Raluca Scarlat

Despite the wide application of molten fluoride salts, such as 2LiF-BeF₂ (FLiBe), as heat transfer fluids, fuel solvents, and fusion blanket materials in advanced nuclear power concepts, the mechanisms of ion transport, speciation, and reaction kinetics in these systems have not been comprehensively characterized. Though electrochemical methods are powerful tools for probing these properties of molten fluorides, current studies are limited by the lack of a reliable thermodynamic reference electrode (TRE) compatible with the fluoride environment. The reference electrode designs used in fluorides can be classified into three main categories: quasi-reference electrodes (qREs), dynamic reference electrodes (DREs), and thermodynamic reference electrodes. qREs and DREs are not well suited to in-line measurements of molten fluorides since the reference redox couple of the noble qRE material cannot be assumed to be constant and the time scale of the DRE’s relaxation of the transient redox system is too short.

Therefore, explorations of the FLiBe system have pursued the development of a TRE which can hold a thermodynamically stable reference potential for an extended time. Current designs using the H₂/HF or Ni/Ni²⁺ couples suffer from either difficult handling of gas electrodes at high temperatures [1] or long equilibriation times. Since pure NiF₂ does not melt at reactor operating temperatures (~500-1000°C), the NiF₂ used must dissolve into a reservoir of supporting FLiBe, a process which takes over 30 hours. In this study, the performance of a reference electrode using the Ag/Ag⁺ redox couple is assessed for long-term electrochemical stability. The Ag/AgF system was chosen to address the limitations of the Ni/Ni²⁺ TRE, as the low melting point of AgF (T_melt = 435°C) allows for the direct use of the pure salt in the electrode, eliminating the time-consuming dissolution step required for NiF₂.

The Ag/AgF TRE was constructed by melting AgF salt into a boron-nitride crucible fitted with a LaF₃ fluoride membrane and inserting a silver wire into the salt reservoir. The stability and reproducibility of the electrode potential were evaluated through long-term open circuit potential measurements against a platinum qRE in a FLiBe melt at 600°C. The reversibility of the Ag/Ag⁺ reaction was assessed using a linear polarization study. Finally, the location of the redox potential was found by comparing the open circuit potential of the Ag/AgF TRE against a beryllium rod electrode. The development of a reliable and easy-to-operate thermodynamic reference electrode will facilitate future studies on the thermochemical properties of molten FLiBe while laying the foundation for advancing in-line monitoring and redox control in nuclear technologies.

[1] Jenkins, Howard W., "Electrochemical Measurements in Molten Fluorides. " PhD diss., University of Tennessee, 1969. https://trace.tennessee.edu/utk_graddiss/3072

The presence and speciation of oxides are relevant to various engineering applications of molten fluoride salts. Here, our studies are directed toward two primary applications: 1) as coolants, fuel salts, or breeding blankets in advanced fission and fusion reactors, and 2) as the reaction medium for the novel direct synthesis of Li-ion battery cathodes from Li-ores.

In an advanced nuclear reactor, oxide might be introduced to the molten fluoride coolant/blanket via ingress of oxygen or moisture from air. Oxygen and moisture act as oxidants, reacting with the metal fluoride to produce metal oxides or hydroxides, along with corrosive HF. By these reactions, the presence of oxide is an indicator of leaks as well as of the progression of corrosion. Quantification of oxides by square wave voltammetry (SWV) has been proposed as a technique for electrochemical oxide sensors [1]. The intensity of the peak current for the oxidation of oxide to oxygen gas is proportional to the quantity of oxide present, so comparison to a calibration curve can indicate how much oxide is present in the melt. True engineering systems, however, would likely contain multiple solutes including corrosion products, fission products, and activation products. Multi-component chemistries are further complicated by differences in solvent-solute interactions; different fluoride solvents have varying degrees of polymerization, so they incorporate solutes into their structure differently. Practically, this means that oxides may exist in different complexes depending on the solvent and other solutes, therefore oxide quantification via SWV is complicated by solvent and solute chemistry.

The synthesis of Li-ion battery cathodes via reaction in molten fluoride salts is currently being studied within a DOE Office of Science BES project as a lower-carbon emissions alternative to traditional cathode manufacturing. This concept involves the dissolution of spodumene (LiAlSi₂O₆) in molten fluorides and the electrochemical deposition of disordered transition metal (M) oxyfluoride anion rocksalt (Li_1+xTM_1-xO_2-zF_z) for use as a battery cathode. The anion disorder in the rocksalt is relevant due to its relation to cathode performance. Energy storage capacity from the transition metal redox states increases as F is substituted for O. Cathode synthesis, therefore, requires tuning of the chemical potentials and transport properties in melts containing fluoride and oxide anions. In this context, a deeper understanding of the structure and speciation of oxide in molten fluorides is critical to formulating the reaction pathway and required composition and configuration of the rocksalt product.

Given these applications, we investigate the speciation of oxide and its existence within oxo-fluoro complexes in molten fluoride salts using electrochemistry. We present results of electroanalytical studies of oxides in molten 2LiF-BeF₂ and 46.5LiF-11.5NaF-42KF, using cyclic voltammetry and SWV. We make connections between solute speciation and solvent structure, allowing for mutually enhanced understanding of solubility and fluoroacidity. Broadly, electroanalytical studies provide a deeper understanding of oxide’s speciation in molten fluoride systems which, in turn, informs electrochemical sensor development and electrochemical synthesis processes. By investigating the fundamental science of oxide and oxide complexes, we hope to advance technologies which enable the transition to clean energy.

[1] L. Massot, L. Cassayre, P. Chamelot, and P. Taxil, “On the use of electrochemical techniques to monitor free oxide content in molten fluoride media,” J. Electroanal. Chem., vol. 606, no. 1, pp. 17–23, Aug. 2007, doi: 10.1016/j.jelechem.2007.04.005.

Fluoride salt-cooled High-temperature Reactors (FHRs) are a subtype of molten salt reactors (MSRs) in which a molten salt fills the role of a low pressure coolant to coated particle fuel. Another use for molten salt is in fusion reactors as a fusion blanket/coolant. The use of a molten salt as the coolant or blanket offers several safety advantages when compared to the current water-based designs due to the low vapor pressure of molten salts and their boiling point being far above the maximum coolant temperature. It also allows to integrate passive safety systems and to use a high-temperature power cycle. Careful control over the chemistry of the molten salt coolant must be kept in FHRs to reduce the corrosion of structural materials. The primary coolant candidate for FHRs is FLiBe (66.7%LiF-33.3%BeF₂), which is not corrosive in its pure form, but impurities in the salt (commonly oxygen or water) and tritium fluoride generated from neutron irradiation of the salt can create an oxidizing environment.¹ This in turn leads to structural corrosion, which degrades the system and and further modifies the properties of the salt. Because of this, redox control of the coolant salt is a key issue in FHRs.² One of the methods for redox control is to introduce excess Be as a redox control agent to the salt, which reacts with tritium fluoride and decreases the redox potential. However, it has been found that when Be metal is contacted with FLiBe containing HF, it does not simply react with the HF, but is also dissolved in the salt. Another method is to add LiH to the FLiBe mixture as a hydrogen donor, which will induce the formation of dissolved metallic Be. This dissolution is not well understood, but could prove to be an elegant way for introducing a reducing agent to the system as the salt could be left in contact with Be metal in one part of the reactor to saturate the salt everywhere or be periodically saturated using LiH. The solubility of Be plays an important role here as it sets the limits for this saturation. Phases with visually different colours at different contents of Be in FLiBe have been reported in the literature, but no comprehensive analysis has been conducted to understand the solubility of metallic Be in FLiBe.³

In this study, we present data on these different phases of dissolved Be in FLiBe using LiH additions to understand the solubility of metallic Be. The crystal structure of different phases will be characterized using powder X-ray diffraction, the elemental composition via inductively coupled plasma mass spectrometry and melting points and latent heats via differential scanning calorimetry. Cyclic voltammetry will be used to determine activity coefficient of BeF₂ in with different amounts of Be dissolved in it and electrical conductivities of different phases of the melt will be measured via electrochemical impedance spectroscopy. Additionally, neutron diffraction PDF data of Li₂BeF₄ with and without 1 mol% of metallic Be dissolved inside will be shown. A better understanding of the characteristics and speciation of dissolved Be in FLiBe will allow for closer redox control of the coolant melt and a longer lifetime for structural elements in FHR coolant loops.

References

(1) Vergari, L.; Scarlat, R. O.; Hayes, R. D.; Fratoni, M. The Corrosion Effects of Neutron Activation of 2LiF-BeF2 (FLiBe). Nuclear Materials and Energy 2023, 34, 101289. https://doi.org/10.1016/j.nme.2022.101289.

(2) Zhang, J.; Forsberg, C. W.; Simpson, M. F.; Guo, S.; Lam, S. T.; Scarlat, R. O.; Carotti, F.; Chan, K. J.; Singh, P. M.; Doniger, W.; Sridharan, K.; Keiser, J. R. Redox Potential Control in Molten Salt Systems for Corrosion Mitigation. Corrosion Science 2018, 144, 44–53. https://doi.org/10.1016/j.corsci.2018.08.035.

(3) Hara, M.; Hatano, Y.; Simpson, M. F.; Smolik, G. R.; Sharp, J. P.; Oya, Y.; Okuno, K.; Nishikawa, M.; Terai, T.; Tanaka, S.; Anderl, R. A.; Petti, D. A.; Sze, D.-K. Interactions between Molten Flibe and Metallic Be. Fusion Engineering and Design 2006, 81 (1), 561–566. https://doi.org/10.1016/j.fusengdes.2005.06.372.

Molten fluoride salts have been studied for decades due to their applications in the production of aluminum as well as in the operation of advanced nuclear reactors. Advanced fission and fusion nuclear reactor designs may include molten fluoride salts as coolants, fuel carriers (fission), or fuel breeding blankets (fusion). Relevant to their nuclear applications are the thermophysical and thermochemical properties of the salt. These include viscosity and thermal conductivity, as well as density, vapor pressure, and heat capacity. Underlying these properties are the interactions of the ions constituting the liquid, characterized by their coordination number, correlation times, and the possible formation of polymeric networks in the melt. The composition and structure of a melt, that is, what ions are present and how they interact, dictates the macroscopic properties relevant to reactor operation. Fluoroacidity, or the activity of dissociated fluoride anions, is one metric that may be used to describe the relationship between salt chemistry and structure. Overall, understanding the relationship between molten fluoride salt chemistry and structure enables deeper understanding and, thus, prediction of property changes that may take place throughout a reactor’s lifetime.

In this poster, we present cyclic and square wave voltammetry studies of chromium speciation and diffusivity in various molten fluoride salts, connecting results with current understanding of the structure of the melts. Chromium is a thermodynamically favored corrosion product in molten salt systems, and in this study, it serves as a probe ion in different fluoride melts. Chromium trifluoride is added to various fluoride solvents and the diffusion coefficients of its various oxidation states are calculated and interpreted in light of salt structure. Experimental apparatus design and know-how for high-temperature molten salt electrochemistry are also highlighted.

Detailed knowledge of the mass transport behavior of hydrogen isotopes in molten 2LiF-BeF₂ salt (FLiBe) is essential to the design of new nuclear fusion technologies that make use of this molten salt to breed tritium to fuel the fusion reaction. However, typical studies of hydrogen transport using macroelectrodes are limited to solely measuring the permeability of an analyte—a property defined as the product of concentration squared and diffusivity. This challenge necessitates the development of microelectrodes capable of dynamically measuring diffusion coefficients when concentrations are variable. Unlike macroelectrodes, microelectrodes can measure diffusivity and concentration independently due to a steady-state current proportional to the product of diffusivity and concentration captured in addition to the Cottrellian current response.

This steady-state current is established due to a continuously expanding diffusion layer from the electrode that cannot expand indefinitely within finite experimental setups. Therefore, COMSOL Multiphysics simulations mirroring experimental conditions were performed to validate the analytical mass transport model based on the solution of Fick’s Second Law in radial coordinates. These simulations employed a 2D axisymmetric model to examine the conditions under which the infinite bulk solution approximation is valid, confirming that our electrode design would yield true steady-state current responses indicative of radial diffusion.

Platinum microelectrodes developed in this study served a dual purpose: they not only addressed previous limitations in high-temperature molten salt experimentation but also enabled dynamic measurements of both the diffusion coefficient and concentration of hydrogen in molten FLiBe introduced by lithium hydride. Square wave voltammetry and chronoamperometry were performed on a three-electrode cell using a platinum working microelectrode, a tungsten counter electrode, and a Ni/Ni²⁺ thermodynamic reference electrode. The successful measurement of hydrogen diffusivity in these experiments not only offers essential insights into molten FLiBe behavior but also sets the stage for future investigations into the effects of redox conditions, impurities, and equilibration time on tritium within nuclear fusion reactors.

The behavior of hydrogen isotopes in molten 2LiF-BeF₂ (FLiBe) is of interest to both advanced fission and fusion reactor designs. Tritium (hydrogen-3) is both a fission product and neutron activation product of lithium-6 fluoride in fission reactors which use FLiBe as a heat transfer fluid. In this case, tritium is an undesirable side product. In fusion reactors, FLiBe may be used as a blanket around the plasma which, upon neutron irradiation, generates tritium by the same neutron–Li-6 reaction. Here, tritium is a necessary and desirable product; the tritium is used to continuously fuel the deuterium-tritium fusion reaction. A thorough understanding of tritium’s behavior and residence time is relevant to managing tritium inventories in fission and fusion systems, which is essential for both the safety cases and functional operation of these reactors. Using hydrogen as a non-radioactive surrogate for tritium, we study the speciation and diffusivity of hydrogen in molten FLiBe at various temperatures relevant to reactor operating conditions.

Here we present results of electrochemical investigations of hydrogen in FLiBe using cyclic and square wave voltammetry. We introduce hydrogen to FLiBe through 1) LiH additions and as 2) H₂ gas, electrochemically desorbed from graphite. The expected solubility of H₂ in FLiBe at 1 atm partial pressure is as low as 1 appm. Previous studies saw a higher apparent solubility of H₂ at atmospheric pressure (up to 8000 appm). The possible presence of the postulated quasi-stable intermediate BeH₂ is explored to explain the high apparent solubility of H₂. The two methods of introducing H (as LiH reacting with BeF₂ to form BeH₂, which decomposes to H₂, and the surface reduction of H₂ on graphite) are compared to investigate the different species in which hydrogen may exist in molten FLiBe. Additionally, we attempt to quantify the diffusivity of these hydrogen species in FLiBe at 500ºC and 800ºC and compare findings with predicted diffusivities from computational models of Be and H additions to FLiBe. Overall, these studies seek to elucidate the speciation of hydrogen in FLiBe to aid in the management and operation of fission and fusion systems.

Controlling tritium is a major issue in developing nuclear reactors which employ molten salts; to aid this, we must understand tritium retention in these reactors. An important factor that must be considered for tritium retention is the effect the molten salt environment has on the interactions between tritium and the graphite moderator. Traditionally, Thermal Desorption Spectroscopy (TDS) is used to study the adsorption of hydrogen and its isotopes onto graphite; however, this is done in a vacuum at a constant heating rate to remove desorbed hydrogen where the gas is then measured for hydrogen gas concentration vs. temperature. To test in a molten salt environment, a method is needed to create a concentration gradient to drive diffusion of hydrogen to the surface of the graphite since a vacuum cannot be applied. This can be done electrochemically using a graphite working electrode submerged in salt by applying a constant oxidizing potential. Desorbed hydrogen gas is oxidized by this potential on the surface of the graphite, creating a concentration gradient which continuously drives diffusion as the graphite and salt is heated. This technique, known as Electrochemical Thermal Desorption Spectroscopy (ETDS) can be used to study tritium interactions with graphite in molten salt using hydrogen as a surrogate.

Previous attempts of ETDS used chronoamperometry with a constant heating rate to determine current peaks and relate them to hydrogen desorption peaks from various traps in graphite identified in TDS literature. The largest problem facing this previous work was the contribution to measured current from sources other than hydrogen oxidation at the surface. Work has been done to use a “blank” sample of graphite to run heated chronoamperometry and act as a baseline to subtract from the “charged” sample data to find current contributions only from hydrogen oxidation. Additionally, characterization of the salt is being done to determine a theoretical background current based on the kinetics of possible reactions and the effects of overpotential, concentration and high temperature.

Current challenges facing ETDS development are being studied. In this work, we examine the contributions of background reactions to measured current, as well as current contributions due to the oxidation of hydrogen within the electrode rather than at the surface. The answers to these questions will increase signal to noise ratios of ETDS, allowing the technique to extract valuable kinetic parameters of hydrogen interactions and quantify hydrogen in graphite in a molten salt environment.

Corrosion and mitigation of environmental degradation of structural materials in high-temperature Gen IV molten salt reactors, are critical issues. However, any corrosion studies have historically been forensic post-test examinations and materials development has occurred mainly by trial and error until recently. Meanwhile, a fundamental understanding can only be achieved by recognizing how the corrosion system of model materials in molten fluoride salts evolves as a function of time. A number of in-situ spectroscopy techniques to identify corrosion product compositions in molten salts have been suggested in recent years. Raman, high-temperature UV-Vis absorption spectroscopy, and X-ray absorption spectroscopy are a few of them [1,2]. These techniques' major limitations include the fact that they are mainly qualitative, and complex hardware requiring a beam focusing inside the glove box is needed. At the same time, electrochemical techniques have been presented which can provide the information on behavior of both metal and its corrosion product in the continuously evolving corrosion system in molten salts. However, use of single isolated methods renders it difficult to obtain mechanistic information on instantaneous, time-dependent processes and phenomena during corrosion. These are needs gaps opportunities. In this work we present a detailed electrochemical analysis of the corrosion of polycrystalline chromium at 600°C in FLiNaK over 50 h including an electrochemical analysis of in-situ near-instantaneous corrosion rates corroborated with other methods.

A variety of corroborating methods including (1) in-situ electrochemical diagnostic techniques, (2) static immersion exposure with gravimetric and SEM analysis, (3) electrochemical impedance spectroscopy, (4) XRD, (5) ICP-OES, was utilized to study the corrosion behavior in unpurified FLiNaK at 600°C. The focus was to evaluate the viability of using an external non-reactive electrode to measure the concentration of corrosion products via redox process analysis in order to determine the corrosion rate during the process of OCP corrosion of Cr in FLiNaK. This work reports the possibility of using of electrochemical techniques such as CV for the instantaneous determination of Cr corrosion rates when Cr is exposed to a 600^oC FLiNaK salts for 50 h. It was found that the concentration of dissolved corrosion products, in the more likely possible forms of CrF₃ ^- and CrF₆ ^3- _, might be calculated by established analytical approaches to CV analysis such as Berzins-Delahay (soluble/insoluble) and Randles-Ševcik equations (soluble/soluble). The calculated values showed the definite concentration of Cr(II)/Cr(III) corrosion products at the distance of the external electrode (due to the static corrosion conditions). These finding supported by XRD and ICP-OES. A semi-infinite diffusion model was employed in this study to adjust the calculated concentrations assuming a stagnant solution and diffusion based one-dimensional concentration diffusion profile in order to calculated the Cr(III) concentration at a fixed position in the Cr(III) concentration gradient. It was shown that the semi-infinite diffusion model helps correctly describe this profile between 10 and 50 h, based on the geometry of the working electrode and corrosion cell. Based on these results the OCP corrosion rate was interpreted to switch over 50 h from initially charge transfer control Cr(II) oxidation at early times with likely HF and Cr(III) reduction, followed by mass transport control regulated by a salt film and then possible stifling due to depletion of HF and Cr(III) solubility limits.

The results of half-cell redox potentials and peak currents pertaining to oxidation/reduction of Cr corrosion products were correlated to the concentrations of particular Cr species and compared with the results of mass loss experiment and ex-situ ICP-OES of post-exposed FLiNaK. The mass loss of the Cr(II)/Cr(III) metal that was approximated by spatially integrating the concentration profiles at a selected time, and the result of Cr mass loss separately obtained from gravimetric measurements showed a reasonable agreement. Furthermore, the rate of accumulation of Cr ions in the residual FLiNaK salt after 50 h of exposure determined by ICP-OES, correlated with the electrochemical CV and EIS analysis. In addition to this, the oxidation and reduction peak potentials pertaining to Cr/CrF₃ ^- and CrF₃ ^-/CrF₆ ^3- redox reactions (measured on Pt wire) were compared with the OCP (measured on Cr) to provide insights on the predominant anodic and cathodic reactions.

Acknowledgments

This research was supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Utilization of the Malvern-Panalytical Empyrean diffractometer was supported by Nanoscale Materials Characterization Facility with National Science Foundation under award CHE-2102156.

References

Y. Liu et al., (2020) Corrosion Science. 169 108636. https://doi.org/10.1016/j.corsci.2020.108636.

Designing and fabricating a stable thermodynamic reference electrode (TRE) for use in molten fluoride salts has proven challenging for several reasons. An ideal TRE will not allow mass diffusion of molten salt across its system boundary while it will allow ionic contact across the system boundary. Appropriate material candidates for the fabrication of a TRE need to remain inert in molten salt environments and be electrically insulating, all while remaining stable at temperatures greater than 700C. UC Berkeley has been working with Ultramet on the Department of Energy SBIR DE-FOA-0002359 to develop such a TRE for 2LiF-Be2F (FLiBe) which is resilient to degradation after long term FLiBe exposure, produces repeatable electrochemical data, and can be incorporated into molten salt systems modularly.

This first testing campaign consisted of two main experimental thrusts which characterized the materials degradation and the electrochemical response of the TRE. The first experimental thrust concerns the degradation of a boron-nitride (BN) coating on a porous graphite substrate under long term exposure to FLiBe at high temperatures. Six (6) unique BN coated graphite foam coupons were exposed to FLiBe held at 700C for 96 hours. The cross sections of each coupon were imaged by macro lens photography for initial qualitative results which help determine the presence and efficacy of the BN coating after the FLiBe exposure. The next phase of research in this thrust requires SEM/EDS imaging of the coupons for quantitative results that will determine the approximate BN thickness remaining on the coupons and indicate if any materials migrated into the graphite foam pores during the exposure. The second experimental thrust characterizes the electrochemical response of the novel TRE in FLiBe. The Ni/NiF2 redox couple was used as a reference reaction for this study since it has been previously studied in FLiBe. Ultramet provided UC Berkeley with open-ended cylinder TRE bodies for electrochemical characterization. Each TRE body consists of graphite foam coated in BN and contained lanthanum-fluoride (LaF3) coated on the inside diameter which acts as an ionic membrane and a mass barrier. Ultramet’s TRE was loaded with NiF2 and assembled with a Ni wire electrode. Open circuit potential (OCP) and polarizability of the TRE were measured using a Gamry Ref 600 potentiostat, where the novel TRE was connected to the working lead, a second Ni wire was connected to the counter lead, and a third Ni wire was connected to the reference lead. Initial polarizability testing confirmed there was ionic contact between the interior TRE body and the exterior bulk salt. Next steps include characterizing the degradation to the LaF3 membrane to ensure it still functions as a mass barrier. Long term OCP drift can be studied after confirmation the LaF3 membrane still functions. These results indicate the novel TRE shows promise for accurately monitoring salt chemistry under high temperature environments for timescales up to 96 hours and beyond.

A consortium of the Massachusetts Institute of Technology, North Carolina State University, the University of California at Berkeley (UCB) and Oak Ridge National Laboratory have initiated a project to build a neutron-irradiated molten-salt forced-circulation loop at the MIT reactor. The loop will use FLiBe (Li₂BeF₄) salt and will duplicate thermal-hydraulics, chemical, and neutronics conditions in a salt reactor.

The SALT Lab at UCB is responsible for the design and application of electrochemical sensors and techniques for redox measurement and tritium transport within the loop. The proposed experiments will investigate redox measurements and control in the loop, tritium retention and diffusion in graphite, and tritium transport in the salt. Redox measurements and control studies will be performed to quantify the corrosive effect of neutron activation reactions and the effect of redox control agents in the loop. Tritium studies are being developed to quantify tritium uptake capacity in graphite, identify tritium desorption mechanisms, and measure tritium solubility, diffusivity, and speciation in the melt. To support these experiments, a variety of electrochemical probes are under development at UCB, including different size, shape, and material of the electrodes.

This talk will present the electrochemical probes under development and discuss the experiments that will be tested off-loop in the Be-gloveboxes of the SALT lab and then implemented in the loop.

A stable reference electrode is essential for electrochemical studies in molten fluoride salts, yet reference electrode design is hindered by challenges such as materials compatibility and high temperature operation. Electroanalytical studies are relevant to sensor development for molten fluoride salt systems as well as fundamental chemistry studies of salt structure and speciation. Dynamic reference electrode techniques have been used to find benchmark reference potentials, but the application of a constant current for the technique may not be feasible for sensing or high throughput studies. A thermodynamic reference electrode which is durable, stable, and reproduceable is urgently needed for both scientific studies and engineering applications of molten fluoride salts.

Fluoroacidity studies are of particular interest for a fundamental understanding of molten salt speciation. The more dissociated fluoride anions present in a melt, the more of a fluorodonor it is. Thus, it is considered to have a higher fluorobasicity due to its Lewis base behavior. Fluoroacidity is related to the coordination of solvent and solute ions and, therefore, solute diffusion coefficients and viscosity. Ion solvation differs in fluoride salts of various fluoroacidities, so solvated ions have different activity coefficients in different fluoride solvents. The relationship between activity coefficients of solvated ions and a melt’s fluoroacidity is not fully understood. The electrochemical determination of these activity coefficients benefits from a stable reference electrode. In addition, quantification of fluoroacidity requires the measurement of the activity of dissociated fluoride, which can be done electrochemically. The measurement of redox potential of a given couple in various fluoride melts can lead to a classification of the solvent melts based on the Nernst equation. Therefore, the study of redox couple behavior in various fluoride melts is relevant to both the development of a robust thermodynamic reference electrode & the quantification and theory of fluoroacidity.

The Ni/NiF₂ and Ag/AgF redox couples have been previously investigated for use in thermodynamic reference electrodes for molten fluoride salt. Ni/NiF₂ has been characterized for its stability in molten fluorides. The Ag/AgCl redox couple has commonly been used for chloride mixtures but literature on the fluoride couple is marginal. Our preliminary studies have shown the stability of the Ag/AgF redox couple in a FLiNaK melt by OCP measurement. Polarization studies will be carried out to test for reversibility, and other redox couples may be investigated. In this presentation, we present results from the Ag/AgF redox couple studies in FLiNaK, as well as their impact on current understandings of fluoroacidity.

We report the results of constant-potential molecular dynamics simulations of the double layer interface between molten 2LiF–BeF2 (FLiBe) and 23LiF–6NaF–21KF (FLiNaK) fluoride mixtures and idealized solid electrodes. Employing methods similar to those used in studies of chloride double layers, we compute the structure and differential capacitance of molten fluoride electric double layers as a function of applied voltage. The role of molten salt structure is probed through comparisons between FLiBe and FLiNaK, which serve as models for strong and weak associate-forming salts, respectively. In FLiBe, screening involves changes in Be–F–Be angles and alignment of the oligomers parallel to the electrode, while in FLiNaK, the electric field is screened mainly by rearrangement of individual ions, predominantly the polarizable potassium cation.

FLiBe has favorable properties for use in molten salt nuclear reactors, and constraints on the thermochemical properties of the salt rely on accurate and precise analysis of the Li/Be ratio.

The next generation of nuclear reactors will expose materials to conditions that, in some cases, are even more extreme than those in current fission reactors, inevitably leading to new materials science challenges. Radiation-induced damage and corrosion are two key phenomena that must be understood both independently and synergistically, but their interactions are often convoluted. In the light water reactor community, a tremendous amount of work has been done to illuminate irradiation-corrosion effects, and similar efforts are under way for heavy liquid metal and molten salt environments. While certain effects, such as radiolysis and irradiation-assisted stress corrosion cracking, are reasonably well established, the basic science of how irradiation-induced defects in the base material and the corrosion layer influence the corrosion process still presents many unanswered questions. In this review, we summarize the work that has been done to understand these coupled extremes, highlight the complex nature of this problem, and identify key knowledge gaps.

Fluoride-salt cooled High-temperature Reactors (FHR) use a 2LiF-BeF₂ salt mixture (FLiBe) as a primary coolant for a graphite pebble-bed-fueled core. Tritium management is a challenge for the design of FHR reactors: tritium is produced through the transmutation of Lithium-6 in a neutron environment. The amount of tritium produced in FHR is about 1000 times larger than that for current commercial pressurize water reactors and it can readily permeate through the heat exchanger walls due to its high diffusivity in metals. Understanding the tritium transport properties from FLiBe solution to the reactor materials as a function of tritium oxidation state is essential for the design of FHR reactors.

Hydrogen isotope solubilities in FLiBe were studied with saturation and stripping experiments. The solubilities values for hydrogen isotopes in molecular and fluoride form were obtained from those experiments [1][2]. Hydrogen isotope diffusivities in FLiBe were studied with permeation experiments. It was observed that hydrogen oxidation state in the salt affects the diffusivity values. The oxidation state is known to be dependent on the salt chemistry. Electrochemistry techniques could allow the study of hydrogen isotope transport properties in FLiBe solution. Hydrogen evolution in high-temperature molten salt has been studied electrochemically for tritium recovery in fusion applications [3][4], for fluorine gas production [5] and for fundamental chemistry studies [6]. However, no electrochemical studies are available in the literature on hydrogen isotopes in FLiBe.

This paper reviews hydrogen studies in high-temperature fluoride salts and introduces the ongoing experimental research at UW Madison. The experimental work focuses on understanding the behavior of tritium in molten FLiBe using hydrogen as a surrogate for tritium. The paper puts emphasis on the experimental choices made to design the electrochemical cell to study hydrogen in FLiBe. Thermodynamic predictions of the hydrogen reduction potential are provided, and preliminary experimental results on hydrogen evolution are discussed.

References

[1] E. Field, H. Shaffer, et al., ‘The Solubilities of Hydrogen Fluoride and Deuterium Fluoride in Molten Fluorides ’.’, vol. 71, no. 10, pp. 3218–3222, (1967).

[2] A. P. Malinauskas and D. M. Richardson, ‘The Solubilities of Hydrogen, Deuterium, and Helium in Molten Li2BeF4’, Ind. Eng. Chem. Fundam., vol. 13, no. 3, (1974).

[3] V. A. Maroni, ‘Process for Recovering Tritium from Molten Lithium Metal’, (1976).

[4] H. Qiao, T. Nohira, et al., ‘Electrochemical Behavior of Hydride Ion in a LiF-NaF-KF Eutectic Melt’, Electrochemistry, vol. 6, no. 67, pp. 643–648, (1999).

[5] H. Groult, C. Simon, et al., ‘Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF – 2HF’, Fluorinated Mater. Energy Convers., vol. 6, no. 3, pp. 1–29, (2005).

[6] S. Pizzini, G. Sternheim, et al., ‘Hydrogen Evolution from KHF2 Melts at Platinum Electrodes’, Electrochem. Acta, vol. 8, no. July 1962, pp. 227–232, (1963).