Molten LiF BeF2 Research Articles

Speciation and Diffusivity of Hydrogen in Molten 2LiF-BeF2 for Nuclear Fission and Fusion Applications

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.

Microelectrode-Based Diffusivity Measurements of Hydrogen in Molten 2LiF-BeF2

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.

Surface Fluorination of Nuclear Graphite Exposed to Molten 2LiF-BeF2 (FLiBe) Salt and Its Cover Gas at 700 °C.

This study demonstrates that the reaction of Li2BeF4 (FLiBe) with graphite both in the liquid phase and the gas phase of the molten salt leads to the formation of covalent and semi-ionic carbon-fluorine bonds at the graphite surface and is accompanied by surface microstructural changes, removal of C-O groups, and deposition of metallic beryllium, based on XPS, Raman, and glow discharge mass spectroscopy characterization. At 700 °C, the observed surface density of C-F is higher after 240 h than after 12 h of exposure to molten FLiBe salt; the kinetics of covalent C-F formation is slower than that of semi-ionic C-F formation, and the relative amount of semi-ionic C-F content increases with depth. The graphite sample exposed to the cover gas exhibits less surface fluorination than the salt-exposed sample, with predominantly semi-ionic C-F. Based on these observations and the observed LiF/BeF2 ratio by surface XPS, the hypotheses that fluorination of the salt-exposed graphite occurs via a gas-phase mechanism or that it requires salt intrusion are refuted; future studies are warranted on the transport of C-F semi-ionic and covalent species in graphite at high temperatures.

Density Measurements of Molten LiF-BeF2 and LiF-BeF2-LaF3 Salt Mixtures by Neutron Radiography.

The densities of eutectic (LiF)2-BeF2 and mixtures of this salt (FLiBe) with LaF3 were measured by dilatometry and by neutron attenuation from 673 K to 1,073 K. Because LaF3 has a limited solubility in FLiBe, it was necessary to determine the amount of LaF3 in solution before the density could be determined. The FLiBe density determination was favorably benchmarked against the literature data. A simple comparison was not available for the LaF3-FLiBe mixtures, so extrapolation of published data was necessary based on analysis using the Molten Salt Thermal Properties Database-Thermochemistry, or MSTDB-TC, developed by the US Department of Energy. Solubilities for LaF3 in FLiBe ranged from 1 to 4 mol % over 673 to 1,073 K. The salt system was heated and cooled over 24 h to evaluate potential changes in composition and hysteresis during the measurement. Changes in the meniscus were observed, and these were included in the correction for density determinations. Salt surface tension may have led to supersaturation of LaF3 in the salt because the solubility curve was nonlinear with respect to the inverse temperature, as would be expected for an ideal system. Surface tension measurements are currently underway to test this hypothesis.

Compositional transferability of deep potential in molten LiF-BeF2 and LaF3 mixtures: prediction of density, viscosity, and local structure.

The accumulation of lanthanide fission products carries the risk of altering the structure and properties of the nuclear fuel carrier salt LiF-BeF2 (Flibe), thereby downgrading the operating efficiency and safety of the molten salt reactor. However, the condition-limited experimental measurements, spatiotemporal-limited first-principles calculations, and accuracy-limited classical dynamic simulations are unable to capture the precise local structure and reliable thermophysical properties of heterogeneous molten salts. Therefore, the deep potential (DP) of LaF3 and Flibe molten mixtures is developed here, and DP molecular dynamics simulations are performed to systemically study the densities, diffusion coefficients, viscosities, radial distribution functions and coordination numbers of multiple molten Flibe + xLaF3, the quantitative relationships between these properties and LaF3 concentration are investigated, and the potential structure-property relationships are analyzed. Eventually, the transferability of DP on molten Flibe + LaF3 with different formulations as well as the predictability of structures and properties are achieved at the nanometer spatial scale and the nanosecond timescale.

Electrochemical behavior of Sm(III)/Sm(II) and extraction of Sm on reactive electrode from molten LiF-BeF2

The electrochemical behavior of Sm(III)/Sm(II) in 2LiF-BeF2 (FLiBe) molten salt is studied by cyclic voltammetry (CV), square wave voltammetry (SWV), chrono amperometry (CA) and Tafel method in the temperature range of 773 to 973 K. The coexistence of Sm(III) and Sm(II) and the concentration ratio of Sm(III)/Sm(II) are determined by SWV test. The redox reaction of Sm(III)/Sm(II) couple on an inert tungsten electrode is demonstrated to be a quasi-reversible process with diffusion controlled mass transfer. The diffusion coefficients of Sm(III) and Sm(II) determined by chrono amperometry are in the range of 1.17 × 10-6 to 1.09 × 10-5 cm2 s−1 for Sm(III) and 2.49 × 10-6 to 3.90 × 10-5 cm2 s−1 for Sm(II) at 773 K to 973 K and the diffusion activation energy is 8.63 and 10.1 kJ mol−1 for Sm(III) and Sm(II) respectively. The coordination and transport characteristics of the Sm(III)-F and Sm(II)-F are also studied by the first principles molecular dynamics simulations. The Gibbs free energy of the SmF2 is calculated to be −1004.4 kJ mol−1 at 873 K by experimental data, and the standard reduction potential of E0SmF2/Sm is about −0.583 V vs E0BeF2/Be at 873 K. Thermodynamic evaluation and experimental results indicates that the metallic Sm could be extracted as M−rich SmxMy intermetallics (M = Cu, Ni, Al) from FLiBe based molten salts.

Diffusion behaviors of HF in molten LiF-BeF2 and LiF-NaF-KF eutectics studied by FPMD simulations and electrochemical techniques

The hydrofluorination reaction has proven to be a very effective approach to remove impurities in the preparation and usage of molten LiF-BeF 2 (Flibe) and LiF-NaF-KF (Flinak) eutectic salts, however, the diffusion behaviors of HF in two molten fluorides are incompletely understood. Here we design two systems, HF+Flibe and HF+Flinak, to reveal the diffusion properties and local structures as well as their correlations by first principles molecular dynamics (FPMD) simulations and electrochemical methods . It is shown that the strong interaction between Be 2+ and F – in molten Flibe reduces their diffusivities while providing more space for Li + migration. The diffusion coefficient of H + in molten Flinak is higher than that in molten Flibe determined by the fast exchange of H + between two F anions, and the larger diffusivity of F – in molten Flinak is related to the irregular movement of free F anions. Besides, the quasi-gaseous transport behavior of HF in two molten fluorides is proposed in this study according to the stable H-F bonds. Moreover, the experimental diffusion coefficient of HF in molten Flinak estimated by the method of convolution linear sweep voltammetry is consistent with our simulation value, and the larger diffusivity is well explained by the higher stretching vibration frequency between H + and F – in molten Flinak. Finally, the ionic diffusion coefficients are descripted by the first sharp diffraction peaks of structure factors, the nearest neighbor peaks of radial distribution functions , and the major bond angels of angular distribution functions, and it is speculated from these FPMD simulations that the peak position has a higher weight on diffusion coefficient. Overall, in-depth understandings of the structure-property relationships of HF+Flibe and HF+Flinak systems provide insights into the molten salt purification technology and the daily operation of molten salt reactors .

Constant-potential molecular dynamics simulations of molten salt double layers for FLiBe and FLiNaK.

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.

The LIBRA Experiment: Investigating Robust Tritium Accountancy in Molten FLiBe Exposed to a D-T Fusion Neutron Spectrum

The Liquid Immersion Blanket: Robust Accountancy (LIBRA) experiment will be a first-of-a-kind experiment to explore and develop the liquid immersion blanket (LIB) concept. The LIB is a radically simple molten–LiF-BeF2 (FLiBe)–salt tritium breeding blanket for deuterium-tritium (D-T)–fueled fusion power plants (FPPs) achieving a high tritium breeding ratio (TBR) in neutronics models. However, tritium breeding in FLiBe is inherently difficult to study experimentally. As a result, the coupled issues of FLiBe radiochemistry and tritium (T) transport are poorly understood. LIBRA approaches this challenge by simulating an FPP blanket environment using a D-T neutron generator and 1000 kg of FLiBe. LIBRA will investigate T breeding, containment, and extraction, coupled with FLiBe redox control and radiochemistry. The primary goal of LIBRA is to demonstrate robust T accountancy in blanket prototypical conditions. Here, T accountancy encompasses accurate predictions of T breeding in the FLiBe; detection and measurement of all T bred in LIBRA; and speciation of the T extracted from the FLiBe. Initial neutronics simulations of LIBRA indicate that a global TBR of 1 is possible, where the TBR is defined as the number of tritons bred and extracted from FLiBe relative to the number of neutrons produced by D-T fusion reactions in the neutron generator. In this paper, we present the LIBRA concept and its scientific goals in the context of T breeding experiments. We also consider the potential impact of the LIB on the future fusion power industry, motivating further development of FLiBe-based T breeding research activities such as LIBRA.

The distribution and behavior of noble metal fission products (99Mo, 103Ru, 105Rh and 132Te) in molten 2LiF-BeF2 mixtures

In order to better understand and predict the behavior of noble metal fission products in molten salt reactor (MSR), the distributions of 99Mo,103Ru,105Rh and 132Te between molten salt, graphite and GH3535 alloy were investigated using γ-ray spectroscopy. Experimental results showed that 99Mo,103Ru,105Rh and 132Te mainly settle at the bottom of molten salt in the form of metallic particles, and the flow of molten salt can easily make the settled metallic particles suspend in molten salt. In addition, 99Mo,103Ru,105Rh and 132Te tend to be deposited more on GH3535 than on graphite, and the deposition behavior of 99Mo is particularly sensitive to the oxidation-reduction state of molten salt, which might be used as a redox indicator for the fuel salt of MSR. More importantly, the deposition characteristics provide new ideas for the separation and extraction of medical isotope 99Mo from MSR in the future.

Corrosion behavior of GH3535 alloy in molten LiF–BeF2 salt

The corrosion behavior of GH3535 alloy in molten LiF–BeF2 salt was investigated in a thermal convection loop. Results indicate that NiF2 in molten LiF–BeF2 salt and O2 from leakage drive the corrosion of alloy. The corrosion of alloy at the hot section is severer than that at the cold one. The BeCr2O4 was detected on the surface of alloy at the cold section while not at the hot one. Reactions between Be and metallic fluorides result in the concentration of impurities in molten LiF–BeF2 salt significantly reduced, and no obvious corrosion occurred in the GH3535 alloy at different temperature.

First-principles molecular dynamics study of the behavior of tritium in molten LiF-BeF2 eutectic

Molten LiF–BeF2 (FLiBe) eutectic (67 mol% LiF and 33 mol% BeF2) plays the fundamental role in the molten salt reactors (MSRs). However, the residual 6Li nuclides capture neutrons and generate the radioactive tritium nuclides, thus the tritium management is essential for the safe operation of MSRs. In this work, first-principles molecular dynamics (FPMD) simulations are applied to the investigation of the behavior of tritium in molten FLiBe. It is proved that the calculated temperature dependence of density and the coordination structure of molten FLiBe agree well with the available experimental information. Thus, it is confident that FPMD can be applied to the investigation of behavior of tritium in molten FLiBe. Finally, the coordination and electronic structures, as well as transport characteristics of various tritium moieties are studied for the understanding of the behavior of tritium in molten FLiBe.

An electrochemical study of hydrogen in molten 2LiF-BeF2 (FLiBe) with addition of LiH

The chemical and transport behavior of hydrogen isotopes in molten 2LiF-BeF2 (FLiBe) is of interest for the design of tritium management systems in nuclear fission and fusion reactors that use FLiBe. The chemical reaction of LiH with FLiBe is used to introduce hydrogen in the molten salt and electrochemical methods are used for in situ studies of hydrogen in FLiBe. LiH reacts with molten FLiBe to generate an electroactive species whose voltammetry peak is proportional to the added quantity of LiH. The cyclic voltammetry reaction potential of 2.009±0.050VvsBe/Be2+ and the electron exchange of n=0.8±0.5 are consistent with the one-electron oxidation of dissolved H in the zero valence state, Ho to H+. The concentration of Ho is estimated by linear sweep voltammetry at 60–80% of the hydrogen introduced by the reaction of 0.42 mol% LiH with FLiBe and after eleven hours it remains above 40%. It is postulated that covalent BeH2 is formed in FLiBe upon LiH addition, as a FLiBe-soluble quasi-stable intermediate product. The results provide an evaluation of LiH as a means of introducing dissolved hydrogen in FLiBe, enabling electrochemical methods as tools to advance the understanding of the chemistry of hydrogen isotopes in FLiBe.

Investigation of impurity driven corrosion behavior in molten 2LiF-BeF2 salt

A 1000 -h corrosion experiment of 316H stainless steel at 700 °C investigated the role of metal fluoride impurities CrF2 and FeF2 in LiF-BeF2 (66-34 mole %) (FLiBe) salt. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) demonstrated the ability to measure changes in the concentration of these fluorides in FLiBe salt. Cyclic voltammetry (CV) showed the potential for real-time quantitative concentration measurement of corrosion products in the molten salt. Materials characterization revealed that both impurity species influence the dissolution of Cr from the alloy. The collective results from these analysis methods advance the understanding of the influence impurities have on materials corrosion.

Solubility and precipitation investigations of UO2 in LiF–BeF2 molten salt

The solubility of UO2 in molten LiF–BeF2 (2:1 mol) (FLiBe) eutectic salt at 600 °C was studied by chemical and electrochemical methods. The results of dissolution experiment showed that the saturated solubility of UO2 in this melt was 2.37 × 10−3 mol/kg and its corresponding apparent solubility product (Ksp’) was approximately 1.67 × 10−5 mol3/kg3. When more Li2O were added to the FLiBe–UF4 system, the cathodic peak current of U(IV) in the electrochemical cyclic voltammetry (CV) curve accordingly decreased because of precipitate formation. The precipitate corresponded to UO2 as determined by stoichiometric ratio (concentration variation of U4+ and O2−) and X-ray diffraction (XRD) analysis. Compared to the chemical analysis method, the CV technique was confirmed to be more feasible for accurate determination of concentration of U(IV). Meanwhile, the Ksp’ value was also obtained to be 1.33 × 10−5 mol3/kg3 during the whole oxide titration procedure, which was highly consistent with that from the dissolution experiment. With the value of Ksp’, the allowable amount of dissolved oxide ions (oxide tolerance) can be theoretically estimated in the FLiBe–UF4 system.

Characterization of molten 2LiF–BeF2 salt impregnated into graphite matrix of fuel elements for thorium molten salt reactor

The impregnation behavior of molten 2LiF–BeF2 (FLiBe) salt into a graphite matrix of fuel elements for a solid fuel thorium molten salt reactor (TMSR-SF) at pressures varying from 0.4 to 1.0 MPa was studied by mercury intrusion, molten salt impregnation, X-ray diffraction, and scanning electron microscopy techniques. It was found that the entrance pore diameter of the graphite matrix is less than 1.0 µm and the contact angle is about 135°. The threshold impregnation pressure was found to be around 0.6 MPa experimentally, consistent with the predicted value of 0.57 MPa by the Washburn equation. With the increase of pressure from 0.6 to 1.0 MPa, the average weight gain of the matrix increased from 3.05 to 10.48%, corresponding to an impregnation volume increase from 2.74 to 9.40%. The diffraction patterns of FLiBe are found in matrices with high impregnation pressures (0.8 MPa and 1.0 MPa). The FLiBe with sizes varying from tens of nanometers to a micrometer mainly occupies the open pores in the graphite matrix. The graphite matrix could inhibit the impregnation of the molten salt in the TMSR-SF with a maximum operation pressure of less than 0.5 MPa.

Application of voltammetry for quantitative analysis of chromium in molten 2LiF-BeF2 (FLiBe) salt

The electrochemical behavior of Cr2+ in molten 2LiF-BeF2 (FLiBe) salt at temperatures between 500°C and 700°C was studied using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). LSV has been used to establish a stable reference electrode with the Be/Be2+ couple. The ability to distinguish between common corrosion products, such as Cr2+ and Fe2+, with CV has been demonstrated using the Be/Be2+ reference reaction. Methods for high-precision, in-situ quantitative measurement of Cr2+ concentration are developed. Models for predicting the concentration of Cr2+ and its diffusion coefficient are approximated by relating faradaic cathodic peak current density to Cr2+ concentration at 600°C and 650°C. The study provides the basis for the development of a sensor for on-line quantification of corrosion products that improves understanding of materials performance in the molten FLiBe salt.

Fluorination of nuclear graphite IG-110 in molten 2LiF-BeF2 (FLiBe) salt at 700 ºC

It is important for the development of advanced nuclear reactors to understand if the exposure to molten fluoride salts changes physical properties of graphite and its ability to absorb tritium. This paper demonstrated that the interaction between high purity nuclear grade graphite IG-110 and molten salt mixture 2LiF-BeF2 (FLiBe) at 700 °C for twelve hours in argon envirnoment resulted in graphite fluorination and changes in graphite microstructure. XPS C1s spectra analysis shows an increase in F functionalization (from 3.5% to 9.0–19.3%), an increase in the sp3/sp2 ratio (from 0.1496 to 0.1798), and F1s spectrum analysis confirms appearance of CF bonds. Raman spectra show an increase in ID/IG ratio from 0.35 to 0.69 and an appearance of peak shoulders characteristic of CF bond formation. GDMS depth analysis from the surface to 5 μm depth shows a higher depth penetration of F compared to that of Li and Be, indicative of surface salt reduction and bulk graphite fluorination. FLiBe reduction by graphite fluorination is not thermodynamically favorable, but certain carbon sites in the graphite are more reactive than the majority of carbon atoms in graphite. We argue that graphite fluorination occurred at these reactive sites, leading to the reduction of salt at the graphite surface. XRD and Raman confirmed microstructural changes. This indicates that the fluorination process may introduce new active sites as existing active sites are consumed, which could be advantageous for tritium chemisorption from FLiBe.

Electrochemical Studies of Hydrogen in FLiBe Salt

This paper provides an overview of the study of hydrogen chemistry and transport in molten LiF-BeF2 (FLiBe) at University of Wisconsin - Madison. Hydrogen was introduced in the salt in the form of LiH solid powders. Preliminary data shows an oxidation peak after LiH addition, attributed to the oxidation of H2 into HF. The hydrogen oxidation reaction is studied by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) and approximate H2 diffusion coefficients in FLiBe solution are calculated with LSV (2.3 x 10-5 cm2/s) and CA (1.6 x 10-5 cm2/s).

Infiltration of graphite by molten 2LiF–BeF2 salt

With the aim of developing neutron moderator and neutron reflector materials for fluoride-salt-cooled high-temperature reactors (FHRs), static infiltration tests were performed on graphite materials in molten 2LiF–BeF2 (FLiBe) salt, which is a potential primary coolant, at 700 °C and various pressures. The weight gain ratios of four grades (NBG-18, IG-110, NG-CT-10, and NG-CT-50) of graphite after infiltration were measured to determine their infiltration curves. The threshold pressure for FLiBe salt infiltration for the ultrafine-grained graphite (NG-CT-50) was greater than 600 kPa and much higher than those of the other three grades (medium-grained/fine-grained graphites), indicating that this graphite grade more probably resists salt infiltration in FHRs than other grades of graphite. However, if the threshold pressure is exceeded, it has the highest potential capacity for infiltrated salt over the pressures tested. The four grades were also characterized using mercury intrusion porosimetry. It was found that the infiltration curves of these two unwetting liquids were very similar. Scanning electron microscopy characterization showed that the FLiBe salt was distributed relatively uniformly in all four grades, indicating the presence of interconnected networks of open pores throughout the samples. X-ray diffraction patterns showed that infiltration test at high pressure led to an improved structural order and a decreased d-spacing in graphite.