Electrochemical deposition of Sr and Ba into liquid metals (Bi and Sb) was investigated in LiCl-KCl-SrCl₂-BaCl₂ electrolytes at 500–650 °C as a means to separate stable alkaline-earth ions from the molten salts (eutectic LiCl-KCl) utilized for recycling used nuclear fuel. Considering the higher stability of Sr and Ba ions relative to the Li and K ions in the chloride system, it would seem impractical to deposit Sr and Ba from the molten chloride solutions by electrochemical means; however, this work will present that the deposition of Sr and Ba into liquid metal electrodes becomes thermodynamically feasible by leveraging the strong chemical interactions between alkali/alkaline-earth metals and liquid metals.

The thermodynamic properties of four binary alkaline earth-liquid metal alloys, (Sr, Ba)-(Bi, Sb), were determined by electromotive force (emf) measurements using solid-state electrolytes to examine the degree of chemical interactions (activity) between alkaline-earth metals and liquid metals. Using a pure alkaline-earth metal A (A = Sr or Ba) as the reference electrode and binary A-B alloys (B = Bi or Sb) as working electrodes in solid-state binary CaF₂-AF₂ electrolyte, emf measurements were conducted over a temperature range of 450–850 °C in 25 °C increments. Reproducible emf values were obtained within ±5 mV accuracy during cooling-heating cycle at alloy mole fractions up to x_{A(in B)} = 0.40. Based on the experimentally determined emf values of alkaline-earths in liquid Bi and Sb electrodes, the electrode reactions involving Sr and Ba elements were found to be thermodynamically feasible due to substantially larger emf values of Sr and Ba in Bi, compared to those of Li and K in Bi. Namely, the chemical interactions of liquid metals (Bi and Sb) were stronger for alkaline-earths (Sr and Ba) than for alkali (Li and K) metals.

Based on thermodynamic measurements, the liquid metal electrodes were subjected to cathodic discharge at a constant current density of 50 mA cm^–2 in eutectic LiCl-KCl with the addition of 5–10 mol% of SrCl₂ and/or BaCl₂. As shown in Figure 1, the use of liquid metals (Bi and Sb) resulted in the deposition of Ba in LiCl-KCl-10mol%BaCl₂ electrolyte. Further study in LiCl-KCl-SrCl₂-BaCl₂ electrolytes also indicated that the use of strongly interacting liquid metal electrodes could result in the co-deposition of Sr and Ba in addition to Li. The results of this work show that alkaline-earth fission products (Sr²⁺ and Ba²⁺) accumulated in molten salts can be recovered into liquid metals by electrochemical separation, which could be employed as a critical step for recycling the process salt (LiCl-KCl) in order to minimize the generation of additional nuclear wastes.

Figure 1. Cross-section analyses of Bi (discharged at 510 °C) and Sb (discharged at 650 °C) electrodes under constant current density at 50 mA cm^–2 in LiCl-KCl-10mol%BaCl₂ electrolyte, using SEM and elemental X-ray mapping.