Abstract—One of the methods for high-temperature treatment (HTT) of nitride spent nuclear fuel (SNF) of fast neutron reactors is its direct oxidation. At this stage of processing, it is desirable to remove radioactive cesium, which is in the form of uranates in an oxidizing atmosphere, from SNF. The behavior of the Cs2U2O7 cesium diuranate upon heating to 1000°C is investigated by of synchronous thermal analysis, isothermal holding, and vacuum sublimation. Samples were analyzed by X-ray diffraction, microstructural, and chemical analyses. The data of thermal analysis and isothermal holding at 150, 400, 700, and 900°C indicate that the decomposition of cesium diuranate to form monouranate and volatile products begins at temperatures of 150–200°C, the complete conversion of diuranate into monouranate ends by 700°C, and complex uranium oxides form. The sample mass loss upon holding at 700°C is 8

Abstract—Molten mixtures of sodium and potassium cryolites KF–(10 wt

The thermophysical properties of molten salts promising for the nuclear industry are crucial, but the available data are limited and contradictory. The thermal diffusivity of the molten mixtures (NaF-KF)eut–UF4 containing 30, 40, and 50 mol

Abstract—The main problems hindering the determination of oxygen in fluoride salts by carbothermic reduction have been identified. They include the volatility of salts when heated to high temperatures, hygroscopicity, and the absence of appropriate standard samples and determination techniques. Most of these problems can be eliminated by selecting the correct analysis conditions and modifying the equipment. A bench setup installed in a glove-box for carbothermic oxygen determination has been created at the Institute of High-Temperature Electrochemistry in cooperation with ZAO Spectroscopic Systems, a Russian manufacturer of glove-box systems, and NPO Eksan, a manufacturer of gas analyzers. Potassium, sodium, and lithium fluorides and the LiF–NaF–KF salt system synthesized by melting the initial components are analyzed using the modified and classical apparatus systems. According to the results of analysis, the oxygen concentration in hygroscopic FLiNaK and KF powders, the sample preparation and analysis of which were carried out in the box-containing setup, is significantly lower than that determined in air.

A technique for measuring the viscosities of cryolite melts by the rotational method using a high-temperature FRS-1600 rheometer has been developed. The viscosities of NaF–AlF3 sodium cryolite melts with a cryolite ratio (CR) of 2.1, 2.3, 2.5 and KF–AlF3 potassium cryolite melts with CR = 1.5, which are used in the electrolytic production of aluminum and its alloys in industrial electrolysis cells and laboratory installations, are determined. The shear rate dependence of viscosity is determined using flow curves, which characterize the relationship between the shear stress and the shear rate, and viscosity curves, which represent the shear rate dependence of viscosity. The shear rate parameter at which a melt behaves like a Newtonian fluid is then chosen. The laminar flow region of the cryolite melts is in the shear rate range 10–16 s–1. The viscosity of the NaF–AlF3 sodium cryolite is measured in the temperature range from the liquidus point to 1293 K at a constant shear rate of 12 s–1. The viscosity of the cryolite melts decreases with decreasing CR. At 1283 K, the viscosity of the NaF–AlF3 melts with CR = 2.1, 2.3, 2.5 is 2.35, 2.43, and 2.50 MPa s, respectively. These values exceed the well-known literature data by 25

A thermodynamic analysis of the equilibrium in the LiF–BeF2–BeO–C system at temperatures below 3000°C is carried out. The conditions and parameters of the carbothermic reduction of beryllium oxide dissolved in FLiBe are chosen on the basis of the analysis results. The occurrence of the reaction BeO(s) + C(s) = Be(s/l) + CO(g) is thermodynamically possible at temperatures above 2500°C. Such a high temperature undesirable for an analysis of a molten salt with a significantly lower liquidus temperature can be decreased by adding a metal that forms alloys with beryllium with a low activity coefficient via the reaction BeO(s) + C(s) = M(Be)(l) + CO(g). A thermodynamic analysis of the system 66 kmol

The possibility of producing aluminum alloys during low-temperature electrolysis in cells with vertical low-consumption metal anodes and wetted cathodes is shown. Aluminum alloys are fabricated by electrolysis of KF–NaF (10 wt

The present paper is dedicated to the quantitative determination of oxygen-containing impurities in the LiF–NaF–KF eutectic using electrochemical (cyclic and square-wave voltammetry) and reduction melting methods. The LiF–NaF–KF melt was analyzed before and after purifying electrolysis. The amount of oxygen-containing impurities removed from the salt during purification was determined. It was found that after electrolysis, the concentration of oxygen-containing impurities decreased by 7 times. The results obtained via electrochemical techniques and reduction melting were well-correlated, which made it possible to evaluate the quality of the LiF–NaF–KF F melt. To verify the analysis conditions, mechanical mixtures of LiF–NaF–KF containing Li2O were analyzed using the reduction melting method. The oxygen concentration in the mixtures varied from 0.672 to 2.554 wt. %. Based on the analysis results, the dependence approximated by the straight line was obtained. These data may be used to draw calibration curves and to further develop the procedure of oxygen analysis of fluoride melts.

The viscosity of molten salt, as an important hydrodynamic property, should be taken into account when creating and operating molten salt nuclear reactors (MSRs). An eutectic FLiNaK is considered to be one of the most suitable for use in MSR designed for the minor actinides transmutation. The dynamic viscosity of the molten mixtures FLiNaK + NdF3, FLiNaK + CeF3 and FLiNaK + LaF3 was measured in a temperature range of 600–700 °C using the high-temperature rotary rheometer FRS-1600. Lanthanide fluorides were considered as analogues of actinide fluorides. It was revealed that the additions of rare earth fluorides (REM)F3 in amount of 15 mol. % significantly impact the viscosity of the system FLiNaK + (REM)F3,but the effect of NdF3, CeF3 and LaF3 was found to be almost the same. In order to calculate the kinematic viscosity of the molten mixture FLiNaK + NdF3, a regression equation depending on several parameters was derived. This model equation can be used for predicting the kinematic viscosity of molten mixtures of FLiNaK with other rare earth fluorides.

The paper presents the results, which are consistent within 2%, obtained both in the simulation of molecular dynamics and in the experiment on the study of the kinetic properties of molten FLiNaK with addition of lanthanide fluorides. The parameters of the Born-Huggins-Meier potential for the interaction of CeF3 or NdF3 with FLiNaK components are first calculated using the ab initio approach. The enthalpy of the system with dissolved CeF3 or NdF3 calculated in the model increases by ∼4.4% over the entire temperature range studied (800 ≤ T ≤ 1020 K). The self-diffusion coefficients of the molten salt components are calculated from the Einstein relation and also estimated from the shear viscosity data. The temperature dependences of the shear viscosity of molten FLiNaK as well as FLiNaK with additions of 15 mol % CeF3 or NdF3 are determined experimentally and by calculation. In addition, the dependence of shear viscosity on the concentration of CeF3 and NdF3 in FLiNaK is measured and calculated. The linear growth of the shear viscosity with the CeF3 and NdF3 concentrations is obtained. Experimental dependence is in good agreement with the simulated results in the case of NdF3, and there is the discrepancy while CeF3 addition. An analytical approximation of the temperature and concentration dependences for the viscosity of molten FliNaK and for the calculated self-diffusion coefficients of constituent elements is proposed. Linear approximation of temperature dependence of the self-diffusion coefficients of similar components in the corresponding extended systems is presented.

The process of electrolytic production of Al–Y and Al–Sc alloys in an electrolyte based on potassium cryolite KF–NaF(10 wt %)–AlF3 with a cryolite ratio (CR) of 1.5, containing Al2O3, Sc2O3, or Y2O3 oxides, in a cell with vertical electrodes has been studied. The Fe–Ni–Cu alloy served as an inert anode. The wetted cathode was a graphite plate coated with the aluminum diboride. The electrolysis was carried out at a cathode current density of 0.2 A/cm2 and a temperature of 830°C. The Al2O3 mass was calculated based on the value of the current efficiency of 60%. The Sc2O3 additive was introduced into the melt in an amount of 1 wt %. The mass of the Y2O3 additive was chosen based on its solubility in the melt under study. For this, the influence of Y2O3 additives on the liquidus temperature of the quasi-binary mixture [KF–NaF(10 wt %)–AlF3 (KO = 1.5)]–Y2O3 was determined and it was found that, in contrast to Sc2O3 additives, which lower the liquidus temperature of the cryolite melt, small additions of Y2O3 lead to its sharp increase. It has been found that the efficiency of the electrolytic reduction of Y2O3 is 10 times higher than that of the aluminothermic reduction. Other things being equal, the efficiency of the electrolytic reduction of Y2O3 is higher than that of Sc2O3. Alloys Al–Y and Al–Sc with a REM content of 0.6 wt % have been obtained. However, the time to reach the maximum recovery of yttrium significantly exceeds the time to recover scandium. Metallographic studies of the obtained alloys indicated the presence of Al3Sc and Al2Y intermetallic compounds. A conclusion is made about the fundamental possibility of low-temperature electrolytic production of Al-REM alloys in cryolite melts based on potassium cryolite in vertical cells with an inert metal anode and a wettable cathode.

Rotational viscometry with the FRS 1600 (Anton Paar, Austria) high-temperature rheometer was used to obtain temperature dependences of the dynamic viscosity of molten lithium and beryllium fluoride salts considered as candidate fuel and coolant compositions for the molten salt reactor (MSR) for burning long-lived actinides from the spent nuclear fuel of the PWR 1000/1200 pressurized water reactor. 0.66LiF–0.34BeF2 and (0.73LiF–0.27BeF2)+UF4 molten salt mixtures containing 1 and 2 mol.% UF4 were investigated with regard to the MSR intermediate and fuel circuits. Salt mixtures were prepared by the direct melting of components and certified using X-ray phase and elemental analysis. The «shear rate» parameter was selected according to the viscosity curves obtained in the studied melts at 700 °C. It was found that the viscosity does not depend on the shear rate in the range of γ = 6÷20 s–1. When measuring the temperature dependence of viscosity, the shear rate was 11 s–1. Viscosity values of LiF–BeF2–UF4 melts obtained from experiments in the temperature range from liquidus to 800 °C are described by the linear equation logη = a + b/t, but their temperature coefficients differ evidently, which indicates a significant dependence of the viscosity of these melts on composition and temperature. Viscosity values obtained for the 0.66LiF–0.34BeF2 melt agree with the available literature data within 7–10 % in the temperature range of 650–750 °C. With an increase in the LiF content, melt viscosity decreases: it is lower by 20 % in the 0.73LiF–0.27BeF2 melt at t = 650 °C. However, when 2 mol.% UF4 is added, the 0.73LiF–0.27BeF2+UF4 fuel salt viscosity increases by 10 % at the same temperature.

The corrosion of structural materials in fluoride media is a key problem on organizing many technological processes and scientific research. In this connection, when the physicochemical properties of fluoride melts are studied, specific requirements are imposed on the structural materials of electrochemical devices. In this work, a method is developed to measure the electrical conductivity of liquid fluoride systems in a conductometric cell with coaxially arranged electrodes. The electrodes are made of glassy carbon, which is resistant to fluoride melts under the action of rf alternating current. The inner electrode is a rod disposed coaxially inside the outer electrode (tube). The electrode system can be dipped to any depth; in this case, the melt volume is constant over wide temperature range. Electrochemical impedance spectroscopy is used to measure the electrical resistance of the melt in the ac frequency range from 1 Hz to 105 kHz at a voltage amplitude of 5 mV. The coaxial cell was calibrated against molten CsCl salt in the temperature range 660–880°C on heating and cooling. The temperature dependence of the cell constant is used to calculate the electrical conductivity of oxide–fluoride systems 0.565KF–AlF3 with a molar fraction ratio xKF/ $${{x}_{{{\text{Al}}{{{\text{F}}}_{3}}}}}$$ = 1.3 and (KF–AlF3)–Sc2O3 with Sc2O3 contents of 1, 2, and 3 wt %. The electrical conductivities of the 0.565KF–AlF3 melt measured in cells of various designs (coaxial, with parallel molybdenum electrodes, and with a BN capillary) coincide within 1%. In the range 590–720°C, the temperature dependences of the electrical conductivity of the (KF–AlF3)–Sc2O3 systems have inflection points corresponding to their liquidus temperatures. The coaxial cell can be used to measure the electrical conductivity of aggressive fluoride and oxide–fluoride systems over a wide temperature range, including a heterogeneous region.

The dynamic viscosity (η) of the molten system (NaF-KF)eut-NdF3 containing NdF3 in an amount from 0 to 15 mol.% was studied by rotational viscometry using a high-temperature rheometer, FRS 1600. Viscosity measurements were carried out in the temperature range from liquidus to 1153 K. The measurement procedure was tested on the (LiF-NaF-KF)eut melt. The choice of the parameter shear rate was carried out according to the viscosity and flow curves. Viscosity does not depend on shear rate, and therefore the investigated melts behave like Newtonian fluids, in the range of 9–19 s−1. The experimentally obtained viscosity values for (NaF-KF)eut-NdF3 melts in a wide temperature range are described by an exponential equation. In the coordinates ln(η) = f(1/T), they are straight lines; however, their temperature coefficients are noticeably different, which indicates significant impacts of composition and temperature.

A eutectic mixture of lithium, sodium, and potassium fluorides is considered as a fuel salt for molten-salt reactors. For this reason, information about the solubility in it of fluorides of fissile materials and fission products is the most important when choosing a salt composition for a molten-salt reactor. In this work, the liquidus and solidus temperatures of the LiF–NaF–KF–CeF3 quasi-binary system are determined by thermal analysis and differential scanning calorimetry. The system has one eutectic point at CeF3 content 3 mol.% and temperature 445°C and three peritectic points – 7.5, 15, and 20.5 mol.% at 472, 512, and 600°C, respectively. The solubility of CeF3 in LiF–NaF–KF is 10 mol.% at 500°C and reaches 25 mol.% at 665°С.

Electrochemical boriding of the graphite plates in the potassium cryolite based electrolytes was studied. The boriding were carried out in a cell with vertical electrodes. The procedure included 2 stages: 1) electrolysis in the KF–AlF3–KBF4 melt (CR=1.3) at low current density (0.01–0.02 A/cm2), required for the boron reduction, at 700 and 750 °C; 2) electrolysis in the KF–AlF3–Al2O3 melt at higher current density (0.2 A/cm2), required for the aluminum reduction. The optimal conditions of electrodeposition for obtaining the borated wettable cathodes were determined. According to the SEM data, a continuous AlB2 layer with a thickness of 7–10 μm was formed on the graphite surface. The borated graphite was tested as a wetted cathode during the low-temperature aluminum electrolysis. Prolonged electrolysis in a vertical cell with the graphite anode and the borated graphite cathode was carried out in the KF–NaF(10 wt.%)–AlF3–Al2O3 electrolyte (CR=1.5) at 830 °C. After 100 h of electrolysis, the thickness of the AlB2 layer on the graphite surface was 5 μm, while the cathode surface was completely wetted with aluminum. Thus, we demonstrated the feasibility of using the borated graphite cathode as a wettable dripping cathode in the low-temperature aluminum electrolysis in the vertical cell.

The study covers the viscosity of NaF–AlF3–CaF2–Al2O3 conventional cryolite-alumina melts with a cryolite ratio CR = 2.3 depending on the CaF2, Al2O3 content and temperature. The viscosity of cryolite-alumina electrolyte samples prepared under laboratory conditions and electrolyte samples of industrial electrolytic cells was measured by the rotary method using the FRS 1600 rheometer («Anton Paar», Austria). The laminar flow region of the melt determined according to the dependence of viscosity on shear rate at a constant temperature was 10–15 s–1 for all the studied samples. The temperature dependence of cryolite-alumina melt viscosity was measured at a shear rate of 12 ± 1 s–1 in the temperature range from liquidus to 1020 °C. It was shown that the change in the viscosity of all samples in the investigated temperature range (50–80 °С) can be described by a linear equation. The average temperature coefficient of linear equations describing the viscosity of cryolite-alumina electrolytes prepared in laboratory conditions was 0.005 mPа· s/°С, which is 2 times less compared to industrial cell electrolytes. Thus, the change in the viscosity of industrial cell electrolytes with increasing temperature is more significant. Both alumina and calcium fluoride additives increase the cryolite melt viscosity. The viscosity of samples prepared with the conventional composition NaF–AlF3–5%CaF2–4%Al2O3 (CR = 2.3) is equal to 3.11 ± 0.04 mPа· s at an electrolysis operating temperature of 960 °C, while the viscosity of industrial cell electrolytes with the same cryolite ratio is 10–15 % higher and falls in the range of 3.0–3.7 mPа· s depending on the electrolyte composition.

A new way to reduce the energy consumption during the operation of powerful aluminum reduction cells is suggested via reducing the resistance of the electrolyte, i.e., increasing its electrical conductivity. The electrical conductivity of molten cryolite mixtures NaF-AlF3-CaF2-Al2O3 with cryolite ratio (CR) of 2.1–3.0 and content of CaF2 and Al2O3, up to 8 wt%, was measured at the temperatures from liquidus to 1300 K. Based on the experimental results, a multifunctional equation for the electrical conductivity of oxide-fluoride cryolite melts was evaluated. The experimental and calculated values of the electrical conductivity agree within 1.5%. The activation energy of the electrical conductivity of the NaF-AlF3-CaF2-Al2O3 melts was estimated. The activation energy of electrical conductivity for molten NaF-AlF3 mixtures with CR 3.0 and 2.1, determined by the most mobile cations Na+, increased from 15.8 kJ/mol up to 18.5 kJ/mol. It was found that CR had a greater impact on the activation energy than the changes in the Al2O3 or CaF2 concentrations. Based on the ratio of the activation energies of the electrical conductivity and the viscous flow, the correlation between the electrical conductivity and viscosity of molten cryolite mixtures NaF-AlF3-CaF2-Al2O3 was illustrated.

The NdF3 solubility in molten eutectic FLiNaK, which is a conceivable medium for a molten salt reactor (MSR), was determined by the quasi-binary phase diagram FLiNaK-NdF3. The eutectic mixture FLiNaK was prepared by direct melting of components LiF, NaF and KF·HF. The acidic anhydrous salt (KF·HF) was used instead of the hygroscopic KF. The NdF3 was sintered by hydrofluorination of Nd2O3. The oxygen impurity in the prepared eutectic FLiNaK, determined by an oxygen analyzer LECO OH836, was 0.036 wt.%, whereas the NdF3 contained 0.04 wt.% of oxygen. A part of the FLiNaK-NdF3 quasi-binary phase diagram was obtained using two thermal analysis techniques: differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The FLiNaK-NdF3 phase diagram in the region of 0–30 mol.% NdF3 contains one eutectic at 2 mol.% NdF3 and 450 °C and two peritectic points: 8 mol.% NdF3 at 500 °C and 22 mol.% NdF3 at 575 °C. The region of the FLiNaK-NdF3 phase diagram below the liquidus line is rather complicated due to the complex structure of the multicomponent system in its molten state, as in its solid state. The NdF3 solubility in FLiNaK is about 5 mol.% at 490 °C and 29 mol.% at 700 °C; this means that the process of the MA transmutation in the MSR can be carried out in molten FLiNaK with a content of actinides as high as 15–20 mol.% in the temperature range of 550–650 °C.

This paper presents experimental data on the dependence on alumina and calcium fluoride concentrations of the viscosity of the industrial electrolytes NaF-AlF3-CaF2-Al2O3 with cryolite ratios (CR) of 2.1, 2.3, 2.5 in the temperature range 1243–1293 K. The temperature dependence of the formation of a solid phase on the graphite crucible wall in cryolite-alumina electrolyte during aluminium electrolysis was studied in a laboratory-scale electrolytic cell. The viscosity of the industrial electrolytes was found to depend more on the temperature and cryolite ratio than on the alumina concentration in the melt. A long period of electrolysis with controlled solid phase formation on the cell wall was carried out, with a constant electrolyte composition (CR = 2.4), a calcium fluoride concentration of 5 wt% and an alumina content of 4.0 wt%. The formation of a stable solid phase layer on the graphite crucible wall was found to depend on the electrolysis temperature. A correlation was observed between the formation of a solid under the influence of temperature and the viscosity of the melt.