Study of the hydrothermal synthesis of ThSiO4, USiO4 and CeSiO4 aiming at determining the conditions of PuSiO4 formation

International audienceThorite, ThSiO4, and coffinite, USiO4, are two naturally occurring phases which were extensively studied because of their abundance in the environment [1]. The supposed formation mechanism of natural coffinite implies the alteration of oxide phases in reductive and silica rich media. It rose important questions about the actinides' behaviour consequently to the alteration of spent nuclear fuel under geologic repository conditions [2]. Indeed, silicate compounds' formation has been suspected for Pu-containing precipitates observed in basic media and for plutonium borosilicate glasses altered by vapour hydration. However, even if PuSiO4 has already been hydrothermally synthesized once [3], the favourable conditions for the formation of this phase and its stability domain are not well constrained. To provide more data on the Pu system and to determine suitable synthesis conditions, three surrogates, CeSiO4, ThSiO4 and USiO4, which crystallize in the same zircon type-structure (space group I41/amd) [3,4] than PuSiO4, were investigated. Optimized conditions of synthesis were determined for these three silicate-based systems by varying several experimental parameters so that they could be transposed to the plutonium system. [1] A. Mesbah et al., Inorganic Chemistry, 54, 6687-6696, 2015 [2] J. Janeczek, R. Ewing, Materials Research Society, Symposium Proceedings, 257, 497-504, 1992 [3] C. Keller, Nukleonik, 5, 41-48, 1963 [4] J. Skakle et al., Powder Diffraction, 15, 234-238, 200