Irradiation damage in nuclear graphite and carbon material

Nuclear graphite and carbon material were irradiated in the JMTR, JRR-2 and HFR at 550 - 1335degC up to a maximum neutron fluence 6.8 x 1025 n/m2 (E > 29 fJ). Changes in dimension, thermal expansivety, thermal conductivity, electrical resistivity and Young's modulus were measured after the irradiation and the successive thermal annealing at each fixed temperature up to 2300degC. Macroscopic dimensional shrinkages were observed, and the shrinkage rate were the smallest value at irradiation temperature of around 850degC, and carbon material showed larger shrinkage rate than that of nuclear graphite. Coefficient of thermal expansion increased in the initial small range of neutron fluence, followed by decrease with neutron fluence. After the irradiation thermal conductivity decreased and electrical resistivity and Young's modules increased. Heat treatment was done for irradiated nuclear graphite and carbon material, and property changes were measured. Significant difference in dimensional changes was observed for the samples of nuclear graphite and carbon material. Increase of thermal conductivity was observed for irradiated carbon material. A model of irradiation defects is given to explain the effects of irradiation and thermal annealing. (author)