Thermodynamic Properties of Strontium-Bismuth Alloys Determined by Electromotive Force Measurements

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 788988 K, which provide an effective justification for using liquid bismuth electrodes as a medium for preferentially removing strontium from contaminated molten salt electrolyte.   

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About The Author

Hojong Kim is an Assistant Professor of Materials Science and Engineering, and Norris B. McFarlane Faculty Professor at the Pennsylvania State University. Dr. Kim received his B.S from Seoul National University in South Korea in 2000 and Ph.D. degree at MIT in 2004 both in Materials Science and Engineering. His doctoral research sought to identify the corrosion mechanisms of constructional alloys in high temperature and high pressure steam environments under Professor Latanision in the Uhlig Corrosion Laboratory at MIT.

After graduate research, Dr. Kim worked as a senior research scientist at Samsung-Corning Precision Glass Co. Ltd. and as a project lead to improve the process yield for thin film transistor liquid crystal display (TFT-LCD) glass melting processes by engineering high temperature materials of refractory ceramics (alumina and fused zirconia), molybdenum, and platinum alloys.

After five years of industrial experience, Dr. Kim returned to MIT as a post-doctoral associate and later as a research scientist to contribute to the growing need for sustainable technology. He conducted research on high temperature electrochemical processes, including molten oxide electrolysis for carbon-free iron and steel production with focus on developing inert anode materials in molten slags as well as molten salt liquid metal batteries for large-scale energy storage.

Dr. Kim’s research interests embrace the development of environment-friendly electrochemical processes for resource extraction/recycling, corrosion-resistant materials, as well as energy storage systems. He is currently leading efforts to separate alkali/alkaline-earth elements from the used nuclear fuel recycling processes in the molten salt electrolytes. Dr. Kim is the lead organizer of the present Rare Metal Extraction and Processing Symposium at the TMS 2017 Annual Meeting and is the chair of the Hydrometallurgy and Electrometallurgy Committee under Extraction and Processing Division at TMS.

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.

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