Etude expérimentale et modélisation, en fonction du pH et de la concentration en NaCl, du système ternaire U(VI)-NaCl-H2O à T = 155°C et pression de vapeur saturante

Abstract : An experimental study on the solubility of U(VI) in concentrated (0.5-6.0 M) NaCl solutions as a function of pH (3-13) has been realized at 155°C and saturation vapor pressure. The obtained results indicate that the initial solid phase UO3 initial solid phase is transformed into multiple solid phases of hydrated sodium uranates with general formula Na2xUyO(x+3y), the Na/U molar ratio of which increases with increasing pH and NaCl concentration. The solubility of U(VI) is directly dependant of three physico-chemical parameters : the pH, the Cl- ligand, and the Na+ cation. The first two parameters strongly favour the U(VI) solubility in the form of aqueous U(VI) complexes with the OH-and Cl- ligands at a pH = 4, while the Na+ cation inhibits the solubility of U(VI) as it precipitates into various low solubility sodium uranates. The results of solubility measurements obtained for U(VI) serve as a reference data base for the calculation of concentration variations of each soluble U(VI) complex as a function of two parameters, pH and [Na], with the use of a negative elliptic paraboloïd exponential. An empirical model has also been developed from the Pitzer equations in order to determine, in a continuous way, the variations of various physico-chemical parameters of the soluble ions present in the concentrated NaCl solutions as a function of four parameters: pH, [Na], T and P of the reaction medium. This model is valid up to [NaCl] = 6M, T = 300°C and P = 100 MPa.
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Christophe Rozsypal. Etude expérimentale et modélisation, en fonction du pH et de la concentration en NaCl, du système ternaire U(VI)-NaCl-H2O à T = 155°C et pression de vapeur saturante. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 2009. Français. ⟨NNT : 2009NAN10127⟩. ⟨tel-01748294⟩

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