ABSTRACT
Reduction in UO2 thermal conductivity with burnup has been experimentally observed using simulated high burnup fuel - termed SIMFUEL. (Thermal conductivity was obtained from thermal diffusivity and specific heat measurements). This reduction could be explained by the mass difference between the fission-product solute atoms in the fluorite lattice and U atoms alone, without a need for a strain term.
The calculation of the misfit in ionic radius shows that the largest distortion effect is obtained for simulated fission products of Mo, Pd, Ru, and Rh. Nevertheless these atoms precipitate outside of the fluorite lattice due to the large radial misfit that limits their solubility in the matrix. This explains well the agreement obtained for SIMFUEL between experimental phonon scattering values and those calculated from the mass differences alone. Note that if all the Mo, Rh, Ru cations would be in solution, the phonon scattering would be enhanced by a factor of three and the thermal conductivity of SIMFUEL reduced further by roughly a factor of 0.6.
