Electrochemical Sensor Development for Fluoride Molten Salt Redox Control

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Nikolas W. Shay

Graduate Program in Mechanical Engineering

The Ohio State University

2017

Master's Examination Committee:

Jinsuo Zhang, Co-advisor

Marat Khafizov, Co-advisor

Copyrighted by

Nikolas William Shay

2017

Abstract

Investment in nuclear technology is experiencing a revitalization as nuclear power becomes uniquely poised to take the burden left by phasing out fossil fuels to meet climate change goals. The United States Department of Energy is investing in research and development of the Fluoride salt-cooled High-temperature Reactor (FHR) with the ultimate goal of a 2030 deployment. One challenge presented by this reactor is corrosion of the reactor’s structural materials by the molten salt due to foreign and generated impurities. These impurities will shift the reduction-oxidation (redox) potential of the salt beyond the equilibrium potential of candidate structural materials, causing accelerated corrosion. This issue demands control of the molten salt’s redox condition in order to prevent unacceptable levels of corrosion. Research has been conducted on methods for redox control and electrochemical measurement techniques. The limited research that has been conducted related to measurement apparatus either lack certain characteristics specific to application for the FHR reactor or appropriate comparison to demonstrate first rate performance.

The primary issue presented by an electrochemical sensor for this application is the selection of an appropriate reference electrode. This report investigates candidate reference electrodes with the purpose of identifying a leading candidate and proposing a holistic electrochemical sensor design which possesses high performance, durability, and

ii ease of use. Candidate reference electrodes are the platinum quasi-reference electrode, dynamic reference electrode, gold/sodium alloy reference electrode, and nickel/nickel(II) reference utilizing a boron nitride sheath. Electrochemical tests show that cyclic voltammetry is a precise technique to measure the concentration of a redox agent.

Experiments also show that a quasi-reference electrode is the best suited reference for this application when paired with dynamic operational techniques. This choice of reference allows a full electrochemical sensor to be designed for placement in a molten salt forced convection loop. The design proposed here utilizes a stainless steel or nickel alloy housing, containing a boron nitride cylinder which serves to locate and electrically insulate the three-electrode cell. This proposed design is uniquely suited to meet the demands of redox control in an industrial molten salt application such as an FHR based on its high performance, durability, and ease of use.

iii

Acknowledgments

Special acknowledgment is necessary for my advisor Dr. Jinsuo Zhang for being an excellent mentor, both academically and professionally, as well as being an irreproachable advocate for his students.

Gratitude is expressed to Dr. Marot Khafizov for his generosity and service to a number of department students, including myself, during a critical transition period.

Thanks are also expressed to Dr. Shaoqiang Guo for his collaboration and significant mentoring of electrochemical theory. I am also very grateful to Evan Wu and

Ryan Chesser for sharing their advice and experience related to experimental electrochemistry.

This study is funded by the NEUP-IRP program (Project number: IRP-14-7476, leading PI: Professor Farzad Rahnema). Professor Mike Short from the Massachusetts

Institute of Technology generously donated the EuF3 which was used to perform this study’s experiments.

Vita

2016...... B.S. Mechanical Engineering, The Ohio

State University

2016-present ...... Graduate Research Assistant, Department

of Mechanical, Aerospace, and Nuclear

Engineering, The Ohio State University

Fields of Study

Major Field: Mechanical Engineering

Table of Contents

Abstract ...... ii

Acknowledgments...... iv

Vita ...... v

Table of Contents ...... vi

List of Tables ...... viii

List of Figures ...... ix

Chapter 1: Introduction ...... 1

Molten Salt Nuclear Reactors ...... 1

Corrosion and Materials ...... 3

Redox Control of Molten Systems ...... 6

Electrochemical Techniques ...... 11

Cyclic Voltammetry ...... 14

Reference Electrodes ...... 17

Project Proposal...... 22

Chapter 2: Methodologies ...... 25

Experimental Methods ...... 25

Sensor Design ...... 30

Chapter 3: Results and Discussion ...... 32

Quasi-reference ...... 32

Dynamic Reference ...... 45

Ni/Ni2+ with Boron Nitride Compartment ...... 50

Gold Alloy Reference...... 53

Thermodynamic Study ...... 59

Sensor Design ...... 63

Chapter 5: Conclusions ...... 68

Reference Electrode Selection ...... 68

Sensor Design ...... 69

Sensor Applications...... 70

References ...... 72

Appendix A: Cyclic Voltammetry Data ...... 76

Appendix B: Chronopotentiometry Data ...... 81

vii

List of Tables

Table 1: Summary of potassium reduction potential from full sweep cyclic voltammetry.

...... 37

Table 2: Summary of species concentration calculation from cyclic voltammetry measurements...... 43

Table 3: Summary of chronopotentiometry measurements with comparison to full sweep voltammetry...... 50

Table 4: Summary of potassium equilibrium potential as found from chronopotentiometry measurements...... 60

Table 5: Summary of apparent standard potential as found from cyclic voltammetry data.

...... 60

viii

List of Figures

Figure 1: Applied potential profile for cyclic voltammetry...... 15

Figure 2: Operational schematic of the three-electrode cell...... 16

Figure 3: Boron nitride enclosed nickel/nickel(II) reference...... 19

Figure 4: Grade A boron nitride custom crucible from Saint-Gobain...... 27

Figure 5: Nickel/nickel(II) reference assembly minus the boron nitride crucible, bottom.

...... 28

Figure 6: Custom quartz lid manufactured by Technical Glass Products...... 29

Figure 7: Cyclic voltammetry of europium couple in FLiNaK at 650°C vs Pt...... 33

Figure 8: Cyclic voltammetry of pure FLiNaK at 650°C...... 34

Figure 9: Cyclic voltammetry of pure FLiNaK at 700°C...... 34

Figure 10: Cyclic voltammetry of pure FLiNaK at 750°C...... 35

Figure 11: Full sweep at 650°C showing potassium reduction and europium couple...... 35

Figure 12: Plot of peak current density vs. square root of scan rate to evaluate diffusion controlled region...... 38

Figure 13: Cyclic voltammetry at 650°C (1)...... 39

Figure 14: Cyclic voltammetry at 700°C (1)...... 39

Figure 15: Cyclic voltammetry at 750°C (1)...... 40

Figure 16: Definition of cathodic and anodic peak current from corresponding linear baselines...... 41

Figure 17: Plot of diffusion coefficient results from Huang et al. fitted with trendlines. . 42

Figure 18: Chronopotentiometry at 650°C (1)...... 47

Figure 19: Chronopotentiometry at 700°C (1)...... 47

Figure 20: Chronopotentiometry at 750°C (1)...... 48

Figure 21: Open circuit potential using the Ni/Ni2+ reference over five days at 700°C. .. 51

Figure 22: Cyclic voltammetry of the FLiNaK plus 1wt% EuF3 salt using a gold working electrode...... 54

Figure 23: Plot of current response to an applied potential of -1V for 2 hours...... 55

Figure 24: Open circuit potential following a 2-hour application of -1V...... 56

Figure 25: Cyclic voltammetry of pure FLiNaK system with a gold working electrode. 57

Figure 26: Current response to an applied potential of -1.0V for 2.5 hours...... 57

Figure 27: Open circuit potential following Au2Na formation at gold working electrode.

...... 58

Figure 28: Redox potential for various redox couples in FLiNaK and FLiBe. Solid line:

-6 metal dissolution at limit activity of 10 ; Dotted line: reduction of oxidants. H2/HF

(100:1): a mole ratio of H2/HF=100 at 1 atm total pressure. Double solid line: redox potential calculated based on measured apparent potential...... 62

Figure 29: Electrochemical sensor housing dimensioned drawing...... 64

Figure 30: Drawing of the complete sensor assembly...... 66

Figure 31: Exploded view of the sensor assembly and flanged connection...... 67

Figure 32: Cyclic voltammetry at 650°C (1)...... 76

Figure 33: Cyclic voltammetry at 650°C (2)...... 76

Figure 34: Cyclic voltammetry at 650°C (3)...... 77

Figure 35: Cyclic voltammetry at 700°C (1)...... 77

Figure 36: Cyclic voltammetry at 700°C (2)...... 78

Figure 37: Cyclic voltammetry at 700°C (3)...... 78

Figure 38: Cyclic voltammetry at 750°C (1)...... 79

Figure 39: Cyclic voltammetry at 750°C (2)...... 79

Figure 40: Cyclic voltammetry at 750°C (3)...... 80

Figure 41: Chronopotentiometry at 650°C (1)...... 81

Figure 42: Chronopotentiometry at 650°C (2)...... 81

Figure 43: Chronopotentiometry at 650°C (3)...... 82

Figure 44: Chronopotentiometry at 700°C (1)...... 82

Figure 45: Chronopotentiometry at 700°C (2)...... 83

Figure 46: Chronopotentiometry at 700°C (3)...... 83

Figure 47: Chronopotentiometry at 750°C (1)...... 84

Figure 48: Chronopotentiometry at 750°C (2)...... 84

Figure 49: Chronopotentiometry at 750°C (3)...... 85

Chapter 1: Introduction

Molten Salt Nuclear Reactors

Nuclear reactors in the United States supplied 19.5 percent of the nation’s total generated electricity in 2015 [1]. These U.S. commercial reactors are light water cooled reactors (LWRs); however other coolants such as liquid sodium and helium gas are coolant candidates for advanced nuclear reactors that are being studied by the U.S. and other countries. The molten salt reactor (MSR) is a reactor concept which utilizes a molten salt primary coolant with dissolved salt fuel rather than water, gas, or liquid sodium with solid fuel. The MSR has its origins directly following the conclusion of

World War II in the Nuclear Energy for Propulsion of Aircraft (NEPA) and Aircraft

Nuclear Propulsion (ANP) programs. As their names imply, these programs held the aim of developing an aircraft which was to be powered by a nuclear reactor. A successful proof of concept test was performed in 1954 at Oak Ridge National Laboratory (ORNL) using a sodium-zirconium fluoride salt with uranium fluoride dissolved into the coolant.

This proof of concept led to the design of a full-scale aircraft reactor in 1956, nicknamed

“Fireball” due to its spherical shape. In 1961 the ANP program was cancelled as the

“Fireball” facilities were nearing completion due to advancements in missile technologies and political complexities [2].

This molten salt technology was able to continue in the form of the Molten Salt 1

Reactor Experiment (MSRE) in the 1960’s at ORNL. The MSRE’s goal was to investigate the application of molten salt reactor technology for civilian purposes. The

MSRE utilized a LiF-BeF2-ZrF4-UF4 liquid fuel with a graphite moderated core. This reactor experiment achieved its first full power operation in 1966, maintaining power for three years. Despite the program’s exceptional performance, the MSRE was terminated in

1973 due to lack of financial support. The Liquid Metal Fast Breeder Reactor being developed at Argonne National Laboratory at the time took precedence over the molten salt reactor developments at ORNL, leaving no funds for the molten salt programs [3].

The termination of the MSRE program in 1973 effectively halted the research and development (R&D) of molten salt technologies for the following decades. It was not until the Generation IV International Forum (GIF) identified molten salt reactors as a next generation (Gen IV) reactor in the early 2000’s that R&D work was revitalized. GIF identified both the Molten Salt Breeder Reactor and the Fluoride salt-cooled High- temperature Reactor (FHR) as candidate MSR’s for future deployment [4].

The U.S. Department of Energy (DOE) has recently invested millions of dollars into advancing FHR technology in support of a 2030 deployment. One of the FHR designs will employ a solid fuel called TRISO (tristructural-isotropic) rather than the dissolved fuel salt used in the MSRE. This TRISO fuel is a small kernel of uranium oxide surrounded by an inner low density pyrolytic carbon buffer, a high density pyrolytic carbon layer, a silicon carbide layer to limit fission product diffusion, and another outer high density pyrolytic carbon layer [5]. The primary coolant will be the fluoride molten salt called FLiBe (2LiF-BeF2) and a secondary coolant called FLiNaK (LiF-NaF-KF) for

2 the intermediate loop.

The FHR concept is advantageous due to its low operating pressure, high temperature power cycle, and passive decay heat rejection. These advantages combine to enhance safety, increase output, and lower costs for electricity and high temperature process heat generation [6]. These qualities will allow FHRs to compete in the current and future markets where low natural gas prices and renewable energy credits for wind and solar are currently driving electricity prices down to levels where Generation II reactors are struggling to compete. This revolutionary reactor will be instrumental in meeting the nation’s energy demands while simultaneously meeting climate change goals and therefore is the motivation of the research further discussed.

Corrosion and Materials

Corrosion is defined as a process in which material degradation occurs as a result of an electrochemical process with its environment. For any metal, M, this reaction takes the following form where the lost electrons in this oxidation reaction are transferred to the coupled reduction reaction:

푀 → 푀푛+ + 푛푒− (1)

+ − 푛푅 + 푛푒 → 푅푛 (2)

Collectively this set of half reactions is termed a reduction-oxidation or redox reaction.

The location where oxidation occurs is termed the anode while the location where reduction occurs is termed the cathode [7].

Corrosion in nuclear systems can have detrimental effects on the lifetime of a

3 nuclear plant’s systems and components. In current reactors, this issue is effectively managed by using stainless steel as the containing material for the water coolant.

Stainless steel naturally forms a passive oxide layer on its surface which inhibits corrosion rates of the metal even when exposed to acidic environments. Molten salt systems however present a much more complex issue for corrosion control due to the coolant’s ionic composition. Neutron reactions with Li6 will produce significant amounts of tritium in the core even with high enrichments of Li7. This is due to the large neutron cross section of Li6 and the production of Li6 from neutron interactions with Be9 [8, 9].

Produced tritium will form hydrofluoric acid, HF, with any free fluorides in the system.

A majority of this tritium will be retained in the salt due to HF’s high solubility in fluoride molten salts [10]. Additional, and perhaps a majority of the generated hydrofluoric acid will be generated by impurities in the salt such as water and oxygen.

Water in particular will react at these elevated temperatures with the fluoride salt constituents to form hydrofluoric acid [11] by:

푥 푀퐹푥 + 퐻2푂(푔) → 푀푂푥 + 푥퐻퐹(푔) (3) 2 2

The generated HF, either by tritium produced by neutronic interactions with Li6 or the existence of impurities, will easily break down any passive oxide layers and cause corrosion of the containing structural materials described by:

2퐻퐹 + 푀 → 퐻2 + 푀퐹2 (4)

This issue is very problematic to the application of fluoride molten salts and has therefore been the inspiration for investigation of candidate structural materials. This was recognized by the scientists at ORNL during the MSRE project who developed a nickel 4 alloy called Alloy N or Hastelloy N. This alloy was a successful performer when paired with redox control as found by examination of MSRE components following its shutdown [12]. Although Hastelloy N has acceptable corrosion resistance, it does have limited mechanical properties at the high temperatures which the FHR demands [6].

Investigations of other candidate materials such as austenitic steels have been performed for fluoride salt applications. Kondo has performed corrosion experiments for material candidates of the liquid blanket of a fusion reactor [13]. He studied the corrosion of stainless steel 304 (SS304) and stainless steel 316L (SS316L) in FLiBe. Exposure to HF at 500°C and 600°C for 1000 hours showed corrosion rates of 10.6 um/year and 5.4 um/year for the general corrosion of SS304 and SS316L respectively. This study noted that corrosion rates increased with temperature and chromium and iron were corroded from the steel by selective intergranular mechanisms. Nagasaka investigated the use of the ferritic steel JLF-1 and the vanadium alloy HEAT-2 for use in FLiBe [14]. His study found that when exposed to HF at 550°C, JLF-1 exhibited a corrosion rate half that of

SS316 while the HEAT-2 alloy showed a corrosion rate 20-30 times greater than JLF-1.

Both of these researchers observed that the main corrosion product present for all metal subjects was a chromium oxide.

It is important to note that the nominal operating temperature of an FHR is 700°C, which is significantly higher than the temperatures at which these studies were conducted. Even the MSRE operated at a lower 550-650°C [12]. It is expected that the borderline performances of these candidate materials will not suffice for the higher temperatures which the FHR demands either because of excessive corrosion rates at

5 elevated temperatures or because of inadequate mechanical strength. ORNL has recently developed more advanced versions of Hastelloy N which exhibit an increased strength as a result of precipitation hardening. It is claimed that this new alloy will still have an effective resistance to corrosion in fluoride salts, however less so than the original.

ORNL is currently working to acquire material qualifications under Section III of the

Boiler Pressure Vessel Code for nuclear power plants [6].

Redox Control of Molten Systems

Although the candidate structural materials are, to varying levels, capable of resisting attack by the pure molten salt, TF will be continuously generated over the course of operation. It is therefore necessary to develop a control mechanism which actively prevents the corrosion of structural materials from being thermodynamically favorable. As the studies of Nagasaka and Kondo have observed [13, 14], chromium is the most thermodynamically unstable element in traditional metal alloys, including

Hastelloy N. The MSRE developed a rudimentary form of corrosion control in which the concentration ratio of UF3/UF4 was maintained within a specific range. This worked to inhibit the chromium corrosion reaction by maintaining greater amounts of UF3 through the following mechanism:

퐶푟 + 2푈퐹4 ↔ 2푈퐹3 + 퐶푟퐹2 (5)

This control was accomplished by the contact of metallic beryllium with the molten salt described by:

퐵푒 + 2푈퐹4 ↔ 퐵푒퐹2 + 2푈퐹3 (6)

Metallic beryllium was added to the salt routinely through a sample tube which also allowed for CrF2 samples to be monitored [12]. Maintenance of these redox reactions is called redox control.

For fluoride molten salts, Olander has identified the definition of the redox potential as the fluorine potential, further defined by the partial Gibbs free energy of the fluorine gas [15]:

∆퐺퐹2 ≡ 푅푇푙푛(푝퐹2) (7)

Equation 7 expresses this relation where R is the gas constant, T is temperature, and p is the gas partial pressure. This result has roots in the general oxidation reaction of Equation

3. There are three mechanisms for controlling the redox potential: gas phase control, major metal control, and dissolved salt control. Gas phase control fixes the fluorine partial pressure through the following mechanism:

퐻2(푔) + 퐹2 → 2퐻퐹(푔) (8) by method of a hydrogen fluoride bubbling system such as that employed by Kondo and

Nagasaka’s corrosion experiments. Major metal control involves addition of a major metal constituent of the salt which has its corresponding fluoride already existing in solution, described by:

퐵푒(푠) + 퐹2 → 퐵푒퐹2 (9)

Dissolved salt control is the use of a fluoride salt with two valence states that are soluble in the molten medium such as:

2퐸푢퐹2 + 퐹2 → 2퐸푢퐹3 (10)

Gas phase control is a useful method of removing initial impurities in the system however

7 introduces significant amounts of corrosive gas to the system and therefore is not desirable during operation. The MSRE utilized major metal control to perform dissolved salt control. This method is particularly beneficial because the additions of major metal can help to maintain the salt at the eutectic as composition changes over service life while the dissolved salt control effectively fixes the fluorine potential through the entire system due to the dissolved salt’s high solubility.

As previously described, the method of redox control used by the MSRE was effective but it was not very well understood. Redox control has been investigated in more depth by the Japan-U.S. Program on Irradiation Tests for Fusion Research

(JUPITER-II). Fusion reactors have proposed the molten salt FLiBe as the liquid blanket material just like the primary coolant of the FHR. This application presents the same issues for redox control because tritium will also be produced in the fusion reactor due to neutronic effects with the coolant. Studies conducted by this program have developed kinetic models, predicted the response of HF following beryllium contact, and assessed the feasibility of using beryllium contact to neutralize the HF in the system sufficiently to inhibit corrosion [9, 16-18].

These studies all have very similar experimental setups in which a gas mixture of

H2 and HF in helium is bubbled through a FLiBe containing crucible. One such study developed a kinetic model based on Henry’s gas law to simulate the conversion of HF over time with beryllium contact [16]. This model assumes that all HF consumption is due to beryllium depletion, therefore HF consumption is a function of the gas mixture flow rate, initial beryllium concentration, time, and the unknown rate constant.

Experiments were then performed at 530°C with initial HF concentrations of 910-1800 ppm. Measurements of the HF in the effluent gas following beryllium contact and HF cutoff were performed. The results of their tests allowed for determination of the rate constant. This single parameter model was able to accurately model the experimental results obtained. This study found that for a beryllium immersion time of 1200-3600 seconds, full HF conversion could be obtained for the next 60 hours. Extrapolation of this model to the much lower HF concentrations expected in a nuclear reactor show that HF can be maintained below 0.02 ppb [16].

The same researchers used this experimental setup and a similar model to further the understanding of the HF response to beryllium contact while HF was still being supplied, which is a better representation of the conditions within a reactor [17].

The enhanced complete mixing model developed was found to fit their experimental data well. The experimental data found that beryllium contact of 5-45 minutes reduced the outlet HF concentration to less than 100 ppm within a couple hours and then returned to initial conditions after periods of slightly greater than 24 hours. It should be noted that the faster HF conversion time found in this study compared to the last is attributed to the active gas flow rate, which reduces residence time, and the fact that full HF conversion does not occur in this case due to continued HF flow. Another important result of this study is that the diffusion of F- ions was found to be the rate limiting step, rather than beryllium’s diffusion into the salt. This was determined by the constant rate reduction of

HF concentration even following beryllium removal.

These studies were then extended to include a materials investigation when

9 coupled with this redox control [9]. This particular study was essentially a joining of the

Nagasaka study previously described and the redox studies performed by Simpson and

Fukada. Exposure of HF gas to JLF-1 samples in FLiBe at 530°C showed elevated levels of chromium and iron in the molten salt. Contact with a beryllium rod was performed for

5 hours, drastically reducing the metallic concentrations in the molten salt as found by an automatic titration system. Continued measurement of the metallic concentrations confirmed a period of redox control extending 40 hours after beryllium removal. After the excess dissolved beryllium was consumed by the HF gas flow, chromium and iron concentrations rose significantly over the course of the remaining 500-hour test.

The JUPITER-II program has made significant progress investigating the major metal form of redox control described above. Although the JUPITER-II program’s focus is for application to the fusion reactor, this type of major metal redox control is currently the proposed method of redox control in the primary loop of the FHR. Due to the ternary loop design of the FHR, redox control will also be necessary in the intermediate loop.

Since beryllium does not exist in the FLiNaK intermediate coolant and these major metals have limited solubility in the salt, it is necessary to explore other methods such as complete dissolved salt control.

Recently a dissolved salt redox control study was conducted for the French

Molten Salt Fast Reactor [19]. This reactor utilizes a liquid uranium fuel dissolved into the molten salt, similar to the MSRE. This study investigates the feasibility of performing redox control with solid uranium rather than through beryllium contact. Uranium contact is made as necessary to control the U4+/U3+ ratio which can be monitored by an

10 electrochemical technique called cyclic voltammetry. This study found that U4+ reached an equilibrium with U3+ after 2.5 hours of contact, as evidenced by the disappearance of the U4+ reduction current. An inactive study using Eu3+/Eu2+ was used prior to the uranium, which exhibited the same results. In summary, this study demonstrates control of the uranium and europium dissolved salt couples and the ability to monitor this process qualitatively.

Electrochemical Techniques

Electrochemistry is a field of study which investigates the relationships between electrical and chemical phenomena. This field has large usage in aluminum production, metal deposition, batteries/fuel cells, and corrosion. The nuclear industry has significant experience in the application of electrochemistry for pyroprocessing. Nuclear pyroprocessing is the process of recycling used nuclear fuel for both the benefit of reducing quantities of high level radwaste and obtaining fissile material for further energy production. Numerous electrochemical studies have been performed to investigate the separation of actinides and fission products from the used fuel [20-25], the most notable and successful being the Experimental Breeder Reactor II (EBR-II) at Argonne National

Laboratory [26].

Electrochemical techniques are extremely useful and extensively employed for investigating quantities and thermodynamic data of actinides and fission products in various molten salt media [20-23, 27]. The high temperatures, aggressive media, and desired accuracy make the systems relevant to pyroprocessing difficult to study, however

11 electrochemical techniques are not affected by aggressive conditions and can provide highly accurate and precise measurements. Uranium and plutonium concentration measurements were studied by Tylka et al to establish best practices for accurate measurements of pyroprocessing systems [22, 23]. She studied uranium and plutonium dissolved in a lithium-potassium chloride salt at 500°C using cyclic voltammetry and square wave voltammetry. This study determined that cyclic voltammetry produced more accurate results when compared with spectroscopy methods (ICP-AES) if a set of experimental methods to enhance accuracy of measured immersed surface area, validate assumptions, and renew the electrode surface were implemented. Cyclic voltammetry was able to measure concentration accurately within a tenth of a weight percent up to two weight percent from:

푛3퐹3퐷푣 푖 = 0.61퐴퐶√ (11) 푝 푅푇 where ip is the peak current, A is the immersed working electrode surface area, n is the number of electrons transferred, F is the Faraday’s constant, D is the diffusion coefficient, v is scan rate, R is the gas constant, T is temperature, and C is concentration.

Above two weight percent, the diffusion coefficient cannot be assumed constant.

Equation 11 is valid for a soluble-insoluble system

As previously mentioned, electrochemical techniques are also useful for determining various thermodynamic data for actinides and fission products. Europium is both a fission product as well as a chemical couple in redox control studies discussed prior. Due to this dual use, studies have been performed on europium in multiple molten

12 salt media. One such study performed by Caravaca, investigated the diffusion coefficients and the standard redox potential of the europium system in LiCl-KCl using cyclic voltammetry and chronopotentiometry at 500°C [20]. The diffusion coefficient of Eu3+ was calculated by convolution of the cyclic voltammetry signal as well as chronopotentiometry. The diffusion coefficients differed by an order of magnitude between the two techniques. The diffusion coefficient of Eu2+ was then calculated by:

푛3퐹3퐷푣 푖 = 0.4463퐴퐶√ (12) 푝 푅푇 which is the equivalent of Equation 11 for the soluble-soluble reaction. The apparent standard potential of the couple was calculated from:

1 푐 푎 퐸푝 + 퐸푝 푅푇 퐷 2 퐸° = + 2.3 log (( 표푥 ) ) (13) 2 푛퐹 퐷푟푒푑

c a where Ep and Ep are the cathodic and anodic peak potentials respectively. This study observed that within the potential window of the salt, between major metal reduction and chloride oxidation, only Eu3+ and Eu2+ states were detectable. The solid Eu state is located outside of the potential window for LiCl-KCl.

The europium couple was also investigated in 700-900°C NaCl-KCl molten salt

[21]. This study implemented linear sweep voltammetry, cyclic voltammetry, and chronopotentiometry to determine the diffusion coefficients and characterize the apparent standard potential over the stated temperature range. Unlike the study previously discussed, calculated diffusion coefficients agreed well between the electrochemical methods employed. Diffusion coefficients were also calculated from Equation 12 for

13 voltammetric methods while the Sands equation:

1 퐼√푣 = (푛퐹퐶퐴√휋퐷) (14) 2 was used for chronopotentiometry data. The standard apparent potential was characterized over the studied temperature range by application of Equation 13 to the voltammetric data. The potential window of NaCl-KCl once again limited the observable states of europium to its Eu3+/Eu2+ states.

Europium was studied by different researchers in the 500-650 °C FLiNaK molten salt to determine its diffusion coefficients [27]. Cyclic voltammetry was once again used to calculate the diffusion coefficients by Equation 12 while the concentration was determined by square wave voltammetry. The same observation of europium’s oxidation states was observed in FLiNaK’s potential window.

Cyclic Voltammetry

The versatility of cyclic voltammetry has made it a popular electrochemical technique. Cyclic voltammetry is capable of investigating redox potentials, reaction thermodynamics, and reaction kinetics. The work that will be presented in this report makes extensive use of cyclic voltammetry, therefore it is necessary to speak on the fundamental principles of this technique. This technique applies an initial potential to the working electrode then increases the applied potential linearly to a limiting potential. The potential is then ramped back down to the initial potential. The rate at which the potential is varied is called the scan rate [28-30]. This procedure is shown graphically in Figure 1.

Figure 1: Applied potential profile for cyclic voltammetry.

The working electrode potential must be measured in reference to a defined potential, therefore it is necessary to include another electrode called the reference electrode. The resistance of the electrolyte being studied can cause a discrepancy between the applied and actual potentials at the working electrode called an ohmic drop. This ohmic drop causes the measured potential between the working and reference electrodes to be slightly less than what is applied. The addition of another electrode, called the counter electrode, passes the current from the working electrode, through the electrolyte, and into the counter electrode which helps reduce the ohmic drop to negligible levels. A commercially available device called a potentiostat serves as both the function generator and data acquisition system as shown in Figure 2. Figure 2 clearly portrays the measurements taking place with this three-electrode system.

Figure 2: Operational schematic of the three-electrode cell.

The kinetics of the voltammogram obtained by cyclic voltammetry are described by the Butler-Volmer equation:

훼푛퐹(퐸−퐸°) (1−훼)푛퐹(퐸−퐸°) 푖 − 퐽 = = 퐶 푘°푒 푅푇 − 퐶 푘°푒 푅푇 (15) 푛퐹퐴 푎 푏 where J is the flux at the electrode surface, alpha is the transfer coefficient, k° is the standard rate constant, C is the concentration of species A and B, and E° is the standard potential. The Butler-Volmer equation models the charge transfer at the electrode surface while Fick’s 2nd law:

휕퐶 휕2퐶 = 퐷 (16) 휕푡 휕푥2 describes the concentration profile in the diffusion layer surrounding the electrode surface. When the charge transfer becomes sufficiently fast, k° greater than 0.1 cm/s, the diffusion of electroactive ions to the electrode surface becomes the limiting step. This case is termed a reversible reaction. For the reversible case the Butler-Volmer equation

16 simplifies to what is called the Nernst equation [28-30].

푅푇 퐶 퐸 = 퐸° + 푙푛 표푥 (17) 푛퐹 퐶푟푒푑

Reference Electrodes

Both the working and the counter electrodes are required to pass current and must not chemically interact with the electrolyte, thereby affecting measurements. This means that the working and counter electrodes may simply be some inert metal rod or wire. The reference electrode on the other hand has a more complex role in the collection of electrochemical data, especially thermodynamic data, because of the need to reference the potential between the working and reference electrode to a thermodynamically defined potential. When selecting a reference electrode for a given study and system there are five qualities which must be considered: reversibility of the electrode system, stability and reproducibility of the potential, stability of the reference electrode materials, impedance of the reference, and compatibility of the reference with the type of measurement [31, 32]. The importance of each of these five qualities highly depend on the application.

There are three classifications of reference electrodes which are based on each’s working principle: half-cell reference electrodes, inner reference electrodes, and quasi- reference electrodes. Half-cell references are electrodes which are separated from the electrolyte by an insulating compartment. This compartment is able to limit mass transport while maintaining electrolytic charge transfer. The inner reference in contrast is placed directly into the electrolyte where the ionic species for the reference reaction has a 17 known concentration. This coupling, solid metal with corresponding ion, establishes a particular half reaction with an associated thermodynamic potential. The quasi-reference is fundamentally different from both the half-cell and inner reference types. This reference comprises of an inert metal directly immersed in the electrolyte, similar to the inner reference with the exception that no half reaction is established with a corresponding ion in the electrolyte. For many inert metals, it is observed that the potential may be stable over time intervals typical of laboratory experimentation. This temporary stability allows these electrodes to be useful when only changes in potential or current are needed but the lack of a thermodynamically defined potential limits knowledge of precise potential locations necessary for thermodynamic studies.

Reference electrodes for fluoride molten salts have been qualitatively studied much less than those for aqueous media and even other molten salts. Despite this shortcoming in the literature, a diverse set of references have been employed with varying results. A popular reference for fluoride molten salts such as FLiNaK and FLiBe is a half-cell reference which utilizes the Ni/Ni2+ couple [31, 33, 34]. This reference is contained in a boron nitride sheath, as shown in Figure 3. Nickel wire is placed in the reference fluoride melt which contains a known concentration of Ni2+ ions. The boron nitride compartment is an electrical insulator in fluoride salts. It can be permeated however by the salt at high temperature to provide ionic contact. It has been reported that this wetting occurs within 24-48 hours in FLiNaK at 500°C and 10-14 days in FLiBe at

550°C [33]. Studies show that this reference has good reproducibility, short term stability, a low junction potential, and is reversible.

Figure 3: Boron nitride enclosed nickel/nickel(II) reference.

The construction of this reference only allows for limited use of a few days due to breakdown of the boron nitride. Reaction with the NiF2 is thought to compromise the integrity of the reference compartment by the following mechanism:

3 3 1 푁푖퐹 + 퐵푁 → 퐵퐹 + 푁푖 + 푁 (18) 2 2 3 2 2 2

A compartment of graphite which was coated in pyrolytic boron nitride except for the bottom has also been investigated [34]. This study found that the reference exhibited a stable potential for 50 hours and wetted in only 3 hours. An additional variation of this reference utilizes a single lanthanum fluoride crystal which provides much better ionic contact between the reference and bulk salts. The lanthanum fluoride crystal has been observed to crack due to thermal cycling and therefore can only be used once. For all of these half-cell type reference designs, diffusion of bulk species into the reference compartment occurs at long exposure times. 19

The Ag/Ag+ couple is another reference that can be used in much the same way as the Ni/Ni2+ couple. A silver-chloride solution can be added to a chloride salt contained in a boron nitride type compartment. This reference has been shown to be both reversible and reproducible however it will succumb to the same limitations due to its containing material [31]. Tylka investigated the use of the Ag/Ag+ couple enclosed in a vycor glass tube but found that the durability at high temperatures was extremely low. A mullite tube was implemented instead but the mullite created additional redox peaks in the study’s voltammograms. Tylka ended up utilizing platinum and tungsten quasi-reference electrodes [22]. The Ag/Ag+ couple is additionally limited by the melting temperature of silver, 960°C.

Quasi-reference electrodes have become popular in recent studies, likely due to their simplicity and a current interest in kinetics of actinides in molten salts for pyroprocessing rather than fundamental thermodynamic data [21-23, 27, 35-37]. There is no container for these electrodes, therefore are very durable and simple yet they have no thermodynamically defined potential as previously described. Studies have shown that quasi-references have been observed to hold a stable potential over several days in some cases. Quasi-reference electrodes in fluoride molten salts can be platinum, tungsten iridium, gold, nickel, or molybdenum. Platinum, tungsten, and molybdenum seem to be the most popular in the literature.

Other less common reference electrodes which have been used in fluoride molten

2+ 2+ salts are the Ni/NiO, Fe/Fe , H2/HF, and Be/Be couples [31]. The Ni/NiO reference can be employed in a FLiBe salt which is saturated with NiO and BeO. The potential at a

20 nickel wire is fixed by the following reaction:

푁푖푂 + 퐵푒2+ + 2푒− ↔ 퐵푒푂 + 푁푖 (19) where the saturated NiO/BeO containing salt is housed within a BeO or silica compartment. The Fe/Fe2+ is another reversible alternative to Ni/Ni2+ or Ag/Ag+ which also requires a containing sheath. A H2/HF reference consists of bubbling a known concentration gas mixture across a metal electrode which is defined by:

− − 퐻2 + 2퐹 + 2푒 ↔ 2퐻퐹 (20)

A Be/Be2+ is particularly useful for molten salts which possess a beryllium major metal.

The beryllium couple has been shown to be reversible with small potential variations across a large range of high temperatures.

Due to the shortcomings of typical references in fluoride molten salts, several novel reference electrode designs have been investigated recently [38-40]. Afonichkin and Duran-Klie have separately investigated dynamic reference electrodes in LiF-NaF-

BeF2 and FLiNaK respectively. Afronichkin found that for beryllium containing molten salts, beryllium is the most easily reduced major metal. Based on this observation, application of a current pulse was investigated to reduce metallic beryllium onto an inert molybdenum working electrode thereby establishing a Be/Be2+ reference electrode. This reference belongs to the inner reference type however the effect will be temporary as the deposited beryllium dissolves back into its thermodynamically more stable ionic form i.e. dynamic reference. A basic optimization test was conducted which found that a longer lasting stable potential is generated by increasing the pulse time and current magnitude.

This dynamic electrode was tested in a convection loop where it was found precise to

+/- 5mV and stable for more than 1200 hours at 600°C. Duran-Klie found that for the

FLiNaK molten salt, potassium is the most easily reduced major metal. A very similar procedure was used in which a current pulse was applied to a tungsten electrode and the voltage response was measured during and after the pulse. For sufficient magnitudes of current, a stable potential plateau was observed corresponding to the K/K+ couple. A more detailed optimization study was performed which found that a current magnitude at least 4 times that of the other cations limiting cathodic current is required to establish this reference. The potassium dynamic reference electrode was then validated by tracking the potential of a nickel wire versus the dynamic reference for varying concentrations of NiF2 in the FLiNaK salt at 500°C. The measured results agreed with that predicted by the

Nernst equation.

An additional study by Goto in FLiNaK at 500°C investigated a possible gold alloy reference [40]. Cyclic voltammetry of a pure FLiNaK salt using a gold working electrode displayed the existence of a pair of redox peaks. Electrolysis of the salt at a potential more cathodic than the cathodic peak and subsequent analysis of the electrode using x-ray diffraction showed the formation of a Au2Na alloy. The stability of this inner type reference was investigated by producing two of these electrodes and measuring the potential between them. A stable potential difference of +/- 1mV was observed for more than 25 hours.

Project Proposal

As presented so far, fluoride molten salts are an important component for nuclear

22 applications such as the FHR concept. These molten salts present a unique challenge to operation of such a reactor because of the corrosive behavior caused by nuclearly generated tritium and the existence of impurities such as water and oxygen. Studies have been performed which investigate candidate structural materials which will resist corrosion and maintain strength however some form of redox control will still be required to inhibit the corrosion potential of the system sufficiently. Electrochemical methods such as cyclic voltammetry are uniquely suited to monitoring this redox potential in real time.

Despite this opportunity, discrepancies still exist in the ability to monitor the redox potential due to the deficiencies of many reference electrodes for use in fluoride molten salts. Some references hold promise, however to date no comparative study has been performed to identify an ideal candidate for an industrial fluoride molten salt application such as the FHR. Based on the presented review of reference electrodes which have found use in a fluoride molten salt study, four hold significant promise for this application: the nickel/nickel(II) reference with boron nitride container, quasi- reference electrode, dynamic reference, and Goto’s gold alloy reference. The other references can be eliminated based on limitations of mechanical durability such as the cracking of lanthanum crystals, temperature limitations such as silver’s melting temperature, and the presence of unnecessary sources of impurities near the point of measurement such as the NiO/BeO and H2/HF couples.

This report seeks to perform a comparative study of the identified candidate reference electrodes with the goal of identifying an ideal reference. Europium has been identified as a method of dissolved-salt redox control, therefore will be the subject of

23 investigation using the candidate reference electrodes. The suitability of cyclic voltammetry to measure the concentration of the europium(III)/europium(II) couple for redox control by the Nernst equation will be accomplished as a by-product of this reference electrode study. Through study of the europium couple by cyclic voltammetry using the stated candidate references, an ideal reference will be identified with the ultimate development of a complete electrochemical sensor design which can perform measurements that will be the input to a redox control procedure. Completion of this objective will effectively address the current gaps to successful application of redox control in an FHR, bringing the FHR significantly closer to deployment.

Chapter 2: Methodologies

Experimental Methods

This study will be performed utilizing the FLiNaK molten salt media. FLiNaK is commonly used as a surrogate for FLiBe because these molten salts have very similar physical and chemical properties; however, FLiBe presents a much more hazardous work environment. This choice is particularly appropriate given the intended FHR application because all results can be directly applied to the intermediate loop and extended to application of the primary loop because of the similar coolant properties. Lithium fluoride, sodium fluoride, and potassium fluoride are procured from Sigma Aldrich at greater than 99% purity. All FLiNaK used in this study is prepared at its eutectic composition, 46.5 mol% LiF, 11.5 mol% NaF, 42 mol% KF, by weighting the powders individually using an analytical balance with an accuracy of 10-5 grams. Anhydrous europium trifluoride (EuF3) and NiF2 are procured from Sigma Aldrich at greater than

99.9% purity where the EuF3 is donated by Professor Mike Short of MIT. The weighed components are then mixed in a pure nickel crucible from Sigma Aldrich. A glassy carbon crucible is commonly utilized for these kinds of experiments because of its inert behavior [20, 38, 41], however nickel will be the main alloy constituent of the FHR structural material and nickel crucibles are found to be significantly cheaper and more disposable than a glassy carbon crucible. These benefits mean that the experiments can be 25 performed in a more representative manner and each test can be completed using a fresh crucible, ensuring quality. Other researchers have utilized nickel based crucibles [22-24], however this choice is validated by electrochemical tests which show no additional effects from a molten salt/nickel crucible interaction. All electrodes are positioned such that no contact exists with the nickel crucible. Salt preparation is performed inside of a custom Inert Technology glovebox under an inert argon atmosphere. All preparations and experiments are performed with water and oxygen impurities less than 5 parts per million

(ppm).

The three-electrode electrochemical cell consists of a tungsten working electrode, tungsten counter electrode, and a variety of reference electrodes. The working electrodes are either 1/16 inch or 1 mm diameter while the counter electrodes are either

5/32 inch or 3 mm diameter. The counter electrode is set much larger than the working in order to promote a negligible ohmic drop due to current flow limitation between the working and counter electrodes [28]. Tungsten is selected because it is much more stable thermodynamically than any of the major alloy constituents which could exist either in these experiments or in the FHR such as nickel, chromium, iron, and the molten salt major metals [37, 38]. Tests show no evidence of any intermetallic compounds forming on either tungsten electrode, validating the material’s choice. The 1 mm diameter platinum quasi-reference is procured from the OSU eStores supply at greater than 99.9% purity. A 0.5 mm diameter gold wire is procured from Sigma Aldrich at greater than

99.9% purity for the gold alloy reference. The nickel/nickel(II) reference is constructed with a 99.9% pure 1 mm diameter nickel wire from Sigma Aldrich and a custom boron

26 nitride Combat Grade A crucible from Saint-Gobain, Figure 4.

Figure 4: Grade A boron nitride custom crucible from Saint-Gobain.

In order to reach the desired depth inside the furnace, it is necessary to fabricate a stainless steel 316 extension. This rod is machined to have a male connection which attaches to the crucible using stainless steel 316 dowel pins. A sequence of holes is machined through the top portion of the rod in order to hold the assembly at the necessary depth using additional stainless steel 316 dowel pins. The entire rod is hollowed to allow the nickel wire to be threaded through its length and into the crucible. The nickel wire is insulated from the stainless-steel extension by a carbon fiber insulating rope. This apparatus is depicted in Figure 5. All additional materials whose sources are unidentified are procured from McMaster-Carr. 27

Figure 5: Nickel/nickel(II) reference assembly minus the boron nitride crucible, bottom.

A high temperature Kerr Electro-Melt furnace is used to heat the salt samples to 650,

700, and 750°C, which is representative of the temperature profile within the primary loop of the FHR. A custom quartz lid is fabricated by Technical Glass Products in order to accommodate the placement of the various electrochemical cells which are studied,

Figure 6. The large port in the middle is specifically designed to accommodate the large bore nickel/nickel(II) reference apparatus. Insulation is packed into the gap between the top and bottom panes to prevent excessive heat escaping the top of the furnace. Upon initial melt after mixing, the temperature is set and held at 650°C for a minimum of 12 hours in order to ensure thermal and chemical equilibrium of the salt and remove any water impurities.

Figure 6: Custom quartz lid manufactured by Technical Glass Products.

Electrochemical measurements are collected by a Gamry Instruments Interface

1000 potentiostat/galvanostat and a PC computer running associated Gamry software.

Equilibrium is verified by open circuit potentiometry before any measurements are collected and an electrode cleaning procedure is implemented which replenishes the electrode surface in between consecutive measurements. The cleaning procedure consists of setting a sufficiently more anodic potential than any observed reactions in order to remove any deposits which may accumulate on the electrode surface during the previous measurements. These practices ensure that each set of measurements are not affected by those that preceded. The immersion depth can be measured in a variety of ways;

29 however, it is decided in this study to directly measure the solidified salt level on the working electrode following cooldown due to its simplicity. It is observed that the salt level was well defined following cooldown, therefore the level of accuracy this method provides is sufficient given that the focus of this study is not highly accurate but rather highly precise concentration results. The measured immersed surface area is therefore not a factor for precision given that a constant value is used for all sets of measurements for a given experiment.

Sensor Design

A sensor design for application in an FHR must be capable of at least measuring the concentration of redox species as stated in the project proposal, however obtaining information which requires both precise current and a thermodynamically defined potential would make it more versatile to plant operators and engineers. Any sensor must also possess a high level of durability and a long service life because the sensor may be placed in plant locations inaccessible for maintenance/replacement during full power operation. The experimental results will be used to identify an ideal reference electrode based on the previously discussed qualities for a reference electrode which are repeated here for convenience: reversibility of the electrode system, stability and reproducibility of the potential, stability of the reference electrode materials, impedance of the reference, and compatibility of the reference with the type of measurement. For application to an

FHR the stability/reproducibility of the potential, stability of the reference materials, and compatibility of the reference with the measurement type are considered most important.

Following the identification of the best reference electrode, a design will be completed which also adheres to the durability demands while simplifying the construction, installation, and use as much as possible. This will require significant materials consideration.

Chapter 3: Results and Discussion

Quasi-reference

The quasi-reference electrode has been a popular choice for recent molten salt studies due to its simplicity. An electrochemical cell is assembled with a 1mm diameter platinum reference, 1mm diameter tungsten working, and 5/32’’ diameter tungsten counter electrode. FLiNaK is weighted and measured in the amount of 30.001 grams. An additional 0.300 grams of EuF3 is added, corresponding to 1 weight percent. Although the

Eu3+ valence state is directly added to the FLiNaK mixture, Eu2+ is also present in equilibrium with Eu3+ at elevated temperatures. This coexistence of Eu3+ and Eu2+ has been observed in both chloride and fluoride molten salts when only Eu3+ was initially added [19-21, 27, 42]. Thermal decomposition of some amount of Eu3+ to the Eu2+ valence state is thought to occur with heating. The study performed by Huang investigated the molten salt after melting by X-ray photoelectron spectroscopy which showed the coexistence of both europium valences states. An equilibrium ratio of these valences states exists which is investigated by cyclic voltammetry.

Cyclic voltammetry is performed at a scan rate of 100 mV/sec to characterize the signal and locate the redox peaks of europium, Figure 7. From Figure 7, the europium peaks are located at -0.05 V and -0.205 V vs Pt. This signal displays a very low magnitude signal. An immersed working electrode surface area of 0.374 cm2 is measured 32 which corresponds to peak currents of 5-7 mA. This low signal means that a large portion of the measured current is being supported by the electrolyte itself rather than the electroactive couple. The background signal of pure FLiNaK must be characterized over

Figure 7: Cyclic voltammetry of europium couple in FLiNaK at 650°C vs Pt.

the potential range of the europium couple for subtraction in order to attain proper values of current. A slightly narrower potential window is scanned around which the europium couple can be observed at 650, 700, and 750° for a range of scan rates, Figures 8-10.

Interestingly, these figures show that the molten salt possesses capacitive behavior as exhibited by the nearly constant current density across the scanned potential window. It is observed that current increases towards the extremities of the scanning window which is indicative of non-perfect capacitance [30]. Current density is strongly dependent upon scan rate while only slightly dependent on temperature. Characterization of the background signal over varying scan rates and temperatures allow for any further

33 measurements to be normalized as a function of scan rate and temperature.

Figure 8: Cyclic voltammetry of pure FLiNaK at 650°C.

Figure 9: Cyclic voltammetry of pure FLiNaK at 700°C.

Figure 10: Cyclic voltammetry of pure FLiNaK at 750°C.

Cyclic voltammetry of the FLiNaK plus 1wt% EuF3 is again performed. A full sweep of the FLiNaK’s potential window shows a sharp reduction current at the cathodic extremity, Figure 11. Based on prior studies [19-21, 27] this is attributed to the reduction of the potassium ions of the salt into potassium metal at the working electrode.

Figure 11: Full sweep at 650°C showing potassium reduction and europium couple.

Conceptually this can be interpreted as the equilibrium potential of K/K+ since it is at this particular potential that potassium ions become thermodynamically less stable than potassium metal. Additionally, since the reduction current is so sharp it has a clearly defined potential. Since quasi-reference electrodes have no thermodynamically defined potential, all measurements performed using this platinum reference are also not defined.

For the purpose of establishing fixed potential coordinates in the results which follow, full sweep voltammograms are conducted prior and following each set of cyclic voltammetry experiments and then referenced to the K/K+ reduction. Due to the limited time elapsed during each set of cyclic voltammetry experiments, 1-2 hours, it is valid to assume that the potential of the platinum reference does not vary considerably and therefore all measurements can be referenced to the potassium reduction found from the full sweep. This procedure will allow for not only the determination of europium species’ concentrations to evaluate suitability of the quasi-reference but also determination of the apparent standard potential later in this report as required by the Nernst equation,

Equation 17. This assumption’s validity will be experimentally tested by the results of the dynamic reference since its working principle relies on establishing the equilibrium potential of a major metal/major metal ion, potassium in the case of FLiNaK.

Two full sweep voltammograms are performed for each set of cyclic voltammetry scans, before and after. Three replicate sets of cyclic voltammetry tests are performed at each temperature. From Table 1, it can be seen that the reduction potential varies very little before and after a set of cyclic voltammetry measurements are taken. Very small variations are observed within a given temperature as well as between differing

36 temperatures. Only variations of a few millivolts are observed which validates the assumption that the potential is stable. The full scan measurements are performed with a scan rate of 200 mV/sec.

Table 1: Summary of potassium reduction potential from full sweep cyclic voltammetry.

Average K/K+ Temperature (°C) K/K+ Reduction Reduction (V vs. Pt) (V vs. Pt) 650 (1) -1.283 -1.279 -1.281 650 (2) -1.277 -1.275 -1.276 650 (3) -1.277 -1.275 -1.276 700 (1) -1.283 -1.277 -1.280 700 (2) -1.277 -1.275 -1.276 700 (3) -1.275 -1.275 -1.275 750 (1) -1.305 -1.289 -1.297 750 (2) -1.279 -1.275 -1.277 750 (3) -1.275 -1.273 -1.274

A full range of scan rates from 20 mV/sec to 1000 mV/sec are investigated.

Plotting the peak current density versus the square root of scan rate will be linear for scan rates in which the electrochemical reaction is diffusion controlled [27]. This is an important preliminary step before presenting the raw cyclic voltammetry because identification of the diffusion controlled region can greatly refine the useful data since application of the Nernst equation requires reversibility. Figure 12 plots this scan rate study. This plot shows deviation from linear behavior for scan rates exceeding 0.7 V/s0.5, corresponding to 500 mV/sec. It is also observed from the cyclic voltammetry signals that current peaks are ill defined for scan rates less than 60 mV/sec, therefore a scan rate range of 60 mV/sec to 500 mV/sec is presented for further analysis. This behavior is

37 observed for all sets at all temperatures.

Figure 12: Plot of peak current density vs. square root of scan rate to evaluate diffusion controlled region.

Figures 13-15 plot the first set of cyclic voltammograms for the relevant scan rates at each temperature in the vicinity of the europium redox couple’s potential window. Each experiment is referenced to the potassium equilibrium potential found before and after that particular experiment. Three replicate tests at three different temperatures result in nine total sets of voltammograms.

Figure 13: Cyclic voltammetry at 650°C (1).

Figure 14: Cyclic voltammetry at 700°C (1).

Figure 15: Cyclic voltammetry at 750°C (1).

Each scan rate was repeated five times before increasing the rate to ensure the electrochemical reaction was in equilibrium and repeatable. These scans converged within the second or third repetition, however the fifth and final converged repetition is plotted for a given scan rate below. The complete set of voltammograms are shown in

Appendix A.

The cyclic voltammograms, Figures 13-15, show a general increase in peak current density with increasing scan rate and peak current potentials which are independent of scan rate. The independence of the difference between anodic and cathodic peak potentials with scan rate indicates reversibility. Peak potential difference is defined by the following relation:

푅푇 ∆퐸 = 2.218 (21) 푝 푛퐹 which is only a function of temperature and the number of electrons transferred. One final characteristic of a reversible reaction is equivalent peak cathodic and anodic current 40 magnitudes. Peak current magnitudes are defined in reference to their corresponding baselines which are linearly fitted to the section of curve which precedes the anodic or cathodic reaction [28-30]. This definition is displayed in Figure 16. When the currents are defined in this way, the ratio of peak oxidation and reduction currents are approximately unity for all tests at all temperatures.

Figure 16: Definition of cathodic and anodic peak current from corresponding linear

baselines.

It can therefore be concluded that the Eu3+/Eu2+ reaction is reversible for scan rates of 60-

500 mV/sec so the Nernst equation is valid for determination of the system’s redox potential.

Since this system is a soluble-soluble reaction, Equation 12, called the Randles-

Sevcik equation, can be used to calculate the concentration of both species. All terms in this expression are known except for the concentration and diffusion coefficients. In the

41 study by Huang [27], diffusion coefficients for europium(III) and europium(II) are reported in FLiNaK at 500-650°C. Diffusion coefficients for a given species are defined by Arrhenius’ law:

퐸 − 푎 퐷 = 퐷표푒 푅푇 (22) where Do is the pre-exponential factor and Ea is the activation energy. In order to extrapolate to the 650-750°C range, the diffusion coefficient data reported by Huang is plotted on a log scale in order to determine the pre-exponential factor and activation energy, Figure 17. The pre-exponential factor is calculated as 0.00557 cm2/s and 0.00257 cm2/s for Eu3+ and Eu2+ respectively from the y-intercept. The activation energy is calculated as 41.28 kJ/mol and 34.05 kJ/mol for Eu3+ and Eu2+ respectively.

Figure 17: Plot of diffusion coefficient results from Huang et al. fitted with trendlines.

Determination of the diffusion coefficients for both Eu3+ and Eu2+ at 650-750°C from Arrhenius’ law allows the Randles-Sevcik equation to be solved for species’

42 concentration. The working electrode immersed surface area is measured as 0.2592 cm2 after the conclusion of experimentation. A summary of all nine data sets, Appendix A, is compiled in Table 2 where the concentration listed for a given scan rate is the average of the three replicates performed at that given temperature. No concentration dependence on scan rate or temperature is observed. The ratio of Eu2+/Eu3+ is found as 0.91±0.03 and appears to increase slightly with increasing temperature. Using the concentration results the total europium weight percent can be calculated from the molar mass of europium and density of FLiNaK in order to verify the validity of using extrapolated diffusion coefficients. The density of FLiNaK is 2.056, 2.019, and 1.983 g/cm3 for 650, 700, and

750°C respectively [43]. It is found that the measured weight percent of total europium is

0.82% ± 0.01%, 0.71% ± 0.02%, 0.76% ± 0.01% at 650, 700, 750°C respectively.

Table 2: Summary of species concentration calculation from cyclic voltammetry measurements.

Scan rate Eu2+ concentration (10-5 mol/cm3) Eu3+ concentration (10-5 mol/cm3) (mV/sec) 650°C 700°C 750°C 650°C 700°C 750°C 60 5.32 4.55 4.86 6.12 5.33 5.35 80 5.33 4.56 4.58 6.05 5.26 5.32 100 5.15 4.42 5.01 5.88 5.23 5.31 200 5.24 4.29 4.81 5.68 5.03 5.02 300 5.20 4.25 4.71 5.79 4.90 4.96 400 5.28 4.37 4.84 5.82 4.85 4.89 500 5.29 4.55 4.88 5.72 4.82 4.86 Average concentration 5.26 4.43 4.81 5.87 5.06 5.10 Percent deviation 1.04% 2.45% 2.01% 2.18% 3.59% 3.81%

Accounting for the mass of fluoride added with the 1 wt% EuF3, 0.73 wt% total europium was targeted for addition which is within that calculated from measurement. This validates the extrapolated diffusion coefficients.

The acceptable agreement of total europium measured to that added indicates that the use of the platinum quasi-reference electrode is able to provide accurate measurements. The low percent deviations shown in Table 2 provide evidence that the quasi-reference is also capable of precise measurements. The results of this europium investigation display that the platinum quasi-reference possesses reversibility and potential stability over limited time scales. A platinum wire/plate is durable from a material and mechanical perspective, the impedance of the reference is negligible, and this reference has been shown compatible with making concentration measurements for redox control. The only desired characteristic of a reference which this electrode lacks is reproducibility of potential due to the non-existent thermodynamically defined reference potential.

Although the platinum quasi-reference electrode does not inherently possess a thermodynamically defined reference potential, the potassium equilibrium observed at the limiting potential for FLiNaK does possess this quality. Referencing the cyclic voltammetry measurements using a quasi-reference with respect to the observed potassium reduction potential, as is performed in the plotting of the cyclic voltammetry data, effectively fixes the potential on a thermodynamically defined potential. Using this method means that the measurements performed using a quasi-reference now possess all of the desired characteristics for a reference electrode. This reference choice looks very

44 promising for application to an industrial fluoride molten salt application, however partially relies on the assumption that the potassium reduction potential is equivalent to the K/K+ equilibrium potential. This assumption will be inherently tested by the dynamic reference experiments presented further. The quasi-reference’s performance must also be compared with the other candidate reference designs to identify a top performer.

Dynamic Reference

A dynamic reference electrode is an attractive idea for an industrial molten salt application because it does not rely on a complex material construction to establish a defined reference potential. Instead, it takes advantage of the molten salt itself to temporarily establish an inner type reference. Afonichkin and later Duran-Klie have studied the use of this reference type in the FLiBe and FLiNaK mediums respectively

[38, 39]. They both found that in each medium a species of the molten salt itself can be deposited onto the working electrode by applying a specified current. This deposition establishes the inner reference consisting of the species’ ionic and metallic forms.

Beryllium is observed to be the limiting species in FLiBe while potassium is for FLiNaK.

Both studies found that for a specified minimum current pulse, a stable potential is observed for some duration of time corresponding to the major metal redox reaction.

After some period of time following the pulse the major metal deposit will dissolve back into its more thermodynamically favorable ionic form. The idea behind the use of this type of reference is that periodically the dynamic reference is created in order to take simple potential measurements of the redox potential over time.

An experimental study mimicking Duran-Klie’s investigation is performed using a FLiNaK salt with the addition of 1 wt% EuF3. Five differing current magnitudes are applied for 5 seconds at the working electrode, -5, -50, -100, -200, -300 mA. This electrochemical technique is called chronopotentiometry. The potential is measured between the working electrode and the platinum reference during and after the current pulse. The same electrode setup is used which was utilized in the quasi-reference investigation. The immersed working electrode surface area is measured to be 0.2592 cm2. Figures 18-20 plot a data set at each temperature. Three replicate sets of data are collected and plotted in Appendix B for each temperature, 650, 700, and 750°C.

Figures 18-20 plot the potential evolution during and following a 5 second current pulse. The first observation to note is that for all data sets, there exists a sufficiently low current whereby a stable potential is not achieved. This is attributed to a working electrode surface area which is not fully covered by the metallic potassium. For the remaining four current pulses, a stable potential is established which possesses two distinct regions. Region I occurs during the 5 second current pulse while region II directly follows as annotated in the plots. Region I possesses a greater magnitude potential compared to region 2. This potential is constant over the 5 second pulse, however it increases with increasing current. The potential drops to a constant value potential following the pulse whose magnitude does not appear to be affected by the pulse’s magnitude. This potential drop is an ohmic drop corresponding to the molten salt’s electrolytic resistance. Comparing the potential difference between differing current pulse magnitudes allows for the determination of resistance from the ratio ΔV/ΔI.

Figure 18: Chronopotentiometry at 650°C (1).

Figure 19: Chronopotentiometry at 700°C (1).

Figure 20: Chronopotentiometry at 750°C (1).

The electrolyte resistance is found as 0.12 Ω∙cm2 at 650°C, 0.11 Ω∙cm2 at 700°C and 0.09

Ω∙cm2 at 750°C. These values agree well in magnitude and trend with the 0.13 Ω∙cm2 at

500°C reported in literature [39].

The stable potential of region II corresponds to the equilibrium potential of K/K+ versus the platinum quasi-reference. The length of time which stability is held increases with increasing current i.e. total charge transfer, however the potential itself is invariant of current magnitude. Table 3 summarizes the potassium equilibrium potential as found from chronopotentiometry where the reported values are an average of the three replicates performed at a given temperature. From these results, it is observed that the equilibrium potential can be measured precisely for all magnitudes of current above a certain threshold. There appears to be a very slight increase of equilibrium potential with increasing temperature. Table 3 also reports the reduction potential from full sweep voltammetry as reported in Table 1 as an average for the tested temperature. The

48 difference between the reduction potential and the equilibrium potential is calculated in the final column of Table 3. The small differences show that the reduction potential measured earlier from cyclic voltammetry is indeed equivalent to the equilibrium potential of potassium. This result validates reporting the cyclic voltammetric measurements with respect to the potassium reduction. Identifying the potassium equilibrium potential can be performed using either full sweep cyclic voltammetry or by chronopotentiometry depending on what the scientist desires for their needs.

The findings from these measurements indicate that the dynamic reference electrode can be a very powerful tool for application to industrial fluoride molten salt measurements, especially if paired with the measurement capabilities the quasi-reference electrode has displayed. It should be noted that physically, the dynamic reference electrochemical cell is identical to the quasi-reference therefore the creation of the dynamic reference periodically can be completed before or after cyclic voltammetry is performed with a single electrochemical cell.

Table 3: Summary of chronopotentiometry measurements with comparison to full sweep voltammetry.

K/K+ Reduction K/K+ Equilibrium Plateau Temperature Current Full Sweep Potential Plateau Difference (°C) (mA) (V vs. Pt) (V vs. Pt) (mV vs. Pt) 650 -5 -1.278 N/A N/A -50 -1.278 -1.270 8 -100 -1.278 -1.272 5 -200 -1.278 -1.275 3 -300 -1.278 -1.279 2 700 -5 -1.277 N/A N/A -50 -1.277 -1.268 9 -100 -1.277 -1.271 6 -200 -1.277 -1.276 1 -300 -1.277 -1.276 2 750 -5 -1.283 N/A N/A -50 -1.283 -1.268 15 -100 -1.283 -1.273 9 -200 -1.283 -1.282 5 -300 -1.283 -1.284 10

Ni/Ni2+ with Boron Nitride Compartment

The Ni/Ni2+ reference is tested by first constructing the electrode assembly shown in Figures 4-5 and described previously. A total of 30 grams of FLiNaK is prepared at its eutectic composition and 1 wt% of EuF3 is added into the nickel crucible just like the setup for the quasi-reference and dynamic reference. An additional FLiNaK mixture is prepared for the boron nitride compartment weighing 1.958 grams with an additional 1 wt% NiF2 added to establish the half-cell reference. 1/16’’ diameter and 5/32’’ diameter tungsten working and counter electrodes are employed to complete the electrochemical cell. The electrochemical cell is heated to 700°C and immersed in the

50 nickel crucible. The goal of this study is to establish the stable reference by allowing the boron nitride compartment to be permeated by the molten salt, thereby establishing electrolytic charge transfer while limiting diffusion of the solute species in either salt.

Completion of the boron nitride permeation will be observable when the open circuit potential becomes stable. Prior studies using a much thinner, 1mm thick, membrane report complete permeation within 24-48 hours [31, 33]. Once the reference is established, a series of cyclic voltammetry experiments will be performed which mimic those completed using the quasi-reference for comparison.

The open circuit potential between the tungsten working electrode and Ni/Ni2+ is monitored over a 24-hour period of time following immersion into the 700°C salt. It is observed that the potential wildly fluctuates from -0.6 V to 0 V. This indicates that permeation has not occurred, therefore the open circuit potential is monitored for an additional 4 days, Figure 21.

Figure 21: Open circuit potential using the Ni/Ni2+ reference over five days at 700°C.

This figure shows a decay in the potential fluctuations between the 35th-86th hours. It appears that permeation is occurring over this time interval, however the fluctuations begin to increase very slightly once again. The experiment was terminated following the fifth day of measurement due to limitations of the furnace’s relay, which may burn out with prolonged usage. Since the experiment was forced to be terminated before the reference could be established, cyclic voltammetry was not able to be performed for comparison with the quasi-reference.

Even though the goals of this experiment were not successful, the results still provide valuable information about the suitability of the Ni/Ni2+ reference with a boron nitride compartment to applications such as the FHR. It has been noted that prior studies of this reference type have utilized a boron nitride thickness of 1mm. This thickness is not appropriate for application to an industrial process due to its minimal mechanical durability. It may be an acceptable reference for the laboratory but will likely break under real world usages. A judgement was made to test the feasibility of the larger 2.54 mm thickness compartment because this increased thickness would likely provide sufficient mechanical durability. The chemical degradation of the boron nitride observed in the literature makes the case for a thicker membrane even stronger. The results show that for a sufficiently thick membrane to ensure durability, permeation is not able to be accomplished within a reasonable period of time, if ever. The amount of time it would take for permeation to occur cannot be determined from this study, however it is plain to see it will be unacceptable for operators of a nuclear plant to wait greater than 5 days to collect data from such a sensor. In summary, there is a direct tradeoff between durability

52 of this reference to the time elapsed before measurements can be recorded.

Gold Alloy Reference

Goto has proposed a gold alloy reference of the inner reference type which seems promising due to the mechanical durability and simplicity it possesses [40]. Cyclic voltammetry was performed on the FLiNaK system at 500°C using a gold working electrode. Goto observed a set of redox peaks which were found to be the formation and dissolution of the Au2Na gold alloy, where the sodium ions are supplied by the molten salt. The reference redox potential is claimed fixed by:

+ − 2퐴푢 + 푁푎 + 푒 ↔ 퐴푢2푁푎 (23)

The potential of this reaction was determined to be 0.535 V versus the K/K+ reduction.

This alloy was generated by applying a potential at the gold electrode which was more negative than the formation peak observed using cyclic voltmametry. Two electrodes were generated by this method and the potential between them was measured to vary within 1mV over 25 hours. This indicates that this reference possesses high potential stability. Applications such as the FHR employ much higher temperatures than previously tested, therefore this reference will be investigated for higher temperatures.

A 0.5mm diameter gold wire of greater than 99.99% purity is used as the working electrode while a 1/16’’ tungsten reference and 5/32’’ counter are assembled. A FLiNaK salt with 1 wt% EuF3 is prepared identically to the prior studies. It is determined that the immersed surface area of the gold electrode caused the potentiostat to overload due to the high current density produced from using a much thinner working electrode than previous

53 studies. This is remedied by looping the wire once in a U-shape in order to double the surface area. Cyclic voltammetry is performed for the system as shown in Figure 22.

Figure 22: Cyclic voltammetry of the FLiNaK plus 1wt% EuF3 salt using a gold working electrode.

This signal shows three sets of redox peaks corresponding to the europium couple, the gold alloy couple, and the potassium couple. Qualitatively, this is the type of signal that is expected. The gold alloy formation appears to occur at about -0.8V versus the tungsten reference. Applying a potential more negative than this redox peak will deposit this gold alloy at the working electrode. Figure 23 plots the current response when a voltage of

-1.0V is applied to the working electrode for 2 hours. This plot shows a constant current of about 2.5 mA is passed for the entire duration of the experiment. It is expected that if the gold alloy formation was completed on the entire electrode surface area, the current would decrease to zero at some point indicating no more charge is transferred. Since

Goto’s investigation was in pure FLiNaK it is thought that the europium may be acting as 54 a buffer to the gold alloy reaction because all of the Eu3+ would need to reduce to Eu2+ before the Au2Na can be formed.

Figure 23: Plot of current response to an applied potential of -1V for 2 hours.

This hypothesis is tested by integrating the current response to calculate the total charge transferred during the experiment. The total charge transferred is calculated to be 14.4 coulombs while 190.7 coulombs is required to reduce all of the europium in the salt. For a current of 2.5 mA, it will take 26.5 hours to accomplish full europium reduction, thereby beginning the gold alloy formation. The open circuit potential is measured following the applied potential, shown in Figure 24.

Figure 24: Open circuit potential following a 2-hour application of -1V.

This figure shows the open circuit potential approaching nearly zero following the conclusion of applied potential, which is where the system was at pre-applied potential.

This shows that no lasting effect on the system and/or electrode was achieved.

Any impurity in the FHR coolant will cause the same buffering behavior exhibited by europium in this study. In fact, multiple impurities and higher concentrations of impurities will further compound the buffering effect because more charge must be transferred before the gold alloy formation can occur. Since the europium in the system is acting as a buffer to the gold alloy formation, a pure FLiNaK system is investigated with the intent that the reference electrode be generated in a laboratory setting and then inserted into the industrial system. Figure 25 plots the cyclic voltammetry signal obtained for this pure FLiNaK system. This system exhibits the same behavior as that of Figure 22 minus the europium couple. The gold alloy formation is again occurring at about -0.8V versus the tungsten reference, so a potential of -1.0V is applied for 2.5 hours.

Figure 25: Cyclic voltammetry of pure FLiNaK system with a gold working electrode.

The current response, Figure 26 shows a region of charge transfer until about 5 minutes when the current drops to zero for the remainder of the experiment. This behavior shows that a reaction occurred and reached completion within 5 minutes. This is the behavior that is expected for the formation of Au2Na on the entire gold electrode surface area.

Figure 26: Current response to an applied potential of -1.0V for 2.5 hours.

The open circuit potential is measured, Figure 27, following termination of the applied potential. Although the current response measurements are expected, the open circuit potential exhibits extreme potential variations which is not expected based on the stability claims made by Goto’s research. It appears that the potential is attempting to stabilize at about -0.22V, however some chemical reaction is greatly disturbing the potential and preventing an equilibrium from being reached. Figure 26 shows that the formation of Au2Na on the gold electrode was completed, therefore the potential disturbances may be the result of sodium dissolution back into its ionic form. This reference was shown to be extremely stable at 500°C, however the results of this study display that for the elevated temperatures of the FHR, this reference does not possess adequate stability.

Figure 27: Open circuit potential following Au2Na formation at gold working electrode.

Thermodynamic Study

The results of the quasi-reference electrode study have shown that concentration measurements can be precisely measured while the results of the dynamic reference electrode study have provided the ability to present the cyclic voltammetry results in reference to a thermodynamically defined potential. This is a powerful combination because now a thermodynamic study can be performed which characterizes the apparent standard potential over the studied range of temperatures. Characterizing the apparent standard potential and possessing the ability to precisely measure a redox species’ concentration allows for the complete use of the Nernst equation, equation 17. Europium has been previously described as a candidate for dissolved-salt redox control so the results of this report can be utilized to determine the Eu3+/Eu2+ concentration ratio which must be maintained to perform redox control in a FHR.

For reversible systems utilizing linear sweep voltammetry, the apparent standard potential is related to the peak potentials, standard potential, diffusion coefficients, and coefficients of activity by the following equation set:

1 c a (퐸p + 퐸p) 푅푇 퐷 2 푅푇 훾 = 퐸° + ln ( Red) + ln ( Ox ) (24) 2 푛퐹 퐷Ox 푛퐹 훾Red

푅푇 훾 퐸∗ = 퐸° + ln ( Ox ) (25) 푛퐹 훾Red

Rearrangement of these two equations produces Equation 13 [21]. The results of the cyclic voltammetry measurements are used to characterize the apparent standard potential referenced to the potassium equilibrium potential found from chronopotentiometry, summarized in Table 4, and are also converted to the fluorine potential. 59

Table 4: Summary of potassium equilibrium potential as found from chronopotentiometry measurements.

퐫퐞퐝퐨퐱 Temperature (°C) 푬퐊+⁄퐊 (V vs. Pt) Standard Deviation (mV) 650 -1.274 4.5 700 -1.273 5.4 750 -1.277 8.3

The apparent potential results are displayed in Table 5. Calculation of the apparent standard potential using the three replicates at each temperature produce precise results, evidenced by the low standard deviations. These results allow for complete understanding of the europium redox potential by the Nernst equation.

Table 5: Summary of apparent standard potential as found from cyclic voltammetry data.

Temperature Average 푬∗ Average 푬∗ Standard Deviation + † − (°C) (V vs. 퐊 ⁄퐊 in FLiNaK ) (V vs. standard 퐅ퟐ⁄퐅 ) (mV) 650 1.168 -3.809 2.6 700 1.166 -3.764 5.5 750 1.172 -3.700 4.3 †The potential of Pt quasi-reference was calibrated using PDRE method for each replicate test.

Metallic corrosion is described by the general electrochemical half reactions of

Equations 1 and 2. This corrosion reaction will occur spontaneously if the Gibbs free energy of the reaction is negative, where the Gibbs free energy is expressed as:

∆퐺 = −푛퐹∆퐸 (26)

푟푒푑표푥 푟푒푑표푥 푟푒푑표푥 푟푒푑표푥 The quantity ∆퐸 = 퐸푐 − 퐸푎 where 퐸푐 /퐸푎 are the cathodic and anodic reaction redox potentials respectively. A spontaneous reaction will occur for cathodic redox potentials more positive than the anodic redox potential. The redox potentials are

60 defined by an alternate definition of the Nernst equation which relates the redox potential to the activities of the oxidized and reduced species, expressed by the following equation set:

n+ redox ° 푅푇 푎M 퐸a = 퐸a + ln (27) 푛퐹 푎M

redox ° 푅푇 푎Ox 퐸c = 퐸c + ln (28) 푛퐹 푎Red

- The standard electrode potential in reference to the standard F2/F reference can be calculated by:

° − ° 퐸M⁄Mn+(vs. F2⁄F ) = −∆퐺f (MXn)⁄푛퐹 (29) for any given species MFx by finding the standard Gibbs free energy of formation from thermochemical property tables [44].

From Equation 29, the redox potential of a variety of redox couples as a function of temperature are plotted in Figure 28. Solid lines plot the redox potential where a metal

M dissolves into the ion Mx+ with an activity of 10-6, chosen as a condition for the onset of corrosion. For potentials lower than this redox potential, the metal M is expected to be stable assuming the activity coefficient of the metallic ion is 1. Dotted lines represent the redox potential of oxidants within the salt such as impurities or major metals of the molten salt itself. It has been observed by material investigations that chromium is the most limiting element in candidate structural materials [13, 14]. Figure 28 also finds that chromium has the most negative equilibrium potential out of elements such as nickel and iron and therefore is most easily corrodible. The double lines shown in Figure 28 represent the equilibrium potential as found using the apparent standard potentials for

61 europium summarized in Table 5 and using chromium as the limiting baseline for corrosion prevention. It is determined that for a Eu3+/Eu2+ concentration ratio lower than

0.05, the redox potential can be fixed below that for which chromium will corrode. The

MSRE maintained a U3+/U4+ ratio of 0.007-0.02 to perform redox control, therefore a europium ratio of 0.05 is a feasible ratio to maintain.

Figure 28: Redox potential for various redox couples in FLiNaK and FLiBe. H2/HF (100:1): a mole ratio of H2/HF=100 at 1 atm total pressure.

Sensor Design

One of the goals of this study is to implement the results of the reference electrode investigation to a full electrochemical sensor design which could be employed in an industrial molten salt application such as the FHR. This proposed sensor must be capable of making accurate and precise measurements while maintaining sufficient durability and service life. Design of the sensor cell is performed under the assumption that only a small rod/wire will be used as the reference electrode based on the results discussed prior. An important quality which is considered as the starting point of design is the determination of the electrode’s surface area. In the studies described above, the immersion depth of the electrode is measured using a ruler as indicated by the solidified salt. Inaccurate quantification of this immersion depth directly relates to less accurate calculations. This method is also problematic because it prevents real time calculations from being made, negating the purpose of the sensor in the first place. Fixing the working electrode’s surface area prior to the sensor insertion is critical to ensuring the desired function and performance. Figure 29 displays a designed component which can achieve this by constraining the exposed electrode depth. The working, counter, and reference electrodes can be inserted into the through holes which correspond to their particular diameters. This component will need to electrically insulate each electrode from each other. This requirement eliminates the use of metallic materials which can withstand fluoride molten salt environment such as a stainless steel or nickel based alloy. Boron nitride is a candidate for this purpose since this ceramic has low electrical conductivity and high durability in low impurity fluoride molten salts.

Figure 29: Electrochemical sensor housing dimensioned drawing.

A high purity boron nitride such as Saint-Gobain’s AX05, which does not contain any oxide binders, is selected here having a thermal expansion coefficient of 3.0E-6 m/m-K.

Metals such as tungsten and platinum have thermal expansion coefficients of 4.5E-6 and

9E-6 m/m-K respectively [45, 46]. At high temperatures, the thermal expansion coefficients difference will limit any existing gap between the electrode and the boron nitride hole, serving to protect the known immersion depth of the electrodes. The electrodes are prevented from slipping through the housing component by brazing of the electrode with a nickel or stainless steel bushing. This bushing fixes the electrode’s vertical position and allows for desired electrical connections to be made to each electrode either by another brazing process or a simple clip. The brazing material must be compatible with the high operating temperatures, possibly nickel, platinum, or gold. 64

The sensor assembly, Figure 30, will be inserted into the molten salt to a level which completely immerses the exposed electrode length. In order to physically accomplish this, the sensor may need to be lowered some distance from an access port, likely flanged.

Figure 31 depicts its interface with a 1.25-inch Schedule 40 pipe with a Class 300 flanged connection. This flanged pipe is to be constructed out of the same structural materials which will be utilized for the FHR, likely Hastalloy-N. Three metallic dowels which possess a nominal fit with holes in the pipe and sensor housing maintain the sensor’s position inside the end of the pipe. These dowels can also be welded to the pipe for robustness. At the high operating temperature, a nominal fit at room temperature will create a slight interference fit because of the difference in the metal and ceramic thermal expansion coefficients. The pipe length can be varied depending on the specific molten salt surface level of the tank or coolant loop for which measurements are desired. The specific dimensions noted in these figures were specified to interface with a 2-inch diameter flanged access port for the Liquid Salt Testing Loop’s sump tank at ORNL, however the dimensions can be altered as needed to fit any given access port or insertion point. This design can easily be utilized in either stagnant or flowing conditions.

Figure 30: Drawing of the complete sensor assembly.

Figure 31: Exploded view of the sensor assembly and flanged connection.

Chapter 5: Conclusions

Reference Electrode Selection

Fluoride molten salts provide a unique challenge to industrial nuclear applications such as the FHR. Based on their chemical composition, fluoride salts are susceptible to tritium generation by neutronic flux as well as chemical reaction with water and oxygen impurities. Redox control of the molten salt will be performed continuously in order to maintain the redox potential below that which corrosion of structural materials occurs.

Electrochemical techniques such as cyclic voltammetry are particularly well equipped to monitor this redox potential by measurement of the redox potential directly or quantification of a redox couple’s concentration ratio. Currently, limitations exist in the awareness of candidate reference electrodes’ performance for industrial fluoride molten salt application which hold back deployment of these advanced technologies.

A literature review of candidate reference electrodes was performed which identified the Ni/Ni2+ reference with boron nitride insulator, platinum quasi-reference, dynamic reference, and a gold alloy reference as credible candidates. A series of electrochemical studies have been performed for the first time which evaluated the performances of each in the context of redox control for an industrial fluoride molten salt application such as the FHR. Boron nitride was observed to be an unrealistic containing material for a half-cell type reference due to the inability of this study to create this 68 reference with a design which would survive an industrial application. The platinum quasi-reference electrode on the other hand displayed excellent performance in the measurement of a redox couple’s concentration by use of cyclic voltammetry. These measurements were shown to be accurate, precise, stable, and reproducible. The performance of the quasi-reference electrode is further bolstered by the dynamic reference’s results, which show that reference to a thermodynamically defined potential can be accomplished using the same electrochemical cell. Reference to the molten salt major metal reduction potential can be performed either by full sweep voltammetry or chronopotentiometry. The results of this study show that the quasi-reference electrode possesses superior capabilities compared to other references on the basis of its ease of use, precision, and versatility.

Sensor Design

Identification of the quasi-reference electrode as the highest performing reference choice allows for its integration to be designed into a holistic sensor design which can be directly inserted into an industrial fluoride molten salt system. A simple yet effective design has been presented in which an insulating ceramic is used to house a three- electrode electrochemical cell consisting of an inert metal working and counter electrodes such as tungsten or molybdenum in conjunction with a quasi-reference such as platinum.

An elegant sensor assembly is presented in which the electrodes are assembled into a complete cell by means of brazing metallic bushings to the electrodes, providing both physical constraints and access to electrical contact with leads for a data acquisition

69 system, Figures 29-30. An example of this sensor’s implementation is also presented by way of a flanged access port depicted in Figure 31. The presented holistic sensor design is capable of simple dimensional modifications to adapt to any available access point or desired measurement location.

Sensor Applications

Use of the platinum quasi-reference electrode for cyclic voltammetry measurements has allowed for the characterization of the europium(III)/europium(II) standard apparent potential in FLiNaK at 650-750°C. This newly identified information is consequential because it has allowed for a thermodynamic study whose results clearly characterize the equilibrium potential of a variety of redox couples which may be present in a fluoride molten salt. Knowledge of the europium couple’s apparent standard potential is able to fully describe its redox potential as a function of temperature and concentration. Assembling this information provides the determination that a Eu3+/Eu2+ concentration ratio of lower than 0.05 is sufficient to inhibit corrosion of even a material’s most easily oxidized metal, chromium. A concentration ratio of 0.05 is in alignment with the U3+/U4+ ratio which the MSRE demonstrated for redox control proving the feasibility of using the Eu3+/Eu2+ couple for redox control. With the application of the full sensor design presented, additional redox couples and molten salt media can be investigated with the methods utilized in this study.

The sensor design presented in this study has been investigated and designed with the intent of application for a FHR system in support of redox control, however could be

70 applied to numerous other applications as is or with only slight modifications. All fluoride molten salts are potential applications including FLiNaK and FLiBe. The primary and intermediate loops of the FHR are clear examples of environments in which this sensor can be utilized, however this sensor could find valuable usage in future fusion systems as well as other MSRs. Additional materials consideration can be performed to even make the sensor useful to chloride molten salts which are used in pyroprocessing of used nuclear fuel. The application of the results of this study mark a significant advancement to the deployment of the Fluoride salt-cooled High-temperature Reactor and will assuredly be applied to further electrochemical investigations and industrial molten salt applications.

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Appendix A: Cyclic Voltammetry Data

Figure 32: Cyclic voltammetry at 650°C (1).

Figure 33: Cyclic voltammetry at 650°C (2).

Figure 34: Cyclic voltammetry at 650°C (3).

Figure 35: Cyclic voltammetry at 700°C (1).

Figure 36: Cyclic voltammetry at 700°C (2).

Figure 37: Cyclic voltammetry at 700°C (3).

Figure 38: Cyclic voltammetry at 750°C (1).

Figure 39: Cyclic voltammetry at 750°C (2).

Figure 40: Cyclic voltammetry at 750°C (3).

Appendix B: Chronopotentiometry Data

Figure 41: Chronopotentiometry at 650°C (1).

Figure 42: Chronopotentiometry at 650°C (2).

Figure 43: Chronopotentiometry at 650°C (3).

Figure 44: Chronopotentiometry at 700°C (1).

Figure 45: Chronopotentiometry at 700°C (2).

Figure 46: Chronopotentiometry at 700°C (3).

Figure 47: Chronopotentiometry at 750°C (1).

Figure 48: Chronopotentiometry at 750°C (2).

Figure 49: Chronopotentiometry at 750°C (3).