Transmission electron microscopy (TEM) is a key characterization tool for studying the microstructure of MOX (mixed uranium-plutonium oxide) fuel, regardless of the degree of homogeneity in the spatial distribution of plutonium. Preliminary results have shown that MOX fuel appearing homogeneous at the electron probe microanalysis scale was not homogeneous at the sub-micron scale of its constituent grains (Siméon et al., Journal of the European Ceramic Society 42 (2022) 7237-7246). One of the objectives of this thesis is to gain a detailed understanding of the formation of the MOX fuel microstructure, paying particular attention to potential plutonium-rich heterogeneities, given their significance for reprocessing (nitric acid dissolution). The impact of the manufacturing process (e.g., the MIMAS approach versus the direct co-milling approach) and sintering parameters (atmosphere, isothermal holding temperature, and holding time) on the development of these plutonium-rich zones must be taken into account. To this end, the student will need to identify existing sintered MOX samples (with a Pu/(U+Pu) content of 11 mol%) from previous studies and/or fabricate specific ones. These samples will differ based on the granular material preparation method used prior to uniaxial pressing and on the sintering conditions (specifically, oxygen partial pressure). In addition to standard microstructural characterization (grain size, pore size, and pore location via scanning electron microscopy; plutonium spatial distribution via electron microprobe and Raman spectroscopy), thin-foil preparation using a focused ion beam (FIB) and TEM characterization will be performed on the samples deemed most significant. Particular attention will be paid to plutonium localization (grain bulk, grain boundaries, space-charge layer formation) and local chemical composition (plutonium content, local O/M ratio) using EDS/EELS analysis. Alongside detailed microstructural characterization, the student will conduct nitric acid dissolution tests to attempt to establish correlations between plutonium heterogeneity (size, volume fraction, location, etc.) and the content of undissolved residue. The experimental work will be carried out at Marcoule (Atalante facility) and Cadarache (LECA-STAR laboratory). The results will be disseminated through scientific publications in peer-reviewed international journals.