Chemical Interactions between Palladium and UCO Nuclear Fuel Kernel

Abstract Tri-structural Isotropic (TRISO) particle fuel is an advanced coated‑particle system engineered to limit fission‑product release through its multilayer containment architecture, yet uncertainties remain on how fission product–fuel chemical interactions influence fuel microstructure and mechanical performance. In this study, a palladium (Pd) and UCO fuel kernel diffusion couple was annealed at 1100°C for 100 hours enabling direct observation of previously uncharacterized diffusion behavior and chemical interactions between fission product and fuel matrix. This study, through Scanning Transmission Electron Microscopy – Energy‑Dispersive X‑ray Spectroscopy (STEM-EDS) mapping and selected area electron diffraction (SAED) revealed a degradation mechanism of UCO under high temperature annealing and Pd diffusion. The annealed UCO showed the emergence of carbon discontinuous precipitates along UC grain boundaries and U 2 C 3 string precipitates within UC grains. The interfacial region between UCO and Pd mainly showed reaction products like UPd 3 and carbon. Nanoindentation technique revealed a decrease in average hardness in regions where UPd 3 and porous areas were present.