Understanding Heavy Rare Earth Effects on UO₂ Lattice with XAS : A Structural Approach to Lanthanides Separation in Nuclear Waste

The MaLaR European project focuses on improving the selective separation of actinides and lanthanides (Ln) from nuclear waste by exploring their interactions with novel materials. This proposal investigates U0.9Ln0.1O2±δ compounds (Ln = Gd, Dy, Tm, Lu) to understand the structural effects of heavy rare earths elements on the UO2 lattice, particularly charge compensation mechanisms and structural distortions. Samples will be prepared via wet chemistry and sintered under controlled atmospheres which have been evidenced to impact redox speciation and microstructure development. Advanced spectroscopies (HERFD-XANES, EXAFS) will allow us to determine U(IV)/U(V) ratios, check Ln valence states, and evidence the presence of oxygen vacancies. It will also evaluate structural distortions generated by HREE with very small ionic radii, that can be below that of U4+. These findings will further be linked to dissolution kinetics, as a first step to Ln separation process.