The fission products that form a solid solution with UO2 and the point defects influence the thermal

conductivity at the atomic scale. Their effect on the thermal conductivity is interpreted in terms of pho­non scattering and is described by the formula l = 1/ (A + BT). The constant A can be expressed as the sum of the thermal resistance due to phonon scatter­ing by substitutional atoms or point defects (impuri­ties, interstitial atoms, vacancies) using the expression obtained by Ambegaoker.10 The main parameters are the Debye temperature, the mean phonon velocity, and the phonon diffusion cross section of the defects. Extended defects such as grain boundaries scatter phonons by limiting their mean free path. The product BT corresponds to the intrinsic lattice thermal resis­tivity caused by phonon-phonon scattering. The value of B can be evaluated from a simplified model by Leibfried and Schlomann,11 but the information avail­able on the physical and thermodynamic data of UO2 is not sufficiently accurate. Therefore, B is usually obtained empirically from the measured thermal resistivity slopes.