Thermal Diffusivity of Ceramics During Neutron Irradiation

Abstract

Several structural ceramic materials possess many superior properties for nuclear applications, such as blanket of future fusion reactor or core material of high-temperature gas cooling fission reactor for hydrogen generation, where they would be exposed to high fluence of neutrons at temperatures up to 1400K [1–3]. High fluence of neutron irradiation introduces various changes in physical properties of the materials, especially in thermal diffusivity. Thermal diffusivity is one of the most important factors for the plant efficiency. Thermal conductivity K(W/K·m) is obtained as K = αCpρ, where Cp (J/K·kg) is specific heat at constant pressure and ρ (kg/m3) is density. Change in density after an irradiation is measured by linear or volume swelling, but the amount is not so large ( < 2% ), and the specific heat keep almost same after an irradiation [4]. So a change in thermal diffusivity almost represents a change in thermal conductivity. In ceramic materials, unlike metals, heat is mainly carried by phonon. Phonon transportation is obstructed by two factors. Even in unirradiated ceramics, phonons are scattered among each other, and this phonon-phonon scattering increases with measurement temperature arise. In addition, phonon is scattered by lattice defects. This phonon-lattice scattering is decided by grain size or concentration of impurity in the case of unirradiated ceramics, but neutron irradiated material contains many lattice defects, especially vacancy scatters phonon severely. Hence, it has also been reported that neutron-irradiated specimens show severe degradation in the thermal diffusivity [5–12]. These post irradiation measurements were performed at room temperature to specimens after the irradiation. Nevertheless, it is very important to estimate the thermal diffusivity during the irradiation. But it is very difficult to measure thermal diffusivity during neutron irradiation directly. So it is required to estimate that from the post irradiation measurement, with consideration of both phonon-phonon scattering and phonon-lattice scattering. Thermal diffusivity α (m2/s) of neutron-irradiated ceramics depend onmeasured temperature T (K) as α = k/Tn, where k is the constant that is related to the absolute value, and n is the constant that represents the state of induced defects. The n constant is usually 1 for unirradiated ceramics, but it decreased with the irradiation, so it is not easy to 3