Thermodynamic Properties of Uranyl-Containing Materials Based on Density Functional Theory

Abstract

The thermodynamic properties of uranyl containing materials, including dehydrated schoepite, metastudtite, studtite, soddyite, rutherfordine, and γ-UO3 were studied. These materials are among the most important secondary phases arising from corrosion of spent nuclear fuel under the final geological disposal conditions, and γ-UO3 is the main oxide of hexavalent uranium. The crystal structures of the dehydrated schoepite and metastudtite were determined by means of density functional theory using a new norm-conserving pseudopotential for uranium atom. The resulting structural properties and X-ray powder patterns were found to be in very good agreement with the experimental data. By using the optimized structures of these materials, as well as of those obtained in previous works for studtite and soddyite, the thermodynamic properties of dehydrated schoepite, metastudtite, studtite, and soddyite were determined, including specific heats, entropies, enthalpies and Gibbs free energies. Finally, the computed thermodynamic properties of these materials together with those reported previously for rutherfordine and γ-UO3 were used to determine their enthalpies and free energies of formation and their variation with temperature. The theoretical results for rutherfordine and γ-UO3 are shown to be in excellent agreement with experimental information, even at high temperatures (up to 700 and 900 K, respectively). The results for dehydrated schoepite are in reasonable agreement with the experimental values obtained from the extrapolation of the measured values for UO2·0.77H2O and UO2·0.85H2O to UO2·1H2O, and in good agreement with previous theoretical data. The corresponding temperature dependent functions and the associated reaction constants for metastudtite, studtite, and soddyite, for which there are no experimental data to compare with, were predicted. The overall thermodynamic data obtained in this work by a theoretical approach can be used for the improvement and extension of the nuclear thermodynamic databases for modeling uranium containing systems and its dynamical behavior under different geochemical environments.