i Integrated Study of Rare Earth Drawdown by Electrolysis for Molten Salt Recycle DISSERTATION ...

203 Pages·2017·4.39 MB·English

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

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy
in the Graduate School of The Ohio State University

By
Evan Wu, B.S.
Graduate Program in Nuclear Engineering

The Ohio State University
2017

Dissertation Committee:
Jinsuo Zhang, Advisor
Marat Khafizov, Co-advisor
Lei R. Cao
Longya Xu

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Copyrighted by
Evan Wu
2017

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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 and eventually affecting uranium recovery efficiency. 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 are calculated during the analysis. La, Nd
and Gd are three RE that we are particularly interested in due to the high ratio of these
elements in UNF (La, Nd), the well-studied properties in dilute solution to provide a base
for comparison, and the highest standard potential among all RE (Gd).
Fundamental properties of La, Nd, Gd in LiCl-KCl eutectic salt are studied at a
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).
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BET model that considers the RE adsorption on the electrode is developed for
diffusion coefficient analysis. 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 electrolysis
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 surface 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
chronoamperometry and chronopotentiometry.

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Acknowledgements

Firstly, I would like to thank my advisor, Prof. Jinsuo Zhang, for his support and
guidance. He helped me get through a lot of difficulties, especially when I am stuck
analyzing all the data I measured. He is always helpful and genuinely cares about us.
I also want to thank Prof. Marat Khafizov for his help when Prof. Zhang is not here.
He is very kind and willing to help all the time.
I also want to thank my wife Chacha for everything she has done for me, thankful
for the discussion we have on my research and everything outside of research, hope she
also has a good time in U-Mich for her Ph.D. study.
I also want to thank Dr. Wentao Zhou for his help on my research, especially on
the electrolysis modeling and phase diagram development. He’s one of the nicest guys I
ever met (He’s also looking for a girlfriend so if anyone is interested, I can help).
I’m thankful for the help from Dr. Sahoqiang Guo for our discussion on EIS data
analyzation, it is a tough job, but he is always on point.
Special thanks to Mr. Yafei Wang and Mr. Jeremy Isler for their help on BET model
development and ICP test on my sample.
I also want to acknowledge all the colleagues in our research group and department.
I’ll remember the good times we spent together doing research and having fun.
The funding support from Nuclear Energy University Program, U.S. Department
of Energy for this research is highly appreciated.
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Finally, I want to thank my family, my parents are always supportive and always
worry about my progress, I’m thankful for it. I also wish my sister have a good time in her
Ph.D. study in South California.

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Vita

2012 ...............................................................B.S. Engineering and System Science,
National Tsing Hua Univeristy, Taiwan

2013 to present ..............................................Graduate Research Associate, Nuclear
Engineering Program, The Ohio State
University

Publication

1. Cohen, William, et al. "Molten fluoride salt and liquid metal multistage extraction
model." Progress in Nuclear Energy 97 (2017): 214-219.
2. Samin, Adib, Evan Wu, and Jinsuo Zhang. "The thermodynamic and transport
properties of GdCl3 in molten eutectic LiCl-KCl derived from the analysis of cyclic
voltammetry signals." Journal of Applied Physics 121.7 (2017): 074904.
3. Zhang, J., E. A. Lahti, and E. Wu. "Thermodynamic properties of actinides and
fission products in liquid bismuth." Progress in Nuclear Energy 81 (2015): 67-77.
4. Samin, Adib, Evan Wu, and Jinsuo Zhang. "The role of correlations in the
determination of the transport properties of LaCl in high temperature molten
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eutectic LiCl–KCl." Radiochimica Acta.
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5. Shaoqiang Guo, Evan Wu, Jinsuo Zhang, “Exchange current density of Gd(III)/Gd
reaction in LiCl-KCl eutectic and fundamental analysis of errors caused by various
methods”, submitted to Electrochimica Acta.

Fields of Study

Major Field: Nuclear Engineering

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Table of Contents
Abstract ............................................................................................................................... ii
Acknowledgements ............................................................................................................ iv
Vita ..................................................................................................................................... vi
Publication ......................................................................................................................... vi
Fields of Study .................................................................................................................. vii
Chapter 1. Background and Importance ....................................................................... 1
1.1 Introduction .......................................................................................................... 1
1.1.1 History of Used Nuclear Fuel Reprocessing and its Importance ............................ 1
1.1.2 Development of Pyroprocessing ......................................................................... 2
1.1.3 Used salt Recycling ........................................................................................... 6
1.1.4 Fundamental Properties of Rare Earth Materials .................................................. 7
1.2 Literature Review .................................................................................................. 7
1.2.1 Electroanalyitcal Methods ...................................................................................... 7
1.2.2 Electrochemical Properties of Lanthanides in LiCl-KCl Eutectic Salt ...................... 12
1.2.3 Research Gap and Motivation .............................................................................. 39
1.2.4 Dissertation organization ..................................................................................... 40
Chapter 2. Fundamental Data Measurement and Analysis......................................... 41
2.1 Experiment Setup ....................................................................................................... 41
2.2 Experiment and Data Analysis Methodology ............................................................... 44
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2.2.1 Square wave voltammetry .................................................................................... 45
2.2.2 Cyclic voltammetry ............................................................................................. 46
2.2.3 Chronopotentiometry and chronocoulometry ......................................................... 53
2.2.4 Tafel method and linear polarization ..................................................................... 56
2.2.5 Optimized fitting of polarization curve using electrode kinetics equation ................ 59
2.2.6 Electrochemical impedance spectroscopy .............................................................. 63
Chapter 3. Experimental Data Analysis ..................................................................... 69
3.1 Lanthanum property in LiCl-KCl eutectic salt .............................................................. 70
3.1.1 Exchange current density of LaCl in LiCl-KCl ..................................................... 70
3
3.1.2 Thermodynamics and transport properties of LaCl in LiCl-KCl ............................. 89
3
3.2 Gadolinium property in LiCl-KCl eutectic salt ............................................................. 99
3.2.1 Exchange current density of GdCl in LiCl-KCl .................................................... 99
3
3.2.2 Thermodynamics and transport properties of GdCl in LiCl-KCl .......................... 124
3
3.3 Neodymium property in LiCl-KCl eutectic salt .......................................................... 134
3.3.1 Exchange current density of NdCl in LiCl-KCl .................................................. 136
2
3.3.2 Thermodynamics and transport properties of NdCl and NdCl in LiCl-KCl .......... 142
3 2
3.4 Evaluation of properties’ temperature and concentration dependency .......................... 153
Chapter 4. Electrolysis Model for RE drawdown..................................................... 160
4.1 Methodology ........................................................................................................... 161
4.2 Chronoamperometry – Constant Potential Electrolysis ................................................ 164
4.3 Chronopotentiometry – Constant Current Electrolysis ................................................ 166
4.4 Conclusion .............................................................................................................. 168
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