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Conference Article: Investigation of morphology and impurity of nuclear-grade graphite, and leaching mechanism of carbon-14

R. Takahashi, M. Toyahara, S. Maruki, H. Ueda, T. Yamamoto

Abstract

Leaching of 14C from irradiated graphite waste seems to depend on its morphology. We have investigated morphology and impurities of nuclear-grade graphite in order to reveal the leaching mechanism of 14C. Observation of SEM photograph revealed that planar graphite sheets existed everywhere in the graphite, thus indicating that graphite blocks contained many single-crystal grains. The degree of graphitization for a nuclear-grade graphite sample was obtained by using powder X-ray diffraction method and estimated to be 0.85. The value was large compared with that of artificial graphite, 0.9. It should be noted that the crystalline quality of nuclear-grade graphite is good. In order to estimate the 14C concentration of nuclear-grade graphite with neutron bombardment, 14N content in nuclear-grade graphite was identified by measuring thermal conductivity of produced gas accompanying melting a graphite sample in tin alloy at 2800C. Also, the sample was prior heated at 400C, and then the total amount of nitrogen was measured by the same method to estimate nitrogen absorbed on the surface of the graphite block sample. Further, by SIMS analysis 26CN- depth profile in graphite block was obtained in order to determine the distribution of nitrogen. Overall nitrogen content of graphite block, is consistent with the nitrogen concentration obtained by calculation of neutron activation using 14C content in irradiated graphite. The results of SIMS analysis indicated that nitrogen content was rather large on the surface. Taking into account the leaching data of 14C and distribution of nitrogen, it was suggested that the bulk of 14C activity remained in graphite matrix which was quite stable and that only 14C localized on the surface of the irradiated graphite was released.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
IAEA Technical Committee Meeting on "Nuclear Graphite Waste Management", held from 18-20 October 1999 in Manchester, United Kingdom
International Atomic Energy Agency, Vienna (Austria)
TCM-Manchester99, pp:176-190