Significance Statement
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.
Reference
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.
Materials Science and Engineering, The Pennsylvania State University, 406 Steidle Building, University Park, PA 16802, United States.
Go To Electrochimica Acta