NaF KF Research Articles

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.

The Composition of the Second Coordination Sphere and Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Halide Melts

A study of the effect of the second coordination sphere on the standard rate constants of charge transfer (k s) for the Nb(V)/Nb(IV) redox couple in chloride-fluoride and fluoride melts is presented. By using cyclic voltammetry, the following series of the standard rate constants of charge transfer has been experimentally determined: k s (KCl) < k s (CsCl) < k s (NaCl–KCl). Quantum-chemical calculations showed that charge transfer activation energies could actually change in a nonmonotonic manner in the sodium-potassium-cesium series, resulting in a nonmonotonic change of charge transfer of rate constants in chloride-fluoride melts. In pure fluoride melts, the following dependence of the standard rate constants has been obtained: ks (CsF) < ks (KF) < ks (NaF–KF). It has been established that ks tend to increase in response to both temperature increase and substitution of a glassy-carbon electrode for a platinum one.

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.

CeF3, PuF3 Solubility in LiF–NaF–KF Melt

The solubility of PuF3 and CeF3 in melt of the salt 46.5LiF–11.5NaF–42KF with eutectic composition at 550–690°C was measured and their solubility was found to be high (~30 mol. fraction at 700°C). It was shown that CeF3 can serve to imitate PuF3 in a wide temperature range.

Fuel Salt for the Molten-Salt Reactor

The solubility of Th, U, Ce, La, and Pr in melted salt 45LiF–12NaF–43KF, which supports a fast neutron spectrum for the molten-salt reactor, is found experimentally to be high in the interval 550–700°C. On the basis of an analysis of the physicochemical and nuclear-physical properties of different fluoride salts, it is proposed that the eutectic 46.5LiF–11.5NaF–42KF be used as the fuel salt for a molten-salt fast reactor with a uranium-plutonium fuel cycle.

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

Surface tension and viscosity of the molten (LiF–NaF–KF)eut–K2ZrF6 system

Surface tension and viscosity of the system (LiF–NaF–KF)eut–K2ZrF6 were measured in its entire concentration range. Surface tension was measured by the maximal bubble pressure method, and viscosity was measured using the computerized torsion pendulum method. Concentration dependencies of both quantities were calculated for three selected temperatures T = 1,123, 1,173, and 1,223 K on the basis of the experimental data. The values of the surface tension lie in the region 96–160 mN m−1 and the values of the viscosity are in the region 0.86–3.06 mPa s.

Transport numbers in the molten system NaF–KF–AlF3–Al2O3

Transport numbers in the molten system NaF–KF–AlF3 (Al2O3, CaF2) were investigated by the Hittorf method. It was confirmed that in molten cryolite, Na3AlF6, 1,010 °C, the current is transported almost exclusively by the Na+ cations (t(Na+) = 0.99). When AlF3 was added to a Na3AlF6 melt, the transport number of sodium cations decreased to 0.74 at the composition corresponding to NaAlF4. In molten K3AlF6, the transport number of K+ cations equals 0.836 at 1,005 °C. In melts containing both Na+ and K+, the cations contribute to the charge transport approximately in the ratio of the squares of their ionic radii. When 5 mass % of CaF2 was added to the molten NaF–KF–AlF3 system, it remarkably influenced the transport numbers of potassium and fluoride anions.

Silicon electrodeposition in molten fluorides

Silicon nucleation process was investigated in molten NaF–KF (40–60 mol%) on silver electrodes in the 820–950 °C temperature range in order to optimize silicon coating operating conditions. Chronoamperometric measurements evidenced that silicon electrodeposition process involved an instantaneous nucleation with diffusion-controlled nuclei growth whatever temperature and Si(IV) ions concentration in the mixture. The overpotential and temperature influence on nucleation sites number was also studied. Silicon deposits were obtained using the same temperature range as nucleation study, for different current densities on substrates: Ni, Ag, C graphite and C vitreous. A sensitive influence of the cathodic substrate on the deposit adherence and roughness was observed and discussed.

Corrosion of oxide dispersion strengthened iron–chromium steels and tantalum in fluoride salt coolant: An in situ compatibility study for fusion and fusion–fission hybrid reactor concepts

Primary candidate classes of materials for future nuclear power plants, whether they be fission, fusion or hybrids, include oxide dispersion strengthened (ODS) ferritic steels which rely on a dispersion of nano-oxide particles in the matrix for both mechanical strength and swelling resistance, or tantalum alloys which have an inherent neutron-induced swelling resistance and high temperature strength. For high temperature operation, eutectic molten lithium containing fluoride salts are attractive because of their breeding capability as well as their relatively high thermal capacity, which allow for a higher average operating temperature that increases power production. In this paper we test the compatibility of Flinak (LiF–NaF–KF) salts on ODS steels, comparing the performance of current generation ODS steels developed at Kyoto University with the commercial alloy MA956. Pure tantalum was also tested for comparative purposes. In situ data was obtained for temperatures ranging from 600 to 900 °C using a custom-built high temperature electrochemical impedance spectroscopy cell. Results for ODS steels show that steel/coolant interfacial resistance increases from 600 to 800 °C due to an aluminum enriched layer forming at the surface, however an increase in temperature to 900 °C causes this layer to break up and aggressive attack to occur. Performance of current generation ODS steels surpassed that of the MA956 ODS steel, with an in situ impedance behavior similar or better than that of pure tantalum.

Electrochemical behavior of silicon compound in LiF–NaF–KF–Na2SiF6 molten salt

The electrochemical reduction and nucleation process of Si4+ on an electrical steel electrode in the eutectic LiF–NaF–KF molten salt were investigated at 750 °C, by means of cyclic voltammetry and chronoamperometry technique. Silicon was electrodeposited on steel, and Fe3Si was formed by the diffusivity of silicon on the electrode surface. The electrochemical reduction of Si4+ process in single-step charge transfer and the cathode process was reversible. The electrocrystallization process of silicon is controlled by progressive three-dimensional mechanism. The diffusion coefficient was calculated to be 5.42 × 10−7 cm2/s by chronopotentiometry at experimental conditions.