Molten LiF NaF Research Articles

Lunar simulant behaviour in molten fluoride salt for ISRU applications

This study investigated the behaviour of a lunar mare crystalline analog dissolved in molten LiF–NaF at 800 °C for the in situ production of metals as a part of In Situ Resource Utilization (ISRU) research. Molten fluorides have the capability to dissolve metallic oxides, and the Hall-Héroult process uses this kind of media to produce Al from Al2O3.The first step was to compare the individual solubility of the main oxides composing the mare lunar soil (SiO2, Al2O3, Fe2O3, and MgO) with the solubility of the crystalline analog using Inductively Coupled Plasma – Atomic Emission Spectroscopy (ICP-AES). The species concentration added jointly are lower than the concentration of the same species added separately. Nonetheless, this study showed that LiF–NaF can be used to dissolve the analog with a maximum solubility of 3.9 wt% at 800 °C. Cyclic voltammograms were also used to verify the electroactivity of all oxide species in LiF–NaF, wherein all the main oxides are electroactive except SiO2 and TiO2. Then electrolyses on different cathodic substrates were performed at different conditions and the obtained cathodic products were analysed with a scanning electron microscope (SEM) coupled with an energy dispersive spectroscopy (EDS). Despite the non-electroactivity of SiO2 and TiO2, they were extracted in an alloyed form through Under Potential Deposition (UPD). Metallic deposition of other metals such as aluminium and titanium was achieved on carbon electrode. Finally, a synthetic mixture made of the different oxide species with the same chemical composition as the simulant, was investigated as a viable substitute for lunar mare soil. Its electrochemical behaviour was identical to the crystalline lunar simulant showing that our original process based on oxides dissolution is not influenced by the amorphous/crystalline state of the raw material.the outputs of LiF–NaF molten process are not critically influenced by the physical state of the lunar regolith.

Corrosion Products Electrochemical Behaviour into Molten LiF-NaF: Investigation of Cr(II) System

In this work, a detailed electrochemical study of the molten LiF-NaF-CrF2 system is provided on Ag electrode in the 750 °C–900 °C temperature range. The first part was dedicated to the electrochemical study, by cyclic and square wave voltammetries, allowing the determination of thermodynamic and physico-chemical data such as CrF2/Cr standard potential, diffusion coefficient, activity coefficient, transfer energy and nucleation overvoltage. The influence of Li2O addition on the electrochemical reduction pathway of dissolved CrF2 on a silver electrode was investigated. The results showed that an increase of Li2O activity in the solution causes CrF2 disproportionation into solid chromium oxide and metallic Cr.

Electrochemical technique for detecting the formation of uranium-containing precipitates in molten fluorides

The electrochemical behavior of U(IV) in molten LiF–NaF–KF (46.5:11.5:42mol) (FLINAK) eutectic salt was studied by cyclic voltammetry and square-wave voltammetry analyses. The reduction of U(IV) proceeded in two steps: reduction of initial U(IV) to U(III), followed by three-electron reduction of U(III) to U metal. Both reactions were reversible and controlled by ion diffusion at a scan rate of 0.02–0.3Vs−1. A linear relationship between the reduction current density of U(III)/U(0) and the concentration of U(IV) was also established within 0.01–0.084molkg−1. When different concentrations of Li2O were added to the FLINAK–UF4 system, the peak current density of U(III)/U(0) was depleted because of the formation of UO2 precipitates. The apparent solubility product (Ksp) of UO2 in FLINAK–UF4–Li2O melts was approximately 4.75×10−6 mol3/kg3, which can be used to evaluate the allowable amount of dissolved oxide ions in uranium-based fluorides.

Electrochemical Investigation on the Stable Iron Species in Molten FLINAK

The electrochemical behavior of Fe(III) in molten LiF–NaF–KF [46.5:11.5:42 (mol%)] [FLINAK] salt was studied by cyclic voltammetry (CV), chronopotentiometry (CP) and square wave voltammetry (SWV) at 600°C. It was found that Fe(III) was initially reduced to Fe(II) at +2.0 V (vs. alkalis) followed by reduction of Fe(II) to Fe at +1.4 V (vs. alkalis) and both reduction processes were controlled by ion diffusion. Diffusion coefficients of Fe(III) in molten FLINAK were calculated through CV and CP. The reduction peak of Fe(III)/Fe(II) was still observed in SWVs even though only FeF2 was added to FLINAK. This result showed that Fe(II) was converted to Fe(III), which exists stably in FLINAK melts.

Electrochemical Investigation on the Stable Iron Species in Molten FLINAK

The electrochemical behavior of Fe(III) in molten LiF–NaF–KF [46.5:11.5:42 (mol%)] [FLINAK] salt was studied by cyclic voltammetry (CV), chronopotentiometry (CP) and square wave voltammetry (SWV) at 600°C. It was found that Fe(III) was initially reduced to Fe(II) at +2.0 V (vs. alkalis) followed by reduction of Fe(II) to Fe at +1.4 V (vs. alkalis) and both reduction processes were controlled by ion diffusion. Diffusion coefficients of Fe(III) in molten FLINAK were calculated through CV and CP. The reduction peak of Fe(III)/Fe(II) was still observed in SWVs even though only FeF2 was added to FLINAK. This result showed that Fe(II) was converted to Fe(III), which exists stably in FLINAK melts.

Impact of Corrosion Test Container Material in Molten Fluorides

The effects of crucible material choice on alloy corrosion rates in immersion tests in molten LiF–NaF–KF (46.5–11.5-42 mol. %) salt held at 850 °C for 500 hrs are described. Four crucible materials were studied. Molten salt exposures of Incoloy-800H in graphite, Ni, Incoloy-800H, and pyrolytic boron nitride (PyBN) crucibles all led to weight-loss in the Incoloy-800H coupons. Alloy weight loss was ∼30 times higher in the graphite and Ni crucibles in comparison to the Incoloy-800H and PyBN crucibles. It is hypothesized galvanic coupling between the alloy coupons and crucible materials contributed to the higher corrosion rates. Alloy salt immersion in graphite and Ni crucibles had similar weight-loss hypothesized to occur due to the rate limiting out diffusion of Cr in the alloys to the surface where it reacts with and dissolves into the molten salt, followed by the reduction of Cr from solution at the molten salt and graphite/Ni interfaces. Both the graphite and the Ni crucibles provided sinks for the Cr, in the formation of a Ni–Cr alloy in the case of the Ni crucible, and Cr carbide in the case of the graphite crucible.

Prospects of subcritical molten salt reactor for minor actinides incineration in closed fuel cycle

Abstract A subcritical molten salt reactor is proposed for minor actinides (separated from spent fuel VVER-1000 light water reactor) incineration and for 233U conversion from 232Th. Here the subcritical molten salt reactor with fuel composition of heavy nuclide fluorides in molten LiF–NaF–KF salt and with external neutron source, based on 1 GeV proton accelerator and molten salt cooled tungsten target is considered. The paper presents the results of parametrical analysis of equilibrium nuclide composition of molten salt reactor with minor actinides feed in dependence of core dimensions, average neutron flux and external neutron source intensity. Reactor design is defined; requirements to external neutron source are posed; heavy nuclides equilibrium and fuel cycle main parameters are calculated.

High-temperature corrosion behavior of Ni–16Mo–7Cr–4Fe superalloy containing yttrium in molten LiF–NaF–KF salt

The corrosion tests of a Ni–16Mo–7Cr–4Fe superalloy containing various yttrium contents were performed in molten FLiNaK (LiF–NaF–KF: 46.5–11.5–42mol%) salt at 850°C for 620h, with the goals of investigating the effects of yttrium on the corrosion resistance of Ni–16Mo–7Cr–4Fe alloy and understanding the corrosion mechanisms. The addition of an appropriate amount of yttrium promotes the formation of a Y-rich layer near the surface and inhibits the diffusion of the susceptive elements.

Electrodeposition of Si film on Ag substrate in molten LiF–NaF–KF directly dissolving SiO2

The electrochemical reduction of SiO2 in a molten LiF–NaF–KF electrolyte containing SiO2 at 873K was studied using cyclic voltammetry and chronoamperometry. The cyclic voltammograms indicated that SiO2 is first dissolved in the electrolyte, followed by reduction on a Ag electrode. Si films were deposited by potentiostatic electrolysis on a Ag electrode, and the specimens obtained were characterized by Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. Analysis of the Raman spectra indicated that the Si film obtained predominantly consists of the crystalline phase.

Elemental mapping of structural materials for a nuclear reactor by means of LA-ICP-MS

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was applied to the study of the interaction of molten LiF–NaF salts with candidate structural materials for a nuclear reactor-transmutor cooling circuit. At a working temperature of 680 °C structural materials of heat exchangers are attacked and the melt–alloy interaction provokes microstructural and composition changes of the contact surface. The resulting corrosion-influenced zones were studied by LA-ICP-MS and the maps of elemental distribution were created on the semiquantitative level using the procedure based on the normalization of particular isotopic signals to the sum of signals of all isotopes present in the examined zone. Consequently, LA-ICP-MS signals were measured for structural material constituents (Ni, Cr, Fe, W, Mo, Ti, and Mn) and for cationic constituents of the cooling medium (Li and Na). The hypothetical fluorine signal was calculated as a sum of Li and Na signals. Corrosion products in the exposed cooling medium were determined by ICP-OES after dissolution. Of the three examined candidate structural materials (pure nickel, nickel-based alloy A071EV and nickel-coated iron) pure nickel exhibits the best resistance.

The EPMA, LA-ICP-MS and ICP-OES study of corrosion of structural materials for a nuclear reactor cooling circuit by molten fluoride salt treatment

Electron probe microanalysis (EPMA), inductively coupled plasma atomic emission spectrometry (ICP-OES), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were applied to study the interaction of molten LiF-NaF salt mixtures with candidate structural materials (alloys) for a nuclear reactor-transmutor cooling circuit. The corrosion of test ampoules and inserted test specimens made of structural materials was brought about by the action of molten LiF-NaF coolant at 680 °C and its extent and character were examined in dependence on the time of exposure. The material corrosion changes were studied by mapping the sections of ampoule walls and inserted specimen surfaces with EPMA, whereas LA-ICP-MS was employed for linear scanning the salt/ampoule wall boundary. Corrosion-released structural material, dissolved in solidified molten salt, was analyzed by ICP-OES after the salt dissolution. The melt activity was proved to induce a surficial modification of a structural material up to the depth of 15–50 µm, which was associated with the coolant contamination. The X-ray maps by EPMA with its 1-µm lateral resolution revealed compositional changes in alloys, such as regular depletion of Cr to the depth of 10–25 µm. While the lateral resolution of LA-ICP-MS with the applied laser spot diameter of 25 µm was not exactly adequate to dimensions of the corroded regions and, consequently, yielded less information in comparison with EPMA, this technique was quite sufficient for the monitoring of the presence of alloy constituents in an adhered salt layer. It was concluded that: i) the EPMA study, involving semi-quantitative elemental mapping/content profiling and detailed spot quantitative analyses makes it possible to obtain quantitative assessment of the corrosion process; ii)LA-ICP-MS profiles can be converted from signal domain to elemental contents on a semi-quantitative level when applying signal normalization to the total sum of signals.

Transport in molten LiF–NaF–ZrF 4 mixtures: A combined computational and experimental approach

The transport properties of several LiF–NaF–ZrF 4 mixtures have been determined. Our work primarily consisted in the determination of the electrical conductivity from experimental measurements and from computer simulations. A good agreement was observed between both approaches. The simulations are based on the molecular dynamics technique and they employ a polarizable interaction potential, which was parameterized from first-principles calculations only. The diffusion coefficients were also determined from the simulations, which allowed us to understand the mechanisms responsible for the variations of electrical conductivity with temperature and composition of the melt.

Electrochemical behaviour of silver and rhenium electrodes in molten alkali fluorides

Electrochemical oxidation of silver and rhenium electrodes in molten LiF–NaF–KF eutectic at 600 °C was investigated by cyclic and convolutional voltammetry. Simulated Cyclic voltammograms were calculated to reinforce understanding of the reaction mechanism. A theoretical relationship was proposed to determine standard potentials from voltammograms of metallic electrodes having an oxidation process governed by Butler–Volmer kinetics. The reversibility of silver oxidation was established and the number of exchanged electrons and the standard potential of the Ag/Ag(I) couple were determined. The rhenium electrode oxidation process was shown to be quasi-reversible and the Re/Re(x) standard potential was estimated.

Electrochemical reduction of solutions of ReF6 in fused LiF-NaF-KF eutectic mixture

Electrolyte was prepared by introducing gaseous ReF6 into the molten LiF–NaF–KF eutectic at 600 °C. The electrochemical properties of the solutions were studied by voltammetric techniques. The reduction of ReF82− to Re occurred via a single irreversible step with diffusion controlled mass transfer. The diffusion coefficient of ReF82− was 8 × 10−10 m2 s−1 and the cathodic transfer coefficient was 0.13. Well-crystallized pure rhenium layers, up to 50 μm thick, were obtained on W, Ag, graphite and vitreous carbon substrates and were examined by SEM and X-ray diffraction techniques.

Solubility and Raman spectra of Ta(V) in LiF–NaF–KF–Na2O melts

Raman spectroscopy and solubility measurements have been used to study Ta(V) complexes in the molten LiF–NaF–KF eutectic (FLiNaK). Na2O(s) was added to melts containing a given amount of K2TaF7, and samples were withdrawn at different ratios of oxide to tantalum. The samples were analysed for oxygen and Ta, and studied by Raman spectroscopy using the windowless graphite cell technique. The solubility was studied at 700 °C. Solubility measurements in a FLiNaK melt containing 0.0956 mol kg−1 K2TaF7 indicated that all the added amounts of oxide and tantalum dissolved in melts with nO/nTa 3 the solubility data were consistent with a dissolution of the AlkTaO3 solid, and formation of a TaO4Fx(x + 3)− species. The tantalum tetraoxofluoride complex was possibly TaO4F25− (C2v). The solubility data indicated precipitation of an oxide rich solid at nO/nTa > 4. The Raman spectra supported the existence of TaO4F25− (C2v) species as well as (TaO5)n or (TaO6)n network structures in these melts.

Electrochemical study of the anodic dissolution of iron and nickel in the molten LiFNaFKF eutectic at 600°C using convolutional voltammetry

The kinetics of the anodic dissolution of iron and nickel are studied in the molten LiFNaFKF eutectic at 600°C by voltammetry. In order to eliminate distortion of the voltammograms by ohmic drop, convolutional techniques are used. The standard potentials of both Fe/Fe(II) and Ni/Ni(II) systems are determined by logarithmic analysis of semi-integrals.

A novel carbon electrode impregnated by molten metal fluoride for the production of fluorine

The catalytic ability of solid LiF or AlF 3 to decrease anode overvoltage and the anode behavior of a carbon electrode impregnated with molten LiFNaF or AlF 3−NaF in KH 2F 3 were investigated. When the graphite-sheet electrode was electrolyzed in a bath containing 2% by weight LiF or AlF 3, the anode effect did not occur, but fluorine-graphite intercalation compounds were formed by catalytic reaction of these solid metal fluorides. Under these conditions, the dispersion term of the surface energy of the electrode was nearly the same as that of the control electrode, however the polar term of the electrode increased 30 times over that of the control electrode. The carbon electrode impregnated with molten LiF-NaF or AlF 3-NaF was operated with a high current density ( e.g. 20 A dm −2) for 40 days without the occurrence of the anode effect; contamination of CF 4 in the F 2 was as low as 2 ppm.

Study on Electrowinning of Intermetallic Compounds Tantalum-Nickel and Niobium-Nickel in Molten LiF-NaF

A study of the mechanism of the electrochemical reduction of K2 TaF7 or K2 NbF7 on a nickel electrode in molten LiF-NaF allows the range of potentials to be defined where Nb-Ni or Ta-Ni alloys are formed. Further static potential measurements in a galvanostatic intermittent electrolysis allow the intermetallic compounds to be identified in each case and the Gibbs energy of the Ta-Ni compounds. Then surface alloys are prepared by electrolysis at a controlled positive potential; only NbNi3 and TaNi3 stable phases are detected in the bulk of the layer. A parabolic law of growth is stated, involving the intermetallic diffusion coefficient D˜ and a parameter α depending on solid composition at the boundaries of the layer; values of both α and D˜ were estimated after combination of electrochemical measurements and diffusion equation.

Electrochemical reduction of niobium ions in molten LiF-NaF

The mechanism of the electrochemical reduction of niobium ions in molten LiF-NaF (1∶1 mol) has been studied in detail at 750 and 800° C by the use of cyclic voltametry, chronoamperometry and chronopotentiometry. In the solution of niobium ions in LiF-NaF, it is concluded that the mechanism for the electrochemical reduction of fluoroniobate is: Nbv+e→NbIV NbIV+4e→Nb0 at potentials of about −0.06 and −0.17 V, respectively (referred to the reliable Ni/NiF2 (1 mol%) electrode [1]. The electrochemical reaction NbIV+4e→Nb0 is a (quasi)reversible and diffusion-controlled process. The conclusion has also been confirmed by analysis of the cathodic product obtained at constant potential with a scanning electron microscope.

Premiere etape de reduction du fluoro titanate de potassium dissous dans les fluorures alcalins fondus

The electrolytic reduction of K 2TiF 6 to trivalent titanium, which has been studied in the molten LiFKF and LiFNaFKF eutectic mixtures in the 500–800°C temperature range by means of semi-integral voltammetry and chronopotentiometry, is shown to proceed reversibly. The diffusion coefficients of TiF 2− 6 species have been determined to be 2.7 × 10 −3 exp (−4458/ T)cm 2S −1. Some kinetic complications to this reversible behaviour can occur due to the precipitation of K 2NaTiF 6 and K 3TiF 6 crystals and to the electrode surface stae at the start of the polarization. Tetravalent titanium, as well as anodizing, produce oxidized layers at the electrode surface which induce the appearance of a catalytic mechanism involving the regeneration of tetravalent titanium ions.