Molten FLiNaK Salt Research Articles

The effect of molten FLiNaK salt infiltration on the strength of graphite

Graphite has been identified as a structural and neutron-moderator material in the molten salt reactor (MSR). In order to support the structural material development for MSR, the influence of molten salt infiltration on the mechanical strength of graphite was studied. The high-temperature mechanical strength of graphite grades IG-110 and NG-CT-10 decrease with the increasing of weight gain ratio during the molten FLiNaK salt infiltration. It shows a noticeable high-temperature mechanical strength degradation, which can be evidenced by the softening coefficients. The smaller softening coefficient of compressive strength compared with that of tensile strength at the same infiltration pressure indicates the compressive strength is more vulnerable to the molten salt infiltration than the tensile strength. The major fragments analysis of the samples after compressive strength tests shows that the molten FLiNaK salt infiltrated graphite samples experience are longitudinal splitting fracture rather than the shear fracture of virgin graphite. The increase of thermal strain after molten salt infiltration indicates an extra stress in infiltrated graphite, which may be one of the reasons for the failure mechanism change.

Improvement of Corrosion Resistance of Hastelloy-N Alloy in LiF-NaF-KF Molten Salt by Laser Cladding Pure Metallic Coatings

The corrosion protection of Hastelloy-N alloy in LiF-NaF-KF (commonly referred to as FLiNaK) molten salt has been developed by pure Ni and Co coatings using the laser cladding technique. An immersion experiment with samples was performed in molten FLiNaK salt at 900 °C for 100 h. It was found that the corrosion rates of the pure Ni-coated specimen and the pure Co-coated specimen are 39.9% and 35.7% of that of Hastelloy-N alloy, respectively. A careful microstructural characterization indicates that a selective dissolution of the elemental Cr occurred in the surface of bare Hastelloy-N alloy, showing a severe intergranular corrosion. For pure metal-coated specimens, in contrast, only metal oxide formed during the laser cladding process dissolved into the molten fluoride salt. The dense pure metal (Ni or Co) coatings exhibit a slightly general corrosion and protect the Hastelloy-N substrate effectively. The possible corrosion mechanism for both coated and uncoated Hastelloy-N under the current experimental condition are discussed in this work.

Effect of SO42− on the corrosion of 316L stainless steel in molten FLiNaK salt

The effect of SO42− on the corrosion behavior of 316 L stainless steel (SS) in molten FLiNaK (LiF-NaF-KF: 46.5–11.5–42 mol.%) salt was studied. Results reveal that the SO42− in the salt remarkably accelerates the corrosion of 316 L SS by promoting the dissolution of Cr, leading to increasing intergranular corrosion of the alloy. On the other hand, the results of structural characterization indicate that the SO42− can react with Mn to form MnS at the grain boundaries of the alloy. The galvanic couple between MnS and steel matrix could further accelerate the intergranular corrosion of 316 L SS in molten fluoride salt environments.

Corrosion performance of Ni-16%wt.Mo-X%wt.SiC alloys in FLiNaK molten salt

The corrosion performance of Ni-16%wt.Mo-X%wt.SiC (X = 0.5, 1.5, 2.0, 2.5 and 3.0) alloys prepared via mechanical alloying followed by consolidation using spark plasma sintering (SPS) from pure Ni, Mo and SiC powders is investigated. Corrosion testing at 650 °C/200 h in FLiNaK molten salt showed that increasing the volume fraction of SiC in the initial Ni-Mo-SiC powder mixture leads to formation of large amount of Mo2C precipitates, which readily dissolve into FLiNaK molten salt. Hence, only the corrosion resistance of NiMo-SiC alloys with a low SiC content (<2 wt.%) is comparable to that of Hastelloy-N® alloy.

Effects of silicon carbide on the corrosion of metallic materials in molten LiF-NaF-KF salt

The effects of silicon carbide (SiC) on the corrosion behavior of metallic materials in molten LiF-NaF-KF (FLiNaK) salt were investigated. Results reveal that the corrosion products of SiC in molten FLiNaK salt can react with nickel-based alloy GH3535, Ni metal, 316L stainless steel, and Cr metal to form Ni31Si12, Ni3Si, CrFe8Si, and Cr3Si, respectively. The formation of silicide enhances the corrosion of GH3535 alloy in molten FLiNaK salt. The depletion of Cr along grain boundaries in alloys can promote the diffusion of silicon into the alloys and increase the corrosion depth.

The effect of cold-rolling on the microstructure and corrosion behaviour of 316L alloy in FLiNaK molten salt

The effect of cold-rolling on the microstructure and molten salt corrosion behaviour of 316L alloy was investigated. Corrosion tests were performed in FLiNaK salt at 600 °C for 300 h. The present results indicate that cold-rolling leads to enhanced corrosion in molten salt despite its beneficial effect on the alloys mechanical properties. It is shown that the corrosion resistance of cold-rolled 316L alloy is largely governed by the presence of high-angle grain boundaries (HAGBs) and geometrically-necessary dislocations (GNDs). Exacerbated corrosion is thus directly related to the increased prevalence of grain boundaries and dislocations within the microstructure.

Tritium release from molten salt FLiNaK under low flux neutron irradiation with an AmBe neutron source

For examination of tritium transport in a self-cooled liquid blanket system using fluoride molten salt breeder/coolants, a series of neutron irradiation experiments have been conducted under the low flux neutron environment with an AmBe neutron source at the OKTAVIAN facility of Osaka University in Japan. Tritium release from molten salt FLiNaK (LiF-NaF-KF) at 773 K is evaluated with the neutron source, energies of which are up to 10.7 MeV and the intensity is ∼2 × 106 n/s. To measure the amount of tritium released from FLiNaK and permeating through an Inconel crucible, TF and/or T2O, and T2 are recovered separately using water bubblers and CuO oxidation beds. The amount of tritium recovered for one week in a steady state, 12.0 Bq, has closed to the calculated value, 10.6 Bq, obtained with MCNP6. More than 98% of the recovered tritium from FLiNaK has been TF and/or T2O. The overall mass transfer coefficients for tritium released from a FLiNaK free surface have been evaluated from the transient changes in recovered tritium and those are comparable to the value for HF released from FLiBe previously reported in the literature.

Electrochemical behavior of dysprosium and lanthanum in molten LiF-NaF-KF (Flinak) salt

Cyclic voltammetry (CV) analysis was performed to determine the behavior of Dysprosium (Dy) and Lanthanum (La) in molten FLiNaK salt (LiF-NaF-KF: 46.5–11.5–42 mol%) and to understand the mechanism driving the electrochemical reactions. The goals of these experiments are to understand the behavior of contaminants and nuclear fission products related to the Fluoride Salt-cooled High-Temperature Reactor (FHR) concept, and to generate a reliable source of electrochemical properties in fluoride salts. The tests were performed at 600–650–700 °C for Dy and 650–700–750 °C for La in a glassy carbon crucible contained within an inert argon atmosphere. A platinum wire quasi-reference electrode exhibited stable potential throughout experiments and allowed for thermodynamic reference between test species and known redox couple (F2/F−). The electrochemical reduction of Dy and La (assumed three-electron, single-step transfer) maintained quasi-reversibility at the solid tungsten electrode for scan rates of 160 mV/s and below during CV. For scan rates of 200 mV/s and above, significant deviation from reversibility was evidenced by increasing separation between anodic and cathodic peaks. For Dy3+, the diffusion coefficient was calculated as D [cm2/s] = 0.0538∗exp(−6193/T(K)) and the activation energy Ea = 51.5 kJ/mol. The diffusion coefficient of La3+ was calculated as D [cm2/s] = 14.252∗exp(−15285/T(K)) and the activation energy Ea = 127 kJ/mol. The apparent potential of the La3+/La0 redox reaction was determined as E0∗[V vs. K+/K] = −0.368 + 0.001119∗T[K] and for the Dy3+/Dy0 reaction as E0∗[V vs. K+/K] = 0.0626 + 0.00088∗T[K].

Carburization effects on the corrosion of Cr, Fe, Ni, W, and Mo in fluoride-salt cooled high temperature reactor (FHR) coolant

Corrosion of structural alloys in fluoride salt cooled high temperature reactors (FHRs) is affected by the presence of carbon. The stability of the carbides of alloying elements augments their corrosion, however the same property can be useful if these stable carbides are used to protect materials. Exposure tests of pure elements Cr, Fe, Ni, W, and Mo in graphite crucibles at 700 °C were conducted in molten FLiNaK salt for 100 h. Carbide particles were detected on the surfaces of Cr, Mo, and W samples after exposure. The ranking of these elements, from least to greatest corrosion resistance by mass loss per unit area was Cr < Fe < W < Mo < Ni. The stability of chromium carbides was further investigated using chromium samples which were pre-carburized in a reducing hydrocarbon environment at 800 °C prior to exposure tests in molten FLiNaK. The chromium carbide layer produced by pre-carburization was resistant to attack by molten fluoride and slowed corrosion of the underlying chromium metal.

Influence of graphite-alloy interactions on corrosion of Ni-Mo-Cr alloy in molten fluorides

In this study, the effects of graphite-alloy interaction on corrosion of Ni-Mo-Cr alloy in molten FLiNaK salt were investigated. The corrosion tests of Ni-Mo-Cr alloys were conducted in graphite crucibles, to examine the differences of test specimens in conditions of electric contact and isolated with graphite, respectively. The corrosion attack is severer with more weight loss and deeper Cr depletion layer in samples electric contact with graphite than those isolated with graphite. The occurrence of galvanic corrosion between alloy specimens and graphite container was confirmed by electrochemical measurement. The corrosion is controlled by nonelectric transfer in isolated test while electrochemical reaction accelerated corrosion in electric contact test.

Corrosion of Incoloy 800H alloys with nickel cladding in FLiNaK salts at 850 °C

The corrosion of 800H alloy with and without Ni-cladding, as well as GH3535 alloy, in molten FLiNaK salts at 850 °C has been investigated. Results show that Cr and Fe are depleted from the alloys surface and grain boundaries, where “diffusion paths” are formed. The corrosion process is mainly controlled by the redox reaction of impurity metal ions with Fe and Cr at the alloy surface, and an alloy or cladding layer chemistry criterion is suggested as Fe + Cr ≤ 11 wt.%. Cr element is found to be hardly depleted from Σ3 type grain boundaries.

Effect of CrF3 on the corrosion behaviour of Hastelloy-N and 316L stainless steel alloys in FLiNaK molten salt

The effect of CrF3 on the corrosion behaviour of Hastelloy-N and 316L austenitic stainless steel alloys in FLiNaK at 700 °C was investigated using synchrotron radiation and other characterisation techniques. The results showed that CrF3 considerably accelerated the corrosion of both tested alloys by promoting the dissolution of Cr, Fe, and Mn from alloy matrices into molten salt. The 316L stainless steel alloys mainly exhibited intergranular corrosion, whereas the Hastelloy-N alloys tended to exhibit intergranular and pitting corrosion. After corrosion, the lattice parameter of both alloys decreased and Cr1.07Fe18.93 was formed on the surface of the 316L stainless steel alloys.

High temperature in-situ synchrotron-based XRD study on the crystal structure evolution of C/C composite impregnated by FLiNaK molten salt

An in-situ real-time synchrotron-based grazing incidence X-ray diffraction was systematically used to investigate the crystal structural evolution of carbon fiber reinforced carbon matrix (C/C) composite impregnated with FLiNaK molten salt during the heat-treatment process. It was found that the crystallographic thermal expansion and contraction rate of interlayer spacing d002 in C/C composite with FLiNaK salt impregnation is smaller than that in the virgin sample, indicating the suppression on interlayer spacing from FLiNaK salt impregnated. Meanwhile the crystallite size LC002 of C/C composite with FLiNaK salt impregnation is larger than the virgin one after whole heat treatment process, indicating that FLiNaK salt impregnation could facilitate the crystallization of C/C composite after heat treatment process. This improved crystallization in C/C composite with FLiNaK salt impregnation suggests the synthetic action of the salt squeeze effect on crooked carbon layer and the release of internal residual stress after heating-cooling process. Thus, the present study not only contribute to reveal the interaction mechanism between C/C composite and FLiNaK salt in high temperature environment, but also promote the design of safer and more reliable C/C composite materials for the next generation molten salt reactor.

Effect of irradiation damage on corrosion of 4H-SiC in FLiNaK molten salt

The corrosion behavior of 4H-SiC in the FLiNaK molten salt (650°C), pre-irradiated with 70-keV Si ions to 2.5dpa at 650°C, was investigated in order to clarify the effect of irradiation on corrosion. The irradiated region of 4H-SiC is peeled off by the molten salt, and the corrosion depth is nearly consistent with the irradiation damage depth. Molten salt corrosion leads to the loss of Si element and the formation of carbon-rich phase in irradiated 4H-SiC surface. The broken of Si-C bonds induced by irradiation plays a key role in the irradiation-assisted corrosion of SiC.

Hydrogen inventory control for vanadium by Ti metal powder mixing in molten salt FLiNaK

Titanium powder mixed molten salt FLiNaK, which has larger hydrogen solubility than that pure FLiNaK, was investigated for its capability to mitigate hydrogen inventory in vanadium, in expectation of molten salt/vanadium alloy liquid blanket. The hydrogen fluence from gas to vanadium plate through molten salt containing μm- and nm- size Ti powder was quantified as hydrogen inventory in the vanadium plate. The experiment was performed at 823K, feeding hydrogen gas for 10–1000Pa. FLiNaK containing nm-size Ti powder showed less hydrogen inventory than pure one, whereas μm-size Ti powder showed less difference. nmTi-FLiNaK was found to be an effective buffer for hydrogen transfer.

Effects of SO42− ions on the corrosion of GH3535 weld joint in FLiNaK molten salt

The present work studied the impact of SO42− ions on the corrosion behaviors of GH3535 weld joint in FLiNaK molten salt. The concentration of SO42− ions in the FLiNaK molten salt was controlled by adjusting the quantity of Na2SO4 added into the salt. Results indicate that the SO42− ions in the FLiNaK salt speed up the corrosion rate remarkably by promoting the dissolution of Cr from the alloy matrix into the salt. With the concentration of SO42− ions in the FLiNaK salt increases from 100 ppm to 1000 ppm, the weight losses and the Cr depletion layer depths of the corroded specimens increase linearly. Even in the case of the heavy corrosion attack caused by the SO42− ions, the corrosion performance is similar between the base zone and fusion zone in the GH3535 weld joint. It is demonstrated that the structural diversity caused by the welding process has little impact on the corrosion performances of GH3535 alloy in FLiNaK molten salt.

Preparation of high-purity molten FLiNaK salt by the hydrofluorination process

The preparation conditions and purification reaction mechanism for H2–HF treatment of molten FLiNaK salt were investigated. The optimal process conditions for preparation of high-purity molten FLiNaK salt were obtained. They are purification temperature of 650°C, liquid height of 0.9m, purification time of 48h, flow ratio of H2/HF=10, flow volume magnitude of 800mL/min, and reaction pressure of 0MPa. The oxygen content of the prepared molten salt was around 80ppm, corrosion impurity elements (Cr, Fe, and Ni) were less than 20ppm, and the S and P contents were below 5ppm. The quality of FLiNaK molten salt prepared by this hydrofluorination process is better than that obtained by adding NH4HF2 as a fluorinating reagent.

Corrosion behavior of TiC–SiC composite ceramics in molten FLiNaK salt

Immersion corrosion tests of TiC0.8, TiC, TiC–20vol% SiC, TiC–40vol% SiC and SiC have been performed in molten FLiNaK salt at 800°C for 25–200h under argon cover gas. All of these five samples showed small mass loss and relatively good corrosion resistance in molten FLiNaK salt. The corrosion patterns of TiC0.8, TiC, TiC–20vol% SiC and TiC–40 vol% SiC were inter-granular corrosion, which were attributed to the depletion of Ti along the grain boundaries. SiC exhibited a general corrosion process in which a carbon-rich layer formed on the surface, resulting from the depletion of Si. The carbon-rich layer protected SiC against further corrosion, hence lowering the corrosion rate. The corrosion results of TiC–20% SiC and TiC–40% SiC revealed the corrosion resistance of TiC could be improved by adding SiC. And the contribution of SiC to better corrosion resistance has been elucidated.

Electrochemical behavior and electrowinning of uranium(IV) from FLiNaK molten salt

The electrochemical behaviors and electrowinning of uranium from LiF–NaF–KF (46.5–11.5–42.0 mol%, FLiNaK) molten salt were investigated. Cyclic voltammetric and chronopotentiometric studies revealed the presence of two steps during the electro-reduction of UF4 on W electode. U(IV) undergoes a one-electron reduction to form U(III) followed by a three-electron reduction to form U(0). The diffusion coefficient (D) of U(IV) ion was determined to be 1.33 × 10−6 cm2 s−1 at 550 °C. Based on the electrochemical behaviors of U(IV) in FLiNaK melt, pulse current electrolysis was carried out using Ni electrode. The black bulks obtained as electrolysis products were characterized to contain metallic U. Consecutive electrolysis experiments demonstrated up to 74.7% U can be separated from the fluoride melt.

Thermal performance analysis of a thermocline thermal energy storage system with FLiNaK molten salt

A thermocline thermal storage unit with a heat transfer fluid (HTF) of high-temperature molten salt is considered as one of the most promising methods of thermal storage due to its lower cost and smaller size. The main objective of this work is to analyze the transient behavior of the available molten salt FLiNaK used as the HTF in heat transfer and heat storage in a thermocline thermal energy storage (TES) system. Thermal characteristics including temperature profiles influenced by different inlet velocities of HTF and different void fractions of porous heat storage medium are analyzed. The numerical investigation on the heat storage and heat transfer characteristics of FLiINaK has been carried out. A comparison between two different molten salts, FLiNaK and Hitec, has been explored in this paper with regards to their charging and discharging operations. The results indicate the system with FLiNaK has a greater energy storage capability, a shorter charging time and a higher output power. The numerical investigation reveals heat storage and heat transfer characteristics of the thermocline TES system with FLiNaK, and provide important references for molten salt selection of the TES system in the future.