Paul Estevenon4 5 Eleonore Welcomme4 Szenknect Stephanie1 6 Adel Mesbah2 6 Philippe Moisy4 Christophe Poinssot4 Nicolas Dacheux3 6

4, CEA, Bagnols-sur-Cèze, , France
5, Ecole Nationale Supérieure de Chimie de Montpellier (ENSCM), Bagnols-sur-Cèze, , France
1, CEA, Bagnols-sur-Cèze, , France
6, Institut de Chimie Séparative de Marcoule (ICSM), Bagnols-sur-Cèze, , France
2, Centre National de la Recherche Scientifique (CNRS), Bagnols-sur-Cèze, , France
3, Université de Montpellier (UM), Montpellier, , France

Silicate species are known to be highly abundant in the environment and even more so in the nuclear waste repository as components of the structural materials and the storage matrix. Therefore the silicate ions need to be considered as potential reactants which may interact with radionuclides and, among others, with actinides. Thorite, ThSiO4, and coffinite, USiO4, are two naturally occurring phases which have been extensively studied because of their abundance in the environment [1,2]. The supposed natural formation mechanism of coffinite, which implies the alteration of oxide phases in reductive and silica rich media, rose important questions about the radionuclides’ behaviour in spent nuclear fuel under geologic repository conditions [3]. Moreover, the formation of oxyhydroxy actinide silicate colloids has been observed for thorium, uranium and neptunium in weakly basic carbonate media at room temperature and dramatically increased the mobility of actinides in the environment [4]. In the case of plutonium (IV), silicate compounds’ formation has been suspected for Pu-containing precipitates observed in basic media [5] and for plutonium borosilicate glasses altered by vapour hydration [6]. Therefore, the synthesis and the determination of the thermodynamic properties of PuSiO4 could be a crucial issue in the assessment of the behaviour of plutonium in the environment.

However, even if PuSiO4 has already been hydrothermally synthesized once [7], 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) [7,8] as PuSiO4, were investigated.

Optimized conditions of synthesis were determined for these three silicate-based systems as a function of several experimental parameters (pH, ligands -including carbonates-, concentration, temperature, reactants, redox state, atmosphere…) and then transposed to the synthesis of the plutonium-silicate system. This comparative study has highlighted the differences between plutonium and its most common surrogates in interaction with silicate ions and allowed to assess in which conditions PuSiO4 may be formed in the environment.

[1] A. Mesbah et al., Inorganic Chemistry, 54, 6687-6696, 2015
[2] S. Szenknect et al., Inorganic Chemistry, 52, 6957-6968, 2013
[3] J. Janeczek, R. Ewing, Materials Research Society, Symposium Proceedings, 257, 497-504, 1992
[4] H. Zänker et al., Chemistry Open Reviews, 5, 174-182, 2016
[5] N. Krot et al., PNNL-11901, UC-2030, 1998
[6] J. Fortner et al., Materials Research Society, Symposium Proceedings, 608, 739-744, 1999
[7] C. Keller, Nukleonik, 5, 41-48, 1963
[8] J. Skakle et al., Powder Diffraction, 15, 234-238, 2000