CHANGE IN PHYSICAL-PROPERTIES OF HIGH-DENSITY ISOTROPIC GRAPHITES IRRADIATED IN THE JOYO FAST-REACTOR

Thirteen kinds of isotropic graphites with different density and maxim um grain size were irradiated in the experimental fast reactor ''JOYO' ' to fluences from 2.11 to 2.86 x 10(26) n/m(2) (E > 0.1 MeV) at tempe ratures from 549 to 597 degrees C. Postirradiation examination was car ried out on the dimensional changes, elastic modulus, and thermal cond uctivity of these materials. Dimensional change results indicate that the graphites irradiated at lower fluences showed shrinkage upon neutr on irradiation followed by increase with increasing neutron fluences, irrespective of differences in material parameters. The Young's modulu s and Poisson's ratio increased by two to three times the unirradiated values. The large scatter found in Poisson's ratio of unirradiated ma terials became very small and a linear dependence on density was obtai ned after irradiation. The thermal conductivity decreased to one-fifth to one-tenth of unirradiated values, with a negligible change in spec ific heat. The results of postirradiation examination indicated that t he changes in physical properties of high density, isotropic graphites were mainly dominated by the irradiation condition rather than their material parameters. Namely, the effects of irradiation induced defect s on physical properties of heavily neutron-irradiated graphites are m uch larger than that of defects associated with as-fabricated specimen s.