Alkali/alkaline-earth fission products such as strontium present a challenge in operating electro-refining cells for used nuclear fuel recycling, as they preferentially oxidize and accumulate in the LiCl-KCl-UCl3 molten salt electrolyte. As a result, the molten salt electrolyte must be periodically replaced, leading to an increased volume of nuclear waste.
The alkali/alkaline-earth elements are difficult to remove from the molten salt electrolyte, as primary electrolyte components Li and K will reduce before Sr, Ba, or Cs; however, by leveraging their strong interactions with liquid metals (e.g. Bi), this order of reduction could be shifted, allowing removal of the alkali/alkaline-earth fission products and recycling of the molten salt electrolyte.
Professor Hojong Kim and colleagues at the Pennsylvania State University have evaluated the thermodynamic properties of strontium-bismuth alloys via the electromotive force (emf) method to determine the feasibility of using a liquid bismuth electrode for separation of alkali/alkaline-earth fission products using CaF2-SrF2 as an electrolyte, pure Sr metal as a reference electrode, and Sr-Bi alloys as working electrodes. The research is now published in Electrochimica Acta.
The emf values were determined over the cooling-reheating cycle of 748–1023 K, and for thirteen Sr-Bi compositions xSr between 0.05 and 0.75. Additionally, the strontium-bismuth alloys were characterized using X-ray diffractometry and energy dispersive spectroscopy to elucidate the constituent phases, as well as using differential scanning calorimetry for determination of phase transitions.
Over the aforementioned temperature range at Sr-Bi mole fractions 0.05 < xSr < 0.30, the emf values decreased linearly as a function of decreasing temperature in the liquid phase and collapsed onto the same line as a function of composition once in the liquid + SrBi3(s) region, as activity (and therefore emf) is invariant in two-phase regions. Measurements had less than 5 mV of deviation between heating and cooling cycles.
Sr-Bi alloy xSr = 0.30 exhibited two phase transitions; [liquid = liquid + Sr2Bi3] at 908 K, and the second, [liquid + Sr2Bi3 = SrBi3 + Sr2Bi3] at 843K. Mole fractions xSr > 0.35 showed an increase in hysteresis (up to 25 mV) during the heating-cooling cycle as a result of increased reactivity at higher Sr concentrations.
A two-phase region (Sr2Bi3 + Sr11Bi10) at 0.40 < xSr < 0.52 gave less reproducible emf values, implying non-equilibrium phase behavior. Similar behavior was observed for xSr > 0.67, leading to the postulation of two meta-stable phases (Sr5Bi3 and Sr4Bi3).
The authors were able to confirm high liquid-state solubility of strontium in bismuth (15-40 mol%) as well as low activity values (as low as 1.2´10-13) at 788–988 K, which provide an effective justification for using liquid bismuth electrodes as a medium for preferentially removing strontium from contaminated molten salt electrolyte.
Smith, N.D., Lichtenstein, T., Gesualdi, J., Kumar, K., Kim, H. Thermodynamic Properties of Strontium-Bismuth Alloys Determined by Electromotive Force Measurements, Electrochimica Acta 225 (2017) 584–591.
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