The equilibrium potentials of platinum in a KCl–PbCl2—PtCl2 melt at various temperatures and platinum dichloride contents are measured by the emf method. The empirical equations of the isotherms and polytherms of the equilibrium potentials of platinum are derived. The conditional standard potentials of platinum in a molten mixture of potassium and lead are calculated. The changes in the Gibbs energy upon platinum dichloride formation from elements in the melt under study are determined. The separation factors of the metals from the alloys are estimated. The obtained results indicate prospects of the separation processes of lead and noble metals in chloride melts.

Evaluation has been made of high-temperature vacuum distillation of multicomponent LiCl-based molten mixtures containing the compounds of fission product imitators (alkali and alkali-earth metal chlorides) with a total concentration of about 5 mole percent. Experiments have been conducted under different conditions (temperatures, times, cells). It has been found that the vacuum distillation of the electrolyte and its volatile components (LiCl, RbCl, CsCl) in the temperature interval 750–900°C at pressure P = 1–2 Pa proceeds rapidly and nearly completely if vapor is continuously evacuated from the open surface of the melt. The volatility and evaporation rate (degree of distillation) of the multicomponent melt rise rapidly with temperature mostly owing to its volatile components (LiCl, RbCl, CsCl). The separation ratio (the vapor/melt concentration (in mol

The possibilities of conducting a direct mass spectral determination of Li, Be, Mg, Sr, Zr, Mo, Ru, Rh, Pd, Ag, Cs, Ba, La, Ce, and Nd in the products of processing spent nuclear fuel based on metallic uranium or its oxides are discussed. The optimum method of sample preparation is selected to ensure a quantitative transition of all elements into a working solution. The influence of the uranium content on the analytical signal intensity and the detection limits of the elements to be detected is estimated. The presence of 1 g/L uranium in a solution is found to decrease the analytical signal intensity by 50 times and worsens the detection limits of individual analytes by two orders of magnitude compared to these indicators in uranium-free solutions. The possibility of using internal standards to correct matrix effects is shown.

The interaction between uranium trichloride and molten LiCl–Li2O in the eutectic LiCl–KCl melt at 773, 823, and 873 K is investigated using potentiometry in order to check the completion of UCl3 precipitation reaction. Measuring the equilibrium U3+/U couple potential is found to allow the precipitation of UCl3 to be checked in real time. Quantitative precipitation is observed at a UCl3 : Li2O molar ratio of 1 : 2; in this case, the uranium electrode potential shifts to the negative region by more than 300 mV relatively to equilibrium. Using the obtained experimental data, the apparent standard potentials of the U3+/U couple are determined, and the activity coefficients of UCl3 in the eutectic LiCl–KCl melt are calculated. The activity coefficients of UCl3 at 773, 823, and 873 K are 5.4 × 10–4, 9.1 × 10–4, and 4.8 × 10–3, respectively.

Abstract—The equilibrium potentials of silver are measured by the emf method in a KCl–PbCl2–AgCl melt at different temperatures and silver chloride concentrations. Empirical equations are derived for the equilibrium potential isotherms and polytherms of silver. The conditional standard potentials of silver in a molten mixture of potassium and lead chlorides are calculated. The changes in the Gibbs energy during the formation of silver chloride from the elements are calculated. The separation coefficients of the metals from the alloys are estimated. The obtained results indicate that the processes of lead and silver separation in chloride melts are promising.

Equilibrium potentials of uranium (U3+/U0) have been measured in the LiF-BeF2-UF3 melt as a function of temperature and uranium fluoride concentration. The empirical equation of uranium potential isotherms and polytherms have been obtained. Cathode polarization of uranium in the molten mixture of beryllium and lithium fluorides has been measured using the current switch off method form the stationary state. Uranium ions were found to have primarily the-valence of three in the LiF-BeF2 electrolyte in the studied temperature and uranium fluoride concentration ranges. A conditional standard potential of uranium (U3+/U0) in the molten mixture of lithium and beryllium fluorides was calculated relative to the reference fluorine electrode according to the experimentally obtained data on the equilibrium potentials of the uranium electrode in the LiF-BeF2-UF3 melt. Standard conditional changes in the Gibbs energy, enthalpy and entropy at the formation of uranium trifluoride from the elements in the form of dilute solutions were determined. The enthalpy of mixing of liquid uranium fluoride and the 0.73LiF–0.27BeF2 melt was calculated.

The purification of molten salts from admixtures as well as the effective control of admixture concentration has attracted researchers’ interests. In the present paper, the possibility of the electrochemical purification of PbCl2 from PbO and the effective control over the oxide ions concentration in molten PbCl2 is studied at the temperature of 520 °C. The PbCl2 melt with the initial addition of 0.5 wt% of PbO was used as a molten salt sample. The method of potentiostatic electrolysis was used to remove the oxide additions from the melt; the linear and square-wave voltammetry dependencies were recorded, and the melt samples were taken for analysis. Based both on the results of the electrochemical measurements and the analysis of oxygen concentration in the electrolyte, we built linear empirical dependencies of the anode peak current of the oxidation of oxygen-containing electroactive anions on the PbO concentration in the studied melt. We demonstrated that the obtained dependencies may be used for direct electrochemical nondestructive in-situ control over the concentration of PbO dissolved in the PbCl2 melt containing up to 0.5 wt% of PbO. The deep electrochemical purification of the chloride PbCl2 melt from molten oxide (up to 0.044 wt% PbO or to 0.007 wt% of oxygen) was achieved by the potentiostatic electrolysis.

A procedure for a quantitative determination of the valent forms of uranium in the case of their presence in the molten salt LiCl–KCl–UCl3–UCl4 is described. The procedure is based on an indirect determination of U3+ and U4+ by titration with a potassium dichromate solution of a sample dissolved in iron(III) chloride. The preliminary determination of the weight fraction of total uranium by inductively coupled plasma mass spectrometry is required for the calculation of concentrations of the valent uranium forms in the sample. The relative measurement inaccuracy for the weight fraction of U3+ does not exceed 7.0

The interaction between CrF 2 and a molten binary LiF-BeF 2 mixture has been studied using the electrochemical methods. The polarization of the chromium electrode relative to the dynamic beryllium electrode was measured. The obtained voltammetry data elucidates that under the experimental conditions, chromium is observed exclusively in one valence form. According to the cathode polarization dependences, it is established that the reduction of chromium ions in the metal occurs by a two-electron electrode reaction. The equilibrium concentrations of chromium were experimentally measured at the temperatures of 923, 973, and 1023 K at different chromium fluoride concentrations in the melt. The oxidation degree of chromium, dissolved in the melt, was calculated according to the concentration dependences of the equilibrium potentials. A thermodynamic analysis of the chromium difluoride dissolution in a molten mixture of lithium and beryllium fluorides was performed. The formation of dilute solutions of chromium difluoride is accompanied by slight deviations of the salt system from Raoult’s laws.

Molten fluorides of alkali metals are considered a technological medium for molten salt reactors (MSRs). However, these media are known to be extremely corrosive. The successful implementation of high-temperature technological devices using molten alkali metal fluorides requires the selection of such structural materials that have high corrosion resistance in melts with compositional characteristic of MSRs. In this research, the corrosion behavior of 12Cr18Ni10Ti steel, the alloy Ni60Cr20Mo15, and the alloy Monel 404 (Ni50Cu50) was investigated in the LiF–NaF–KF eutectic melt, containing additions of CeF3 and NdF3 from 0 to 5 wt.% as imitator fluorides of actinides in an inert argon atmosphere at 550 °C for 100 h. Gravimetry, energy-dispersive X-ray (EDX) microanalysis of surfaces and cross-section of samples, and ICP-MS were used to establish the corrosion behavior of the investigated alloys. Corrosion resistance of the studied materials was found to decrease in a row from Monel 404 > Hastelloy C2000 > 12Cr18Ni10Ti. The addition of cerium fluoride into the melt resulted in the additional etching of the alloy surface. The addition of neodymium fluoride resulted in the formation of the point/inter-crystalline corrosion damages in the sample bulk. The samples of steel 12Cr18Ni10Ti were subjected to local cracking corrosion. The austenitic nickel-based alloys suffered specific local corrosion with formation of subsurface voids. Excellent corrosion resistance of the Monel alloy under the test conditions was found.

The equilibrium potentials of iron in a LiF-BeF2-FeF2 melt were measured using the EMF method and were dependent upon the temperature and iron fluoride concentrations. The empirical equations for the isotherms and equilibrium polytherms of the iron fluoride concentration were obtained. The cathode polarization of iron fluoride in the molten mixture of lithium and beryllium fluoride was measured using the current switch off method from the stationary state. It was found that in the studied temperature and concentration ranges of iron fluoride in the LiF-BeF2 electrolyte, the valence state of iron in the melt is mainly +2. According to the experimental values of the equilibrium potentials of the iron electrode in the LiF-BeF2-FeF2 melt, the conditional standard potentials of iron were calculated relative to the fluoride reference electrode in the molten mixture of lithium and beryllium fluoride. The conditional standard values of the Gibbs energy change were calculated at the formation of iron difluoride from the element in the form of dilute solutions, as were the thermodynamic values (enthalpy and entropy) when iron difluoride was mixed with LiF-BeF2.

An installation for purifying alkali-metal chlorides, by means of zone recrystallization, for use in the basic operations of the pyrochemical processing of spent fuel from fast reactors is considered. The main technological operations for purifying chlorides at the proposed installation, which contains units for preparation, primary remelting, zone recrystallization, and packaging of finished salts, are described. The installation allows alkali-metal chlorides to be obtained very efficiently and with the lowest possible content of oxygen-containing impurities because all purification operations are performed in a way where the salt to be purified does not come into contact with a humid atmosphere. By way of an example, experimental confirmation was obtained for the purification of 100 kg lithium chloride and the optimal parameters for the purification of the salt were determined.

Platinum anodes are widely used for metal oxides reduction in LiCl–Li2O, however high-cost and low-corrosion resistance hinder their implementation. NiO–Li2O ceramics is an alternative corrosion resistant anode material. Anode processes on platinum and NiO–Li2O ceramics were studied in (80 mol.%)LiCl-(20mol.%)KCl and (80 mol.%)LiCl-(20 mol.%)KCl–Li2O melts by cyclic voltammetry, potentiostatic and galvanostatic electrolysis. Experiments performed in the LiCl–KCl melt without Li2O illustrate that a Pt anode dissolution causes the Pt2+ ions formation at 3.14 V and 550°С and at 3.04 V and 650оС. A two-stage Pt oxidation was observed in the melts with the Li2O at 2.40 ÷ 2.43 V, which resulted in the Li2PtO3 formation. Oxygen current efficiency of the Pt anode at 2.8 V and 650°С reached about 96%. The anode process on the NiO–Li2O electrode in the LiCl–KCl melt without Li2O proceeds at the potentials more positive than 3.1 V and results in the electrochemical decomposition of ceramic electrode to NiO and O2. Oxygen current efficiency on NiO–Li2O is close to 100%. The NiO–Li2O ceramic anode demonstrated good electrochemical characteristics during the galvanostatic electrolysis at 0.25 A/cm2 for 35 h and may be successfully used for pyrochemical treating of spent nuclear fuel.

Crystallization temperatures (liquidus and solidus) in the LiCl-Li2O and (LiCl-KCl)-Li2O systems with the KCl content of 10 and 20 mol.% were obtained with independent methods of thermal analysis using cooling curves, isothermal saturation, and differential scanning calorimetry. The linear sweep voltammetry was applied to control the time of the equilibrium establishment in the molten system after the Li2O addition, which depended on the composition of the base melt and the concentration of Li2O. The fragments of the binary LiCl-Li2O and quazi-binary [LiCl-KCl(10 mol.%)]-Li2O and [LiCl-KCl(20 mol.%)]-Li2O phase diagrams in the Li2O concentration range from 0 to 12 mol.% were obtained. The KCl presence in the LiCl-KCl-Li2O molten mixture in the amount of 10 and 20 mol.% reduces the liquidus temperature by 30 and 80°, respectively, but the region of the homogeneous molten state of the system is considerably narrowed, which complicates its practical application. The Li2O solubility in the molten LiCl, LiCl-KCl(10 mol.%) and LiCl-KCl(20 mol.%) decreases with increasing the KCl content and is equal to 11.5, 7.7 and 3.9 mol.% at 650°С, respectively. The LiCl-KCl melt with 10 mol.% KCl can be recommended for practical use as a medium for the SNF pyro-chemical reprocessing at temperature below 700 °C.

The phase diagrams of the binary LiCl–Li 2 O system and the quasi-binary [LiCl–KCl]–Li 2 O systems containing 10 and 20 mol % KCl have been built using thermal analysis of cooling curves and isothermal saturation curves. The solubility of Li 2 O in LiCl–KCl melts is determined in the temperature range 500–800°C. The solubility of Li 2 O in the melts LiCl, LiCl–KCl (10 mol %) and LiCl–KCl (20 mol %) decreases as the KCl content increases;at 650°C, it is 11.5, 7.7, and 3.9 mol %, respectively.

Application of protective aluminum-based coatings is one of the ways to increase the oxidation resistance of low-carbon steel. The electrolytic deposition of aluminum in the NaF33-KF15.2-(AlF3)51.8 wt.% melt at 920°C and the current density of 0.8-1.0 A·cm-2 (0.25 A·h·cm-2) have provided a continuous aluminide coating based on Fe2Al5 with a good adhesion to the steel substrate due to dissolution of oxide film from the surface of the treated products in fluoride melt. The resistance of steel samples to sulfide corrosion was investigated in a lab-scale three-electrode cell at 900°C in contact with a “Soderberg” carbon anode, which was obtained by carbonization of the coal tar pitch containing 2.0-2.5 wt.% of sulphur. The IRC (ohmic voltage drop in contact layer) growth rate was 3.3 times higher for uncoated steel due to formation of the oxide-sulfide layer based on FeS. The electrolytically aluminized steel with preliminarily formed α-Al2O3 layer possessed more stable value of the IRC in comparison with the uncoated steel because of the higher chemical resistance.

Electrolytic aluminizing of low-carbon steel in NaF44.4-(AlF3)(55.6) (wt.%) melt implies formation of a multi-phase diffusion zone consisting of iron aluminides including FeAl3, Fe2Al5, Fe3Al and Al solid solution in alpha-Fe (Fe(Al)). The influence of current density on morphology of the aluminide coating was investigated and the most uniform and continuous intermetallic layer with fewer inclusions of electrolyte obtained at the current density of 0.8 A.cm(-2). An aluminum oxide layer formed on the surface of the steel during oxidation treatment at elevated temperature (700, 900 degrees C) in air acts as a diffusion barrier and protects the material from further oxidation.