Structural and Microstructural Characterization of Fission Product Phases in MOX Nuclear Fuel for Fast Neutron Reactors

In the context of spent fuel recycling and the valorization of plutonium, (U,Pu)O₂ mixed oxides (MOX) have been developed for use in French Pressurized Water Reactors (PWR). They are also leading candidates for some GEN IV reactor concepts, such as sodium-cooled fast reactors (SFR). One of the critical challenges in the nuclear industry is the mastery of the nuclear fuel cycle, specifically plutonium multirecycling. In order to achieve this goal, it is crucial to identify the secondary phases created during irradiation. In this work, (U,Pu)O₂ MOX have been doped with 11 stable fission products (FP) (Sr, Y, La, Nd, Ce, Zr, Mo, Pd, Rh, Ru, Ba) to reproduce FP-based precipitates existing in the real spent fuel. The structural and microstructural properties of these secondary phases were characterized by coupling scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), Electron Probe MicroAnalysis (EPMA), and synchrotron techniques such as X-ray Absorption Spectroscopy (XAS) and Synchrotron Powder X-ray Diffraction (SP-XRD). This analysis highlights the relationship between the partial segregation among metallic FP (Mo, Pd, Rh, Ru) and their crystallographic structures, as well as the speciation shift of several FP induced by the addition of Ba. The synthesized SIMMOX samples present a secondary phase representative of irradiated MOX and can be used as an effective model material to study spent nuclear fuel and its reprocessing.