LiF NaF KF Research Articles

Joint solubility of PuF3 and UF4 in a melt of lithium, sodium, and potassium fluorides

The solubility of PuF3 and UF4 added jointly into a melt of the molar composition 46.5LiF–11.5NaF–42KF was studied in the temperature range 823–1073 K. Anhydrous PuF3 and UF4 were synthesized. The isothermal saturation method was used in studying the dissolution of pressed PuF3 and UF4 pellets in the melt in an inert (argon) atmosphere. The temperature dependences of the solubilities of jointly added PuF3 and UF4 in the LiF–NaF–KF melt of the chosen composition were determined. The solubilities of PuF3 and UF4 added into the LiF–NaF–KF melt jointly are lower than the solubilities of the same fluorides added separately.

Prospects of subcritical molten salt reactor for minor actinides incineration in closed fuel cycle

Abstract A subcritical molten salt reactor is proposed for minor actinides (separated from spent fuel VVER-1000 light water reactor) incineration and for 233U conversion from 232Th. Here the subcritical molten salt reactor with fuel composition of heavy nuclide fluorides in molten LiF–NaF–KF salt and with external neutron source, based on 1 GeV proton accelerator and molten salt cooled tungsten target is considered. The paper presents the results of parametrical analysis of equilibrium nuclide composition of molten salt reactor with minor actinides feed in dependence of core dimensions, average neutron flux and external neutron source intensity. Reactor design is defined; requirements to external neutron source are posed; heavy nuclides equilibrium and fuel cycle main parameters are calculated.

Corrosion behavior of ZrC–SiC composite ceramics in LiF–NaF–KF molten salt at high temperatures

The suitability of ZrC–SiC-based ceramics was assessed as candidate materials for fluoride. An immersion corrosion with ZrC–SiC composites was performed in molten FLiNaK salts (LiF–NaF–KF: 46.5/11.5/42mol%) at 850°C. Results indicated that ZrC–SiC-based ceramics were more corrosion resistant than the ZrC ceramic. ZrC–40SiC ceramic exhibited 50% weight-loss to ZrC. This carbide exhibited minimal corrosion attack because of strong resistance to dissolution in molten fluorides. Furthermore, XRD results were consistent with weight-loss and SEM results. The corrosion behavior of ZrC–SiC-based ceramics was affected by impurities, particularly oxygen content. Oxygen and/or oxygen ion participated in corrosion of ceramic. Corrosion of carbide ceramics is mainly due to C4− in ceramics. This ion is oxidized to elemental C by oxidants or impurities in the molten salt. The addition of C may inhibit corrosion of molten salts and thus improve the corrosion resistance of carbide ceramics.

High-temperature corrosion behavior of Ni–16Mo–7Cr–4Fe superalloy containing yttrium in molten LiF–NaF–KF salt

The corrosion tests of a Ni–16Mo–7Cr–4Fe superalloy containing various yttrium contents were performed in molten FLiNaK (LiF–NaF–KF: 46.5–11.5–42mol%) salt at 850°C for 620h, with the goals of investigating the effects of yttrium on the corrosion resistance of Ni–16Mo–7Cr–4Fe alloy and understanding the corrosion mechanisms. The addition of an appropriate amount of yttrium promotes the formation of a Y-rich layer near the surface and inhibits the diffusion of the susceptive elements.

The effect of corrosion product CrF3 on thermo-physical properties of FLiNaK

FLiNaK (LiF–NaF–KF: 46.5–11.5–42 mol %) is a promising candidate as the secondary loop coolant in molten salt reactor. The thermo-physical properties of pure FLiNaK and FLiNaK containing up to 6000 ppm (equivalent to mg/kg) corrosion product CrF3 were measured. The results indicate that the effects of CrF3 on melting point, enthalpy, specific heat capacity, density and thermal diffusivity of FLiNaK in liquid state are negligible within the allowable error range, meanwhile the change of thermal diffusivity is significant for FLiNaK in solid state. This work provides fundamental knowledge for the thermo-physical properties of coolant in molten salt reactor.

Electrodeposition of Si film on Ag substrate in molten LiF–NaF–KF directly dissolving SiO2

The electrochemical reduction of SiO2 in a molten LiF–NaF–KF electrolyte containing SiO2 at 873K was studied using cyclic voltammetry and chronoamperometry. The cyclic voltammograms indicated that SiO2 is first dissolved in the electrolyte, followed by reduction on a Ag electrode. Si films were deposited by potentiostatic electrolysis on a Ag electrode, and the specimens obtained were characterized by Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. Analysis of the Raman spectra indicated that the Si film obtained predominantly consists of the crystalline phase.

High-temperature corrosion of helium ion-irradiated Ni-based alloy in fluoride molten salt

A comparison of the effect of helium-ion-radiation damage on a Ni–Mo–Cr–Fe alloy exposed to high-temperature (750°C) corrosion in eutectic LiF–NaF–KF molten salt has been made. Microstructural examination showed that both the corroded-only and irradiated and corroded samples exhibit the characteristics of intergranular corrosion. However, helium ion irradiation introduces defects such as helium bubbles and cavities into the microstructure, greatly increasing intragranular corrosion of the irradiated sample. The thickness of the remaining corrosion zone for the irradiated sample was much greater than that for the unirradiated sample.

Viscosity of LiF–NaF–KF eutectic and effect of cerium trifluoride and uranium tetrafluoride additions

Kinematic viscosity of the 46.5LiF–11.5NaF–42KF eutectic (mol%) has been measured in the temperature range 727–1144K by the method of torsional oscillations damping of a cylinder with the liquid under study. Found that the kinematic viscosity of this eutectic has some deviations from the exponential law, that may be explained by the existence of clusters in the melt. The volume fraction of the clusters in the eutectic as a function of its temperature was estimated. The kinematic viscosity of the 46.5LiF–11.5NaF–42KF eutectic (in mol%) with additions of (1) 5 and 10mol% CeF3 as well as (2) 20mol% UF4 and 10mol% CeF3 was also measured. It is experimentally proved that the addition of 5mol% CeF3 significantly reduces the viscosity at low temperatures and slightly increases it – at high temperatures. Reduction of viscosity at low temperatures can be explained by the fact that the molecules of CeF3 destroy clusters. Additions of 20mol% UF4 also decreased kinematic viscosity of the molten salt mixture compared to pure 46.5LiF–11.5NaF–42KF eutectic (in mol%). Note, that all additions used significantly increased the liquidus temperature of the molten salt mixture. Particularly, additions of 20mol% UF4 without and with 10mol% CeF3 increased the liquidus temperature up to 923 and 1023K, respectively.

Molten salt fast reactor with U–Pu fuel cycle

In the last two years it was discovered that solubility of PuF3, UF4 and AmF3 in the eutectics LiF–NaF–KF are unexpectedly high (30 mol %, 45 mol % and 43 mol % correspondingly at 700 °C). This result opens the way for the development of the molten salt fast reactor with U–Pu nuclear fuel cycle (UPu-MSFR). The first calculations of the critical UPu-MSFR and subcritical MSR-burner of Am are presented.

Galvanic corrosion of pure metals in molten fluorides

The corrosion of structural materials is a great challenge for the development of a molten salt reactor using molten fluorides as fuel or coolant. The corrosion of materials in molten fluorides occurs mainly through the dissolution of alloying elements into the melt. In the present investigation, the galvanic corrosion behavior of pure Ni, Fe, and Cr as the common alloying constituents in an eutectic LiF–NaF–KF melt at 700°C has been investigated. The experimental results indicate that the corrosion potential of the three metals decreases significantly by the order of Ni, Fe and Cr, with the largest potential difference observed between Ni and Cr. Their great potential differences lead to the occurrence of significant galvanic corrosion. The galvanic corrosion effects of the couples Ni/Cr, Ni/Fe and Fe/Cr are significantly larger than 1, with the most remarkable effectiveness obtained for the couple Ni/Cr, then Fe/Cr and Ni/Fe.

Failure analysis of 316L stainless steel crucible by molten fluoride salt interaction with clay bonded silicon carbide

Detailed investigation of a recently failed static corrosion test involving molten eutectic LiF–NaF–KF salt at 850°C contained in 316L stainless steel crucibles shows that a single sealed crucible leaked molten salt into the electric furnace due to mechanical failure, causing a chemical reaction between the molten salt and the clay bonded silicon carbide insulation used in the furnace. As a result, corrosive vapors of Na2SiF6, K2SiF6, SiF3, and F2, were formed. These vapors reacted with the stainless steel crucibles and lead to the formation of a porous corrosion crust and the eventual catastrophic structural failure of all nine sealed 316L stainless steel crucibles.

Thermal Conductivity of Molten Alkali Metal Fluorides (LiF, NaF, KF) and Their Mixtures

The thermal conductivities of molten alkali fluorides (LiF, NaF, and KF) and their mixtures (LiF-NaF, LiF-KF, and NaF-KF binaries and LiF-NaF-KF ternary) are predicted using molecular dynamics simulation with the Green-Kubo method. A polarizable ion model is used to describe the interionic interactions. All the systems except LiF-KF and LiF-NaF-KF mixtures follow a scaling law: it is proportionnal to mA (-1/2)(N/V)(2/3), where mA is the arithmetic average of the ionic species masses in a given melt and N is the total number of ions included in the system volume V. In LiF-KF and LiF-NaF-KF mixtures a significant departure from the scaling law is observed. By examining separately the effects of the cation mass and size asymmetry in LiF-KF mixtures, we show that both of them account for half of the deviation. Finally, we observe that the temperature dependence of the thermal conductivity is very small in these molten fluorides.

First-principles molecular dynamics modeling of the molten fluoride salt with Cr solute

Fluoride salts and their interactions with metals are of wide interest for the nuclear community. In this work, first-principles molecular dynamics (FPMD) was employed to study both pure molten fluoride salt and fluoride salt with dissolved solute Cr ions (a common corrosion product) at high temperature (823–1423K). Two types of molten fluoride salts, namely flibe (LiF–BeF2) and flinak (LiF–NaF–KF), with the Cr0, Cr2+ and Cr3+ ions were chosen as a target system for the FPMD modeling. The prediction of thermo-kinetic properties of pure fluoride salt, such as the equilibrium volume, density, bulk modulus, coefficient of thermal expansion, and self-diffusion coefficient, provide useful extensions of existing data and verify the accuracy of the FPMD simulation in modeling of fluoride salts. The FPMD modeling of solute Cr in fluoride salt shows the effect of Cr valence on diffusivity and local structure in the salt.

High temperature corrosion studies in molten salt-FLiNaK

FLiNaK (LiF–NaF–KF: 46·5–11·5–42 mol.-%) is a promising candidate as high temperature coolant for next generation nuclear reactor coolants. Major technical challenge in implementation is the selection of structural material that can withstand corrosive nature of FLiNaK. Corrosion study of different alloys such as SS-316L, SS-317L, Inconel-625, Incoloy-800H, Hastelloy-B and Ni-201 was performed in FLiNaK salt. The tests were carried out in the temperature range of 650–850°C in graphite crucibles for 60 h under inert atmosphere. Weight loss of the alloy sample due to corrosion was measured, and corrosion rate was estimated. The salt composition after corrosion was analysed by inductively coupled plasma spectroscopy coupled with atomic emission spectroscopy. The surface morphology of the alloy samples was analysed by scanning electron microscopy. Ni-201 and Hastelloy-B showed better resistance to corrosion than other alloys. Corrosion rate was found to increase with increase in Cr content in the alloy.

Long-term corrosion behaviors of Hastelloy-N and Hastelloy-B3 in moisture-containing molten FLiNaK salt environments

This study investigated long-term corrosion behaviors of Ni-based Hastelloy-N and Hastelloy-B3 under moisture-containing molten alkali fluoride salt (LiF–NaF–KF: 46.5–11.5–42%) environment at an ambient temperature of 700°C. The Hastelloy-N and Hastelloy-B3 experienced similar weight losses for tested duration of 100–1000h, which was caused by aggregate dissolution of Cr and Mo into FLiNaK salts. The corrosion rate of both alloys was high initially, but then reduced during the course of the test. The alleviated corrosion rate was due to the depletion of Cr and Mo near surface of the alloys and thus the long-term corrosion rate was controlled by diffusion of Cr and Mo outward to the alloy surface. The results of microstructural characterization revealed that the corrosion pattern for both alloys tended to be intergranular corrosion at early stage of corrosion test, and then transferred to general corrosion for longer immersion hours.

PuF3, AmF3, CeF3, and NdF3 Solubility in LiF–NaF–KF Melt

The temperature dependence of the PuF3, AmF3, CeF3, and NdF3 solubility in the eutectic 46.5LiF–11.5NaF–42KF was measured in the interval 550–700°C. Anhydrous plutonium, americium, cerium and neodymium trifluorides were synthesized. The method of isothermal saturation with dissolution of pressed PuF3, AmF3, CeF3, and NdF3 pellets in melt in an inert argon atmosphere was used for the measurements. It was found that the solubility of trifluorides is high and strongly dependent on the melt temperature. The following values of the solubility were found 700°C: PuF3 30, AmF3 43, CeF3 32, and NdF3 37.

Molten-salt subcritical transplutonium actinide incinerator

The results of calculations of a high-capacity subcritical molten-salt reactor burning transplutonium elements and operating with a fast-intermediate neutron spectrum and the salt LiF–NaF–KF are presented. It is shown that this reactor makes it possible to burn ~300 kg/GW of americium per year, i.e., long-lived wastes from approximately 30 VVER-1000.

Electrochemical separation of uranium in the molten system LiF–NaF–KF–UF4

This article is focused on the electrochemical investigation (cyclic voltammetry and related studies) of possible reduction of U4+ ions to metal uranium in the molten system LiF–NaF–KF(eut.)–UF4 that can provide basis for the electrochemical extraction of uranium from molten salts. Two-step reduction mechanism for U4+ ions involving one electron exchange in soluble/soluble U4+/U3+ system and three electrons exchange in the second step were found on the nickel working electrode. Both steps were found to be reversible and diffusion controlled. Based on cyclic voltammetry, the diffusion coefficients of uranium ions at 530 °C were found to be D(U4+) = 1.64 × 10−5 cm2 s−1 and D(U3+) 1.76 × 10−5 cm2 s−1. Usage of the nickel spiral electrode for electrorefining of uranium showed fairly good feasibility of its extraction. However some oxidant present during the process of electrorefining caused that the solid deposits contained different uranium species such as UF3, UO2 and K3UO2F5.

Phase analysis and volume properties of the (LiF–NaF–KF)eut–K2ZrF6 system

The influence of the addition of K2ZrF6 to the eutectic system of LiF–NaF–KF on temperatures of primary crystallization and volume properties was investigated in the entire composition range. Thermal analysis and Archimedean method were used for the determination of the phase equilibrium and density. Molar volume and excess molar volume were calculated on the basis of the experimental density data. Temperatures of primary crystallization do not change considerably in a wide concentration range $$ x_{{\rm{K_{2} ZrF_{6} }}} = 0.20 - 0.70 $$ and concentration dependences of densities and molar volumes show monotonous behavior for selected temperatures (t = 850, 900, 950 °C). “Compressibility parameters” of K2ZrF6 and (LiF–NaF–KF)eut were calculated in order to discuss the volume contraction or expansion in the investigated system at selected temperatures (t = 850, 900, 950 °C). On the basis of the negative values of the calculated compressibility parameters volume contractions can be considered for both components.

LiF–NaF–KF Molten salt reactor with a fast neutron spectrum

LiF–NaF–KF Molten salt reactor with a fast neutron spectrum