Chloride Molten Salt Research Articles

Electrochemical Methods for Analysis of Hydroxide and Oxide Impurities in Li, Mg/Na, and Ca Based Molten Chloride Salts

Due to their highly hygroscopic nature, molten chloride salts are known to resist complete thermal dehydration, resulting in incomplete water removal prior to use in high temperature applications. At temperatures required to melt these salts, this can result in hydrolysis, leading to formation of oxides or hydroxides. Depending on application, these hydrolytic impurities can cause further adverse effects through the formation of insoluble oxides and oxychlorides. In this paper, previously published work is reviewed to discuss electrochemical methods used for identification and measurement of hydrolytic impurities in LiCl and CaCl2 as well as their effect on electrolytic processes. Cyclic voltammetry shows that thermal dehydration of LiCl and CaCl2 promotes LiOH and CaO formation, respectively. The presence of CaO in a CaCl2-CeCl3 salt meant to emulate a typical actinide-chloride electrorefining salt causes insoluble Ce2O3 and CeClO to form, reducing electrochemical signals as the active species is precipitated as an insoluble phase.

Neutron irradiation of alloy N and 316L stainless steel in contact with a molten chloride salt

Capsules containing NaCl–MgCl2 salt with 316L stainless steel or alloy N samples were irradiated in the Ohio State University Research Reactor for 21 nonconsecutive hours. A custom irradiation vessel was designed for this purpose, and details on its design and construction are given. Stainless steel samples that were irradiated during exposure had less corrosive attack than samples exposed to the same conditions without irradiation. Alloy N samples showed no significant effect of irradiation. This work shows a method for conducting in-reactor irradiation–corrosion experiments in static molten salts and presents preliminary data showing that neutron irradiation may decelerate corrosion of alloys in molten chloride salts. Index TermsMolten salt reactor, Corrosion, Irradiation, High-temperature experiment.

Th–U cycle performance analysis based on molten chloride salt and molten fluoride salt fast reactors

The recent development of molten salt fast reactors has generated a renewed interest in them. As compared to traditional solid fuel fast neutron systems, it has many unique advantages, e.g., lower fissile inventory, no initial criticality reserve, waste reduction, and a simplified fuel cycle. It has been recognized as an ideal reactor for achieving a closed Th–U cycle. Based on the carrier salt, molten salt fast reactors could be divided into either a molten chloride salt fast reactor (MCFR) or a molten fluoride salt fast reactor (MFFR); to compare their Th–U cycle performance, the neutronic parameters in a breeding and burning (BB at a reprocessing rate of 40 L/day, using LEU and Pu as start-up fissile materials, the doubling time (DT) of an MFFR and MCFR were 88.0 years and 48.0 years, and 16.5 years and 16.2 years, respectively. Besides, an MCFR has lower radio-toxicity due to lower buildup of fission products (FPs) and transuranium (TRU), while an MFFR has a larger, delayed neutron fraction with smaller changes during the entire operation.

Corrosion behaviors of 316 stainless steel and Inconel 625 alloy in chloride molten salts for solar energy storage

AbstractCorrosion behaviors of 316 stainless steel (316 ss) and Inconel 625 alloy in molten NaCl–KCl–ZnCl2 at 700°C and 900°C were investigated by immersion tests and electrochemical methods, including potentiodynamic polarization and electrochemical impedance spectroscopy. X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy were used to analyze the phases and microstructures of the corrosion products. Inconel 625 alloy and 316 ss exhibited high corrosion rates in molten chlorides, and the corrosion rates of these two alloys accelerated when the temperature increased from 700°C to 900°C. The results of the electrochemical tests showed that both alloys exhibited active corrosion in chloride molten salt, and the current density of 316 ss in chloride molten salt at 700°C was 2.756 mA/cm−2, which is about three times the value for Inconel 625 alloy; and the values of the charge transfer resistance (Rt) for Inconel 625 were larger than those for 316 ss. The corrosion of these two alloys is owing to the preferred oxidation of Cr in chloride molten salt, and the corrosion layer was mainly ZnCr2O4 which was loose and porous and showed poor adherence to metal.

Preparing High-Purity Anhydrous ScCl3 Molten Salt Using One-Step Rapid Heating Process

In this study, a one-step rapid heating novel process was used to prepare high-purity anhydrous scandium chloride molten salt with low-purity scandium oxide. High-purity anhydrous ScCl3 molten salt was used as the Sc-bearing raw material for preparing the Sc-bearing master alloy. Inert gas was used to enhance the purity of anhydrous scandium chloride and reduce the hydrolysis rate of scandium. The results show that high-purity scandium chloride (purity, 99.69%) with the scandium content of 29.61%, was obtained, and the hydrolysis rate of scandium was 1.19% under the conditions used: removing ammonium chloride; residual crystal water temperature of 400 °C; m(Sc2O3):m(NH4Cl) = 1:2.5; holding-time of 90 min; heating-rate of 12 °C/min; and argon flow of 7.5 L/min. XRD, SEM, and EPMA analyses further verified that anhydrous scandium chloride crystallization condition was relatively good and the purity of high-purity anhydrous scandium chloride approached the theory purity of anhydrous scandium chloride.

Potential scalability of a cost-effective purification method for MgCl2-Containing salts for next-generation concentrating solar power technologies

Next-generation concentrating solar power (CSP) technology requires a high-temperature heat-transfer fluid and thermal energy storage media. Molten MgCl2-KCl-NaCl is considered a potential candidate salt due to its thermophysical properties. However, MgCl2 presents various challenges because of its hygroscopic nature. To keep corrosion under control, this molten chloride needs to remain free of hydrates and other impurities. Here, we have developed an effective purification method for MgCl2-containing salts by 1) combining known laboratory-scale, batch-style thermal and chemical purification processes to enable scalable, continuous-style processes for commercial next-generation CSP operation; 2) improving overall efficiency and minimizing the major corrosive impurity MgOHCl by optimizing key engineering parameters such as heating temperature/time and amount of elemental Mg addition; and 3) investigating the addition of halite (NaCl) to carnallite (KMgCl3) to reduce the liquidus temperature. Laboratory-scale results suggest that 1) adding 6.5 wt% of halite and less than 0.1 wt% of elemental Mg to commercial carnallite and 2) following a heating schedule to at least 650 °C with ~3 h of holding time at that temperature can produce a ternary MgCl2-KCl-NaCl salt composition with a low liquidus temperature of about 400 °C. It also reduces the presence of the corrosive impurity, MgOHCl, from ~1 to 2 wt.% to ~0.1 wt%. Findings of these key engineering parameters should provide a pathway toward a scalable, continuous-style salt-purification process at a scale of metric tons per hour that can produce a corrosion-controlled chloride molten salt for CSP applications.

Cathodic Protection Using Aluminum Metal in Chloride Molten Salts as Thermal Energy Storage Material in Concentrating Solar Power Plants

The new generation of concentrated solar power (CSP) plants to be developed presents a great challenge related to the increase in maximum operating temperature since molten salt CSP technologies require alternative salt chemistries such as chloride. The cathodic protection strategy involves the addition of a sacrificial metal to prevent corrosion of the alloy tested as container material in a CSP plant. In this paper, aluminum (Al) metal was analyzed as a corrosion inhibitor in OCT and HR224 alloys, obtaining corrosion rates of 4.37 and 0.27 mm/y, respectively. It has been confirmed that the use of Al metal can reduce the anodic current which is directly related to the corrosion rate. The formation of protective alumina scales (Al2O3) was assessed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), confirming the corrosion model results from electrochemical impedance spectroscopy monitoring tests.

Thermal charge/discharge performance of iron–germanium alloys as phase change materials for solar latent heat storage at high temperatures

In this study, an iron–germanium alloy (Fe–Ge alloy) was examined as a phase change material at temperatures exceeding 800°C for thermal energy storage in solar thermal applications. The cyclic properties of the thermal charge/discharge of the Fe–Ge alloy were examined at various thermal cycles. A thermal reliability test was performed in the form of a multi-cyclic performance assessment to evaluate the short- and long-term thermal stability of this alloy. Eutectic and hypereutectic chemical compositions of the Fe–Ge alloy were placed in a graphite container under vacuum and an inert atmosphere. The Fe–Ge alloy was experimentally examined for its compatibility with the container material of graphite-carbon. The element distribution of each Fe–Ge alloy was examined via a cyclic performance test. The heat storage capacities were evaluated and compared with those of chloride molten salts. The Fe–Ge alloy exhibits significant potential as a latent heat storage material in next-generation solar thermal applications as it demonstrates various advantages, namely: a higher storage capacity than that of chloride molten salts, temperature followability to the phase diagram, a quick thermal response, satisfactory cyclic behavior of charge/discharge modes, a thermodynamically stable metallographic structure, and non-reactivity with capsulation materials of graphite carbon.

Fundamental Analysis for Electrochemical Extraction and Monitoring of Impurities from Lead Lithium and Lithium with Chloride Molten Salt

Liquid Li-Pb eutectic alloy and lithium, assumed to be used in a liquid blanket system for fusion reactors, can easily contain light element impurities. It is thought that these impurities have bad influences on compatibility with structural materials. However, an effective reducing method for impurities has not yet been established. In this work, an electrochemical method is focused on this application. The experimental results of our work suggest the transfer of hydrogen and nitrogen impurities. From the results, a possible way to remove the impurities by electrochemical method is shown.

A multiphysics model for analysis of inert gas bubbles in Molten Salt Fast Reactor

Molten salt reactors (MSRs) have gained worldwide interest in recent years due to their appealing safety and resource utilisation characteristics. These reactors have a unique feature, i.e. the presence of nuclear fuel in the form of a molten fluoride or chloride salt containing the fissile and fertile materials. The fuel salt also acts as the coolant, and this dual role results in a complex, highly coupled multiphysics system which poses a challenge in modelling and simulation of MSRs. This paper presents the development of a simulation model for the Molten Salt Fast Reactor (MSFR) to predict the behaviour of inert gas bubbles in the core and to quantify their impact on the reactivity. Inert gas bubbles in MSFR have been modelled using a multiphysics approach coupling computational fluid dynamics for fluid flow and heat transfer with neutron diffusion equation for neutronics and a balance equation with diffusion and advection terms for taking into account the drift of the delayed neutron precursors. The two-phase flow has been modelled using a simplified Euler–Euler model for small volume fraction of the dispersed phase, i.e. for small bubble fraction, which combines the momentum and continuity equation of the liquid and gas phases and adds a gas-phase transport equation to track the void fraction. Simulations reveal that the bubble distribution in the core has a significant impact on reactivity resulting in a difference in the bubbling feedback coefficient compared to studies using a homogeneous distribution.

Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part 2: Materials screening performance

The operating temperature of a steam turbine is limited to 565 °C by the molten nitrate heat-transfer fluid; therefore, molten-salt CSP technologies require alternative salt chemistries such as chloride. The prevention of high-temperature corrosion on containment materials plays a critical role, and a corrosion mitigation plan is needed to achieve the target plant lifetime of 30 years. This paper performed a materials screening test, using a eutectic ternary chloride molten salt, composed by 20.4 wt. % KCl + 55.1 wt. % MgCl2 + 24.5 wt. % NaCl, in stainless steel (AISI 304) and two Ni base materials (Inconel 702 and Haynes 224). The corrosion mechanism and corrosion rates were obtained through electrochemical impedance spectroscopy (EIS). Ni base alloys showed a protective scale layer during 8 hours of immersion with a corrosion rate of 6.34 mm/year (In702) and 3.12 mm/year (HR224). Monitoring corrosion results were confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), obtaining alumina and chromia protective layers.

High temperature molten salt corrosion of structural materials in UCl3–LiCl–KCl

ABSTRACT High temperature corrosion behaviour of 2.25Cr–1Mo, 9Cr–1Mo and SS 410 in molten UCl3–LiCl–KCl salt at 500 and 600°C under inert and reactive atmospheres for 24 h were studied by thermogravimetry. The results indicate marginal weight gains in all the alloys. In all conditions, except at 600°C under reactive atmosphere, the weight gains were higher in initial period of exposure. The weight gain was continuous for samples studied at 600°C under reactive atmosphere. The 2.25Cr–1Mo showed higher weight gain in an inert atmosphere, whereas SS 410 showed more weight gain at 500°C and 9Cr–1Mo at 600°C under reactive atmosphere. The weight gains were attributed to the formation of γ-FeOOH, α-Fe2O3, Fe3O4, FeCr2O4 and Cr2O3 as confirmed by Raman spectra analysis. The oxide films formed are neither uniform as evidenced from SEM-EDS analysis nor protective, attributed to the corrosive and aggressive nature of molten chloride salts.

Fundamental analysis for electrochemical removal and monitoring of oxide impurities in lead lithium using LiCl-KCl melt

• The liquid LiPb alloy is expected to be used in the blanket system. • The oxide impurities in the LiPb is forcused in this research. • LiCl-KCl is useful for the transfer of oxide impurities. • The oxide impurities in the molten salt can be extracted by applying voltage. The electrochemical treatment using chloride molten salt was focused as a method for removal and monitoring of oxide impurities in the liquid breeder materials. The anodic reactions on a glassy carbon electrode in a LiCl-KCl eutectic melt contacting with Li-Pb eutectic melt without and with Li 2 O addition were investigated. The transfer of oxide impurities to the molten salt from liquid metal was proved by the change of the concentration of oxide ions in the molten salt estimated from the cyclic voltammetry. The content of oxide impurities in the Li-Pb can be estimated by calculating backward from the transfer ratio and this technique may also be applied to monitoring. The visual observation and the weight loss of the anode indicated that the oxide impurities transferred to the molten salt could be removed as gases such as CO 2 by applying a voltage between an anodic glassy carbon rod in the molten salt and the Li-Pb used as a cathode.

Effect of $$^{{\textbf{37}}}{\textbf{Cl}}$$ enrichment on neutrons in a molten chloride salt fast reactor

A molten chloride salt fast reactor (MCFR) is well suited to fuel breeding and the transmutation of transuranium (TRU) elements owing to its advantageous features of fast neutron spectrum and high TRU solubility. However, the neutron absorption cross section of $$^{35}{\hbox {Cl}}$$ is approximately 1000 times greater than for $$^{37}{\hbox {Cl}}$$, which has a significant impact on the neutron physical characteristics of a MCFR. Based on an automatic online refueling and reprocessing procedure, the influences of $$^{37}{\hbox {Cl}}$$ enrichment on neutron economy, breeding performance, and the production of harmful nuclides were analyzed. Results show that $$^{37}{\hbox {Cl}}$$ enrichment strongly influences the neutron properties of a MCFR. With natural chlorine, $$^{233}{\hbox {U}}$$ breeding cannot be achieved and the yields of S and $$^{36}{\hbox {Cl}}$$ are very high. Increasing the $${^{37}{\hbox {Cl}}}$$ enrichment to 97% brings a clear improvement in its neutronics property, making it almost equal to that corresponding to 100% enrichment. Moreover, when $${^{37}{\hbox {Cl}}}$$ is enriched to 99%, its neutronics parameters are almost the same as for 100% enrichment. Considering the enrichment cost and the neutron properties, a $$^{37}{\hbox {Cl}}$$ enrichment of 97% is recommended. Achieving an optimal neutronics performance requires 99% $$^{37}{\hbox {Cl}}$$ enrichment.

Heat and mass transportation properties of binary chloride salt as a high-temperature heat storage and transfer media

Molten binary chloride salts are considered as promising heat transfer and storage mediums in a Concentrating Solar Power (CSP) Station. The properties of them over the entire operating temperature are required to be calculated, since they are difficult to be measured under high-temperature conditions. Reliable numerical method is an alternative tool to accurately predict thermal properties of molten salts from the viewpoint of micro-structure. Based on the effective Born-Mayer-Huggins (BMH) pair potential model, the local structures and thermodynamic properties of lithium-potassium chloride were computed by using Molecular Dynamics simulations (MD) and the results are consistent with available experimental literature data. The simulation results prove that BMH force field is reasonable to be adopted to predict the properties of various molten chloride salts and their mixtures. The minimum errors of thermal properties above are all within 11.1%, which is accurate and reliable. The results show that Li+ promotes the diffusion of the system efficiently. In addition, the specific heat capacity and thermal conductivity are significantly improved with the increase of Li+. The database of thermal properties and structures of this work are significant for accurate design of material structure and regulation of thermal storage system performance.

Anodic Protection Assessment Using Alumina-Forming Alloys in Chloride Molten Salt for CSP Plants

The generation of a natural protective coating in alumina-forming alloys was analyzed using a ternary chloride molten salt as a thermal energy storage material for concentrated solar power (CSP) technology. The formation of the protective layer was monitored using electrochemical impedance spectroscopy (EIS). A protective layer model for the OCT alloy and a localized corrosion model for the OCI and OC4 alloys were obtained after 5 h of immersion. The corrosion rates calculated using the linear polarization technique (LPR), were 8.03, 21.55, and 7.61 mm/year for OC4, OCI, and OCT alloys, respectively. These results were confirmed by scanning electron microscopy and X-ray diffraction. Our analysis showed that MgAl2O4 was the main protective coating generated by the alumina-forming alloys.

Selection About Molten Salts Used For The Electrochemical Reduction of Porous TiO2 pellets

Diagrams of the equilibrium potential E (relative to the standard chlorine electrode) against the activity of oxide ion ( expressed as its logarithm, pO2−) in systems of Ti-O-Cl-M (M=Li, Na, Mg, Ca, Sr and Ba) were plotted to investigate the electro-deoxidation of porous TiO2 pellets in different chloride molten salts, including LiCl, NaCl, MgCl2, CaCl2, SrCl2 and BaCl2. From the diagrams, the reduction pathways of TiO2 in different molten salts are similar; and the applied potential (vs Cl−/Cl2) on the cathode and O2− ion activity influence greatly on the reduction pathway of TiO2 and the deoxidation limit. To test and verify the results of the thermodynamic analysis, electro-deoxidation experiments in different molten salts were done to determine the actual pathways in the electrochemical reduction of TiO2 and the possible deoxidation limits in the molten salts. The results indicated that the thermodynamic analysis is helpful in predicting and understanding the electrochemical reduction process of titanium oxides though the experimental results differ slightly from the thermodynamic analysis in some systems. Low-oxygen Ti can be obtained only in molten CaCl2.

Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part 1: Thermal treatment assessment

Abstract The operating temperature of a steam turbine is limited to 565 oC by the molten nitrate heat-transfer fluid; therefore, a new molten salt chemistry is needed to increase the maximum operating temperature in the new generation of CSP plants and improve the thermal-to-electrical energy conversion efficiency in the turbine block, such as chloride molten salts. Nevertheless, the prevention of high-temperature corrosion on containment materials using chlorides plays a critical role and a corrosion mitigation plan is needed to achieve the target plant lifetime of 30 years. This paper presents a corrosion mitigation strategy focused on different thermal treatments performed in the eutectic ternary chloride molten salt composed by MgCl2/NaCl/KCl (55.1 wt.%/24.5 wt.%/20.4 wt.%). Corrosion rates were obtained through linear polarization resistance technique in a conventional commercial stainless steel (AISI 304) at 720 oC during 5 h of immersion after the different thermal treatments carried out. Scanning electron microscopy and XRD analysis were used to confirm the corrosion rates and corrosion layer proposed by electrochemical techniques, obtaining a minimum corrosion rate of 6.033 mm/year for the best thermal treatment performed.

Study of optical behaviour of Eu3+ and Tb3+ doped zirconate perovskite phosphors prepared by molten salt technique

This paper gives a keen insight of the study of spectroscopic and optical behavior of Eu3+ and Tb3+ doped calcium zirconate, barium zirconate and strontium zirconate perovskite phosphors synthesized by molten salt synthesis (MSS) technique. Here, the low temperature MSS technique uses molten chloride salt sodium chloride NaCl that increases solid–solid diffusion rate for creating an active reaction medium. These phosphors were subjected to sintering at a temperature of 950 °C with Eu3+ and Tb3+ ions doping concentration varying from 0 to 2 mol%. The Powder X-ray diffractometry technique was used to study of crystallographic behavior of synthesized phosphors. In order to study the morphological and topographical aspects, field-emission scanning electron microscopy and transmission electron microscopy analyses were done. The Fourier transform infrared analysis generates the fingerprint of the synthesized phosphors confirming the presence of all constituent elements. The photoluminescent (PL) characteristics of synthesized phosphors were studied on exciting CaZrO3:Eu3+, Tb3+ with 225 nm, BaZrO3:Eu3+, Tb3+ with 229 nm and SrZrO3:Eu3+, Tb3+ with 235 nm excitation wavelength. The PL emission spectra show variable emissions over a broad spectral region corresponding to 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 hypersensitive electronic transitions of Tb3+ ions along with 5D0–7F0, 5D0–7F1 and 5D0–7F2 induced electric dipole and magnetic dipole transitions of Eu3+ ions. These color tunable phosphors with increased PL output confirm their suitability for photonics, optoelectronics and display device applications.

Characterization of corrosion resistance of C/C\u2013SiC composite in molten chloride mixture MgCl2/NaCl/KCl at 700\u2009\xb0C

Due to their high thermal stability and low cost, molten chlorides are promising high-temperature fluids for example for thermal energy storage (TES) and heat transfer fluid (HTF) materials in concentrated solar power (CSP) plants and other applications. However, the commercial application of molten chlorides is strongly limited due to their strong corrosivity against commercial alloys at high temperatures. The work addresses on a fundamental level whether carbon based composite ceramics could be potentially utilized for some corrosion critical components. Liquid silicon infiltration (LSI) based carbon fiber reinforced silicon carbide (called C/C–SiC) composite is immersed in a molten chloride salt (MgCl2/NaCl/KCl 60/20/20 mole%) at 700 °C for 500 h under argon atmosphere. The material properties and microstructure of the C/C–SiC composite with and without exposure in the molten chloride salt have been investigated through mechanical testing and analysis with scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) scanning. The results reveal that the C/C–SiC composite maintains its mechanical properties after exposure in the strongly corrosive molten chloride salt. The oxidizing impurities in the molten salt react only with residual elemental silicon (Si) in the area of the C/C–SiC matrix. In comparison, no indication of reaction between the molten chloride salt and carbon fiber or SiC in the matrix is observed. In conclusion, the investigated C/C–SiC composite has a sound application potential as a structural material for high-temperature TES and HTF with molten chlorides due to its excellent corrosion resistance and favorable mechanical properties at high temperatures.