Investigation of metal-H2O systems at elevated temperatures: Part III. solubility data and new Zr Pourbaix diagrams at 298.15 K and 373.15 K.

Abstract The fuel bundles in coolant systems of CANDUTM reactors operate between 533 to 583 K (260 to 310°C). Given these extreme conditions and because of its neutronic properties, zirconium and Zircaloy-4 are used in these applications which require corrosion resistance at elevated temperatures. However, thermodynamic and hydrolysis properties of aqueous zirconium species have not been measured above standard conditions, making prediction using standard thermodynamic tools, such as the Pourbaix (E-pH) diagram difficult. This lack of information is addressed through solubility measurements and the development of elevated temperature Pourbaix diagrams for zirconium and Zircaloy-4. These Pourbaix diagrams of zirconium and a multi-element diagram (Sn, Zr, Cr) of Zircaloy-4 were developed at 373.15 K (100°C) and are presented in this work. Solubility measurements were made using a batch-style pressure vessel and concentration measurements were made using ICP-OES and ICP-MS, for zirconium and Zircaloy-4, respectively. For Zr(OH)6(2-); Zr(OH)4 (aq); Zr(OH)3+; and Zr(OH)2(2+) the Gibbs energy of formation (ΔG(°f,373.15 K)°) were found to be: -2177.4±8.5 kJ/mol, -1704.9±1.5 kJ/mol, -1808.8±8.9 kJ/mol, and -1095.1±2.7 kJ/mol, respectively.