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Electrochemical and Spectro-electrochemical Studies of Uranium and Lanthanides in High Temperature LiCl-KCl Molten Salt

  • 김대현 (대학원 화학과 물리화학전공)

초록/요약

Approximately 23.8% of the electricity generated in Korea comes from nuclear power, and four additional nuclear power plants are being constructed to supplement the shortfall in electricity supply. Nuclear power generation rarely produces carbon dioxide, making it a low carbon-generating energy resource, but it has the critical disadvantage of producing a high volume of spent nuclear fuel. Since spent nuclear fuel becomes highly active radioactive waste after the nuclear power generation, there is a lot of focus on the management and recycling of nuclear waste. Pyroprocessing, in particular, has received much attention as a recycling method. Pyroprocessing recycles useful elements contained in spent nuclear fuel by means of a fused-salt at high temperature in an electrochemical manner. Currently, pyroprocessing has been selected as an approach for the recycling of spent nuclear fuel in Korea and intensive studies have been carried out. However, as the process is complicated due to the inclusion of multiple elements from spent nuclear fuel, further understanding of the process is required. In particular, an understanding on the chemical and electrochemical behavior of the high temperature molten salt for nuclear fuel recycling is lacking. In this study, the chemical and electrochemical behavior of actinide and lanthanide elements from spent nuclear fuel in a used-salt environment at high temperature was investigated through electrochemical and spectroscopic methods. Chapter 1: This chapter outlines the motivation for this study. Nuclear power generation contributes to approximately 23.8% of power produced in Korea, and four additional nuclear power plants are being constructed to support the electricity supply. Nuclear power generation rarely produces carbon dioxide, making it a low carbon-generating energy resource, but it has the critical disadvantage of producing a high volume of spent nuclear fuel. As the spent nuclear fuel includes highly active radioactive waste after the nuclear power generation, much attention has been paid to the management and recycling of nuclear waste. To recycle spent nuclear fuel, there are two types of approaches: the wet process and dry process. The pyrochemical process, which is an effective dry process for nuclear non-proliferation, has been selected in Korea and intensive studies are being conducted to complete a comprehensive Korea Advanced Pyroprocessing Facility (KAPF) construction by 2030. However, pyroprocessing uses a chloric fused-salt as an electrolyte at high temperature up to 500C, and as very little is known from domestic research, further chemical and electrochemical research related to the process is required. Thus, we studied the chemical and electrochemical behavior of actinide and lanthanide elements from spent nuclear fuel within a used-salt at high temperature. Chapter 2: Pyroprocessing is an extreme environment to the reactive cell or to the electrode as Cl-based salts are used as the electrolyte after dissolution at a high temperature of over 500℃. Thus, it is essential for the electrode material used in pyroprocessing to maintain stability under extreme Cl-based media even at high temperature. For related studies, the stability of a number of electrode materials was investigated using electrochemical SEM and EDX methods in a high-temperature fused-salt environment. Furthermore, information on the stable electrode material and its potential window depending on the electrode material was obtained by investigating the characteristics of the electrode such as the changes in the surface of the electrode after various electrochemical reactions. Chapter 3: For efficient management of the pyroprocessing technique, safety of the nuclear material, measurement of the nuclear material, and the concentration of the nuclear material within the fused-salt of high temperature during the pyroprocess need to be monitored real time. For this, studies on the electrochemical reaction of the solute were carried out in a high-temperature fused-salt environment using a number of electrochemical measurement methods. Cyclic voltammetry, a method for obtaining electrochemical measurements, revealed linearity for the ratio of the solute concentration within a high-temperature fused-salt higher than 9wt.% to the current or electric charge obtained by integrating the current amount. Chronoamperometry showed good linearity between the initial constant current and the concentration of the highly concentrated solute. However, the measured current was not linearly proportional for concentrations higher than 4wt.%, in square wave voltammetry and normal pulse voltammetry, which are commonly used methods. Based on these results, we suggested a novel method to monitor the concentration within a high-temperature fused-salt real time. The new repeating chronomaperometry method revealed good linearity between the concentration and electronic charge up to 9wt.% Neodynium (Nd) cation concentration. Given these results, it is expected that this method can be an efficient electrochemical method to measure uranium concentration during pyroprocessing and can be directly applied to the process. Chapter 4: Spectroscopic measurements not only monitor the oxidation-reduction state of the solute in a fused-salt in a real-time manner, but can also provide information on the electron structure depending on the oxidation state of the element. Furthermore, this method is a good tool for investigating the changes of the oxidation state in the solute inside the total fused-salt beyond the limit of electrochemical measurements, which can only assess reactions taking place on the electrode surface. In this study, we investigated the behavior of a solute within a high-temperature fused-salt depending on the changes in the electrochemical environment in a spectroscopic manner by combining the electrochemical- and spectroscopic measurements. First, electrochemical behavior of the Yb cation as a lanthanide element inside a high-temperature fused-salt was studied using the electrochemical/spectroscopic measurement. In a high-temperature fused-salt, the Yb ion stays at a +2 and +3 oxidation state. From cyclic voltammetry electrochemical measurements, the oxidation-reduction reaction of Yb2+/3+ was observed at approximately -0.48 V. From UV-Vis absorption spectroscopy, the absorption band of Yb3+ was observed between 228 nm and 268 nm, whereas Yb2+ was observed between 249 nm and 369 nm. Furthermore, a number of parameters were investigated including molar absorptivity of the Yb cation at a high-temperature fused-salt, the number of reactive electrons, and formal potential.

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TABLE OF CONTENTS

ABSTRACT……………………………………………………………………………….i
TABLE OF CONTENTS………………………………………………………………..vii
LIST OF FIGURES……………………………………………………..……….….……. x
LIST OF TABLES……………………………………………………………..……….xiv

Chapter 1. Overview
1. Nuclear spent fuel in Republic of Korea…………………………………………....2
2. Recycling of the Nuclear spent fuel………………………………………………..6
3. Pyrochemical process…………………………………………………………......9
3.1. Head-end process…………………………………………………………....9
3.2. Electro-reduction process……………………………………………….....11
3.3. Electro-refining process…………………………………………………......14
3.4. Electro-winning process…………………………………………………......17
4. Motivation for this work…………………………………………………………..19

Chapter 2. Stability of Electrode Materials during Electrolysis in LiCl-KCl Melt
1. Introduction………………………………………………………………………..25
1.1. The stability of the graphene electrode material………………………….....25
2. Experimental…………………………………………………………………........28
3. Results and discussion…………………………………………………………......31
3.1. Characterization of various working electrode materials…………………....31
3.1.1. Cyclic voltammetry of electrode materials………………………....31
3.1.2. Open Circuit Potential measurements………………………….......33
3.1.3. Electrodeposition of metal……………………………………........36
3.1.4. Scanning electron microscopy…………………………………........36
3.2. Characterization of graphene electrode material………………………….....44
3.2.1. Cyclic voltammetry of EuCl2 for various electrode materials………44
3.2.2. Cyclic voltammetry of UCl3 for carbon electrode materials…….....47
3.2.3. Scanning Electron Microscopy for uranium deposited on graphene
surfaces………………………………………………………….....53
3.2.4. Cyclic voltammetry of GdCl3 for graphene electrode material……..55
4. Conclusions………………………………………………………………………..60

Chapter 3. Real time monitoring of metal ion concentration in LiCl-KCl melt using electrochemical techniques
1. Introduction………………………………………………………………………..62
2. Experimental…………………………………………………………………........64
3. Results and discussion…………………………………………………………....65
3.1. Electrochemical behaviour of NdCl3 in LiCl-KCl melt……………….......65
3.1.1. Cyclic voltammetry ………………………………………………..65
3.1.2. Chronoamperometry ………………………………………………...69
3.2. Real time monitoring technique of NdCl3 in LiCl-KCl melt………….......71
3.2.1. Repeating chronomaperometry……………………………………71
3.2.2. Real time monitoring of Nd ions concentration……………………. .76
4. Conclusions……………………………………………………………………....78

Chapter 4. Investigation of the Electrochemical Behavior of Lanthanide Cations in LiCl-KCl Melt Using Spectro-Electrochemical Methods
1. Introduction………………………………………………………………………..80
2. Experimental…………………………………………………………………........82
3. Results and discussion…………………………………………………………....83
3.1. Electrochemistry of YbCl3 in LiCl-KCl melt………………………….......83
3.1.1. Electrochemical behavior of Yb cations……………………………..84
3.1.2. The electrode depth dependence…………………………………...86
3.2. Specto-electrochemistry of YbCl3 in LiCl-KCl melt………………….......90
3.2.1. Spectroscopy behavior of Yb cations………………………………90
3.2.2. Spectro-electrochemical behavior of Yb cations …………………. ..97
3.2.3. Spectroscopy characteristics of the Yb cations for the electrochemical
oxidation state……………………………………………………..99
4. Conclusions……………………………………………………………………....102.
References………………………………………………………………………………103

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