Dissertation Defense: Integrated Study of Rare Earth Drawdown by Electrolysis for Molten Salt Recycle

Committee Members

• Dr. Marat Khafizov, Chair
• Dr. Jinsuo Zhang
• Dr. Cao Lei
• Dr. Longya Xu

Abstract

Pyroprocessing is an electrochemical method that is capable of separating uranium (U) and minor actinides from LiCl-KCl eutectic salt where used nuclear fuel (UNF) is dissolved. During the process, fission products including rare earth metals (RE) continually accumulate in the salt. In order to reduce the salt waste after uranium and minor actinides recovery, electrolysis is performed to drawdown rare earth materials from molten salt to restore salt initial state. Present research focus on the development of RE fundamental physical properties in LiCl-KCl eutectic salt. These properties includes apparent potential, activity coefficient, diffusion coefficient and exchange current density. Additional properties including charge transfer coefficient and reaction rate constant is also calculated during the analyiss. La, Nd and Gd are three RE that we are particularly interest in due to the high ratio of these element in UNF, the well-studied properties in dilute solution to provide a base for comparison, and the accessibility of these elements. Fundamental properties of La, Nd, Gd in LiCl-KCl eutectic salt are studied at temperature ranging from 723 K to 823 K and RE concentration ranging from 1 wt% to 9 wt%. These properties are studied by electroanalytical methods including Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), Tafel method, Chronocoulummetry (CC) and Chronopoentiometry (CP). Additional properties including charge transfer coefficient, reaction rate constant of RE reaction in LiCl-KCl are also being studied. New analysis method BET model is developed for diffusion coefficient analysis and electrode kinetic model is developed to account for mass transfer effect during the analysis of exchange current density. Correlations of diffusion coefficient, apparent potential, exchange current density with temperature and concentration are developed. These fundamental data are integrated with a kinetic model to predict the electrolysis process for RE drawdown from LiCl-KCl salt. The model considers both the diffusion in electrolyte and Faraday process on the electrode surface and a buffer layer is introduced to account for the fact that diffusion current is not necessarily equal to the current due to the Butler-Volmer equation. The model is validated by the experiment with both constant current and constant voltage experiment.