For future generation nuclear fuel cycle, actinide co-conversion processes are one option for the co-management of actinides. Oxalic acid is a well-known reagent to recover actinides thanks to the very low solubility of An(IV) and An(III) oxalate compounds in acidic solution. Therefore, considering mixed-oxide fuel or considering minor actinides incorporation in ceramic fuel materials for transmutation, oxalic co-conversion is convenient to synthesize mixed oxalate compounds, precursors of oxide solid solutions.

 

Up to now, only U(IV)-Ln(III) oxalate solid solutions have been completely structurally described started from single-crystals. In these oxalate compounds, a mixed-crystallographic site which accommodates both elements in spite of their different charges has been established. The extent of this study on powder compounds allowed to examine the influence of An(IV) and An(III) nature. The switch of actinide’s nature causes unexpected structural modifications underlining the complexity of specific transuranium elements physical chemistry and the need to pursue studies on single crystal on these actinides.

 

As the existing oxalate single crystal syntheses are not adaptable to the actinide-oxalate system, several original crystal growth methods allowing the formation of mixed actinide oxalate crystals were first developed. Applied to the mixed actinide systems, they lead to the formation of the first mixed An(IV)-An(III) oxalate single crystals. The results, including the different structural resolutions, are presented.