Molten Fluoride Salts Research Articles

Structure and Kinetic Properties of a Molten FLiBe Mixture with Tritium

A study is performed of the self-diffusion of tritium and fluorine atoms, and the change in the structure of molten FLiBe upon raising the temperature of the system from 873 to 1073 K. The interaction between neutrons and both lithium and beryllium in molten-salt reactors (MSR) using FLiBe as a fuel salt results in the formation of large amounts of tritium. Tritium, which easily penetrates metallic structural materials at high temperatures, is a radionuclide hazard. Predictive models for the behavior of tritium in a molten fluoride salt must therefore be developed to solve the problem of MSR safety. The emergence of tritium in the system increases the average energy of interatomic bonds upon raising the temperature and concentration of tritium in the system. A rise in temperature is also accompanied by fluorine atoms creating a closer short-range order in the environment of tritium atoms. This is expressed in the formation of a high first peak of radial distribution function gT-F(r), an increase in the number of probable geometric neighbors, which is shown by Voronoi polyhedra, and fluorine atoms giving priority to fourth-order rotational symmetry in the environment of tritium atoms.

Effects of reducing impurity addition on corrosion and mechanical properties of structural materials in molten LiF-NaF-KF

A proposed method for preventing corrosion in molten fluoride salts is adding certain reducing impurities that can control the oxidizing impurities. The effectiveness of many potential reducing impurities in preventing corrosion and their effects on mechanical properties of structural materials have not been studied. This work studies the effectiveness of six different reducing impurities on preventing corrosion of Hastelloy N and SS316H in molten LiF-NaF-KF (FLiNaK) at 700 °C. Our results indicate that addition of reducing metals can be effective in controlling the corrosion of alloys depending on the testing conditions but may affect the mechanical behavior of SS316H.

Molten salt-assisted carbonized zeolite imidazolate framework on nickel foam for highly efficient iodide capture in fluoride molten salts

Removing radioactive iodide ions from molten salt nuclear fuel in fourth-generation molten salt reactors is essential for nuclear and environmental safety. In the present work, a synthesis strategy as synthesised in molten salt was utilized by employing. A two-dimensional nanoscale carbon membrane with a hierarchical pore structure (HNPC@NF) was successfully prepared via molten salt-assisted carbonized three-dimensional ZIF-8 grown in situ on nickel foam, which showed the maximum adsorption capacity of 55.6 mg/g for iodide ions in LiF-BeF2 molten salt within 8 h under static adsorption experiments. HNPC@NF was capable of effectively adsorbing iodide ions even in the presence of various cations and anions such as Cs+, Sr2+, Nd3+, Sm3+and Br- and also adsorbed coexisting ions. The different-sized pores of HNPC@NF act as filters, sieving, and occluding for these ions. Radioactive tracer experiments have confirmed that HNPC@NF could adsorb 131I in simulated spent nuclear fuel salt and captured radionuclides 99mTc, 99Mo, 239Np, and 132Te within 4 h without interacting with 237U. The adsorption mechanism was determined by combining kinetic fitting, XRD, XPS, and simulation calculations using the Adsorption Locator and CASTEP module of BIOVIA Materials Studio based on density functional theory (DFT). On the macroscopic scale, iodide ions covered by molten salt diffused the surface of HNPC@NF through physisorption and occluded in the pores. On the atomic scale, charge-shift bonds formed between carbon atoms and iodide ions, allowing for stable adsorption at the hollow sites within the carbon network. The coexistence of cations increased the adsorption energy of iodide ions, which benefited the reaction. Based on these results, HNPC@NF showed the potential application as an adsorbent for radioactive iodide in the fluoride molten salt system.

Depletion Calculations for an Integral Small Molten Salt Reactor With Serpent

Abstract The molten salt reactor (MSR) concept is among the Generation IV designs considered feasible for providing clean, safe, sustainable, and economical energy supplies to the world's population. The depletion of fuel for a small modular fluoride molten salt reactor (sm-FMSR) with a closed fuel cycle based on the integral molten salt reactor concept has been investigated using Serpent. The Monte Carlo transport code Serpent has burnup capability and flow features that can be used to model fuel circulation and online fuel addition in an MSR. Three fueling schemes to control Serpent depletion cycles have been simulated and compared: step fueling (SF), continuous fueling with all fission products (FPs) accumulating in the reactor system (CFA), and continuous fueling with insoluble FPs separated from fuel (CFS). CFA and CFS require fewer depletion cycles that are longer in duration than the cycles required by SF, in order to maintain the effective multiplication factor (keff) within a working range over the seven years of the reactor fuel cycle. sm-FMSRs with SF and with CFA require similar quantities of “top-up” fuel, consume similar fuel (fissile) amounts, and result in similar fuel isotopic concentrations if keff is kept within a similar range. However, with separation of insoluble FPs from the circulating fuel, CFS gains a large reactivity worth due to the removal of FP poisons. This allows for reduction of fuel enrichment in both initial and total top-up fuel and leads to savings of a considerable fissile quantity in fueling MSR and in spent fuel. The Serpent depletion calculations require manual arithmetic calculations for adjustment of the Serpent built-in settings before the start of every calculation cycle for all three fueling schemes. Implementation of additional Serpent flow features in changing material volumes and flow constants would facilitate the simulation of the fuel depletion process and allow for more realistic simulations of fuel circulation.

MD Simulation and Transport Property Analysis of Silicon in High-Temperature Molten Fluoride Salts Electrolyte

Silicon is the dominant solar material because of its abundance, low cost, and high solar efficiency. But manufacturing high-purity silicon required for solar energy is very complex, hard to scale, and unsafe since it involves dealing with toxic flammable gases. Therefore, a new solar silicon production technology based on molten salt electrolysis has been proposed. To comprehensively study the self-diffusion and deposition mechanism of silicon ions in high-temperature molten salt electrolyte and their composition and temperature dependence, both interatomic potential molecular dynamics (IPMD) and ab-initio molecular dynamics (AIMD) are introduced. Thermodynamics and transport properties of molten salt electrolyte such as density, heat capacity, viscosity etc. are investigated and compared to the existed experimental results. Also, local structural information and coordination number analysis are obtained and discussed.

Electrochemical and Spectroscopic Study on Electrochemical Processing of Silicon in Fluoride Melt

For sustainable space utilization on the Moon and the Mars, it is essential to establish in-situ resources and energy utilization systems in space. Because of the large amount of silica in the regolith which composed of metallic oxides on the Moon and the Mars, the technology development for the covering the silica to silicon and oxygen is indispensable. The silicon can be used as a key material for silicon photovoltaics. Against this background, the authors have studied the electrochemical reduction of silica to silicon in fluoride melt. We propose a one-step process to obtain a silicon film on the cathode by electrolysis using molten salts containing silica. To determine the feasibility of the proposed process, it is necessary to understand the effects of the physicochemical properties such as the coordination structure of Si ions in the fluoride melt since the difference in the melt structure affects the nucleation and growth mechanisms of the silicon electrodeposition. However, there are few reports on the dissolved state of silicon ions in fluoride melt. The author thus clarified the relationship of the electrochemical behavior of coordination structure of Si ions in fluoride melts composed of LiF, KF, and NaF.The coordination structure of Si ions in the fluoride melt was investigated by high-temperature Raman spectroscopy and quantum chemical calculations based on the density functional theory (DFT). The vibrational modes of the Si ions were detected through high-temperature Raman spectroscopy and the Raman bands were simulated by DFT calculations. We clarified the interaction between Si ions and cation and anion species such as Li+, Na+, K+, O2−, and F−. In addition, the electrochemical behavior of Si ions was investigated by electrochemical measurements. The effect of O2− ions on the dissolution of SiO2 and the electrodeposition process of Si in fluoride molten salts revealed a significant increase in the reduction current of Si ions as well as in the film thickness and current efficiency of deposited Si. The molten fluoride system, which can fabricate Si films from SiO2 at low temperatures, has the potential to develop environmentally friendly and sustainable electrolytic systems.

(Invited) Rare Earth Metal Production Via Chloride Based Molten-Salt Electrolysis

The extraction and purification of metals such as aluminum has relied on the electrowinning process for decades. The Hall-Héroult process, developed in 1885, utilizes a molten salt electrolyte to electrochemically produce aluminum metal (Al) and carbon dioxide (CO2) from aluminum oxide (Al2O3) and carbon.1–3 Similar molten salt techniques have been developed for a variety of other metals including the rare earth metal Neodymium (Nd). Neodymium is of particular interest recently with significant increases in demand being driven by the increased production of technologies such as wind turbines, electric vehicles, and hard disk drives that require neodymium in the form of neodymium–iron–boron (Nd–Fe–B) permanent magnets. 4,5 The current procedure for neodymium processing uses a neodymium and lithium fluoride molten salt electrolyte with a sacrificial carbon anode to convert neodymium oxide (Nd2O3) and carbon to neodymium metal and CO2.4,6 As an unfortunate byproduct of the fluoride containing molten salt, perfluorocarbons (PFCs) can also be produced simultaneously. The formation of PFCs combined with the emission of a significant amount of other greenhouse gases (CO,CO2) make the current process for neodymium electrowinning is dangerous and undesirable from an environmental standpoint. However rare earth metal production remains profitable enough that illegal processing operations make up a significant portion of the world’s supply of rare earth elements like neodymium. 7,8 Due to this, few places currently produce neodymium metal and virtually none is produced in the United States, creating supply chain risks with dire consequences.5 An alternative molten salt process has been proposed in which the fluoride salts have been replaced with chloride salts consisting of lithium chloride (LiCl) and potassium chloride (KCl). Rather than directly converting neodymium oxide to neodymium metal, the oxide is first converted to chloride salt form by reaction with hydrochloric acid. The neodymium salt is then dissolved into the LiCl-KCl molten salt and neodymium is electroplated via the below set of reactions.9,10 Cathode: 2NdCl3 + 6e- → 2Nd(solid) + 6Cl- Anode: 6Cl- → 3Cl2 + 6e- Overall: 2NdCl3 → 2Nd(solid) + 3Cl2 This process has several distinct advantages. Utilizing the chlorine reaction eliminates the need for a sacrificial anode material as well as the production of carbon dioxide. The chloride based molten salt also eliminates the formation of PFCs. The chlorine produced could then be recycled to make more hydrochloric acid for use in converting neodymium oxide to chloride.This talk focuses on recent advances to improve purity and lower energy consumption (kWh/kg). Our work evaluates anode and cathode behavior during this neodymium chloride molten salt process in order to determine its viability. Overpotential and stability of various anode materials are investigated in order to minimize energy consumption and ensure long life of process materials. The effect of various plating conditions such as current density and substrate material are investigated to determine impact on deposit quality, coulombic efficiency, and metal purity. Additional purification techniques such as vapor distillation procedures are developed to ensure a product that is viable for industrial use. This proof of concept work aims to develop a safe, sustainable and environmentally friendly path towards large scale production of rare earth elements. Reactor design and cathode efficiency results are based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Advanced Manufacturing Office, Award Number DE-EE0009434. Anode design work was supported through the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Portions of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. T. R. Beck, Electrochem. Soc. Interface, 23, 36–37 (2014).G. G. Botte, Electrochem. Soc. Interface, 23, 49–55 (2014).W. E. Haupin, J. Chem. Educ., 60, 279–282 (1983).M. F. Chambers and J. E. Murphy, Electrolytic production of neodymium metal from a molten chloride electrolyte.B. Sprecher, R. Kleijn, and G. J. Kramer, Environ. Sci. Technol., 48, 9506–9513 (2014).V. S. Cvetković et al., Met. 2020, Vol. 10, Page 576, 10, 576 (2020).H. Vogel, B. Friedrich, H. Vogel, and B. Friedrich, Int. J. Nonferrous Metall., 6, 27–46 (2017).J. C. K. Lee and Z. Wen, Nat. Sustain. 2018 110, 1, 598–605 (2018).R. Akolkar, J. Electrochem. Soc., 169, 043501 (2022).D. Shen and R. Akolkar, J. Electrochem. Soc., 164, H5292–H5298 (2017).

Synergistic regulation of the formation of yttrium oxyfluoride layer on yttrium oxide substrate by fluoride-chloride molten salts

Fluoride molten salt (KAlF4) can modify Y2O3 to form YOF, but the reaction degree of this process is difficult to control and tends to form a large amount of yttrium fluoride (YF3) with low-strength. The reaction degree of the modification process of fluoride molten salt with Y2O3 can be suppressed by the synergistic effect of chlorides (KCl-CaCl2), which is mainly determined by the diffusion of fluorine-containing complex ions (AlF4-) in the fluoride-chloride mixed molten salts (FCMMS). The low diffusion rate of AlF4- in FCMMS directly leads to the decrease of the fluorination reaction rate and delays or even prevents the deeper fluorination process. After the synergistic modulation of Y2O3 by FCMMS, only YOF is formed, and the formation of YF3 is completely avoided. The strategy and mechanism may be applied to the economical preparation of YOF protective layer or YOF powder on low-cost Y2O3 template.

Method for the Determination of Oxygen in FLiBe via Inert Gas Fusion.

In nuclear reactors that use molten fluoride salts, either as coolants or as the medium for the fuel, the purity of the salts is critical for controlling salt chemistry and mitigating corrosion. Water is a particularly important contaminant in this regard, as it participates in a number of important corrosion reactions, so the careful measurement of oxygen, which is principally present in the salts due to water contamination, is a critical step in salt characterization. Here, we present an analytical method for quantifying oxygen contamination in Li2BeF4 (FLiBe), a technologically important and suitably representative fluoride salt, with a detection limit of 22 μg of oxygen, or 110 ppm in a 200 mg sample. To test the method, four FLiBe samples from different batches were tested. Two of these showed oxygen concentrations below the method detection limit, while two showed concentrations above it. In particular, the difference in the oxygen concentration between purified and un-purified batches of material from Kairos Power showed the efficacy of this method in characterizing the degree of oxygen removal obtained from purification methods.

High temperature tribological properties of the D-gun WC-12Co coating in fluoride molten salt

Development of anti-corrosion and wear-resistant materials in FLiNaK molten salt is vital to the service safety and reliability of the various mechanical systems for the molten salt reactor. In this work, a WC-12Co coating was fabricated by detonation gun spraying technique. Its microstructure, mechanical properties and tribological behaviors in FLiNaK molten salt were studied. A ball-on-disk rotary configuration was selected for the tribological test, in which the WC-12Co coating disks slid against Al2O3, SiC, Si3N4 and steel counterface materials in FLiNaK molten salt and low-oxygen environment at 600 °C. The results showed that the WC-12Co coating displays a low porosity of 0.71%, a bonding strength of above 44 MPa and good anti-corrosion in molten salt. FLiNaK molten salt provides the good lubricity and inhibits the oxidation wear of the WC-12Co coating, contributing to the low friction coefficient of 0.097–0.17 and wear rate of (4.95–8.40) × 10−7 mm3N−1m−1.

Weld corrosion of GH3535 in molten (Li,Na,K)F

The corrosion of structural materials in molten fluoride salts restricts the development of a molten salt reactor (MSR), with local galvanic corrosion of the weld joint being one of the major failure modes associated with the structural materials in the operating environment of an MSR. In this paper, in situ electrochemical measurements, combined with scanning electron microscopy (SEM/EDX) and metallographic microscope testing methods, are used to study the weld corrosion of GH3535 at 700 degrees C in molten (Li,Na,K)F. The results show that each part of the welding areas is in active dissolution state at the open circuit potential. The electrochemical behavior of the heat affected zone is similar to that of the GH3535 parent material. When the weld zone of the GH3535 is coupled with the parent alloy, galvanic corrosion occurs and the galvanic corrosion effect is larger than 1.

Application of the Solute-Solvent EMF Cell to Measure Activity of NiF2 in Molten FLiNaK

Electromotive force (emf) measurements made using a combination of solute- and solvent-based electrodes were used to determine the activity of NiF2 in molten FLiNaK eutectic at 823 K across a concentration range of x NiF2 = 5.2 × 10–4–1.0 × 10–2. The solute emf values were measured using electrodes consisting of Ni wires immersed in FLiNaK with dissolved NiF2 contained in graphite crucibles. The measured emf values were then converted to the FLiNaK Eutectic Potassium Electrode (FEKE) potential and used to quantify the activity of dissolved NiF2. This quantification was based upon comparative measurements of a reference solvent electrode consisting of a K-Bi alloy immersed in pure FLiNaK contained in a boron nitride crucible and a solute electrode. Short cell lives were characteristic of the measurements due to the corrosive nature of the fluoride salts. Quantifying the activity of NiF2 will improve the utility of Ni2+/Ni reference electrodes in molten fluoride salts, which are notoriously difficult electrolytes to work with because of their reactivity. This work demonstrates the general nature of the solute-solvent approach as a repeatable, easily employed method for measuring the activity values of electroactive species in a variety of molten salts to improve understanding of the electroactive species behavior in these systems.

Corrosion characteristics of monolithic SiC materials in beryllium-bearing molten fluoride salt

Chemical vapor deposited SiC single crystal and polycrystals were exposed to a static beryllium-bearing molten fluoride salt, 2LiF–BeF2 for 500 and 1000 h at 750 °C. Overall, the single crystal and polycrystalline SiC materials experienced slight weight loss and nominal surface etching regardless of the material type. Localized pitting and nonuniform Si removal from surface regions were associated with trace metal impurities and oxygen. Cross-sectional observation revealed minimal subsurface changes in the microstructure. The corrosion damage processes were similar to those reported for molten LiF–KF–NaF salt with trace impurities, indicating that the impurities significantly affect SiC corrosion.

Novel pyrolytic boron nitride coating to reduce graphite interactions in molten fluoride salt

Future salt reactors like the Fluoride-salt-cooled-High Temperature Reactor require varying material flavors. Graphite is required for neutron moderation and potential fuel forms, while structural metals are required for containment. Studies show graphite/metal interactions exist when both materials share a salt medium. These interactions may accelerate the corrosion of the metal and change the surface of graphite materials. A novel pyrolytic boron nitride coating on graphite is investigated for reducing graphite/metal interactions. This is the first known application of such a coating, and salt exposure tests reveal the pyrolytic boron nitride coating reduces the interactions between stainless steel and uncoated graphite.

High temperature thermo-chemical properties evolution of NaF-NiF2 system

In Molten Salt Reactor (MSR), Ni based alloys are more suitable for handling of the molten fluoride salts because of formation of corrosive resistive NiF2 layer over the structural material components. A ternary mixture of LiF-NaF-KF (FLiNaK) can be used as coolant salt for transfer of heat from the reactor core. NiF2 layer of the Ni-based alloys shows reactivity towards NaF at reactor operating temperature. NaNiF3(s) is the single complex ternary compound in the NaF-NiF2 system. The compound has been synthesized and characterized. The thermodynamic stability of NaNiF3(s) has been experimentally measured in terms of Gibbs energy of formation using Galvanic cell technique and its measured ΔfG°m(T) was obtained as.ΔfG°m(T) [NaNiF3(s)]/kJ mol−1 = (−1242.9 ± 2.3) + (0.2312 ± 0.0039) × (T/K).The heat capcity and high temperature enthalpy increment was also experimentally measured using DSC and high temperature calorimetric technique, respectively and the values were obtained as.Cp,m(J·mol−1·K−1) = 117.74 + 0.0498 × T −1.07351 × 106/T2(303 ≤ T/K ≤ 828).(HT-H298)(J·mol−1) = −40270.6 + 120.4 × T + 0.0234 × T2+717669.7/T.Effect of oxygen impurity over the stability of the compound has been investigated by calculating the chemical potential diagram of Na-Ni-F-O system. The binary phase diagram of NaF-NiF2 system and the ternary phase diagram of Na-Ni-F2 system has been calculated based on Gibbs energy minimization technique using the experimental thermodynamic data.

Viability of Cathodic Protection for Preventing Corrosion of Stainless Steel 316H in Molten LiF-NaF-KF

Molten fluoride salts are candidate heat transfer fluids in a number of applications such as generation IV molten salt nuclear reactors and concentrated solar power plants. However, a chief concern in the design of these systems is the corrosion of structural materials that come in contact with these molten salts. Redox control methods such as the purification of salt, the addition of active elements, and applied electrochemical potential can be efficient methods for preventing the corrosion of structural materials in molten fluoride salts. Applied electrochemical potential as a redox control method for application in molten fluoride salts has rarely been explored. This study seeks to understand the viability of impressed current cathodic protection (CP) at various currents as a redox control method to prevent corrosion of stainless steel 316H in molten LiF-NaF-KF (FLiNaK) salt. Results show that application of CP can be an effective method to prevent corrosion of SS316H in molten FLiNaK salt, but the applied current will have to be optimized to prevent undesirable side effects such as reduction of salt constituents, salt deposition on electrodes, etc.

Kinetics of Corrosion and Oxidation of Fe- and Ni-Based Alloys by Molten Fluoride Salt

Kinetics of Corrosion and Oxidation of Fe- and Ni-Based Alloys by Molten Fluoride Salt

Effects of simultaneous proton irradiation on the corrosion of commercial alloys in molten fluoride salt

Molten salt fission and fusion reactors hold promise for large-scale, carbon-free power generation, but only if the combined effects of simultaneous irradiation and corrosion on their structural materials can be predicted. Here commercial alloys (SS316L, Incoloy 800 H, Inconel 600) are corroded in molten fluoride salt at 650 ℃ while partially subjected to simultaneous proton irradiation. By comparing corrosion feature depth distributions, it is found that simultaneous proton irradiation accelerates the corrosion of SS316L and Incoloy 800 H, while Inconel 600 does not undergo radiation-accelerated corrosion. Simultaneous proton irradiation accelerates or decelerates corrosion proportional to the ratio of Ni to (Fe+Cr) content.

Tribocorrosion of stainless steel sliding against graphite in FLiNaK molten salt

The molten salt reactor (MSR) concept recently gained renewed interest in developing Generation IV nuclear reactors. One MSR design uses fluoride molten salts to cool tri-structural isotropic particle fuel encapsulated by a carbon matrix into spherical pebbles, which would inevitably contact the reactor's stainless steel container wall during salt circulation. Characterizing this interaction is crucial for reactor safety. Here we report the tribocorrosion behavior of graphite sliding against Type 316H stainless steel lubricated by a FLiNaK (LiF:NaF:KF; 46.5:11.5:42 mol %) molten salt in an argon environment. Accelerated wear loss was observed at a higher temperature because of a lower molten salt viscosity and a higher corrosion rate. The graphite had a more rapid material loss at a higher sliding speed than stainless steel because of its higher vulnerability to vibration-induced microfracture. The salt-starved condition caused more material loss than either the no-salt or the salt-flooded condition because neither a graphite transfer film nor stable boundary lubrication could be established at salt starvation. An interesting dual-layer surface film was discovered on the stainless steel worn surface: a chromium-rich top film and a nickel-accumulated but chromium-depleted interlayer. The film composition and structure provide fundamental insights to the mechanochemical interactions between stainless steel and graphite in a molten salt environment.

High temperature tribological properties of D-gun Al2O3 coatings in fluoride molten salts

Anti-corrosion and wear-resistance coating is important to the reliability of the mechanical systems served in molten salt media. In this work, the Al2O3 coating was prepared on the stainless steel surface by a detonation gun (D-gun) spraying technique and its high temperature tribological properties in FLiNaK molten salt were studied systematically by sliding it against SiC, Al2O3 and steel materials. The results indicated that the Al2O3 coating has a high bonding strength of above 50 MPa with a porosity of 0.8 %. The tribological properties of the Al2O3 coating is significantly influenced by the counterface materials. The friction coefficients are between 0.14 and 0.45, and the wear rate increases from 2.2 × 10−5 mm3/N·m to 19.1 × 10−5 mm3/N·m by sliding it against SiC, Al2O3 and steel materials. The worn surface morphologies and compositions were specifically studied and correlated to these distinct behaviors.