Characterizing Thermal Pd Transport in SiC-Coated Tristructural Isotropic Particles

High Temperature Gas-cooled Reactors are part of the next generation of nuclear reactors, with proposed nuclear fuel in the form of Tristructural Isotropic (TRISO) particles. However, uncertainties remain in the mechanism of fission product diffusion within these particles; Pd is one such fission product that may impact irradiation performance. Additional work is required to fully capture bulk Pd diffusion in the TRISO inner and outer pyrolytic carbon layers (IPyC and OPyC) and the silicon carbide (SiC) layer. Pd has been identified in the IPyC-SiC interface and associated with U within SiC grain boundaries, and has been found in greater quantities within the SiC layer if the IPyC layer is fractured and compromised. In this study, Pd-SiC diffusion couples were annealed at 1100 °C for different lengths of time. Scanning electron microscopy was used to observe microstructure of post-annealed system. Energy-dispersive X-ray spectroscopy and Raman spectroscopy were administered to characterize the Pd compounds formed. Micro- and nanoindentation for mechanical testing of the bulk SiC was conducted, especially near the interfaces. This experiment was then repeated using SiC coated surrogate TRISO particles coated with Pd using physical vapor deposition. Electron backscatter diffraction was additionally used to examine Pd transport within the particles. The resulting data will support computational models describing how the mechanical properties of SiC may impact the transport of Pd.