Saturation in degradation of thermal diffusivity of neutron-irradiated ceramics at 3 × 1026 n/m2

Unlike metals, heat in ceramic materials is mainly carried by phonons, and neutron-irradiated ceramics show serious degradation in thermal diffusivity. In this work, typical structural ceramics were neutron-irradiated in the fast reactor JOYO, to fluences of 0.4–8.0 × 1026 n/m2 at 646–1039 K, and thermal diffusivity was measured by the laser flash method at room temperature. It showed that the thermal diffusivity of neutron-irradiated ceramics saturated at neutron dose around 3 × 1026 n/m2, and was simply determined by the irradiation temperature. All tested materials showed this behavior, with the saturation value different for each material. In ceramic materials thermal diffusivity is proportional to the mean free path of phonons. From the saturation value of thermal diffusivity at the lowest irradiation temperature of 775 K, the calculated mean free path of phonons related to neutron induced defects was roughly equal to the lattice constant of each material.