Résumé
A better understanding of the formation of actinide (IV) silicates from the alteration of oxides in spent nuclear fuel under geologic repository conditions, and above all their stability in contact with groundwater are crucial to evaluate the possible release of radionuclides into the environment. To this aim, the formation of thorite, ThSiO4, and coffinite, USiO4 and their thermodynamic properties have already been extensively studied [1;2]. If hydrothermal syntheses appear to be an efficient way to obtain most of silicates, the aim of this study is to adapt and determine optimal synthesis conditions in the case of the plutonium system. In fact, little is known about PuSiO4 (zircon type structure); Keller's work being the only one to describe its synthesis [3]. Furthermore, no hydrothermal synthesis of orthosilicates has been performed using reactants in nitric acid medium which is usually used to stabilize PuIV.Therefore, the first step of this study was to perform and optimize the syntheses of ThSiO4 and CeSiO4, starting from ThIV and CeIV in nitric acid media, in order to determine suitable conditions for PuSiO4 synthesis. On the one hand, CeIV is usually used as a surrogate for PuIV, because of the really close ionic radii of Ce4+ and Pu4+ and the possible stabilization of their III/IV oxidation states under close conditions. CeSiO4 hydrothermal synthesis is reported [4] but the protocol is different from Keller's PuSiO4 synthesis. On the other hand, ThIV speciation in solution would be more representative of PuIV than CeIV. The first ThSiO4 hydrothermal synthesis was performed by Frondel and Collette [5], since improvements of the protocol was achieved by Costin et al. [6;7].Based on these last developments, experiments were thus performed to determine the influence of the nature of the medium, its acidity and the duration of the hydrothermal treatment on the yield of CeSiO4 and ThSiO4 formation prior to the transposition to PuSiO4.[1]A. Mesbah et al., Inorg. Chem., vol. 14, no. 54, pp. 66876696, 2015.[2]S. Szenknect et al., Inorg. Chem., vol. 52, no. 12, pp. 69576968, 2013.[3]C. Keller, Nukleonik, vol. 5, no. 2, pp. 4148, 1963.[4]J. M. S. Skakle et al., Powder Diffr., vol. 15, no. 4, pp. 234238, 2000.[5]C. Frondel and R. L. Collette, Am. Mineral., vol. 42, no. 378, pp. 759765, 1957.[6]D. T. Costin et al., Inorg Chem, vol. 50, no. 21, pp. 1111711126, 2011.[7]D. T. Costin et al., Prog. Nucl. Energy, vol. 57, pp. 155160, 2012.