Effect of high temperature annealing on the grain size of CVD-grown SiC and experimental PBMR TRISO coated particles

Abstract

Recent research and development activities of the PBMR Fuel Characterization and Optimization Programme focused on the high temperature characteristics of the PBMR fuel and more specifically on the SiC layer. PBMR fuel consists of Tri-Isotropic (TRISO) Coated Particles (CPs) in a graphite matrix with the SiC layer being the main barrier to fission and transmutation products. The integrity of the CP three layer system namely, Inner Pyrolytic Carbon- Silicon carbide- Outer Pyrolytic Carbon (IPyC-SiC-OPyC) has a direct impact on fission product release. Grain boundary diffusion is identified as a possible mechanism by which 110mAg, one of the activation products, might be released through intact SiC layer. Temperature is evidently one of the factors/parameters amongst others known to influence the grain size of SiC and therefore it is important to investigate the effect of high temperature annealing on the SiC grain size. The ASTM E112 method as well as EBSD was used to determine the grain size of various commercial SiC samples and the SiC layer in experimental PBMR CPs after annealing at temperatures ranging from 1600 to 2100 oC. The high standard deviation results obtained for CVD-grown SiC are discussed and explanation for the observations are given.