FUNDAMENTALS AND APPLICATIONS OF LIQUID METALS TOWARDS ELECTROCHEMICAL SEPARATIONS
Open Access
- Author:
- Lichtenstein, Timothy Thomas
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 05, 2019
- Committee Members:
- Hojong Kim, Dissertation Advisor/Co-Advisor
Hojong Kim, Committee Chair/Co-Chair
Zi-Kui Liu, Committee Member
Ismaila Dabo, Committee Member
Thomas Mallouk, Outside Member
John C Mauro, Program Head/Chair
James L. Willit, Special Member - Keywords:
- Electrorefining
Thermodynamics
barium-alloys
electromotive force
Electrochemical separation
Phase diagram - Abstract:
- The electrorefining of used nuclear fuel is an important process being developed to close the nuclear fuel cycle and minimize the amount of nuclear waste generated. In the process, uranium metal is recovered from used nuclear fuel through electrochemical deposition onto an inert cathode in a molten LiCl-KCl-UCl3 electrolyte. During this process, the fission products that are more stable than uranium as chlorides accumulate into the process salt. Fission products such as 90Sr have high heat densities and produce large amount of highly ionizing radiation. Additionally, SrCl2 is significantly more stable than the constituent salt elements (LiCl-KCl) which means that it cannot be removed by electrodeposition onto an inert cathode. Unless fission products are removed, the process salt will need to be replaced and disposed of, which contributes to the overall radioactive wastes generated. This dissertation focuses on evaluating the thermodynamics of electrochemically stable alkaline-earth (Sr,Ba) fission products when interacting with liquid metal electrodes and subsequent application of those liquid metals towards separating out those species. Thermodynamic properties of Ba-Bi, Ba-Sb, and Ba-Sn alloys were determined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and electromotive force (emf) measurements. New intermetallic compounds were identified by XRD and the phase transition temperatures were determined by DSC, which were used to delineate phase boundaries for constructing experimentally-determined phase diagrams of all three systems. In addition, thermochemical solution properties were calculated by measuring emf values of Ba alloys using the following electrochemical cell: Ba(s) | CaF2-BaF2 | Ba(in M), including activity and partial molar quantities of Gibbs energy, entropy, and enthalpy. By integrating solution properties from emf measurements with the phase behavior by DSC and XRD, reliable thermodynamic descriptions of the Ba-Bi, Ba-Sb, and Ba-Sn systems were established. Electrochemical deposition of Sr and Ba into liquid Bi metal in LiCl-KCl-SrCl2-BaCl2 electrolytes at 500 °C was accomplished by leveraging the strong chemical interactions between alkaline-earth metals and liquid Bi. The use of Bi resulted in complex electrode reactions, leading to co-deposition of Sr (2.0–6.5 mol%), Ba (4.1–12.8 mol%), and Li (5.9–16.2 mol%), and coulombic efficiencies of 63–67% were achieved. The observed co-deposition was also supported via thermodynamic analyses of electrode potentials by incorporating the experimentally determined activity values of each alkali/alkaline-earth metal in Bi. The results of this work suggest that the two major alkaline-earth fission products accumulated during electrorefining in molten salts (Sr2+ and Ba2+) can be recovered into liquid Bi 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. Further experimentation explored the role of concentration and current density on depositing Sr and Ba into liquid Bi electrodes at 500 °C by dissolving 0–5 mol% alkaline-earth chloride into the LiCl-KCl eutectic and cathodically discharging into Bi electrodes. The Bi electrodes were capable of separating alkaline-earths out of molten salt electrolytes in concentrations as dilute as 0.4 mol%. Additionally, the role of current density was found to not appreciably affect deposition into or removal from liquid metal electrodes. The results of this work will hopefully provide useful data for reducing the waste generated during the electrorefining of used reactor fuel.