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

Thermal conductivities of stoichiometric SIMFUEL - simulated high-burn up UO2 fuel - with equivalent burnups of 1.5, 3 and 8 at%, deduced fro m thermal diffusivity and specific heat measurements, are modelled as a function of burnup, taking into consideration two key microstructura l features of irradiated high-burnup fuel: (i) the precipitated fissio n-product phases, and (ii) the dissolved fission products in the matri x. The degradation of the UO2 matrix thermal conductivity due to the d issolved fission products (after corrections for the precipitated-prod uct phases) was analyzed by considering their effect on the phonon hea t current. The degree of the scattering of the phonons by dissolved ad ditives in the matrix (scattering parameter) is a function of the phon on 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 addi tives. 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 fu el thermal conductivity, in the absence of fission-gas bubbles, can be explained by the phonon scattering from the dissolved fission product s.