MODELING OF UO2-BASED SIMFUEL THERMAL-CONDUCTIVITY - THE EFFECT OF THE BURNUP

Thermal conductivities of stoichiometric SIMFUEL - simulated high-burnup UO2 fuel - with equivalent burnups of 1.5, 3 and 8 at%, deduced from thermal diffusivity and specific heat measurements, are modelled as a function of burnup, taking into consideration two key microstructural features of irradiated high-burnup fuel: (i) the precipitated fission-product phases, and (ii) the dissolved fission products in the matrix. The degradation of the UO2 matrix thermal conductivity due to the dissolved fission products (after corrections for the precipitated-product phases) was analyzed by considering their effect on the phonon heat current. The degree of the scattering of the phonons by dissolved additives in the matrix (scattering parameter) is a function of the phonon frequency, and was determined using phonon heat current theory. The scattering parameter followed the theoretically predicted square root dependence on the temperature and concentration of the dissolved additives. The thermal conductivity of the SIMFUEL matrix, predicted from the model based on the phonon scattering by dissolved fission products taken into account their mass difference, was in good agreement with the measured values. These results indicate that the degradation of fuel thermal conductivity, in the absence of fission-gas bubbles, can be explained by the phonon scattering from the dissolved fission products.