Résumé
Uranium-Plutonium mixed oxides U1-yPuyO2-x (MOX) are considered as the reference fuels for Sodiumcooled Fast neutron Reactors (SFRs). The fissile nuclear core is composed of dense ceramic pellets. Their plutonium content and their Oxygen/Metal ratio (O/M with M = U + Pu), must range between 20 to 35mol. % and 1.94 ≤ O/M < 2.00, respectively. In the reactor, a thermal gradient along the fuel pellets radius(typically ̴4 mm) occurs, with a temperature reaching 500 °C close to the cladding while the center istypically in the 1800-2000°C range1. At the beginning of the irradiation, this thermal gradient induces aredistribution of plutonium and oxygen atoms along the pellet radius. Therefore the pellet is enriched inplutonium at its center with a local Pu content in the 30-70 mol. % range (depending on the location ofthe pellet inside the nuclear core) and a reduced O/M ratio (< ̴1.95). The region close to the cladding isdepleted in plutonium with a Pu content around 20 mol. %, and an O/M ratio close to the stoichiometry( ̴2.00). This phenomenon leads to significant heterogeneities in the fuel thermo-physical propertiesalong the pellet radius. As instance, the local melting point will be modified as it depends upon both theO/M ratio and the Pu content. An enriched local plutonium content induces a decrease in the meltingtemperature and thus a decrease in the safety margin of the reactor. Studying the thermodynamic andstructural properties of U-Pu mixed oxides with high plutonium content is thus mandatory.The main difficulty lies in the lack of data (enthalpy, heat capacity, conductivity or melting temperature,…) for MOX fuels with Pu content greater than 45 mol. % and even more for MOX fuels with Pu > 60 mol.% 2-6. These data would improve the modelling of the U-Pu-O phase diagram needed for the fuel behaviorsimulation codes7. To fill this knowledge gap, manufacturing single-phase and dense MOX pellets with Pucontents between 60-70 mol. % is required. However, determining the appropriate manufacturingparameters is quite challenging due to the complexity of the U-Pu-O phase diagram in the oxygenhypostoichiometric domain (O/M < 2.00) 8.In this frame, we will detail manufacturing processes based on powder metallurgy that are suitable toobtain such dense and monophasic MOX pellets with Pu content of 60, 65 and 70 mol. %. as well as multiscale characterizations results allowing to check the targeted properties. These research are carried out as part of the European PuMMA project (Plutonium Management for More Agility), funding from theEuropean Union's Horizon 2020 research and innovation program under the grant agreement No 945022.