Defense Date

2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Mechanical and Nuclear Engineering

First Advisor

Supathorn Phongikaroon

Second Advisor

Sama Bilbao y Leon

Third Advisor

Jessika V. Rojas

Fourth Advisor

Julio C. Alvarez

Fifth Advisor

Vivek Utgikar

Abstract

Understanding the characteristics of special nuclear materials in LiCl-KCl eutectic salt is extremely important in terms of effective system operation and material accountability for safeguarding pyroprocessing technology. By considering that uranium (U) is the most abundant and important element in the used nuclear fuel, measurements and analyses of U properties were performed in LiCl-KCl eutectic salt. Therefore, the electrochemical techniques such as cyclic voltammetry (CV), open circuit potential (OCP), Tafel, linear polarization (LP), and electrochemical impedance spectroscopy (EIS) were conducted under different experimental conditions to explore the electrochemical, thermodynamic, and kinetic properties of U in LiCl-KCl eutectic. The ultimate goal of this study was to develop proper methodologies for measuring and analyzing the exchange current density (i0) of U3+/U reaction, which has not been fully studied and understood in literature.

In the preliminary study, cerium (Ce) was selected as a surrogate material for uranium and its behavior was being explored with the developments of experimental methods. CV was performed to evaluate Ce properties such as the diffusion coefficients (D), apparent standard reduction potential (E0*), Gibbs free energy (DG), and activity coefficient (g). In addition, EIS methods were adapted and specific experimental procedures were established for the proper i0 measurements providing repeatable and reproducible data sets. The i0 values for Ce3+/Ce pair were ranging from 0.0076 A cm-2 to 0.016 A cm-2, depending on the experimental conditions. These preliminary results give insight in developing the experimental setups and methods to evaluate the properties of U in LiCl-KCl. Plus, Ce is one of the lanthanide (Ln) fission products in electrorefiner (ER) system; therefore, the resulting data values yield useful information of the fundamental behaviors of Ln elements in the system.

Based on these developed methodologies, the experimental designs and routines were established to explore the main properties (e.g., D, E0*, etc.) of UCl3 in LiCl-KCl eutectic salt under different concentrations (0.5 wt% to 4 wt% UCl3) and temperatures (723 K to 798 K). Specially, the i0 values of U3+/U were evaluated via EIS, LP, Tafel, and CV methods. All i0 values had linear trends with the change of concentration and temperature; however, these values measured by LP, Tafel, and CV methods were greatly influenced by the change in electrode surface area. Overall, the i0 values agreed within 33% relative error range with the EIS method being the most consistent and accurate in comparison to reported literature values. The measured values of i0 were ranging from 0.0054 A cm-2 to 0.102 A cm-2. Therefore, an extremely reliable database for i0 was provided and it is feasible to anticipate the i0 kinetics in other experimental conditions by using the provided equation models. Furthermore, GdCl3 was added to the LiCl-KCl-UCl3 system to explore the effects of other elements on the U properties such as the diffusion coefficients, thermodynamic properties, and i0 kinetics. The diffusion coefficient was generally decreased by 12 ~ 35% with addition of GdCl3 in LiCl-KCl-UCl3; however, the apparent standard potentials and exchange current density follow the same trends with data obtained without GdCl3 additions. Hence, the results indicate that the thermodynamic and kinetic values for U3+/U reaction in LiCl-KCl eutectic salt are not greatly influenced by the presence of GdCl3.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

12-14-2016

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