Electrochemical separation is one of the most promising methods for recycling used nuclear fuel, in which uranium is isolated from other fission products using a molten salt electrolyte¹. The used fuel is immersed in molten salt electrolyte in an electrochemical cell referred to as an electrorefiner. In an ideal electrorefiner cell the used fuel is dissolved anodically, releasing uranium ions into a ternary chloride salt (LiCl-KCl-10wt%UCl₃). The uranium ions are then reduced at an inert cathode, forming uranium metal for reuse in new uranium fuel pellets². However, in actual electrorefiner systems, uranium is not the only element from the used fuel that is oxidized into the electrolyte. Alkali/alkaline-earth elements, such as cesium, barium, and strontium, are also oxidized because they have lower standard reduction potentials than uranium in the chloride system³. As alkali/alkaline-earth elements accumulate in the electrolyte, they change the physical and chemical properties of the electrolyte due to their short half-lives and high heat densities. The electrolyte is periodically removed from the electrorefiner to maintain the recycling process, thereby generating excessive volumes of nuclear waste. Lichtenstein et al. have demonstrated that alkali/alkaline-earth elements exhibit low thermodynamic activity (6.6 × 10^-16 at x_Ba = 0.05, 773 K) in liquid metals such as bismuth, implying strong atomic interactions⁴. These interactions cause a shift in the elements’ reduction potentials and can be leveraged to selectively remove the accumulated elements from the electrolyte by depositing them into liquid metal electrodes. To determine the viability of removing alkali/alkaline-earth elements using liquid metals such as lead, the thermodynamic properties of Sr-Pb alloys were determined by electromotive force (emf) measurements using a solid state electrolyte (binary CaF₂-Sr­F­₂, 97-3 mol%). Emf values were measured at 723 K-1073 K using a Sr(s)|CaF₂-SrF­₂|Sr(in Pb) electrochemical cell and were used to determine the thermodynamic properties of Sr-Pb, including activity, partial molar entropy, and partial molar enthalpy for x_Sr = 0.05-0.75. At 973 K, activity values of Sr in Pb were as low as 5 × 10^-9 at x_Sr = 0.05, implying strong atomic interactions between Sr and Pb. According to Okamoto et al., the liquid-state solubility of Sr in Pb is approximately 37 mol% at 973 K⁵. With the high solubility of Sr in Pb in the liquid state, combined with strong Sr-Pb atomic interactions, Pb has demonstrated promise for use as a liquid metal electrode material to remove Sr²⁺ ions from molten salt electrolytes.

[1] Electrometallurgical Techniques for DOE Spent Fuel Treatment: Final Report, National Academy Press Washington, D.C, 2000.

[2] Willit, J. L., Miller, W.E., Battles, J.E. Electrorefining of uranium and plutonium – A literature review, J. Nucl. Mater. 195 (3), 229-249 (1992).

[3] Roine, A., Outokumpu HSC Chemistry® 5.1; Chemical Reaction and Equilibrium Software with Extensive Thermochemical Database, (2002).

[4] Lichtenstein, T., Smith, N.D., Gesualdi, J., Kumar, K., Kim, H. Thermodynamic Properties of Barium-Bismuth Alloys Determined by emf Measurements, Electrochim. Acta. 228, 628-635 (2017).

[5] Okamoto, H. Sr-Pb (Strontium-Lead), 2^nd ed., ASM International, Materials Park, (1990).