Determination of Fundamental Thermodynamic Properties of Alkali/Alkaline-Earth Elements in Liquid Metals for Recovery from Molten Salt Solutions

Restricted (Penn State Only)
Author:
Smith, Nathan Douglas
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 21, 2018
Committee Members:
  • Hojong Kim, Dissertation Advisor
  • Hojong Kim, Committee Chair
  • Zi-Kui Liu, Committee Member
  • Allison Michelle Beese, Committee Member
  • Thomas E Mallouk, Outside Member
Keywords:
  • Electrochemical Separations
  • Liquid Metals
  • Alkali/Alkaline-Earths
  • Sr-Bi
  • Sr-Sb
  • Sr-Sn
  • Electromotive Force Method
Abstract:
In the nuclear industry, uranium is recovered from used nuclear fuel using a process known as electrorefining in which a metallic used nuclear fuel anode is oxidized into molten LiCl-KCl-UCl3 electrolyte and pure U is preferentially reduced onto an inert cathode. While electrorefiner systems facilitate the recycling of substantial amounts of uranium from used nuclear fuel, they also contribute to the production of nuclear waste due to the build-up of dangerous elements including 90Sr and 137Cs in the molten salt electrolyte as they are more active in the salt than U and will therefore oxidize out of the anode before U. The accumulation of Sr and Cs in the electrolyte presents a problem as Sr and Cs isotopes have high heat densities and produce large amounts of highly ionizing radiation; these hazards combined with difficulty in removing the highly stable Sr and Cs from the electrolyte necessitates frequent replacement and disposal of the electrolyte, which then contributes to the overall volume of nuclear waste. This dissertation focuses on evaluating the viability of using liquid metal electrodes as a method for electrochemical separation of Sr and Cs from LiCl-KCl-based molten salts due to their strong atomic interactions with alkali/alkaline-earth elements, which cause a shift in relative stability of the alkali/alkaline-earths in the electrolyte. Thermodynamic properties, including activities, partial molar entropies, and partial molar enthalpies, were determined using electromotive force measurements for the Sr-Bi, Sr-Sb, and Sr-Sn binary systems in order to elucidate the strength of interactions between Sr and each liquid metal. By combining the fundamental thermochemical data with phase characterization of each of the binary systems using X-ray diffraction (XRD) and differential scanning calorimetry (DSC), a comprehensive understanding of both thermodynamic phase behavior was developed for all three binary systems. Activities as low as aSr = 10-13 at xSr = 0.04 at T = 888 K were measured as well as liquid state solubilities as high as 40 mol% at 988 K. Experimental data was used as input data towards computational efforts involving first-principles calculations as well as the CALPHAD technique in the case of the Sr-Sb system to develop an improved Sr-Sb phase diagram and provide further basis for the use of computational models in elucidating strongly interacting binary systems. Attempts to remove Sr from molten salt electrolyte using an electrochemical cell with liquid metal (Bi, Sb, Sn) cathodes were successful, with post-mortem elemental analysis of the electrodes confirming significant quantities of Sr (4.0-6.5 mol%) deposited into Bi and Sb. Furthermore, deposition results correlated well with the deposition behavior predicted from the aforementioned electromotive force measurements, inviting the possibility of using liquid metal electrodes as a method for selectively removing Sr from molten LiCl-KCl electrolyte and for reducing the total volume of nuclear waste left accumulating in on-site repositories throughout the US.