An empirical method for calculating thermodynamic parameters for U(6) phases, applications to performance assessment calculations

Uranyl minerals form by oxidation and alteration of uraninite, UO2+x, and the UO2 in used nuclear fuels. The thermodynamic database for these phases is extremely limited. However, the Gibbs free energies and enthalpies for uranyl phases may be estimated based on a method that sums polyhedral contributions. The molar contributions of the structural components to Δf Gm0 and Δf Hm0 are derived by multiple regression using the thermodynamic data of phases for which the crystal structures are known. In comparison with experimentally determined values, the average residuals associated with the predicted Δf Gm0 and Δf Hm0 for the uranyl phases used in the model are 0.08 and 0.10%, respectively. There is also good agreement between the predicted mineral stability relations and field occurrences, thus providing confidence in this method for the estimation of Δf Gm0 and Δf Hm0 of the U(VI) phases. This approach provides a means of generating estimated thermodynamic data for performance assessment calcination and a basic for making bounding calcination of phase stabilities and solubilities. (author)