Diffusion of Silver and Cesium in Silicon-Carbide Coatings of Fuel Particles for High-Temperature Gas-Cooled Reactors

Abstract

Fractional releases of silver and cesium from irradiated silicon-carbide-layered coated particles have been measured during isothermal anneals in the temperature range between 1273 and 1773 K. The release rates measured have been evaluated with the aid of a statistical numerical treatment based on a simple diffusion model in multizone geometry. The resulting diffusion coefficients can be described byDSiCAg = 4.5 × 10−9 m2·s−1 exp(−217.7 kJ·mol−1/RT) for silver and DSiCCs = 3.5 × 10−9 m2·s−1 exp(−236.6 kJ·mol−1/RT) for cesium, respectively.A statistical treatment of the data gives corresponding 95% confidence limits. It is argued that the pathway of cesium and silver transport in siliconcarbide layers is grain boundary diffusion. This explains the large scatter found in the data for coating layers because the defect structure depends on the individual manufacturing conditions and varies from coating to coating. Comparison with data from the literature shows the superiority of silicon carbide with respect to silver retention relative to diffusion rates in pyrocarbons, while cesium data indicate no distinct improvement.