The effect of rare earths on thermal conductivity of uranium, plutonium and their mixed oxide fuels

The effect of solid fission products on the thermal conductivity of oxide fuels has been studied systematically by simulating them with their stable isotopes. (U, R)O2.00 (R=Nd, Sm, Eu, Gd, Y; RO1.5=0-15 mol.%), (Pu, R')O2.00 (R'=Nd, Y; R'O1.5=0-10 mol.%) and (M, R'')O2.00 (M=U0.8+Pu0.2, R''=Nd, Eu; R''O1.5=0-10 mol.%) were prepared and thermal conductivities were determined in the temperature range of 700-1900 K from thermal diffusivities measured by a laser flash method. The decreases in the thermal conductivities of solid solutions by adding rare earth elements to the oxide fuels are governed mainly by the lattice defect thermal resistivities. These can be reasonably explained by the lattice defect model for dielectric solids considering U4+, U5+, Pu4+, Pu5+, R3+ or O2- ions as the phonon scattering centres. (orig.)