The comprehensive understanding of a fuel’s microstructure is necessary when trying to predict its performance under varying conditions. As MOX fuel is burned up in the reactor, the production of fission products leads to microstructural changes with the formation of new phases and precipitates. At higher sample burn-ups, the grey phase forms mid-radially and eventually concentrates in the central region of the fuel pellet. Due to the lower thermal conductivity of the grey phase with respect to the bulk of the fuel, the accumulation of it at the hot center of the fuel generates concern of localized melting under accident conditions. This project aims to build a reconstruction of the mid-radial region possessing the grey phase in a MOX fuel sample irradiated to a burn-up of 13.7% fissions per initial metal atom (FIMA). Focused ion beam (FIB) tomography coupled with electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) will be used to gather microstructural and microchemical data while serially sectioning through a volume of material. The micrographs, EBSD, and EDS data will be used to build a comprehensive reconstruction of the three-dimensional (3D) microstructure. Once complete, the reconstruction will be provided to modelers to aid in their development of advanced fuel performance simulations that can predict the fuel’s behavior at increased temperatures. This project will deliver an accurate microstructural and microchemical model of the grey phase found in high burn-up MOX fuel. With an expected timeline of 4 months, the completion of this work will significantly advance the current understanding of the grey phase and its effect on MOX fuel’s properties.