99 16:15

20 Liters of Uranium Solution in the Precipitation Basin Missing!

Science and Technology Agency (STA) announced on October 21st to the Nuclear Safety Committee that about 20 liters of the uranium solution in the precipitation basin was missing. This fact was discovered when JCO employees went into the conversion building to collect a sample of the uranium solution on October 20th. According to one of the three exposed employees that were directly connected with the accident, the employees had added 40 liters of uranium solution into the precipitation basin. On October first, boric acid was poured into the precipitation basin through a hose that was attached to a fire truck. The hose was not collected after the liquid was poured in and the criticality had stopped. It was reported on October 15th that this hose was contaminated. (See report on contaminated hose) This hose was retrieved when the sample was collected on October 20th. Since 17 liters of boric acid liquid was added to the precipitation basin to contain the criticality, it was assumed that there would be 57 liters of liquid in the basin. However, there was only about 37 liters of liquid in the precipitation basin.

On the day of the accident, the JCO employees put the uranium solution into a bucket, then put it through a filter and into a beaker, and finally into the precipitation basin. It has been reported that the uranium solution was left in the beaker, the bucket, and the filter. Some are speculating that the lost 20 liters of solution is still in these containers and particularly in the filter. This filter was regulated to be used for liquid waste containing no uranium, and was not meant to be used for uranium solution. This filter is made out of stainless steel. There was no mention of this filter in the manual, and of course the filter was not approved by the government to be used for uranium solution. Others speculated that the solution had flown backwards out of the hose that was used to pour in boric acid solution on October 1st to contain criticality.

The solution was taken to the Nuclear Fuel Cycle Safety Engineering Research Facility at Tokai at around 4:00 p.m. on October 20th when the sample was taken. The sample is still being analyzed, but STA reported on October 28th at the Nuclear Safety Commission that seven types of fission products were found in the 50cc of sample taken from the solution in the precipitation basin. The seven fission products found were:

barium 140
iodine 131
cesium 137
zirconium 95
molybdenum 99
cerium 144

Further report will follow soon.


Citizens' Nuclear Information Center
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Shinjuku-ku, Tokyo, 162-0065, Japan