Abstract
The diffusion coefficients of plutonium in uranium/plutonium mixed oxide single crystals at varying oxygen to metal ratios have been determined. The diffusion coefficients of plutonium in UO2 single crystals in the temperature range 1500°C--1900°C have also been determined. The measurements on the mixed oxides were made at temperatures of 1500°C, 1600°G and 1700°O. The oxygen to metal ratios were controlled by annealing either either flowing H2/H2O atmospheres with known water content, or under CO/CO2 atmospheres of known ratio. One face of the sample was highly polished, and then coated with a very thin layer of tracer material (Pu-238), using an electron-gun apparatus. The alpha-particle spectrum from the coated layer was measured using a solid-state detector coupled to a multi-channel analyser. Samples were annealed in pairs with polished faces together (sandwich arrangement), under the appropriate conditions for a specified time period. Following annealing the alpha-particle spectrum was re-measured, and from the new tracer distribution a value of Dt was calculated (D - diffusion coefficient, t - time of anneal). The depth of penetration of tracer into the sample was determined using the 'alpha energy degradation method', which uses the principle that the further an alpha-particle travels in a particular medium the more energy it loses. Thus the residual energy of the particle on reaching the detector is proportional to the distance it has travelled. Alpha-particle spectra were not measured over the complete surface area as is conventional for this type of experiment, instead they were, measured over restricted areas of the surface (avoiding the edges), since it was found, during the course of this work that evaporation effects caused distortion of the surfaces. This distortion gave rise to misleading alpha spectra, in turn leading to incorrect values for the diffusion coefficient. The values of the measured diffusion coefficients are appreciably lower than existing literature data would have indicated.