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

The fuel bundles in coolant systems of CANDU™ reactors operate between 250 and 310 °C (523 and 583 K). 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 100 °C (373.15 K) and are presented in this work. Solubility measurements were made using a batch-style pressure vessel and concentration measurements were made using inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS), for zirconium and Zircaloy-4, respectively. For $Zr(OH)62−$⁠; Zr(OH)_{4 (aq)}; Zr(OH)³⁺; and $Zr(OH)22+,$ the Gibbs energy of formation (⁠$ΔGf, 100 °C°$⁠) was 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.