Diffusion of Actinides in Pyrocarbons: Thin-Layer Experiments

Abstract

The diffusion of uranium and thorium in columnar pyrocarbons between temperatures of 1450° and 2065°C has been studied using the usual thin-layer depletion technique. Constant coefficients are obtained that are in good agreement with results reported elsewhere for uranium diffusion in the 〈a〉 direction at actinide concentrations less than about 0.15-mg actinide/cm3. Diffusion coefficients increase at higher actinide concentration. The anisotropic pyrocarbons are well oriented with the basal planes aligned parallel to the deposition surface. Diffusion coefficients parallel to the deposition surfaces are about three orders-of-magnitude greater than those in the perpendicular direction. The difference is due primarily to the pre-exponential term in the usual Arrhenius equation. Autoradiographic studies of specimens in which diffusion occurred perpendicular to the deposition surface show segregation of actinides to growth cone centers or boundaries depending on whether diffusion occurs along or opposite the direction of growth of the pyrocarbon. The segregation is attributed to rapid diffusion parallel to the deposition surface coupled with the roughened nature of the surface. The results are discussed in terms of previously proposed models for actinide diffusion in columnar pyrocarbons.