(418e) Thermodynamic Properties of Strontium-Lead Alloys Determined By Electromotive Force Measurements

During electrorefining of metallic used nuclear fuel, alkaline-earth fission products (Sr and Ba) are oxidized at the anode and accumulate in the molten salt electrolyte. To evaluate the viability of electrochemically separating these stable ions from the electrolyte using a liquid Pb cathode, the thermodynamic properties of Sr-Pb alloys were investigated by electromotive force (emf) measurements. A Sr(s)|CaF₂-SrF­₂|Sr(in Pb) electrochemical cell using a solid state electrolyte (binary CaF₂-Sr­F­₂, 97-3 mol%) was used to measure emf values at 773 K-1073 K for ten Sr-Pb alloys at mole fractions 0.07 ≤ x_Sr ≤ 0.45. These emf measurements were used to determine thermodynamic properties, including activity, partial molar enthalpy, and partial molar entropy. At 873 K, activity values of Sr in Pb were as low as 1.69 × 10^-9 at mole fraction x_Sr = 0.07, implying strong atomic interactions between Sr and Pb. Phase characterization utilizing X-ray diffraction (XRD) was conducted on alloy compositions 0.07 ≤ x_Sr ≤ 0.75 and thermal analysis was conducted on compositions up to x_Sr = 0.34 using differential scanning calorimetry (DSC). Combining phase analysis and transition temperatures, the Sr-Pb phase diagram was refined. Based on the measured thermodynamic properties, the electrochemical deposition into a liquid Pb cathode was conducted in molten LiCl-KCl-SrCl₂ at 500°C and the Pb cathode was characterized to examine the deposition products (e.g., Sr, Li, and K).