Tue Oct 5 09:47:29 1999
Date: Tue, 5 Oct 1999 15:36:03 (lokal tid) +0900
From: hosokawk@cc.saga-u.ac.jp (Hosokawk)
Subject: MagpieNews #991005 (accident-13rd report)

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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

13rd REPORT

ACCIDENT POSSIBLY LEVEL 5;
MORE THAN 100 TERA-BECQUEREL FALLOUT;
POSSIBILITY OF FURTHER RELEASE REMAINS

5 October 1999

Several groups of independent scientists have carried out survey around the JCO Tokai plant, mainly in the 350m zone. Radioactive fallout of cesium-137, iodine-131, strontium-91 have been confirmed. Also sodium-24, which was produced by neutron radiation by the accident, has been extensively detected both by STA and indenpendent groups.

The following is an edited version of a tentative calcuration of the amount of fission products release by the JCO Tokai accident.

MagpieNews is grateful to CNIC and Dr Jin Takagi for their kind permission of citation.

There are also a couple of graphs on the Japanese webpage of CNIC, indicating neutron dose rate. http://pub.jca.apc.org/mirror/cnic/news/tokai_critical/index.html#repo3

English readers can have a look at these figures, disregarding the Japanese text around.

Dr Takagi reserves that these graphs are tentative ones, based on provisional calculations based on the limited date available at the moment. Updated assessment will be publisized shortly. So keep tuned.

==========EDITED TEXT BEGINS==========

Of the uranium poured into the precipitation basin (= "sediment tank" as reported in the previous MagpieNews) that caused the criticality accident, CNIC assumes that roughly 1-10grams of uranium 235 underwent fission, and 1E+16 to 1E+17 Bq (becquerel) of fission products were produced. Assuming the radioactive materials released at Chernobuyl was roughly 1E+19 Bq, the Tokai release may amounts to 1/1000 to 1/100 of that.

IF 1/100 of the produced fission products (radioactive materials) was released, based on the assumption that all the radioactive gas was released and some of the other radioactive materials were released as well, THEN the amount of radioactive materials during the JCO Tokai accident was 1E+14 to 1E+15 Bq, i.e. one hundred teraBq to one thousand teraBq.

According to IAEA International Nuclear Event Scale (INES), an accident involving a release of this much radioactivity is categorized as Level 5. STA announced that the Tokai accident was Level 4. In reality, however, it is a Level 5 accident.

CNIC assumes that an amount of radioactive materials corresponding to the amount produced in the facility were released to the outside of the facility, and that the radioactive materials with short halflives have already attenuated. However, it is certain that no small amount of radioactive materials are still left inside the precipitation basin.

It has been reported that the gamma ray emitted from the remaining radioactive materials are kept inside because the JCO company put a shield consisting of layers of aluminum around their premises. However, this shield is an unreliable tentative treatment, and there are serious doubts for its efficiency.

There is a possibility that some of the remaining radioactive materials are still being released at the moment, and the possibility of such materials being released in the future cannot be denied.

==========EDITED TEXT ENDS==========

More analysis will follow.

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Sidst opdateret 12. oktober 1999