Multi-layer coating of silicon carbide and pyrolytic carbon on UO2 pellets by a combustion reaction

The coating layers of silicon carbide and pyrolytic carbon on UO2 pellets were prepared by using a combustion reaction between the carbon and silicon layers. The pyrolytic carbon and silicon were deposited by thermal decomposition of propane at 1250°C in a chemical vapor deposition unit and microwave pulsed electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) using silane at 500°C. Microstructural observation of the layers with scanning electron microscopy (SEM) showed that an inner layer existed following the surface contour of the pellet and the outer layer had a small number of fine pores inside. Chemical analyses with Auger electron spectroscopy (AES) and X-ray diffractometry (XRD) revealed that the inner and outer layers were pyrolytic carbon and silicon carbide, respectively. From the transmission electron microscopy (TEM) observation, the silicon carbide formed during the combustion reaction was identified as fine crystalline β-SiC. The temperature distribution of the specimen during the combustion reaction was estimated by a finite element method, which showed that preheating above 1300°C was required for the combustion reaction between silicon and carbon to propagate well through the specimen.