High Temperature Fluoride Research Articles

Stability of a Ni/NiF2 Reference Electrode with a Metallic Membrane for Use in Molten Fluoride Salts

In this study, an experimental high-temperature fluoride salt reference electrode (RE) developed by HiFunda LLC was tested in molten FLiNaK at 550 °C. The high-temperature reference electrode (HTRE), based on the Ni/NiF2 redox couple, was tested over a period of 13 d for short-term stability, long-term stability, and electrochemical analysis capabilities. The HTRE was tested using open circuit potentiometry (OCP), cyclic voltammetry (CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS). The HTRE reference potential was measured against changes in salt composition by increasing FeF2 concentrations in the melt across six additions from 0 to 0.1 mol% FeF2. The long-term stability of the electrode was then tested over ten days at a constant composition of 0.1 mol% FeF2. OCP, SWV, and CV were used to calculate an average potential drift between 7–10 mV per day. Using CV and SWV, the number of electrons transferred for iron reduction and the diffusion coefficient of Fe2+ were measured.

Molten Salt Pump Journal-Bearings Dynamic Characteristics Under Hydrodynamic Lubrication Conditions

Abstract A reliable high-temperature molten salt pump is critical for the development of Fluoride-salt-cooled, High-temperature Reactors (FHRs). By supporting the rotating journal, the appropriate journal bearing can ensure that the high-temperature molten salt pump runs smoothly and efficiently in the high-temperature fluoride salt over a long period of time. However, many bearing candidates served well for only a short period and experienced several issues. Moreover, the alignment of the molten salt pump journal bearings is a key factor for the molten salt pump's long-term steady running. In the long-term operation, a misalignment in the journal bearing can result in vibrations and excessive wear on the bearing surface of the molten salt pump. The journal bearing dynamic characteristics can be used to accurately assess the journal misalignment. Therefore, it is necessary to investigate the detailed journal-bearing dynamic behavior under the high-temperature hydrodynamic fluoride salt lubrication conditions for FHR applications. In this study, a small amplitude vibration is superimposed on a steady-running journal bearing to simulate the molten salt operating conditions. A Fortran 90 code has been written for the journal-bearing dynamic behavior analysis. The code was verified using the numerical data reported in the literature. The code is then employed to predict the dynamic coefficients of high-temperature fluoride salt hydrodynamic lubricated journal-bearing with various Sommerfeld numbers. These journal-bearing dynamic coefficients can be used to provide guidelines in the design of molten salt pumps.

Numerical Investigations of Molten Salt Pump Journal Bearings Under Hydrodynamic Lubrication Conditions for FHRs

Fluoride salt-cooled, High-temperature Reactors (FHRs), featuring particle fuel, graphite moderator, and molten fluoride salt coolant, are used for electricity generation and process heat applications. The primary loop of an FHR is a closed loop that operates slightly above the atmospheric pressure with the fluoride salt temperature over 600°C. Reliable high-temperature molten salt pumps are critical to the successful deployment of FHRs. To stabilize rotating shafts and reduce the associated friction coefficients, well-designed bearings are required for molten salt pumps. Therefore, it is necessary to investigate the detailed hydrodynamic performance of bearings under high-temperature molten salt conditions. In this study, a computational fluid dynamics software package, i.e., STAR-CCM+, was used to predict the performance of fluoride salt–lubricated bearings. The numerical models were verified and validated respectively based on an analytical solution derived from the Reynolds equation and experimental data published in the literature. Good agreement was observed between the simulation results and the analytical solution and experimental data with a maximum relative discrepancy of less than 5%. The validated numerical model was then employed to predict the pressure distributions, applied static loads, and power losses of high-temperature fluoride salt–lubricated bearings with various Sommerfeld numbers. In addition, a parametric analysis was performed to investigate the influence of the axial and helical grooves of bearings on applied static load and power loss. It is found that under the same salt lubrication conditions, the bearings with helical grooves and axial grooves respectively yield 20% off and 14% off power loss compared with the bearing without grooves.

Transition from 2D to 3D SBA-15 by High-Temperature Fluoride Addition and its Impact on the Surface Reactivity Probed by Isopropanol Conversion.

A systematic variation of the SBA‐15 synthesis conditions and their impact on the structural and chemical characteristics are reported. An incremental alteration of the hydrothermal aging temperature and time was used to induce changes of the highly ordered SBA‐15 structure. Any effects on the total surface area, mesopores size, micropore contributions, and pore connectivity are amplified by a combined incremental increase of the NH4F concentration. Based on changes of the unit‐cell parameter as a function of the mesopore size, and a feature in the low‐angle XRD pattern, useful descriptors for the disorder of the corresponding SBA‐15 are identified. An additional analysis of the Brunauer–Emmett–Teller (BET) surface area and pore size distributions enables investigations of the structural integrity of the material. This systematic approach allows the identification of coherencies between the evolution of physical SBA‐15 properties. The obtained correlations of the surface and structural characteristics allow the discrimination between highly ordered 2D SBA‐15, disordered 3D SBA‐15, and highly nonuniform silica fractions with mainly amorphous character. The fluoride‐induced disintegration of the silica structure under hydrothermal conditions was also verified by TEM. A direct influence of the structural adaption on the chemical properties of the surface was demonstrated by isopropanol conversion and H/D exchange monitored by FTIR analysis as sensitive probes for acid and redox active surface sites.

The local structure of Ta(v) aqua ions in high temperature fluoride- and chloride-bearing solutions: Implications for Ta transport in granite-related postmagmatic fluids

Abstract The local structure of Ta(V) in high-temperature fluoride- and chloride-bearing acidic solutions was investigated using in situ X-ray absorption spectroscopy (XAS). All XAS spectra were collected from two solutions, designated A and B, at beamline ID-20-C at the Advanced Photon Source, Argonne National Laboratory. Spectra were collected from solution A at 350 and 400 °C and from solution B at 25, 360, and 400 °C after the solutions were sealed in a hydrothermal diamond anvil cell. Solution A was prepared by dissolving Ta2O5 powder in 5% HF solution; solution B consisted of TaCl5 dissolved in 2% HF. The dominant tantalum species in solution A at elevated temperatures was TaF83–. In contrast, TaCl6–, which was the dominant complex in solution B at room temperature, disappeared as hydroxide complexes with an average ligand number between 5 and 7 became the dominant species at 350 and 400 °C. The XAS results confirm the previously recognized effect of fluoride activity on Ta speciation in hydrothermal fluids and suggest that both fluoride and hydroxide complexes play an important role in the transport of Ta in acidic fluoride-bearing solutions involved in the formation of mineralized mica-rich replacement units in granitic pegmatites.

A comparative study of surface layer formation in Ni-based alloys with varying Cr contents exposed to high temperature fluoride environment

Microstructural and surface characterization of Ni-based alloys viz. Alloy 690, Alloy 693 and Ni-17 wt.% Mo- 7wt.% Cr exposed to molten FLiNaK salt under air atmosphere at 973 K was carried out. Patches of mixed oxide layers of type NiO + Cr2O3, NiO + Al2O3 +Cr2O3 and MoO3 + NiO + Cr2O3 were observed for Alloy 690, alloy 693 and Ni-Mo-Cr alloy, respectively. Corrosion susceptibility index and calculated Cr diffusivity in the alloys followed the order: Alloy 690 > Alloy 693>Ni-Cr-Mo alloy. The oxide layers developed are only partially protective as they form a galvanic micro-couple with exposed surface which makes the later more prone to attack. The lower Cr depletion in case of Ni-Mo-Cr alloy as compared to Alloy 690 and 693 can be attributed to the fact that MoO3 layer is formed over the areas devoid of oxide patches which may prevent/reduce galvanic coupling between Ni-,Cr-rich oxides and exposed surface.

Electrochemical Studies of Hydrogen in LiF-BeF2 (FLiBe)

Fluoride-salt cooled High-temperature Reactors (FHR) use a 2LiF-BeF2 salt mixture (FLiBe) as a primary coolant for a graphite pebble-bed-fueled core. Tritium management is a challenge for the design of FHR reactors: tritium is produced through the transmutation of Lithium-6 in a neutron environment. The amount of tritium produced in FHR is about 1000 times larger than that for current commercial pressurize water reactors and it can readily permeate through the heat exchanger walls due to its high diffusivity in metals. Understanding the tritium transport properties from FLiBe solution to the reactor materials as a function of tritium oxidation state is essential for the design of FHR reactors. Hydrogen isotope solubilities in FLiBe were studied with saturation and stripping experiments. The solubilities values for hydrogen isotopes in molecular and fluoride form were obtained from those experiments [1][2]. Hydrogen isotope diffusivities in FLiBe were studied with permeation experiments. It was observed that hydrogen oxidation state in the salt affects the diffusivity values. The oxidation state is known to be dependent on the salt chemistry. Electrochemistry techniques could allow the study of hydrogen isotope transport properties in FLiBe solution. Hydrogen evolution in high-temperature molten salt has been studied electrochemically for tritium recovery in fusion applications [3][4], for fluorine gas production [5] and for fundamental chemistry studies [6]. However, no electrochemical studies are available in the literature on hydrogen isotopes in FLiBe. This paper reviews hydrogen studies in high-temperature fluoride salts and introduces the ongoing experimental research at UW Madison. The experimental work focuses on understanding the behavior of tritium in molten FLiBe using hydrogen as a surrogate for tritium. The paper puts emphasis on the experimental choices made to design the electrochemical cell to study hydrogen in FLiBe. Thermodynamic predictions of the hydrogen reduction potential are provided, and preliminary experimental results on hydrogen evolution are discussed.

Corrosion Behavior of SiCf/SiC Composites in High Temperature Fluoride Salt Environment

Corrosion Behavior of SiC_f/SiC Composites in High Temperature Fluoride Salt Environment

Hydrogen Diffusion and Trapping in Fluoride Salt Melts and Graphite Electrodes

This paper presents an approach currently under development at University of Wisconsin Madison to study tritium transport in high temperature fluoride salt systems and absorption in graphite. This work has applicability to tritium management in Fluoride Salt Cooled High Temperature Nuclear Reactors (FHRs). The applicability of the Double Potential Step Method (DSPM) and Electrochemical Impendence Spectroscopy (EIS) originally developed for the study of hydrogen transport in metals from water solutions is discussed for the salt-graphite system. Mechanistic models for hydrogen transport and trapping in graphite are reviewed. The experimental setup and electrochemical cell used with 2LiF-BeF2 (flibe) salt at 500-700°C is described. The design and preliminary characterization data for a Ni/Ni(II) flibe thermodynamic reference electrode are presented.

Thermal conductivity of high temperature fluoride molten salt determined by laser flash technique

The thermal conductivity of high temperature liquid fluoride salt was measured by laser flash technique for the first time. A graphite crucible was specially designed to exclude the dissolved gas and avoid overflowing of molten salt, meanwhile, a special treat process was used for preparing homogeneous sample. The combined model is feasible for data analysis. The reliability of laser flash method involving crucible structure, sample preparation and data analysis is verified by distilled water and pure KNO3 molten salt. Further, the thermal diffusivity of 46.5mol%LiF–11.5mol%NaF–42mol%KF (FLiNaK) molten salt is determined to be 1.64×10−3–2.48×10−3cm2/s at the temperature range of 773–973K with the blanket atmosphere of helium. Based on specific heat capacity determined by differential scanning calorimetry (DSC) as a constant of 1.88±0.08J/gK and density measured by our group, the thermal conductivity of FLiNaK molten salt is calculated to be 0.652–0.927W/mK with the uncertainty of ±0.023W/mK in the temperature range of 773–973K.

Electrochemistry of metals and semiconductors in fluoride media

The electrochemical surface transformations and diverse applications of a variety of metals and semiconductors in a wide range of fluoride media such as aqueous, non-aqueous media, liquid HF media, room temperature fluoride melts and molten fluoride media with a melting range covering 50–1000°C are reviewed. Nickel shows excellent corrosion resistance in the absence of water. The anodic performance of this metal in electrochemical perfluorination and NF3 production is discussed. Compact carbon materials serve as anodes in fluorine generators. In high temperature melts, they perform as consumable anodes. Graphitic carbon undergoes intercalation/de-intercalation process and related battery applications. Cu/CuF2 couple is a good reference electrode. Pt and vitreous carbon materials are the inert electrodes of choice for electro analytical applications. Electrodeposition of Lithium as a non-dendritic uniform phase is important in Lithium metal based secondary batteries. High temperature fluoride melts are used in electro-deposition of valve metals such as Nb, Ta, and Ti. The stability and decomposition of fluoride complexes in these media are of interest.

Experimental studies of the solubility of cassiterite and the extraction of tin from granitic melts

Experiments indicate that the solubility of cassiterite can be enhanced by increasing either acidity or alkalinity in hydrothermal solutions as a consequence of the duality of tin. The minimum solubility of cassiterite is found in neutral solutions. F- and CL-coordination compounds of Sn can alternate with hydroxy coordination compounds with changing pH in the solutions. In this case, F- and Cl- and OH- can be substituted with each other. The dissolution reaction of cassiterite is of reducing nature. High temperature and acidic reducing environment are favorable for the dissolution of cassiterite and the transport of Sn2+ compounds in fluids or solutions. High-temperature fluoride and chloride fluids can all dissolve, extract and enrich Sn to form F- and /or Cl-coordination compounds, However, F plays a more important role than G. F-coordination compounds are more stable and efficient than Cl-coordination compounds during the transport and enrichment of Sn in melts or solutions. The solubility of cassiterite and the amount of Sn extracted from granitic melt depend not only on T, P, pH and Eh in the fluids or solutions, but also on the amounts of dissociated F- and Cl- in the fluids.