The Gibbs free energies and enthalpies of formation of U6+ phases: An empirical method of prediction

Uranyl minerals form as a result of the oxidation and alteration of uranini te, UO2+x. These uranyl phases are also important alteration products of th e corrosion of UO2 in used nuclear fuels under oxidizing conditions. Howeve r, the thermodynamic database for these phases is extremely limited. The Gi bbs free energies and enthalpies for uranyl phases are estimated based on a method that sums polyhedral contributions. The molar contributions of the structural components to Delta G(f)(0) and Delta H-f(0) are derived by mult iple regression using the thermodynamic data of phases for which the crysta l structures are known. In comparison with experimentally determined values , the average residuals associated with the predicted Delta G(f)(0) and Del ta H-f(0) for the uranyl phases used in the model are 0.08 and 0.10%, respe ctively, well below the limits of uncertainty for the experimentally determ ined values. To analyze the reliability of the predicted Delta G(f)(0) valu es, activity-activity diagrams in SiO2-CaO-UO3-H2O and CO2-CaO-UO3-H2O syst ems at 298.15 K and 1 bar were constructed using the predicted Delta G(f.29 8.15)(0) values for the relevant U6+ phases, There is good agreement betwee n the predicted mineral stability relations and field occurrences, thus pro viding confidence in this method for the estimation of Delta G(f)(0) and De lta H-f(0) of the U6+ phases.