Résumé
Sodium-cooled fast neutrons reactors (SFR) are among the concepts of reactors developed in the Generation IV International Forum. The capacity of these reactors to split transuranic elements allows for better resources management, for control of plutonium stocks and for the reduction of waste toxicity. Fuels for these SFR reactors are Mixed OXide with higher plutonium content than in the MOX fuels dedicated to LWR reactors. They induce significant challenges for the reprocessing step due to the refractory nature of plutonium dioxide in nitric acid medium. One part of the research developed on these SFR MOX fuels relies on the development of modeling codes able to reproduce the dissolution behavior of these mixed oxides. In particular, Pu-enriched heterogeneities present in small quantities in the SFR oxide fuels are expected to significantly partici-pate in the final balance of residues of dissolution. Furthermore, SFR spent fuels also contain refrac-tory metallic inclusions (mainly composed by Ru, Rh, Pd, Tc and Mo) which can impact the dissolu-tion at the solid/liquid interface and in solution. For these reasons, characterization of such Pu enriched heterogeneities and metallic inclusions was required in order to study their impact on the dissolution behavior of SFR oxide fuels and then to improve the current modeling codes regarding dissolution. Using thorium as a plutonium surrogate, heterogeneous U1-xThxO2 samples including platinum group elements (Ru, Rh, Pd) and Mo have been synthetized by wet chemistry route. Several U1-xThxO2 pow-ders containing various thorium contents have been prepared by hydroxide precipitation from mix-tures of cations in solution [1]. The powders were mixed then sintered through a two-step procedure involving uniaxial pressing at room temperature followed by a heating treatment at 1600 °C under reducing conditions. Through this sintering step, metallic inclusions were formed in the materials while the pellets also contained heterogeneities in terms of U/Th mole ratios. The heterogeneity, microstructure and metallic inclusions distribution were then characterized using image analysis and geostatistical tools (a set of mathematical techniques well suited for image analy-sis of spatialized phenomena like heterogeneities distribution [2]) for validation of their ability to mim-ic SFR spent fuel. Finally, several dissolution tests were conducted in concentrated nitric acid.In this presentation, the synthesis of the samples as well as the method used to quantify the hetero-geneity of the synthetized mixed oxides will be developed. The prepared materials will be compared to spent fuel reprocessing. Finally, results of dissolution tests of U1-xThxO2 pellets doped (or not) with metallic inclusions will be presented.