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Magpie Country Nukes Headliner

nuclear issues news brief from Japan
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Criticality accident at Tokai uranium processing plant

22nd REPORT

URANIUM SOLUTION EXTRACTED, BUT IN AN INADEQUATE WAY;
MYSTERY AS TO THE QUANTITY OF SOLUTION IN THE TANK;
UNRELIABLE FIGURES OF FISSION PRODUCT RELEASE

24 October 1999

JCO Company plans to shield the accident site (i.e. building) with concrete blocks 30cm thick and 300cm high. The radiation level (mainly gamma) is still abnormally high around the building and the off-site residents are rightfully worried about it.

On Friday 22 October, JCO and STA (Japanese Government's Science and Technology Agency) extracted 40cc (or 50cc according to certain insider information) of the uranium nitrate solution out of the precipitation tank (sediment basin) in which the uncontrolled criticality occurred. This would provide crucial data regarding the nature and extent of the chain reaction that took place in the tank (see 21st Report).

It must be pointed out, however, that JCO did not stir the solution before they took sample of it. They claim that the stirring mechanism (which is a part of the precipitation tank in question) was malfunctional. They only managed to extract the sample from the surface of the uranium solution that was in the tank. Whatever the reasons or technical constraints may be, the fact flatly means that the result of the analysis of the extracted sample may be incorrect (most probably an underestimation) as regards the number of fissions and the quantity of the fission products generated.

It was also confirmed that there was only less than 20 liters of the uranium solution in the tank. It had been estimated that there were some 40 litters in there. The JCO workers said they had poured 7 bucketfuls of the uranium solution into the tank, each bucket containing 6.5 liters, unexpectedly triggering the chain reaction. If the volume of the solution in the tank (i.e. the substance which went into critical) was really less than 20 litters, it means that the workers could have been unlawfully handling nitrate solution with much higher fissile uranium concentration than the prescribed 18.8%.

As a matter of fact, the JCO plant has an STA license to deal with up to 20kg of 50% U-235. It is just incredible (and outrageous!) that a plant with such a license had no safety measure or contingency plan for a criticality control failure. The responsibility of STA as supervisor is grave.

As to the radioactive iodine-131 concentration that were going out of the accident site (= reconversion test building), JCO admitted that the data they had previously publicized was incorrect. On 14 October, JCO stated that they were releasing 0.51x10E-6 Bq/cm^3 of I-131 (which figure is below the STA limit of 10E-6 Bq/cm^3). On 15 October, JCO said that the figure was actually 1.05x10E-6 (which is above the limit). This figure was based on the exhaust air they had monitored the previous day. The figure with the air sampled on 14 Oct was 1.4x10E-6 8 (i.e. increasing leak!)

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