Characterization of the Phases Formed During the High Temperature Oxidation of (U,Pu)O₂ Mixed Oxides

The aim of the present study is to investigate the oxidation behavior of model compounds simulating the 'pseudo phases' present in the MIMAS MOX microstructure: UO₂, U_0.89Pu_0.11O₂ and U_0.72Pu_0.28O₂. To complete our understanding of the phenomena, a compound with a higher Pu content (U_0.55Pu_0.45O₂) is also being studied. Such an understanding requires the study of the effects of Pu content, temperature and pO₂ on the nature of the phases formed during high temperature oxidation, i.e. their crystallographic structure, the oxidation states and local environment of U and Pu in each phase. To achieve this goal, a multimodal and multi-scale approach is used, employing a wide range of characterization techniques: X-ray Diffraction (XRD, sample-scale), μ-Raman spectroscopy (μm-scale), Scanning Electron Microscopy (SEM) and X-Ray absorption spectroscopy (HERFD-XANES and EXAFS). The oxidation experiments were carried out between 350 C and 500 C under controlled atmosphere (Ar-O₂ mixture containing 20 % to 80 % O₂). The starting and resulting materials were first characterized at the ATALANTE facility (CEA Marcoule). The X-ray Diffraction pattern of the U_0.89Pu_0.11O₂ starting sample shows only peaks belonging to a fluorite structure (Fm3-barm space group 225), with a lattice parameter of a = (5.466 ± 0.001) Angstroms. This pattern is compared with that of the U_0.89Pu_0.11O₂ sample oxidized for 3 hours at 500 C in an Ar-80 % O₂ mixture. An increase in weight of about 3.7 % and the appearance of numerous supplementary diffraction peaks confirm the formation of over-oxidized phases. These peaks are attributed to an orthorhombic (U,Pu)₃O₈ phase and a fluorite phase. For the latter, a decrease in the lattice parameter compared to the starting material is illustrated by a shift towards larger diffraction angles compared is observed. This could be caused either by an excess of oxygen (presence of (U,Pu)O_2+x or (U,Pu)₄O₉ phase) or by an increase in the Pu content in this phase. However, considering the detection limit of laboratory XRD (a few percent), transmission synchrotron Powder X-Ray Diffraction (SPXRD) was performed in December 2023 to achieve a comprehensive identification of the oxidized phases formed. Thanks to a better signal to noise ratio, such measurements also provide information on the nature and the crystallographic structure of the minority phases formed during oxidation. Moreover, due to its ability to reveal information at the molecular level (local range order and oxidation states of each cation), HERFD-XANES and EXAFS experiments at M4,5 and L2,3 edges of Pu and U are planned in March and April 2024. These analyses will be carried out at BM20-ROBL (ESRF, Grenoble, France) and MARS beamlines (SOLEIL, Saint-Aubin, France) thanks to their ability to accept highly radioactive samples