FROM URANOTHORITES TO COFFINITE: A SOLID SOLUTION ROUTE TO THE
THERMODYNAMIC PROPERTIES OF USiO4
Stéphanie Szenknect 1), Dan T. Costin1), Nicolas Clavier1), Adel Mesbah1), Christophe Poinssot2), Pierre
Vitorge3) and Nicolas Dacheux1).
1)
ICSM, UMR 5257 CEA/CNRS/UM2/ENSCM, Site de Marcoule – Bât. 426, BP 17171, 30207
Bagnols-sur-Cèze cedex, France
2)
CEA, Nuclear Energy Division, DRCP/DIR, CEA Marcoule, Bât. 400, BP 17171, 30207
Bagnols-sur-Cèze cedex, France
3)
CEA, Nuclear Energy Division, DPC/SECR, Site de Sclay, Bât. 391, 91191 Gif-sur-Yvette,
France
Coffinite (USiO4) and associated solid solutions are expected to play an important role in the field
of direct storage of spent nuclear fuels in underground repository since they could control the
concentration of actinides in the groundwater. However, the thermodynamic properties associated with
coffinite, especially the solubility constant, remain poorly defined. The few thermodynamic data
related to coffinite formation or solubility reported in the literature are hardly reliable since none of
them are determined experimentally from solubility measurements [1]-[4]. Solubility studies require
pure single-phase USiO4. Most of the natural samples contain coffinite as very fine grain crystals ( 5
µm) [5] and in intimate intergrowths with large amounts of associated minerals. Moreover, for several
decades persistent difficulties have been encountered in the preparation of pure single-phase synthetic
coffinite. Thus, an indirect method based on solubility measurements of Th1-xUxSiO4 samples was
envisaged in this study.
The preparation of synthetic Th1-xUxSiO4 uranothorite solid solutions was successfully undertaken
under hydrothermal conditions (T = 523 K) [6], [7]. The formation of a complete solid solution
between x = 0 (thorite) and x = 0.8 was evidenced by XRD and EDS analyses. Lattice parameters
refinement in the I41/amd space group confirmed the formation of solid solutions between thorite and
coffinite, according to the Vegard’s law. Analysis systematically evidenced the formation of pure Th1xUxSiO4 for 0≤ x≤ 0.4. For higher values of x, a mixed oxide (Th 1-yUyO2) second phase was detected.
Phase quantification by the Rietveld method showed an increase in the amount of the silicate phase
with the hydrothermal treatment time, indicating a slow-rate phase transition of the oxide to silicate.
The kinetics of this transition was found to slow with the increase in the uranium bearing.
Experiments on the solubility of intermediate members of the Th1-xUxSiO4 solid solution were
carried out to determine the impact of Th-U substitutions on the thermodynamic properties of the solid
solution and then allow extrapolation to the coffinite end-member. The ion activity products in
solutions equilibrated with Th1-xUxSiO4 (0 ≤ x < 0.5) were determined by dissolution experiments
conducted in 0.1 mol L-1 HCl under Ar atmosphere at several temperatures ranging from 298 K to
346 K. For all experiments, dissolution was congruent and a constant composition of the aqueous
solution was reached after 50 to 200 days of dissolution. The solubility product of thorite was
determined (log*KS,ThSiO4 = −5.62 ± 0.08) whereas the solubility product of coffinite was estimated
(log*KS,USiO4 = −6.1 ± 0.2). The stoichiometric solubility product of Th1-xUxSiO4 reached a
maximum value for x = 0.45 ± 0.05. In terms of the variation of the standard Gibbs free energy of
dissolution, solid solutions are more likely to dissolve than the end-members. Variations of standard
Gibbs free energy associated with the formation of thorite, coffinite and intermediate members of the
series were then evaluated. Variations of standard Gibbs free energy of formation were found to
increase linearly with the uranium mole fraction. Our data at low temperature clearly show that
uranothorite solid solutions with x > 0.26, thus coffinite, are less stable than the mixture of binary
oxides, which is consistent with qualitative evidence from petrographic studies of uranium ore
deposits.
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Amsterdan, The Netherlands, 1992; Vol. 1, p 715.
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Update on the chemical thermodynamics of uranium, Neptunium, Plutonium, Americium and
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[4] Hemingway, B. S. Thermodynamic properties of selected uranium compounds and aqueous species
at 298.15K and 1 bar and at higher temperatures. Preliminary models for the origine of coffinite
deposits.; US Geological Survey: 1982; p 89.
[5] Deditius, A. P.; Utsunomiya, S.; Ewing, R. C., Chemical Geology 2008, 251, 33-49.
[6] Costin, D. T.; Mesbah, A.; Clavier, N.; Szenknect, S.; Dacheux, N.; Poinssot, C.; Ravaux, J.; Brau,
H. P., Progress in Nuclear Energy 2012, 57, 155-160.
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Inorganic Chemistry 2011, 50, 11117-11126.