Beyond the Limit—Investigating Microstructural and Compositional Evolution at the FCCI in High Burnup UO₂

A comprehensive understanding of the fuel-cladding chemical interaction (FCCI) in high burnup UO₂ is needed to ensure cladding integrity and predict fuel performance beyond the current burnups of commercial nuclear fuel. This study investigates the FCCI and high burnup structure (HBS) regions in a discharged light water reactor (LWR) UO₂ fuel pellet irradiated to a burnup of approximately 73 MWd/kgU. A combination of analytical techniques—focused ion beam (FIB) tomography, energy dispersive spectroscopy (EDS), transmission Kikuchi diffraction (tKD), and transmission electron microscopy (TEM)—was employed to examine the microstructural and chemical evolution in these regions. The 3D structural and compositional reconstructions of the FCCI reveal critical changes in material microstructure and behavior, including porosity, fuel cracking, and chemical alterations. Simultaneously, lower-length scale analyses provide detailed insights into grain character, fission gas precipitation, and phase transformations. These findings will inform and validate mechanistic meso-scale material models, such as those used in MARMOT, to accurately predict key thermomechanical properties and assess fuel and cladding performance at higher burnups.