Molten Fluoride Salts Research Articles

Molecular Structure and Phase Equilibria of Molten Fluoride Salt with and without Dissolved Cesium: FLiNaK–CsF (5 mol %)

Molecular Structure and Phase Equilibria of Molten Fluoride Salt with and without Dissolved Cesium: FLiNaK–CsF (5 mol %)

Technical Note: Corrosion Behavior of 316 Stainless Steel With and Without an Ni Coating in Solid-State Fluoride Salt

With the rapid development of molten salt reactors, the problem of post-treatment of spent fuel salt discharged from the core becomes increasingly prominent. Like molten fluoride salts, solid fluoride salts are also highly corrosive, which poses a serious challenge to container materials. This paper describes the corrosion behavior of 316 stainless steel (316 SS) with and without Ni coating in low-temperature solid fluoride salt. Results show that 316 SS without Ni coating experiences pitting corrosion with the prolongation of corrosion time. 316 SS with Ni coating experiences intergranular corrosion. In addition, a loose hollow interlayer is formed between Ni coating and matrix. Corrosion of both samples increases with the extension of exposure time with solid fluoride salts.

The curious temperature dependence of fluoride molten salt thermal conductivity

To optimize the efficiency and safety of molten salt-based energy applications, accurate molten salt thermophysical property data are required. For molten fluorides, existing thermal conductivity results have large uncertainties and contradict the current theory by eliciting a positive temperature coefficient. Transient grating spectroscopy (TGS), a technique previously deemed reliable by the theoretical community, has been used to measure the thermal conductivity of fluorides (FLiNaK) for the first time. Results show a fairly flat but slightly increasing thermal conductivity as a function of temperature. The technique has been shown to not suffer from contributions from convection and radiation, an explanation used to discount the results of alternative experimental techniques. In addition to thermal conductivity, sound speed data as a function of temperature have also been obtained for the first time in FLiNaK. The use of accurate sound speed data in theoretical models of thermal conductivity provides better but not complete agreement with the results from TGS. The continued existence of a positive temperature coefficient highlights the need for new mechanistic proposals for why TGS, or current theoretical models, are unable to capture the correct temperature dependence for fluoride molten salt thermal conductivity.

Preparation of a chromium carbon coating on 316L by a disproportionation reaction in molten salts and its corrosion resistance in molten fluorides

Molten fluorides have been widely used in specific nuclear reprocessing and molten salt reactors due to their unique pyrophysical and pyrochemical properties. However, the severe corrosion of structural materials in highly corrosive molten fluorides at high temperatures limits their commercial use. In view of the high thermodynamic stability of chromium carbides in molten fluoride salts, and their good Cr-diffusion-blocking ability, it is proposed to develop a CrxCy coating, by electrodeposition coupled with a disproportionation reaction in molten salts, for application in molten fluorides A carbon coating is first prepared on 316L stainless steel by electrodeposition in molten LiCl-KCl-K2CO3, followed by a disproportionation reaction of Cr2+ on the carbon coating to obtain a CrxCy coating. Furthermore, the growth mechanism of the CrxCy coating and its corrosion resistance in molten fluorides are also investigated.

Activity gradient driven mass transport in molten fluoride salt medium

The molten salt-cooled reactor concept has garnered significant interest and one of the current challenges limiting the deployment of these reactor concepts is the complex corrosion phenomenon observed in molten salt environments. One of these phenomena is activity gradient mass transport, which has been shown to affect dissimilar materials submerged in the same salt medium even when best efforts have been made to electrically isolate dissimilar materials from one another. This mechanism while shown experimentally, has not been predictively studied through a modeling approach. In this study, activity gradients in several 316L-X materials systems have been modeled and the mass transport predicted by the model has been confirmed through static isothermal corrosion testing in a molten fluoride salt medium.

Interaction of beryllium with 316H stainless steel in molten Li2BeF4 (FLiBe)

The increased demand in renewable energy resources has led to a renewed interest in nuclear reactors including a new generation of molten salt reactors. One of the issues encountered in handling of molten chloride or fluoride salts is the inability of metals to form and retain a protective oxide surface layer, generally chromium oxide. The low chromium content, nickel-based alloy INOR-8, later marketed as Hastelloy N, was specifically developed to contain molten fluorides. This alloy had some deficiencies, but its composition resulted in it incurring far less chromium depletion by the molten fluoride salts than alloys like 316 stainless steel. For the next generation of molten salt reactors, an alloy with pressure vessel code approval and higher temperature capability than Hastelloy N is required. Consequently, several reactor designers have reportedly chosen to build the reactor containment vessel from 316H stainless steel. In order to minimize corrosion of the stainless steel by the molten fluoride salt (2LiF-BeF2), it is proposed to add beryllium metal to the salt to react with impurities and provide a means to lower the oxidation potential thus making the salt less corrosive. However, there is a concern whether the beryllium would react with components of the stainless steel. To address this concern, static capsule tests were conducted in which selected amounts of beryllium were added to capsules containing 2LiF-BeF2 (FLiBe) salt and 316H stainless steel and Hastelloy N samples. Following exposure, the samples were cleaned and examined using a variety of analysis techniques including, optical microscopy, SEM-EDS, XPS, LIBS, XRD and EPMA. Tensile samples were also exposed in capsules, and those were subjected to tensile testing. It was found that for the level of beryllium additions used in this study, intermetallic compounds were formed which could be detrimental to the long-term performance of the 316H stainless steel.

Electrochemical co-reduction of Y(III) and Al(III) in a fluoride molten salt system and electrolytic preparation of Y–Al intermediate alloys

In this study, a molten salt co-reduction method was proposed for preparing Y–Al intermediate alloys and the electrochemical co-reduction behaviors of Y(III) and Al(III) and the reaction mechanism of intermetallic compound formation were investigated by transient electrochemical techniques. The results show that the reduction of Y(III) at the Mo electrode is a reversible electrochemical process with a single-step transfer of three electrons, which is controlled by the mass transfer rate. The diffusion coefficient of Y(III) in the fluoride salt at a temperature of 1323 K is 5.0238 × 10−3 cm2/s. Moreover, the thermodynamic properties associated with the formation of Y–Al intermetallic compounds were estimated using a steady-state electrochemical method. Y–Al intermediate alloy containing 92 wt% yttrium was prepared by constant current electrolysis at 1323 K in the LiF–YF3–AlF3–Y2O3 (6 wt%)–Al2O3 (1 wt%) system at a cathodic current density of 8 A/cm2 for 2 h. The Y–Al intermediate alloy is mainly composed of α-Y2Al and Y phases. The development and application of this innovative technology have solved major technical problems, such as a long production process, high energy consumption, and serious segregation of alloy elements at this stage.

Corrosion behaviors of Ni-WC cemented carbide in high temperature molten fluoride salt and vapor

Ni-WC cemented carbides has a series of excellent properties, such as mechanical properties, thermal stability and wear resistance, but its corrosion resistance in molten fluoride salt is unknown. In this work, the corrosion behaviors of a Ni-WC cemented carbide (86.7W-8C-5Ni-0.3Cr wt.%) in molten FLiNaK salt (LiF–NaF–KF: 46.5–11.5–42 mol.%) and salt vapor were investigated by static immersion tests at 700 °C. The microstructure and phase composition of the material after corrosion were characterized by scanning electron microscopy (SEM), electron probe micro-analysis (EPMA) and scanning transmission electron microscopy with energy dispersive spectrometry (STEM/EDS). The results show that in the liquid salt, Ni-rich particles are deposited on the material surface, and Cr from the binder of the specimens is dissolved. The oxidative impurities in the salt vapor induce the formation of flaky Cr2O3 on the material surface. The relatively little corrosion in both salt and vapor indicates that the application of this material in molten fluoride salts environment is promising.

Effects of Simultaneous Proton Irradiation on the Corrosion of Commercial Alloys in Molten Fluoride Salt

Effects of Simultaneous Proton Irradiation on the Corrosion of Commercial Alloys in Molten Fluoride Salt

ИССЛЕДОВАНИЕ ТЕПЛОГИДРАВЛИКИ ИМИТАТОРА РАСПЛАВА СОЛИ ПРИ ТЕЧЕНИИ В КАНАЛЕ В УСЛОВИЯХ НЕОДНОРОДНОГО ОБОГРЕВА

The possibility of using molten salts based on fluorides as a coolant in promising developments in nuclear power systems requires solving several key scientific and technical problems. These problems are associated with development of reliable structural materials and study of thermophysical characteristics of coolant. Solution to the latter problem was constrained by lack of reliable systematic data on physical and chemical properties, specificity of heat transfer processes, and operation technology of promising compositions of fluoride molten salt. In this regard, a comprehensive study, consisting of studying properties of promising fluoride salt melts, as well as creating an experimental base for verification of engineering calculations, is of particular interest for design of reactor facilities. In course of the work, specifics of interaction of electromagnetic forces and buoyancy forces during MHD heat transfer of molten salt, which can manifest itself in the formation of local overheating points or quasi-stationary flow regimes accompanied by low-frequency temperature fluctuations, were experimentally investigated. For first time, dependences of heat transfer coefficients in a stream under influence of a magnetic field in an extended range of operating parameters were experimentally obtained. The results are compared with known patterns and numerical modeling.

One-step calcination synthesis of accordion-like MXene-derived TiO2@C coupled with g-C3N4: Z-scheme heterojunction for enhanced photocatalytic NO removal

This work explored an effective treatment method for removing NO from simulated flue gas at normal temperature. We synthesized accordion-like MXene-derived TiO2@C coupled with g-C3N4 (TiO2@C/g-C3N4) via one-step calcination utilizing Ti3AlC2, fluoride salts and melamine as raw materials in CO2 atmosphere. One-step calcination process simultaneously achieved the etch of Al layers of Ti3AlC2 by molten fluoride salts, the generation of g-C3N4 with melamine as precursor and the oxidation of Ti3C2 by CO2. TiO2@C/g-C3N4 for photocatalytic activation of hydrogen peroxide (H2O2) had as high as 94.0% removal rate of NO at parts per million level in a normal temperature environment. This efficiency was 5.3 times and 8.0 times that of TiO2@C/g-C3N4 and H2O2 individually, due to the synergy between photocatalysis and H2O2 oxidation. Meantime, TiO2@C/g-C3N4 exhibited an enhanced performance compared with g-C3N4 (40.3%) and TiO2@C (64.1%) for Z-type heterojunction accelerating the separation of electron-hole pairs. The mechanism was proposed that Z-type TiO2@C/g-C3N4 heterojunction activated H2O2 under visible light to generate hydroxyl radical and superoxide radical for oxidation of NO.

Accelerated corrosion testing in molten fluoride salts: Effect of additives and the crucible material

New aspects of the corrosion resistance/attack of Incoloy 800 H/HT in FLiNaK at 680 °C were studied. Effect of the addition of various impurities like CrF3, CrF3∙3 H2O, Cr2O3, Na2O, KF and KHF2 to the melt were analyzed in respect to the concentration. The corrosion increases by: (1) using acidic or hydrated salts, (2) using a GC crucible, (3) the concentration of the additives. It appears to be generally essential for the use of the melts to avoid the presence of hydrogen after purification, as it triggers a corrosion process together with various substances.

Effect of yttrium content on the corrosion behavior of nickel-based UNS N10003 alloy in high-temperature molten LiF-NaF-KF salt

The corrosion behavior of nickel-based UNS N10003 alloy with different yttrium contents in molten fluoride salt environment was investigated. The results showed that after an 800 °C/1000 h corrosion test, the alloy contained 0.05% yttrium experienced the least weight loss than other alloys without/with higher yttrium contents. Microstructure, surface micro-chemistry analysis indicated that a YF3 and Y2O3 mixed layer formed in the near surface region for this specific alloy. The retardation of active element dissolution because of the present of YF3 was suggested to be the main reason for its superior corrosion resistance in the molten salt environment.

Effect of Li metal addition on corrosion control of Hastelloy N and stainless steel 316H in molten LiF-NaF-KF

Molten fluoride salts are used in various energy harvesting applications such as solar collectors and molten salt nuclear reactors (MSRs). Thermodynamically driven selective dissolution of alloying elements due to impurities present within the fluoride salts have been identified to be the main corrosion mechanism for structural alloys in molten fluoride salts. Impurities in these salts, during salt preparation as well as during operation, can be difficult to control. One way to prevent the selective dissolution of active alloying elements is to make the molten salt reducing by either purifying it to remove impurities or to add some reactive metal that will react with the impurities and make the salt reducing. This study is focused on the corrosion behaviour of Hastelloy N and 316H stainless steel in molten LiF-NaF-KF eutectic mixture (FLiNaK) at 700°C and how this behaviour changes with the addition of Li metal as a reducing impurity. Our results indicate that the active metal addition like Li can be very effective in controlling the redox behaviour of salt, which in-turn can mitigate corrosion. Even the addition of small quantities of Li metal can mitigate the selective dissolution of active alloying elements for the tested structural materials in FLiNaK. Our results also suggest that the presence of excess amounts of Li, above the minimum concentration required to react with the impurities in FLiNaK, does not have any detrimental effect in terms of formation of new intermetallic phases at the surface of the selected alloys under tested conditions.

Proving the Availability of Ni–Fe Anode for Electro-Reduction of Solid V2O3 in Molten Fluoride Salts

The availability of casting Ni–Fe alloy as inert anode for direct electro-reduction of V2O3 in molten Na3AlF6-K3AlF6-AlF3 was investigated. The electrochemical oxidation behavior of anode as well as microstructural evolutions of formed oxide scale were systematically studied. The electrochemical characterization and reaction mechanism of the cathode oxide were also investigated to evaluate the influence of alloy anode on cathodic reduction process. The in situ formed three-layered oxide scale is compact and coherent, which is composed of an outermost Fe2O3 + FeAl2O4 skin layer, a Fe2O3 middle layer and a FeAl2O4 inner layer. The skin layer has a continuous, smooth structure and shows electrochemical activity. The Fe2O3 layer with a compact structure prevents inward diffusion of electrolyte and outward migration of metal cations. The innermost FeAl2O4 layer shows a loose structure and functions as a buffer layer to improve the peeling resistance of oxide scale. With the continuous extension of polarization time, the inner FeAl2O4 layer is slowly oxidized and becomes thinner, simultaneously, the dense Fe2O3 layer becomes thicker. Ultimately, metal vanadium product with fine rod-like particles can be obtained and the oxygen content in the metal vanadium is below 0.3 mass% within electrolyzing time of 2 h. The corresponding current efficiency is around 63%.

Experimental investigation of single helium bubbles rising in FLiNaK molten salt

An experimental investigation of single helium bubbles rising in a stagnant molten fluoride salt mixture was conducted in a 25.4 mm quartz tube at 600 °C and atmospheric pressure. The fluoride salt chosen for this research was the eutectic mixture of LiF-NaF-KF (46.5–11.5–42%), also known as FLiNaK. The images obtained using a high-speed camera were processed to estimate the bubble size, rising velocity, trajectory and shape. The shape of the bubble showed typical characteristics of wobbling bubble corresponding to published classifications. The trajectory of the rising bubble showed complex path oscillations and combinations of rectilinear, zig-zag and helical motion. The projected area-equivalent diameter and experimental terminal velocity varied from 5.26 to 6.23 mm and 232.35–259.57 mm/s, respectively. Several important dimensionless numbers were calculated and reported based on the experimental results, which are closely related to the phenomena involving a gas bubble rising in liquid. The measured terminal velocity was compared with the predictions of existing correlations that include the dimensionless numbers and showed reasonable agreement. The Particle Image Velocimetry (PIV) technique was applied to complement the experimental data set with high-fidelity measurements of the velocity field of the liquid molten salt surrounding the bubble. The paper presents a unique set of PIV and two-phase experimental data capturing the behavior of rising helium bubbles in molten FLiNaK and its surrounding flow field.

Pulse/Pulse Reverse Deposition of NiMo Alloys for Improved Molten Salt Reactor Performance

The use of molten fluoride salts as a primary coolant within next generation nuclear reactors has the potential to improve efficiency and nuclear reactor safety by operating at low pressures and high temperatures without boiling. However, these coolants will require the development of new corrosion resistant material systems that will have to meet or supersede existing standard codes for these systems. Therefore, state of the art reactors require validation and testing of new material systems that can produce robust component structures and enable them to withstand these corrosive environments. Furthermore, any next generation process to apply protective coatings must allow for a conformal, thick, dense, and bonded materials to be formed onto any low cost ASME material (like 316H SS) that comprises the structure of heat exchanger and containment vessel components.Within this context, Faraday Technology Inc. is working on developing low cost and high value corrosion-resistant alloy coatings for next generation molten salt reactors with the goal of increasing their functional lifetime, while reducing the component cost. The manufacturing process involves electrodeposition of functionally graded NiMo alloys onto ASME certified substrates and their subsequent high temperature corrosion evaluation in molten fluoride salt electrolytes. A wide array of electrolytes and processing parameters were evaluated in order to understand these effects on the deposit composition, structure, and corrosion resistance properties towards the goal of developing an ideal alloy coating. Specifically, we demonstrated the potential to apply graded NiMo coating to the 316H SS surface (Figure 1) and found that a coating composition of ~25% Mo has the potential to improve the components lifetime. Acknowledgements: The financial support of DOE Contract No. DE-SC0019602 is acknowledged. Figure 1: Top) show a cross-sectional image of a NiMo coated 316HSS component. Bottom) shows composition of Ni (Left/Dashed) and Mo (Right/Solid) as measured by EDX trace from the SEM along the red line (Top image). Figure 1

Recent Progress in Molten Salt Electrolysis Using a Liquid Metal Cathode and Vacuum Distillation for the Production of Ultra-High-Purity Mg Metal from MgO

In order to produce ultra-high-purity magnesium (Mg) metal from magnesium oxide (MgO), a novel and environmentally-sound process via molten salt electrolysis using a liquid metal cathode and vacuum distillation has been developed. The molten salt electrolysis of MgO in magnesium fluoride (MgF2) – lithium fluoride (LiF) molten salt is conducted at 1053 K using a tin (Sn), copper (Cu), or silver (Ag) cathode and a carbon (C) or platinum (Pt) anode for the production of Mg alloys. Subsequently, vacuum distillation of the obtained Mg alloys is carried out at 1200 – 1300 K to produce Mg metal with a purity of 99.999 %. In this study, the influence of the electrolysis temperature, concentration of Mg in the Mg alloys, and current density of the cathode on the faradaic efficiency was investigated. In addition, the reaction between the Mg alloys cathode and alumina (Al2O3) in contact with the cathode was analyzed. Recent results obtained by scale-up of the developed electrolytic and vacuum distillation processes and the use of secondary resources containing MgO as a feedstock for robustness and extension of the process will also be discussed. This study demonstrates the feasibility of an effective and environmentally-friendly Mg production process using MgO. Figure 1

Effect of high temperature molten salt corrosion on the microstructure of a Co-Mo-Cr-Si wear resistant alloy

The corrosion behavior of a novel low Cr content Laves phase strengthened wear-resistant alloy (Co-27.3Mo-2.9Cr-3.1Si) in molten FLiNaK salt (LiF–NaF–KF: 46.5–11.5–42 mol.%) was investigated by static immersion test at 700 °C. The microstructure and phase composition of the alloy after corrosion were characterized by scanning electron microscopy (SEM), electron probe micro-analysis (EPMA) and transmission electron microscopy with energy dispersive spectrometry (TEM/EDS). Results show that the alloy surface exhibit either pitted or roughened morphology after corrosion. The preferential corrosion of the Laves phase contributes to the pitting, whereas the formation of Co 3 Mo particles and Co layers contributes to the roughening. Carburization occurs in both the Laves phase and alloy matrix in the corrosion affected zone, forming Co 3 Mo 3 C carbides of different sizes. In addition, the thermal exposure effect during corrosion causes phase transformation in the entire alloy matrix and form Co 3 Mo stripes. The observed non-uniform corrosion is mainly induced by the trace amount of H 2 O in the salt, and it suggests that the corrosion resistance of the alloy is not satisfying in the test condition. • A novel low Cr content wear-resistant alloy was fabricated and its corrosion behavior in molten fluoride salt was tested. • Complex changes in microstructure induced by molten salt corrosion were characterized by XRD, SEM, TEM, EDS and EPMA methods. • The non-uniform corrosion of the alloy was mainly induced by the trace amount of water presented in the salt.

Steady-State and Transient Benchmarks of GOTHIC to the Molten Salt Reactor Experiment

The Molten Salt Reactor Experiment (MSRE), which operated at Oak Ridge National Laboratory from 1965 to 1969, was an experimental reactor that used UF4 fuel dissolved in molten fluoride salt. Criticality was achieved when the fuel salt mixture passed through the graphite-moderated core region. Therefore, because the fuel and fission products flowed through the system, delayed neutron precursors were not confined to the core, and decay heat was released outside the core, which is a unique challenge relative to more traditional reactor designs with solid fuel. Therefore, research and demonstration reactors such as MSRE have become a valuable source of information for benchmarking modeling and simulation tools for advanced reactor designs. One such tool being considered is GOTHIC, which is a coarse-grid computational fluid dynamics multiphysics software package. GOTHIC includes attributes and physical phenomena needed for modeling these advanced, non–light water reactor designs. For example, GOTHIC includes fluid property tables for various molten salts; a tracer-tracking module for modeling fission products and the radioactive decay and heat release by delayed neutron precursors locally in the fluid outside the core; and other necessary capabilities for modeling molten salt reactor (MSR) designs, including the ability to model dissolved gases. GOTHIC is used to benchmark steady-state and transient conditions from the MSRE. Zero-power physics testing included fuel salt pump start-up and coast-down transients with a control rod automatically moving to maintain criticality. The control rod motion calculated by GOTHIC is a reasonable match to measured data from these transients. Further, low-power testing included a natural convection transient with no control rod motion such that reactor power was responding to heat load demand from the radiator. The reactor power and fuel salt and coolant salt temperatures calculated by GOTHIC exhibit good agreement with measured data. These results confirm GOTHIC capabilities for modeling MSR designs with circulating fuel.