Surface relaxation kinetics of uranium dioxide

Abstract

Surface relaxation techniques have been used to investigate surface and vapor-phase diffusion in stoichiometric uranium dioxide. A series of sinusoidal profiles, with wavelengths in the range 8–40 μ, were produced on the (100), (110) and (111) surfaces of UO 2. The relaxation to flatness of these profiles, due to capillary forces, was measured at temperatures from 1660 to 2035°C in a helium atmosphere. From the wavelength dependence of the decay constant, it was established that the relaxation kinetics were controlled predominantly by the diffusion of UO 2 vapor in helium. The vapor-phase diffusion coefficients were determined from the relaxation times associated with surface smoothing, and were compared with the predictions of kinetic theory. The agreement is within a factor of 2–4, and is within the limits of uncertainty of various kinetic parameters. The anisotropy of the measured relaxation rates and the relative importance of surface, volume and vapor-phase diffusion in UO 2 are discussed.