Molten Fluoride Salts Research Articles

Improving molten fluoride salt and Xe135 barrier property of nuclear graphite by phenolic resin impregnation process

A densification process has been conducted on isostatic graphite (IG-110, TOYO TANSO CO., Ltd., Japan) by impregnating phenolic resin to get the densified isostatic graphite (D-IG-110) with pore diameter of nearly 11 nm specifically for molten salt reactor application. The microstructure, mechanical, thermophysical and other properties of graphite were systematically investigated and compared before and after the densification process. The molten fluoride salt and Xe135 penetration in the graphite were evaluated in a high-pressure reactor and a vacuum device, respectively. Results indicated that D-IG-110 exhibited improved properties including infiltration resistance to molten fluoride salt and Xe135 as compared to IG-110 due to its low porosity of 2.8%, the average pore diameter of 11 nm and even smaller open pores on the surface of the graphite. The fluoride salt infiltration amount of IG-110 was 13.5 wt% under 1.5 atm and tended to be saturated under 3 atm with the fluoride salt occupation of 14.8 wt%. As to the D-IG-110, no salts could be detected even up to 10 atm attempted loading. The helium diffusion coefficient of D-IG-110 was 6.92 × 10−8 cm2/s, significantly less than 1.21 × 10−2 cm2/s of IG-110. If these as-produced properties for impregnated D-IG-110 could be retained during MSR operation, the material could prove effective at inhibiting molten fluoride salt and Xe135 inventories in the graphite.

Measurement of europium (III)/europium (II) couple in fluoride molten salt for redox control in a molten salt reactor concept

The fluoride molten salt such as FLiNaK and FLiBe is one of the coolant candidates for the next generation nuclear reactor concepts, for example, the fluoride salt cooled high temperature reactor (FHR). For mitigating corrosion of structural materials in molten fluoride salt, the redox condition of the salts needs to be monitored and controlled. This study investigates the feasibility of applying the Eu3+/Eu2+ couple for redox control. Cyclic voltammetry measurements of the Eu3+/Eu2+ couple were able to obtain the concentrations ratio of Eu3+/Eu2+ in the melt. Additionally, the formal standard potential of Eu3+/Eu2+ was characterized over the FHR's operating temperatures allowing for the application of the Nernst equation to establish a Eu3+/Eu2+ concentration ratio below 0.05 to prevent corrosion of candidate structural materials. A platinum quasi-reference electrode with potential calibrated by potassium reduction potential is shown as reliable for the redox potential measurement. These results show that the Eu3+/Eu2+ couple is a feasible redox buffering agent to control the redox condition in molten fluoride salts.

Scaled experiment investigating sonomechanically enhanced inert gas sparging mass transfer

One of the leading advanced reactor concepts is the fluoride salt high temperature reactor (FHR) currently under investigation. This design utilizes a fluoride molten salt, flibe (2LiF/BeF2), as the primary coolant. One challenge of employing flibe as a coolant is the production and release of tritium. The FHR community is currently investigating various techniques to solve this tritium management challenge. One of the methods investigated is inert gas sparging which has been investigated during the Molten Salt Reactor Experiment (MSRE) in the 1960’s and 1970’s. To enhance the efficiency of this technique, high power ultrasonics can be employed to sonomechanically enhance the mass transfer performance. Initial experimental studies coupling ultrasonics and inert gas sparging have been performed in the scaled ultrasonic mass transfer (SUMATRA) experiment. The scaling was done by matching the Sc between a water/glycerol mixture and flibe at FHR temperatures. The SUMATRA experiment evaluated the performance of inert gas sparging with and without ultrasonic enhancement. The results show a significant performance enhancement due to the sonomechanical effect which is contributed to an increase in the diffusive mass transfer contribution due to bubble breakup and the subsequent increase interfacial area.

Hydrogen Permeation Through Fluoride Molten Salt Mixed with Ti Powder

H2 permeation behavior in a new molten salt of Flibe mixed with 0.5 wt% Ti powder is experimentally and analytically investigated. Ti powder included in Flibe can suppress H2 permeation for 70 hours lower than the H2 detection limit after the start of experiment. H2 diffusion behavior in the Flibe+Ti system is analyzed using an unsteady-diffusion equation. The time delay of overall H2 diffusion in the Flibe+Ti system is over 200 times larger than that in Flibe. The effective diffusivity and solubility in the Flibe+Ti system are determined from fitting the analytical solution to experimental H2 permeation rates.

Effects of bubbles on high-temperature corrosion of helium ion-irradiated Ni-based alloy in fluoride molten salt

Samples of a Ni-Mo-Cr-Fe-Si alloy subject to helium ion irradiation damage were corroded in a eutectic mixture of FLiNaK at 750°C. It was found that the helium bubbles that formed increased the number of surface defects so greatly increasing the physical contact area between the alloy and the molten salt, resulting in acceleration of corrosion damage. Segregation and depletion of chemical elements at bubble surfaces were also identified, and became more severe with increasing bubble size. Significant segregation of Si resulted in the formation of Ni-Si precipitates at large bubbles. This segregation enhanced the chemical corrosion damage to the alloy.

Synthesis of UF4 and ThF4 by HF gas fluorination and re-determination of the UF4 melting point

Basic thermodynamic and electrochemical data of pure actinide fluorides and their mixtures are required for the design and safety assessment of any presently studied molten salt reactor concept based on molten fluoride salt fuel. Since the actinide fluorides are usually not produced commercially, they have to be prepared from the available input materials, typically oxides. In this work, a specially designed facility for synthesis of pure actinide fluorides using pure HF gas is described, as well as a complete procedure of synthesis and characterisation of pure UF4 and ThF4. The fluorination installation consists of a glove box kept under a purified argon atmosphere, a high temperature horizontal fluorination reactor and a HF supply gas line connected to the glove box. The fluorides were synthesised from high specific surface oxides prepared from the respective oxalates by low temperature calcination. The fluorination was partly stationary and partly in a HF gas flow, based on a heterogeneous powder-gas reaction at high temperatures. The products were characterised by X-ray diffraction and differential scanning calorimetry, which confirmed high purity products obtained by this method. Moreover, the melting point of UF4 was revised using a pure sample and a new value is suggested.

Use of Nitrogen Trifluoride To Purify Molten Salt Reactor Coolant and Heat Transfer Fluoride Salts

The molten-salt-cooled nuclear reactor is one of the Generation IV reactor types. One of the challenges of implementing this reactor is purifying and maintaining the purity of the various molten fluoride salts that will be used as coolants. The method used for Oak Ridge National Laboratory’s molten salt experimental test reactor was to treat the coolant with a mixture of H2 and HF at 600 °C. In this Article, we evaluate thermal NF3 treatment for purifying molten fluoride salt coolant candidates based on nitrogen trifluoride’s (1) past use to purify fluoride salts; (2) other industrial uses; (3) commercial availability; (4) operational, chemical, and health hazards; (5) environmental effects and environmental risk management methods; (6) corrosive properties; (7) thermodynamic potential to eliminate impurities that could arise due to exposure to water and oxygen. Our evaluation indicates that nitrogen trifluoride is a viable and safer alternative to the previous method.

Electrochemical Extraction of Rare Earth Metals in Molten Fluorides: Conversion of Rare Earth Oxides into Rare Earth Fluorides Using Fluoride Additives

In the present research on rare earth extraction from rare earth oxides (REOs), conversion of rare earth oxides into rare earth fluorides with fluoride fluxes is investigated in order to overcome the problem of low solubility of the rare earth oxides in molten fluoride salts as well as the formation of oxyfluorides in the fluorination process. Based on thermodynamic calculations, a series of experiments were performed for converting the rare earth oxides into rare earth fluorides using AlF3, ZnF2, FeF3, and Na3AlF6 as fluorinating agents in a LiF–Nd2O3 system. The formation of neodymium fluoride as a result of the reactions between these fluxes and neodymium oxide is confirmed. The rare earth fluoride thus formed can subsequently be processed through the electrolysis route in the same reactor, and rare earth metal can be produced as the cathodic deposit. In this concept, the REO dissolution in molten fluorides would become unnecessary due to the complete conversion of the oxide into the fluoride, REF3. The results of XRD and EPMA analysis of the reacted samples indicate that AlF3, ZnF2, and FeF3 can act as strong fluorinating agents for the neodymium oxide giving rise to a complete conversion of neodymium oxide into neodymium fluoride.

Molten fluoride salt and liquid metal multistage extraction model

A multistage extraction model based on the distribution of actinides and rare earths between two phases of lithium activated bismuth and molten LiFBeF2 (66 mole % LiF) has been developed. The model shows that the distribution across the cascade is affected by the concentration of Li in the metal phase, system temperature, flow rates of each phase, and the presence and concentration of other elements in the molten salt phase. Based on the model, it is predicted that an online molten salt coolant purification system could be used to separate both rare earths and actinides from the primary salt coolant in fluoride salt-cooled high-temperature reactors (FHRs). Additionally, the results provide sufficient evidence that this extraction method can separate primarily actinides prior to extraction of other rare earth elements, which would allow for fission products and other reactor poisons to be discarded, while fuel materials (U and Pu) can be recycled back into a reactor system.

The corrosion behavior of CVI SiC matrix in SiCf/SiC composites under molten fluoride salt environment

High temperature corrosion behavior and microstructural evolution of designed chemical-vapor-infiltrated SiC matrix in SiC fiber reinforced SiC ceramic matrix composites in 46.5LiF-11.5NaF-42.0KF (mol. %) eutectic salt at 800 °C for various corrosion time was studied. Worse damage was observed as extending the exposure time, with the mass loss ratio increasing from 0.716 wt. % for 50 h to 5.914 wt. % for 500 h. The mass loss rate showed a trend of first decrease and then increase with the extended corrosion exposure. Compared with the near-stoichiometric SiC matrix layers, the O-contained boundaries between deposited matrix layers and the designed Si-rich SiC matrix layers were much less corrosion resistant and preferentially corroded. Liner relationship between the mass loss ratio and the corrosion time obtained from 50 h to 300 h indicated that the corrosion action was reaction-control process. Further corrosion would lead to matrix layer exfoliation and higher mass loss ratio.

Graphite Foil-Incorporated PAN/Pitch/Phenolic-Derived Carbon/Carbon Composite and Preliminary Hermetic Sealing Test in Molten Fluoride Salt

Graphite Foil-Incorporated PAN/Pitch/Phenolic-Derived Carbon/Carbon Composite and Preliminary Hermetic Sealing Test in Molten Fluoride Salt

Electrochemical Behavior of Dy(III) and Dy-Fe Alloys Preparation by Co-Reduction in Molten Fluoride Salts

This work focuses on the electrochemical behavior of Dy(III) and the preparation of Dy-Fe alloy by its co-reduction with Fe(III) on inert tungsten electrode in fluoride molten salts at 1223 K. Electrochemical techniques, such as cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry were carried out to investigate the cathodic reduction mechanism of Dy(III) ions in LiF-CaF2DyF3 melts. The results prove that the electrochemical reduction process of Dy(III) ions in the melts is a reversible process within the scan rate of 300 mV·s−1 in a one-step process exchanging three electrons.. The diffusion coefficient of Dy(III) in the melts at 1223 K is 1.55 × 10−5 cm2·s−1. Small amount of dispersed deposits composed of DyFe2, DyFe3 and Dy were obtained by potentiostatic electrolysis at 0.1 vs (Li/Li+)/V. Galvanostatic electrolysis at 8 A·cm−2 was also carried out on a W plate in LiF-CaF2-DyF3-Dy2O3 (3 wt%)-Fe2O3 (1 wt%) system for 30 min. Spherical Dy-Fe alloys were obtained with uniform Dy and Fe elements distribution and high molar ratio of Dy. The results indicate that it is feasible to prepare Dy-Fe alloys by electrochemical co-reduction of Dy(III) and Fe(III) on inert tungsten electrode in fluoride melts at 1223 K.

Recovery of Zirconium from Spent Pickling Acid through Precipitation Using BaF2 and Electrowinning in Fluoride Molten Salt

Zirconium(Zr) nuclear fuel cladding tubes are made using a three-time pilgering and annealing process. In order to remove the oxidized layer and impurities on the surface of the tube, a pickling process is required. Zr is dissolved in HF and HNO3 mixed acid during the process and pickling waste acid, including dissolved Zr, is totally discarded after being neutralized. In this study, the waste acid was recycled by adding BaF2, which reacted with the Zr ion involved in the waste acid; Ba2ZrF8 was subsequently precipitated due to its low solubility in water. It is very difficult to extract zirconium from the as-recovered Ba2ZrF8 because its melting temperature is 1031 °C. Hence, we tried to recover Zr using an electrowinning process with a low temperature molten salt compound that was fabricated by adding ZrF4 to Ba2ZrF8 to decrease the melting point. Change of the Zr redox potential was observed using cyclic voltammetry; the voltage change of the cell was observed by polarization and chronopotentiometry. The structure of the electrodeposited Zr was analyzed and the electrodeposition characteristics were also evaluated.

Radiation Heat Transfer in the Molten Salt FLiNaK

The molten fluoride salt eutectic LiF-NaF-KF (FLiNaK) has been proposed as a coolant for use in Generation IV reactors designed to operate at temperatures at which radiation heat transfer (RHT) may be significant. Little research has been performed into the absorption coefficient of FLiNaK as it pertains to thermal RHT. An estimate of the spectral absorption coefficient for FLiNaK has been generated using informed assumptions and existing data for the constituent salts. The effect of heat transfer, as it pertains to flowing salt in circular cross-section pipes with heated walls, has been investigated for laminar flow using a mathematical model. The combined energy equation, in various geometries, was solved for laminar flow, with the radiative heat flux calculated using the differential approximation. The percentage of energy transferred by radiation to the salt was found to be primarily a function of pipe diameter, wall temperature, and the salt absorption coefficient. A map of temperatures and pipe diameters has been generated, which indicates where RHT is significant. A correlation has been proposed, based on the mathematical model, to account for increase in Nusselt number due to radiation. Additional discussion is included on the effects of wall emissivity and high Reynolds flows.

Formation of Al2O3 diffusion barrier in cold-sprayed NiCoCrAlY/Ni multi-layered coatings on 304SS substrate

Abstract This work deals with a double-layer coating for the corrosion protection of stainless steel (SS) structures used in molten fluoride salt reactors for energy production. It aims to mitigate the dissolution of chromium from stainless steel structures into the molten salts. On 304SS substrates, three coating layers consisting of NiCoCrAlY, Ni(O), and the nickel layer on the top were deposited by cold spraying. The Ni(O) coating was made by using a powder containing active oxygen and prepared by mechanical-alloyed pure Ni powders in ambient atmosphere. The Al 2 O 3 diffusion barrier was evolved by annealing the cold-sprayed coating system in argon at temperatures ranging from 800 to 1000 °C at the interface between NiCoCrAlY and Ni(O) layers. During the annealing, the Al 2 O 3 layer formed with a growth following a parabolic law, resulting from the reaction of the oxygen in the Ni (O) layer with the aluminium in the NiCoCrAlY layer. The coating test conducted at 900 °C for 100 h in argon showed that the Al 2 O 3 layer changed little during annealing and could effectively prevent the diffusion of Fe and Cr from the SS substrate to the upper Ni layer.

Development of a MCNP5 and ORIGEN2 based burnup code for molten salt reactor

The Molten Salt Reactor (MSR) is one of the six advanced reactor nuclear energy systems for further research and development selected by Generation IV International Forum (GIF), which is distinguished by its core in which the fuel is dissolved in molten fluoride salt. Because fuel flow in the primary loop, the depletion of MSR is different from that of solid-fuel reactors. In this paper, an MCNP5 and ORIGEN2 Coupled Burnup (MOCBurn) code for MSR is developed under the MATLAB platform. Some new methods and novel arrangements are used to make it suitable for fuel flow in the MSR. To consider the fuel convection and diffusion in the primary loop of MSR, fuel mixing calculation is carried out after each burnup time step. Modeling function for geometry with repeat structures is implicated for reactor analysis with complex structures. Calculation for a high-burnup reactor pin cell benchmark is performed using the MOCBurn code. Results of depletion study show that the MOCBurn code is suitable for the traditional solid-fuel reactors. A preliminary study of the fuel mixture effect in MSR is also carried out.

Obnašanje superzlitine na osnovi niklja pri rekristalizaciji

The thermomechanical processing of a nickel-based superalloy is the way to considerably influence the grain size. A uniform coarse-grain size increases the creep strength and the crack-growth resistance. In this work, the processing for achieving a uniform recrystallized-grain structure with a variation in the thermomechanical parameters is investigated. The MoNiCr alloy is intended for modern designs of nuclear reactors, in which molten fluoride salts are used as the coolants in the primary and/or secondary circuit. It represents a material alternative with a high corrosion resistance in the area of fluoride salts and it has very good creep properties in the temperature range of 650–750 °C as well. The manufacture of vessels and fittings from the MoNiCr alloy requires the mastering of the technology for forming this high-alloyed material. The key step seems to be the transition of the cast state of the material to the state of a cast recrystallized microstructure with homogenous fine grains. A particular stress condition is, besides the temperature, very important during hot forming. Nickel alloys are able to accept a significantly higher deformation level if the compressive stress prevails. A forming process involving the compression state of stress increases the probability that the material will reach, without failure, a level of deformation that allows recovery and recrystallization processes. A particular deformation process was carried out on a physical simulator. The preceding cold deformation essentially accelerates the recrystallization process of a deformed cast structure.

Electrochemical Study of Corrosion in High Temperature Molten Salts

Concentrating solar power systems are projected to need to produce energy with a cost of $0.06/kWh when implemented on a very large scale. It is generally believed that temperatures of the heat transfer fluids will need to be higher than 800oC to drive an advanced Brayton cycle or Rankine cycle with superheated steam to reach this target. However, current state-of knowledge of corrosion science indicates a lack of published data on the impact of molten salts at temperatures above 800°C on corrosion of molten salt contacting materials such as steels or high temperature alloys and ceramics. This work investigates the corrosion mechanism of selected alloys and ceramics in chloride and fluoride molten salts used for high operating temperature heat transfer fluids using a combination of electrochemical techniques, theoretical calculations, and thermodynamic predictions. The electrochemical characterizations included linear polarization resistance scans, electrochemical impedance spectroscopy, and cyclic potentiodynamic polarization. Results show that the Fe-Ni-Cr alloys tested have high general corrosion and require corrosion inhibition. Advanced materials, such as SiC-SiC and TZM, have been shown to have much lower corrosion and good passivation behavior. Experimental tests using coupon immersion in the molten salts support the mechanism determined from electrochemical testing that the main element being oxidized is Cr. Analysis of samples also indicates selective Cr oxidation concentrates along grain boundaries and creates porous networks in the sample. The electrochemical impedance tests suggest that the corrosion is diffusion controlled and this assumption correlates well with corrosion modeling studies. The electrochemical testing data combined with thermodynamic modeling led to the development of a reaction mechanism for high temperature corrosion in molten salts.

Electrochemical Fluorination of Uranium to UF6

The development of efficient and environmentally benign methods to reprocess used nuclear fuel will be enabling technologies for the nuclear renaissance. The primary large scale method for processing used nuclear fuel is aqueous and generates large volumes of high level radioactive waste. The use of electrochemical and fluorination methods for reprocessing used nuclear fuel have been proposed and researched by various groups, but combinations of electrochemical and fluorination methods have not been investigated. Fluorinating agents are extremely potent oxidizers and may electrochemically fluorinate metallic used nuclear fuel with no input of electricity. A galvano-fluorination process has been developed at Savannah River National Laboratory. This electrochemical non-aqueous process enables efficient gas phase uranium recovery while allowing for control of the reaction. Thermodynamic modeling of the electrochemical fluorination process has shown UF6 to be the first gaseous fission product produced during fluorination. Further thermodynamic modeling was used to determine the feasibility of oxide reduction required for oxide fuel before electrochemical fluorination and for experiments. A chemical reactor system for electrochemical fluorination of U with NF3 in a fluoride molten salt was constructed along with vessels for product collection and analysis. This work has proven that an electrochemical reaction happens between U and NF3 which confirms the feasibility of electrochemical fluorination and the development of a processing scheme.

Development of Electrochemical Methods for High Temperature Molten Fluoride Salt Systems

Molten salt processes were developed several decades ago as a promising technology for material processing and industrial process heat applications. This technology is still seen today as extremely suitable for thermal energy storage and power production. Liquid salts can indeed be considered legitimate candidates for several heat transfer applications thanks to favorable physical properties such as high boiling point, low vapor pressures, large specific heat, and large thermal conductivity. In December 2015 a “Molten Salt Electrochemistry Short Course” was organized at University of Wisconsin Madison. The course was taught by Professor Digby MacDonald, a renowned expert in electrochemistry techniques for corrosion studies in aqueous environments. The course concentrated on the fundamentals of electrochemistry, with specific references to processes such as corrosion, electro-deposition, passivity and passivity breakdown, stress corrosion cracking, and corrosion fatigue. The course also included a discussion of the specialized techniques employed to study hydrogen in metals and carbon (graphite). This paper will discuss the applicability of each of these electrochemistry techniques to molten fluoride salts and will provide an overview of the current status of these techniques in fluoride ionic liquids. The definition of a thermodynamically stable reference electrode (RE) has emerged as one of the critical points required to rigorously apply electrochemistry analytical techniques to fluoride salt systems. Therefore, this paper will review the fluoride molten salt RE designs developed in the 1960s and 1970s and in recent years. Additionally this paper will report designs and testing of the fluoride salt REs under development at University of Wisconsin -Madison.