FLiNaK Salt Research Articles

Unexpected accelerated corrosion of Cr in Ni-xW-6Cr alloy with W content increasing

The influence of W content upon corrosion of Ni-xW-6Cr alloys in molten FLiNaK salts at 850 °C was examined. The results indicate a significantly greater depletion of Cr and W from Ni-xW-6Cr alloys with higher W contents in the molten FLiNaK salt, which mainly results from the increasing activities of W and Cr in the alloy with the W content increasing. The high concentrations of oxygen, Ni and Fe impurities further accelerate the corrosion of W and Cr from the alloy by providing more corrosion vacancies as diffusion paths in the alloy's near surface area.

Environmental degradation of electroplated nickel and copper coated SS316H in molten FLiNaK salt

Corrosion of nickel and copper electroplated coatings on SS316H in molten FLiNaK salt was studied. 500- and 1000-hour static corrosion tests were performed in molten FLiNaK at 700 degrees Celsius. Characterization of the coatings following corrosion indicated that they did not degrade in the molten salt during the corrosion experiment and fully protected the underlying SS316H from chromium dissolution. Interdiffusion between the copper coating and SS316H was extremely limited, interdiffusion between the nickel coating and SS316H. Diffusion modelling revealed that the nickel coatings still significantly reduced chromium dissolution over 60 years despite the interdiffusion.

Corrosion Behaviour of Plasma-Sprayed Nickel Coating on Type 316L Stainless Steel in High-Temperature Molten FLiNaK Salt

Corrosion Behaviour of Plasma-Sprayed Nickel Coating on Type 316L Stainless Steel in High-Temperature Molten FLiNaK Salt

Design, fabrication, testing and operation of a simulation facility for high temperature molten salt studies

Purpose Concentrated solar power and molten salt reactors use molten salts for heat energy storage and transfer. FLiNaK salts are being proposed to be used in these plants. However, structural material compatibility is the main hurdle for using molten salt in these systems. Hence, it is essential to study the degradation of materials in high temperature molten FLiNaK salt environment. In view of this paper aims to describe, a simulation facility which was established and operated for carrying out high temperature static corrosion studies of materials under molten FLiNaK salt. Design/methodology/approach This paper describes about the design criteria, method of designing using ASME codes, material selection, fabrication, testing, commissioning and operation. Also, a few experimental results have been illustrated. Findings A simulation facility could be designed, fabricated, commissioned and is being successfully operated to carry out corrosion experiments under static molten FLiNaK environment. Research limitations/implications The facility has been designed for 800°C and maximum temperature of experiment would be restricted to 750°C. The materials tested in this facility can be validated only up to 750°C temperature. A maximum of four exposure periods can be studied at a time with around ten specimens for each exposure. Originality/value Selection of compatible material for the facility and design certain unique features like extracting exposed specimens of intermediate periods without actually shutting down the autoclave and measuring the level of molten salt at high temperature.

Tensile Behavior of Incoloy-800HT Exposed to High-Temperature Molten FLiNaK Salt and Thermal Aging

Tensile Behavior of Incoloy-800HT Exposed to High-Temperature Molten FLiNaK Salt and Thermal Aging

Electrochemical study of the dissolution of oxide films grown on type 316L stainless steel in molten fluoride salt

The corrosion behavior of oxide films grown on Type 316L stainless steel (SS) in molten FLiNaK (LiF-NaF-KF: 46.5−11.5−42 mol.%) salt was investigated. The results show that the oxide film formed on Type 316L SS is unstable and can only temporarily protect materials from corrosion in molten FLiNaK salt. Based on the electrochemical impedance spectroscopy, the oxide dissolution rate is calculated to be 0.85 nm/h at 700 °C in molten FLiNaK salt. After the oxide film dissolved, Cr and Fe are selective dealloyed from the steel, leading to intergranular corrosion of Type 316L SS in molten fluoride salt.

Ni/AlN Composite Coating for Corrosion and Elements Interdiffusion Resistance in Molten Fluoride Salts System

The Ni/AlN composite coating was prepared for increasing corrosion and elements interdiffusion resistance of GH3535 alloy in molten fluoride salts, and the effect of mechanical interlocking on adhesion strength between AlN layer and nickel coating was also studied. Results indicated that the adhesion strength between AlN layer and nickel coating could be significantly enhanced through mechanical interlocking effect, which effectively prevented the nickel coating from flaking off at elevated temperature. Through an etching pre-treatment of AlN layer, the corrosion resistance of the Ni/AlN coated GH3535 alloy in molten FLiNaK salt was further improved, and elements interdiffusion between the substrate and nickel coating was completely suppressed. AlN layer as a diffusion barrier remained compact and continuous in Ni/GH3535 system after high-temperature molten salt corrosion. Moreover, a Ni–P layer consisting of Ni3P and Ni phases formed in the Ni coating after corrosion.

Influence of SiC microstructure on its corrosion behavior in molten FLiNaK salt

The influence of the microstructure on the corrosion rate of three monolithic SiC samples in FLiNaK salt at 900 °C for 250 h was studied. The SiC samples, labeled as SiC-1, SiC-2, and SiC-3, had corrosion rates of 0.137, 0.020, and 0.043 mg/cm2h, respectively. Compared with grain size and the presence of special grain boundaries (i.e., Σ3), the content of high-angle grain boundaries (HAGBs) appeared to have the strongest influence on the corrosion rate of SiC in FLiNaK salt, since the corrosion rate increased six times as the concentration of high-angle grain boundaries increased from 19 to 32% for SiC-2 and SiC-1, respectively. These results stress the importance of controlling the content of HAGBs during the production process of SiC.

Effect of SO42− ion impurity on stress corrosion behavior of Ni-16Mo-7Cr alloy in FLiNaK salt

In FLiNaK salt, acid radical ions impurities can cause aggravated corrosion for Ni-based alloys at elevated temperatures, and would result in deleterious effect on the SCC (stress corrosion cracking) resistance of structural alloy in the reactor. In this study, the corrosion behavior of Ni-16Mo-7Cr alloy under loading stress in FLiNaK salt with sulfate radical ion (SO42−) was investigated by the method of static immersion corrosion at 700°C for 400 h. The results show that the SO42- ion can remarkably accelerate the corrosion process of alloy. The corrosion products mainly consist of Cr2O3 and MnS, which are formed on and near the surface of the alloy. However, whether SO42- ion exists or not in the FLiNaK salt, the stress has little effect on the depth of Cr depletion along the grain boundary, and no SCC phenomenon appears in this alloy.

Parametric study on minor actinides transmutation in a graphite‐moderated thorium‐based molten salt reactors

Transmutation of minor actinides (MAs) is an effective solution to reduce the long-term radiotoxicity of nuclear waste. Online refueling and no fuel rod fabrication in thorium-based molten salt reactors (TMSR) are two remarkable advantages of MA transmutation. However, MA solubility limit of the fuel salt and the neutron spectrum in the core are two key parameters that have a strong impact on MA transmutation capability. In this paper, three types of carrier salt with different MA solubility limits and various salt volume fractions (SVFs) are introduced in a TMSR core to evaluate the MA transmutation characteristics. The results indicate that the Flibe salt with the best neutron economy can obtain the highest MA transmutation rate (TR), while the Flinak salt with the largest MA inventory can achieve the highest specific MA transmutation consumption (STC). STC and TR for the case with the Flibe salt and 5% SVF are about 200 kg/GWth.yr and 82%, respectively, which indicates that it can consume the annual MA yields from about 3.37 typical pressurized water reactors (PWRs). Meanwhile, the STC and TR for the case with the Flinak salt and 40% SVF are about 366 kg/GWth.yr and 75%, respectively, which indicates that it can consume the annual MA yields from about 50 typical PWRs. In addition, the total radiotoxicity of MAs after 50-year operation in TMSR can be lowered by as high as about 50%. The significant production of Pu isotopes can be reused as nuclear fuel in fast reactors. In particular, Pu-238, with the mass fraction of more than 60% in Pu isotopes, is a useful material for many nuclear applications. In conclusion, it is feasible to transmute MAs in TMSR and thus achieve the goals of reduction of MA's long-term radioactive hazards.

Complexation of Mo in FLiNaK Molten Salt: Insight from Ab Initio Molecular Dynamics.

Online extraction of fission products, such as the medical isotope Mo-99, is a key advantage of the proposed molten salt nuclear reactor design. The chemical and structural behavior of Mo solvated in fluoride salt has been relatively unknown. Ab initio molecular dynamics simulations were employed to examine the behavior of molybdenum in the molten salt FLiNaK (LiF-NaF-KF) for oxidation states between 0 and 6+. Mo complexation was found to vary with the Mo oxidation state, with lower oxidation states tending to result in complexes with more molybdenum ions. Complexes containing multiple Mo ions were observed for all Mo oxidation states studied except 5+ and 6+. A relationship between the solubility of a complex and electronic isolation of a complex in a molten salt is explored using the Bader atoms in molecule electron density partitioning scheme, with more volatile complexes exhibiting greater electronic isolation. The impacts of UF4 and H2O on the predominant molybdenum species are also considered. While no impacts on Mo behavior by UF4 were observed, Mo-O interactions may inhibit the formation of complexes containing multiple Mo ions.

Electrochemical measurements of the corrosion of pure Fe and JLF-1 steel in HF-containing molten FLiNaK salt

In order to establish a liquid blanket using a Li-containing molten fluoride salt, the corrosion rate and mechanism of pure Fe and JLF-1 steel in FLiNaK (LiF-NaF- KF = 46.5-11.5-42.0) was investigated by one of the electrochemical methods, the potential step method. Simulating the TF breeding in the blanket, HF gas was injected into FLiNaK at 773 K. The partial pressure of HF gas (pHF) was varied from 8.59 to 2.46 × 103 Pa. The current-potential curves of pure Fe and JLF-1 steel were measured in an electrochemical cell and fit by the Butler-Volmer equation with limited current density. The corrosion current density (icor) of pure Fe increased with pHF as icor=0.6pHF. The corrosion rates of pure Fe and JLF-1 steel were close. In order to keep the uniform corrosion rate of JLF-1 steel below 0.1 mm/year in the blanket, HF concentration in FLiNaK is required less than 1.2 × 10−4 molHF/m3. The cathodic current density (iLc) was proportional to pHF as iLc=-0.11pHF, and, therefore, the rate-limiting process of HF corrosion of pure Fe is predicted to be HF or H+ diffusion in FLiNaK.

Metallic impurities induced corrosion of a Ni-26W-6Cr alloy in molten fluoride salts at 850 oC

The effect of metallic impurities on corrosion of a Ni-26W-6Cr alloy in molten FLiNaK salts at 850 °C was investigated. Results show that the Ni-26W-6Cr alloy exhibits the same excellent compatibility with fluoride salts as Hastelloy N alloy. Fe impurities in molten salts accelerate the diffusion of Cr in the alloy, which determines the corrosion degree of the alloy. Unexpected depletion of W from the alloy was observed, which mostly results from the presence of Mo impurity in the salts. Mo impurity should be limited because of the significant effect of W on the neutron spectra of reactors.

Electrochemical Techniques to Monitor the Concentration of Oxide in Molten FLiNaK Salt

Molten fluoride salts have attracted interest as fuel and/or coolant for several different molten salt reactor (MSR) designs. Fluoride salts have many advantageous properties for these applications, such as high ionic conductivity, good heat transfer capacity, and high solubility for actinides. However, the impurities of the fluoride salt corrode the metals of MSR such as H2O, HF, and metal impurities. Therefore, it is important to monitor the oxides in molten fluoride salt. In this study, LiF-NaF-KF (FLiNaK) salt was used as a less toxic surrogate for FLiBe because of the toxicity of beryllium. In this work, we focused on measuring the concentration of oxide by adding Li2O in the FLiNaK salt using two electrochemical methods: cyclic voltammetry (CV) and square waver voltammetry (SWV). Moreover, after the electrochemical tests, the salts were analyzed for concentration of oxygen by LECO in order to compare with electrochemical results. We observed an oxidation peak corresponding to oxide using CV with a tungsten working electrode. Then SWV was used to measure the oxidation peak for oxide at various concentrations (0.1 to 2.0 wt.% of Li2O). SWV can measure the low concentration, because SWV is more sensitive to the oxidation current than CV through eliminating the non-faradaic current.

Corrosion behavior of a wear resistant Co-Mo-Cr-Si alloy in molten fluoride salts

Evaluating the corrosion performances of wear resistant alloys is important for designing the molten salt-wetted bearings and valves in molten salt reactors. In this study, the corrosion behavior of wear-resistant Tribaloy T-400 alloy (Co-29Mo-8.5Cr-2.8Si) in molten FLiNaK salt (LiF–NaF–KF: 46.5–11.5–42 mol.%), with and without the presence of UNS N10003 alloy, were investigated through static immersion tests at 650 °C. After 400 h corrosion, T-400 alloy showed greater weight loss than UNS N10003 alloy, and the presence of UNS N10003 alloy further increased the weight loss of T-400 alloy. Electron microscopic and salt chemistry analyses showed that Cr and Co were selectively dissolved from T-400 alloy matrix after corrosion, resulting in a porous structure. On the other hand, the Laves phase precipitates were hardly corroded but showed signs of carbide formation on the surface. In addition, when tested with UNS N10003 alloy, some of the Co dissolved from T-400 alloy transferred to the surface of UNS N10003 alloy, hence promoted the corrosion of T-400 alloy. The results have indicated that even T-400 alloy has a relatively low Cr content among high temperature wear-resistant hardfacing alloys, its corrosion resistance is still inadequate in a typical molten salt reactor environment where UNS N10003 alloy and graphite are also present.

Electrochemical Techniques to Monitor the Concentration of Oxide in Molten FLiNaK Salt

The effect of oxide concentration in molten FLiNaK (LiF-NaF-KF) salt on square wave voltammetry (SWV) oxidation peak heights using tungsten working electrodes was determined and compared to the results of acid-base titration of solid salt samples. SWV oxidation current was observed to be more sensitive than that of cyclic voltammetry (CV), possibly due to the elimination of non-faradaic current. The in-situ measurements of the O2- concentrations in the molten salt using SWV showed good agreement with the results obtained from the acid-base titration method.

Hot Isostatically Pressed (HIPed) fluorite glass‐ceramic wasteforms for fluoride molten salt wastes

AbstractMolten pyroprocessing salts can be used to dissolve used nuclear fuel from a reactor allowing recovery of the actinides. Previously, ANSTO have demonstrated hot isostatically pressed (HIPed) sodalite glass‐ceramic wasteforms for eutectic (Li,K)Cl salts containing fission products, but this system cannot be used for the analogous molten alkali fluoride salts (eg, FLiNaK), which have utility in the application of the next generation of nuclear reactors. In this work, a novel glass‐ceramic composite wasteform has been prepared by HIPing, as a candidate for the immobilization of fission product‐bearing FLiNaK salts. The wasteform has been tailored to immobilize the high fluoride content of the waste within fluorite, whereas the waste alkali elements are incorporated in a durable sodium aluminoborosilicate glass, with total waste loadings of ~17‐21 wt% achieved. It was also demonstrated that the speciation of Mo‐ and Sb‐simulated fission products was altered by adding Ti metal due to a controlled redox environment. The resulting candidate wasteform has been studied by X‐ray diffraction and scanning electron microscopy, including the HIP canister‐wasteform interaction zone, and its performance assessed via leaching studies using the PCT and ASTM C1220 leaching protocols. Dr Vance very much enjoyed the challenge of wasteform design for problematic nuclear wastes, for which fission product‐bearing FLiNaK salts are a clear example. His ability to hone in on a wasteform solution with viable waste loadings that meet performance requirements was testament to his nearly 40 years experience in nuclear waste immobilization. The samples discussed in this work represent the last wasteform materials that he prepared.

Kinetic study of hydrogen transport in graphite under molten fluoride salt environment

In this work, a kinetic model, which describes the reactions occurring during the hydrogen charging process on a graphite surface, was optimized on Electrochemical Impedance Spectroscopic (EIS) to study the entry of hydrogen into graphite in high temperature molten FLiNaK salt. The results show that the rate constants of k1 increases with decreasing applied potential (becoming more negative), and k2 and k3 are independent of the applied potential. At the same charging potential, all the three rate constants are independent of the water concentration in the FLiNaK salt. The surface coverage of absorbed hydrogen increases with decreasing the charging potential and increasing the moisture content in the salt. The adsorption efficiency of hydrogen increases with increasingly charging potential and decreases with increasing moisture content of the FLiNaK salt. These results demonstrate that EIS is a promising technique for measuring the kinetic parameters for hydrogen transport in graphite under molten fluoride salt environment.

The structure of molten FLiNaK

The structure of the molten salt (LiF)0.465(NaF)0.115(KF)0.42 (FLiNaK), a potential coolant for molten salt nuclear reactors, has been studied by ab initio molecular dynamics simulations and neutron total scattering experiments. We find that the salt retains well-defined short-range structural correlations out to approximately 9 Å at typical reactor operating temperatures. The experimentally determined pair distribution function can be described with quantitative accuracy by the molecular dynamics simulations. These results indicate that the essential ionic interactions are properly captured by the simulations, providing a launching point for future studies of FLiNaK and other molten salts for nuclear reactor applications.

Galvanic corrosion of Type 316L stainless steel and Graphite in molten fluoride salt

The effect of Graphite on the corrosion of Type 316L stainless steel (SS) was investigated in molten FLiNaK (LiF-NaF-KF: 46.5−11.5−42 mol.%) salt using electrochemical methods. Results show that Type 316L SS and Graphite, when ohmically-coupled, are susceptible to galvanic corrosion in molten fluoride salts. The Graphite, with the more positive electromotive potential, acts as the cathode in the Type 316L SS/Graphite galvanic couple and accelerates the corrosion of the steel. The galvanic effect of Graphite on Type 316L SS in molten FLiNaK salt increase linearly with increasing Graphite/Type 316L SS surface area ratio, temperature, and moisture impurities of the salt.