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Transient electrochemistry techniques (cyclic and square wave voltammetry) were applied to studying the kinetics of reaction of oxygen with solutions of samarium chloride in molten alkali chlorides. The experiments were performed in LiCl–KCl and NaCl–KCl–CsCl eutectic mixtures at 550–750 oC and in NaCl–KCl equimolar mixture 750 oC. The course of the reactions was followed by recording voltammograms at the certain time intervals during bubbling excess oxygen through the melt. Treating samarium containing melts with oxygen resulted in decreasing concentration of samarium ions in the electrolyte. The reaction had first-order kinetics in respect to samarium chloride. Reaction rate constants and activation energy of the reaction were determined.

The effect of electrolysis conditions on the composition of cathodic products formed in Li2MoO4–K2MoO4–MoO3–UO2MoO4 melts was investigated. The experiments were performed in the melts based on (1–x)(0.605Li2MoO4–0.395K2MoO4)–xMoO3 mixtures (x = 0.3–0.5) at 550–800 oC. Electrolysis was performed in the galvanostatic mode on platinum cathode. The initial current density was varied between 0.05–1 A/cm2. Depending on the electrolysis conditions the cathodic deposits consisted of UO2+x, U4O9–y, U1,5Mo10O32, Mo2UO8 and MoO2 phases. Phase composition and oxygen-to-uranium ratio were determined for the products obtained. Conditions allowed obtaining uranium oxide with minimal molybdenum contamination were determined, and confirmed by the bulk electrolysis runs.

The processes that occur during the reactions of barium and strontium chlorides with sodium orthophosphate in the melts based on LiCl–KCl (550°C) and NaCl–KCl–CsCl eutectic mixtures (650°C), a NaCl–KCl equimolar mixture (750°C), and a NaCl–KCl–CsCl–LaCl3 melt (650°C) in an inert atmosphere are studied. The compositions of the formed poorly soluble phosphates are determined by chemical and X-ray diffraction analyses. The influence of the molar ratio of phosphate ions to Sr2+/Ba2+ on the phase composition of the precipitated phosphates is studied. The conditions required for the quantitative precipitation of strontium and barium as phosphates from the melt are determined.

The reactions of oxygen and oxygen-containing gas mixtures (Ar–O2, O2–H2O, and Ar–O2–H2O of various composition) with chloride melts containing uranium(III) ions are studied. Experiments are carried out in molten LiCl at 750°C and a LiCl–KCl eutectic mixture at 550 and 750°C. The uranium content in the melts was varied in the range 0.3–3.5 wt %. The effects of the oxygen to uranium molar ratio and the gas phase composition on the degree of precipitation of uranium from the melt and the composition of the formed products are studied.

Reaction of oxygen with solutions of uranium tetrachloride in molten alkali chlorides was studied. The experiments were performed in LiCl–KCl, NaCl–KCl–CsCl, and NaCl–CsCl eutectic based melts at 450–750oC. Pure oxygen and argon–oxygen mixtures (containing ca. 1 and 10% O2) were used. Amount of oxygen passed through the melt varied from less than one to over 100 moles per mole of uranium present. In addition the reaction of oxygen with the melts containing a mixture of uranium(IV) and neodymium chlorides was investigated. The course of the reactions was followed by in situ electronic absorption spectroscopy measurements with the spectra recorded at the certain time intervals. Depending on temperature, cationic melt composition, and oxygen-to-uranium molar ratio, the reaction resulted in oxidation of uranium(IV) to soluble uranyl chloride and/or precipitation of uranium oxide. Analysis of the spectra provided the information on kinetics of U(IV) concentration change and the reaction rates were determined. Increasing temperature, O2 : U(IV) molar ratio or decreasing mean radius of alkali cations of the solvent melt resulted in faster decrease of U(IV) concentration in the melt.

Molten halides can be effectively employed for electrorefining of refractory and radioactive metals with relatively negative reduction potentials and high melting temperatures. In the present study electrorefining of uranium was studied for developing pyrochemical reprocessing technology of metallized spent nuclear fuels. 3LiCl-2KCl molten salt was chosen as the working electrolyte. Uranium containing electrolytes were prepared by dissolving UCl3 in the solvent salt melt in an argon filled glove box to obtain various desired concentrations of uranium in the electrolyte. The working temperature in the refining experiments varied between 450 and 750 °C. The experiments on uranium electrorefining were carried out in a stainless steel semi-industrial water-cooled electrolyser. Crude uranium metal was loaded in a specially designed molybdenum anodic basket. Glassy-carbon crucible was used to hold the melt. Molybdenum rod was employed as a cathode. The main goal of the electrolytic refining process is obtaining high-purity uranium metal. Specific energy consumption is also very important. The results of the electrolytic refining depend on various parameters. In the present study current density, temperature, uranium concentration in the electrolyte, and specific quantity of electricity passed through the cell were selected as variables, and the current efficiently was chosen as a response factor. Dense and compact uranium deposits were obtained under low current densities, while application of relatively high current densities led to the formation of uranium dendrites. Intermediate values of current density allowed producing more coherent deposits that could be easily scraped off the cathode. Increasing temperature led to a sharp change of the cathodic deposits morphology. The reason for this is the formation of tetragonal beta-uranium instead of the low-temperature orthorhombic alpha modification. The values of cathodic current efficiency in the most of experiments confirmed the three-electron scheme of uranium reduction. Cathodic current density under certain conditions exceeded 95 %. The amount of salt retained in the cathodic deposit varied from 7 to 25 %, values typical for dendrite electrolytic metals.

Molten fluoride salts can be used as the fuel and coolant for molten salt reactors (MSR) and electrolytes for spent nuclear fuel (SNF) reprocessing. 46.5%LiF-11.5%NaF-42%KF (FLiNaK) melt is the prospective media for these purposes due to their desirable thermophysical and nuclear properties. Finding construction materials with sufficient corrosion and mechanical resistance is the most challenging task for practical implementation of MSR concept. In the present study the corrosion and mechanical properties of different types of construction materials were investigated. The materials included various low carbon Ni-Cr-Fe-Mo, Ni-Cr-Mo, and Ni-Mo alloys, and metals with relatively positive electrode potentials. The corrosion experiments were performed in FLiNaK melt at different temperatures (from 550 to 750 °C) in the specially designed stainless steel cells under high-purity argon atmosphere. Corrosion properties of studied materials were investigated under static conditions, and the duration of each test was 100 h to enable the comparison of the experimental data. In a special series of experiments fluorides of typical fissile nuclides and fission products were added to the salt electrolyte to estimate the influence of the red-ox potential on the corrosion resistance of the materials. Corrosion rates were determined from the weight loss measurements and chemical analysis of quenched melts. Surface and microstructure of corroded samples was examined by various microscopic techniques. Mechanical properties of investigated materials were also studied at the ambient temperature and at 600 °C. Advantages and limitations of different types of construction materials were evaluated on the basis of data obtained. The effect of temperature on corrosion and mechanical properties of the studied materials were determined. Possible mechanisms of corrosion of various materials in fluoride melts were proposed. The alloys based on Ni-Cr-Mo and Ni-Mo systems, and molybdenum and its alloys were selected for further long-time tests under dynamic conditions to determine the resource of materials in contact with molten fluorides for MSR and SNF recycling technologies.

Solubility of rare earth oxides in mixtures of alkali or alkaline earth halides was determined by the method of isothermal saturation at the temperatures up to 1400oC. The melts studied included CaCl2–CaF2 (20 or 75 mol. % CaF2), BaCl2–BaF2 (15 or 73 mol. % BaF2), equimolar CaF2–BaF2 and eutectic NaCl–NaF mixtures. Melting points of the salt mixtures were determined by differential thermal analysis and effect of added rare earth oxide on the melting point was considered. Solubility of yttrium, lanthanum, cerium, praseodymium, neodymium, and samarium oxides was determined. The effect of temperature, melt composition, and presence of a mixture of rare earth oxides on solubility was assessed.

Cyclic voltammetry and cathodic polarization measurements were employed to study zirconium and uranium electrochemical behavior in LiCl–KCl eutectic based melts on solid (tungsten) and liquid (zinc, gallium, indium, Gz–Zn eutectic alloy) working electrodes. Deposition potentials of zirconium and uranium were determined. Ga–Zn alloy showed a difference of ca. 0.9 V in U and Zr deposition potentials. Thermodynamically achievable Zr/U separation factor in a “LiCl–KCl salt melt – liquid Ga–Zn alloy” system was experimentally determined.

Molten salt nuclear fast nuclear reactor (MSNFR) is one of the prospective design in frame of Generation IV concept. This technology is also required for reprocessing of spent nuclear fuel (SNF) in molten salts to extract valuable components. Recycling SNF consists of extraction uranium and plutonium with the disposal of minor actinides and fission products. These stages need the materials that retain their corrosion and mechanical properties for long time under the influence of high temperatures, radiation fields and contact with molten salts. Various steels and alloys were widely studied to assess their use under such extreme conditions. Alternative materials include ceramics, composite and carbon materials, which have high corrosion resistance in various media and can be used up to 1000 °C. In the present work the corrosion and mechanical properties of nitride ceramics (Si3N4 and BN) and carbon-based materials (carbon-carbon composite material (C/C) and high-density carbon) were studied in a molten mixture of lithium, sodium, and potassium fluorides (FLiNaK) in the temperature range of 550–750 °C under inert atmosphere. Corrosion tests were performed under static conditions, and the duration of each test was 100 h to enable the comparison of the experimental data. It was found that carbon-containing materials (C/C and high-density carbon) showed high corrosion resistance in the melt. C/C samples had lower corrosion rates (less than 0,1 mm/year in the entire temperature range), while the high-density carbon showed better mechanical properties. The main disadvantage of these materials is their impregnation by molten salt due to relative high porosity. However, C/C composite and high-density carbon are promising structural materials for salt media based on FLiNaK in the selected temperature range. Tested nitride ceramics (BN, Si4N3) demonstrated relatively low corrosion resistance in fluoride systems compare to carbon-containing and metallic materials. The corrosion rates of these materials in FLiNaK exceeded the value of 1 mm/year. The plastic properties of nitride ceramics were also poor. Possible application of such materials for MSNFR and SNF reprocessing is under further investigation.

Speciation and behavior of uranium (III) chloride in the ternary low melting LiCl–KCl–CsCl eutectic was studied by electrochemistry and spectroscopy techniques. Cathodic reduction of U(III) ions on inert (tungsten) and reactive (gallium, cadmium) electrodes was investigated at 623–923 K using cyclic and square wave voltammetry. The potential scan rate was changed from 0.075 to 0.5 V s−1 in all experiments. It was established that the electrochemical reduction process on the inert electrode was irreversible, proceeded in one stage, and was controlled by the charge transfer. Formation of uranium alloys with gallium and cadmium was studied using active liquid Ga and Cd electrodes. Reduction of uranium ions of the reactive electrodes proceeded with considerable depolarization. The effect of current density on the composition of the cathodic product was considered. Conditions for the electrochemical production of alloys of a given composition were determined.

Activity of β-lanthanum were determined in gallium and gallium-aluminum alloy (12 mol. % Al) at 573–1081 K and 573–999 K, respectively, using the electromotive force measurements method. Partial molar thermodynamic functions of lanthanum in the said alloys were calculated. Solubility of lanthanum in liquid gallium was measured between 368 and 1073 K and activity coefficients of β-lanthanum in liquid Ga and Ga–Al alloy calculated. Comparison of results obtained for La–Ga and La–Ga–Al alloys showed that addition of aluminum to Ga–La alloys had an insignificant influence on thermodynamic properties of lanthanum.

Thermodynamic properties of rare earth elements (Y, La, Nd) and uranium were experimentally determined in a number of quaternary alloys at 573–1073 K. Gallium based eutectic mixtures (Ga–Al, Ga–In and Ga–Zn) were chosen as the solvents. The systems studied were La–U–Ga–Al, La–U–Ga–In, La–U–Ga–Zn, La–Nd–Ga–In and La–Y–Ga–In. Activity of the metals in the alloys were determined employing the electromotive force measurements method with the electrolytes based on the ternary LiCl–KCl–CsCl eutectic containing chlorides of the elements studied. Solubility of uranium and lanthanum in La–U–Ga–In and La–U–Ga–Al alloys was determined from the chemical analysis of the saturated alloys, and activity coefficients were calculated from the difference of the temperature dependencies of activity and solubility. Composition of the intermetallic compounds formed in the alloys was assessed using X-ray diffraction analysis. The results obtained for the quaternary systems were compared with the corresponding ternary and binary systems.

Molten salts have a number of potential applications including metals’ electrowinning and electrorefining, pyrochemical spent nuclear fuels’ reprocessing. Considerable efforts were devoted over decades to investigation of various electrochemical processes in molten salts, especially in fused chlorides. Silver chloride electrode is one of convenient types of reference electrodes often employed in potentiometry measurements. This electrode has several advantages (especially compactness) compare to the standard chlorine reference electrode, potential of which in chloride melts is taken as zero. Surprisingly little attention was paid so far to the systematic study (covering the effect of temperature and concentration) of silver electrode potentials in molten chlorides, with majority of the works dating back to 1950-s and 60-s. Molten LiCl–KCl eutectic was, on obvious reasons, the best studied, and several other melts (NaCl, NaCl–KCl, LiCl–KCl–CsCl, NaCl–CsCl) were also looked at. The present study was directed at the systematic study of silver electrochemistry in fused alkali chlorides and determining silver electrode potentials. Here we present the results of the experiments performed in the melts based on the ternary NaCl–KCl–CsCl (30–24.5–45.5 mol. %) eutectic mixture at 823–1073 K. Electrode potentials of silver were determined using the electromotive force measurements method and the following galvanic cell: Ag|(NaCl–KCl–CsCl)+AgCl||(NaCl–KC1–CsCl)|Cl2 (C) Concentration of silver in the electrolyte varied from 0.1 to 5 wt. %. Silver electrode quickly equilibrated with the electrolyte and the potential remained stable over long time, Figure. Experimentally obtained electrode potentials were then used to calculate silver formal standard electrode potentials. Figure. An example of silver electrode potential evolution over time in (NaCl–KCl–CsCl)–AgCl (5 wt. %) melt at 823 K. 2Cl–/Cl2 reference electrode. Figure 1

The behavior of zirconium in the eutectic melt of lithium and potassium chlorides on solid (tungsten) and liquid (gallium, indium, zinc) cathodes is studied in a temperature range of 550–750°C.

This work presents the results of a study of apparent standard potentials of La-(Ga-In) and U-(Ga-In) alloys vs. Cl−/Cl2 reference electrode at the temperature range (723–823) K in fused Me(Ga-In)/LiCl-KCl (3:2 mol ratio) system by potentiometric method at zero current. Lanthanum and uranium activity coefficients and La/U separation factor in gallium-indium alloys containing 40 and 70 wt% indium were determined. Partial excess free Gibbs energy, partial enthalpy of mixing and partial excess entropy of La-(Ga-In) and U-(Ga-In) alloys were calculated. The efficiency of separation of La from U was estimated.

The Ga–Al eutectic melt saturated with praseodymium was studied in the temperature range 572–1076 K by electromotive force (emf) method relative to the reference electrode (InL + PrIn3, where L is the liquid phase) in a LiCl–KCl–CsCl eutectic electrolyte. The partial molar thermodynamic functions (enthalpy, entropy, and Gibbs energy) of praseo dymium in a Ga–Al eutectic melt were calculated. According to the emf measurements of the two-phase Pr–Ga–AlL + intermetallic compound alloys equilibrated with the Ga–Al eutectic melt saturated with praseodymium, there are intermetallic compounds PrGa6, PrGa4, and Pr0.22Ga0.78(PrGa2) in the temperature ranges 572–741, 741–883, and 883–1076 K, respectively.

A usual oxidation state of rare earth metals in chloride melts is +3. A number of lanthanides (particularly europium, ytterbium, samarium, and thulium) can also form thermodynamically stable ions in the oxidation state +2. Ln(II)/Ln(III) redox processes are normally investigated employing transient electrochemical techniques where the formation of Ln(II) ions is limited to the near electrode surface layer. The present work was aimed at studying the reduction of Yb(III) ions in the bulk of the melt and assessing the stability of Yb(II) chloro species in molten chloride media. The experiments were carried out in the melts based on NaCl–KCl–CsCl eutectic and NaCl–KCl equimolar mixtures at 550–850 and 700–850 oC, respectively, employing stationary and transient electrochemistry and high temperature spectroscopy methods. Concentration of ytterbium in the melt was varied from 1 to 10 wt. %. Ytterbium(II)/(III) formal standard redox potentials were obtained from the results of electrochemistry measurements (cyclic voltammetry and potentiometry). Electronic absorption spectra of Yb(II) species were recorded both in molten and quenched electrolytes. Reduction of Yb(III) to Yb(II) can be achieved by shifting the equilibrium of the reaction YbCl6 3– + Cl– ↔ YbCl6 4– + ½ Cl2 to the right by lowering partial pressure of chlorine in the atmosphere above the melt. Here, zirconium, as a metal with high affinity for chlorine, was used as a getter. The reduction process was followed by measuring the absorption spectra. As the reaction proceeded, the color of the melt changed from colorless of YbCl6 3– to brown. Analysis of the resulting melts showed that the mean oxidation state of ytterbium was below three, the value depended on temperature, duration of the reduction and melt composition. Increasing temperature or reduction time expectedly resulted in a greater degree of reduction. Ytterbium(III) can also be reduced with a suitable reductant and hydrogen was chosen here as an example. Sparging the melt containing Yb(III) ions with hydrogen resulted in decreasing mean oxidation state of ytterbium: YbCl6 3– + Cl– + ½ H2 ↔ YbCl6 4- + HCl. The course of the reduction process was followed by measuring the redox potential. The efficiency of hydrogen as a reducing agent was comparable to the thermal decomposition in the presence of zirconium getter. Hydrogen reduction was also a very convenient way of preparing the melts with a low Yb(II) content for spectroscopy measurements. Another way of reducing Yb(III) to Yb(II) is electrolysis: YbCl6 3– + e– ↔ YbCl6 4-. In a series of preliminary experiments Yb(II)/Yb(III) redox potentials were determined from the results of cyclic voltammetry measurements. Reduction of Yb(III) to Yb(II) was carried out by potentiostatic electrolysis under an inert (argon) or a reducing (hydrogen) atmosphere with and without agitating the melt. The lowest mean oxidation state of ytterbium in the melt thus obtained was 2.33 showing that two thirds of Yb(III) could be reduced to Yb(II). Acknowledgement. This work was supported by the Ministry of Education and Science of the Russian Federation (project No. 4.5062.2017/8.9).

Alkali chloride based melts can be employed as working media in spent nuclear fuels pyrochemical reprocessing. Oxygen is on of common impurities in technology and oxygen species can seriously influence speciation of metals in molten electrolytes. From the other hand, one of the possible ways of separating uranium from rare earth fission products by selective precipitation of uranium oxide. The present work was aimed at studying reactions taking place in chloride melts containing neodymium (III) and uranium (III and IV) ions upon addition of oxygen species. In particular, reactions with oxygen and oxide ions (introduced in form of lithium oxide) were considered. Thermodynamic modeling of possible reactions was first performed to estimate the possibility of separating uranium from rare earths. The experiments were then conducted in the melts based on LiCl–KCl eutectic, NaCl–KCl equimolar mixture and individual LiCl at 550 oC (for LiCl–KCl) and 750 oC for all the melts). Reaction of the melts containing NdCl3, UCl3 or UCl4 with Li2O (at various O2– to Nd3+ (U3+, U4+) mole ratios) and with O2 was investigated. High temperature electronic absorption spectroscopy was employed to determine the kinetics of the reactions taking place. Phase composition of the solid phases was characterized by X-ray powder diffraction analysis. Size of the particles in the precipitates formed at various conditions was also determined to assess the feasibility of subsequent separation of the solids from the technological melts.

Phosphate precipitation in molten chlorides is considered as a possible method for removing rare earth and alkaline earth fission product elements from technological melts at a final stage of pyrochemical reprocessing of spent nuclear fuels. Phosphates can be incorporated into a glass matrix in a high level waste vitrification process in preparation for a long term storage or disposal. There are a number of studies considering the formation of rare earth phosphates in the melts of technological interest, i.e. LiCl–KCl, NaCl–KCl, and NaCl–CsCl mixtures, but some uncertainty still remains concerning the conditions when different types of phosphates are formed. While the reaction in LiCl–KCl melts always produces normal orthophosphates (REPO4), double alkali metal-rare earth phosphates, e.g. M3RE(PO4)2 and M3RE2(PO4)3, can be formed in the melts not containing LiCl. The aim of the present study was investigating the effect of the initial phosphate-to-rare earth mole ratio on the outcome of the reaction. The experiments were performed in the melts based on the equimolar mixture of sodium and potassium chlorides at 750 oC. The rare earth elements included Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (heavier lanthanides not representing fission products were included for completeness). Sodium orthophosphate was used as a source of PO4 3– ions and the initial PO3 3– : RE3+ mole ratio was varied from 0.3–0.5 to 10. Formation of the phosphates was studied in the melts containing individual rare earth elements as well as a mixture of rare earths in the proportion simulating their abundance in the thermal neutron reactor spent fuel. The experiments were performed under static conditions and the parameters analyzed included degree of RE precipitation, phase composition of the precipitate and size of particles in the solid phase. Analysis of the experimental results showed that at PO3 3– : RE3+ mole ratios below five only normal REPO4 phosphates were formed, while at the ratios above 8 double Na3RE(PO4)2 and/or K3RE(PO4)2 double phosphates were produced. Particle size of the rare earth phosphates varied from 0.1 to hundredths of microns and increasing the initial PO3 3– : RE3+ mole ratio resulted in increasing particle size. Complete conversion of RECl3 to phosphate required 2–5 times molar excess of the phosphate to the rare earth element.

Separation of lanthanides and actinides can be achieved in a unique “molten chloride – liquid metal” system. Electrode potentials were recorded vs. Cl−/Cl2 reference electrode and the temperature dependencies of the apparent standard potentials of La-(Ga-In) and U-(Ga-In) alloys were determined. Thermodynamic properties and separation factor of lanthanum and uranium were calculated. The obtained data show the perspective for using this system in future innovation method for recovery of nuclear waste.

The corrosion of austenitic 12Kh18N10T, 10Kh17N13M2T, and 03Kh17N14M3 steels and their components (Cr, Fe, Ni, Mo) in NaCl-KCl-VCl2 melts with 5 wt % V at 750°C is studied. The rates and mechanisms of corrosion of the materials under these conditions are determined. The processes that occur during contact of the metals and steels with vanadium-containing chloride electrolytes are investigated.

The processes that occur during the reactions of 15 rare-earth element (yttrium, lanthanum, and all lanthanides except for promethium) chlorides with sodium orthophosphate in the melts based on the NaCl–KCl equimolar mixture are studied in an inert atmosphere at 750°C. The phase and granulometric compositions of the rare-earth element phosphates are determined. The influence of the molar ratio of phosphate to rare-earth element on the composition and particle size of the phosphates is studied. The conditions necessary for the quantitative precipitation of both individual rare-earth elements and a mixture of the rare-earth elements from the melt are determined. The mixture of rare-earth elements imitates the content of rare-earth fission products in the spent nuclear fuels to be processed.

The anodic dissolution of metallic tungsten in the eutectic 3LiCl–2KCl (723–1023 K) and NaCl–2CsCl (823–1023 K) melts was studied by high-temperature electrochemistry and spectroscopy. W(IV) ions are formed in the melt. The conventional electrode potentials of tungsten were determined.

The experimental data on determining the equilibrium potentials of a number of rare-earth metals, namely, lanthanum, neodymium, praseodymium, gadolinium, holmium, and erbium, in a eutectic mixture of lithium, potassium, and cesium chlorides are used to find the temperature dependences of the conventional standard potentials of these metals and to calculate the thermodynamic characteristics of formation of their chlorides in a melt.

The effect of alloy components on the thermodynamic properties of f elements is studied on Zn‑containing alloys. The activity of lanthanum in the La–Zn, La–U–Zn, and La–U–Ga–Zn systems and the activity of uranium in the U–Zn system in the range 573–1073 K are determined. The effect of a second low-melting metal (gallium) and a second f element (uranium) on the activity of lanthanum in zinc-containing alloys is analyzed.

Solubility of neodymium oxide was determined in molten salt systems based on CaF2–BaF2 (50 mol. %), CaCl2–CaF2 (20 mol. %), CaCl2–CaF2 (75 mol. %), BaCl2–BaF2 (15 mol. %), BaCl2–BaF2 (73 mol. %) and NaCl–NaF (34 mol. %) mixtures at 700–1400 °C. Temperature dependencies of the solubility were calculated. Maximum solubility was observed CaF2–BaF2 (50 mol. %) and CaCl2–CaF2 (75 mol. %) based melts.

Rubidium and cesium uranates of various composition (M4UO5, M2UO4, M2U2O7 and M2U3O10, M = Cs, Rb) were prepared by reacting triuranium octaoxide with the corresponding alkali metal carbonates. The infrared spectra of the uranates were recorded in the temperature range of 25–700 °C.

Precipitation of strontium phosphates from LiCl–KCl–SrCl2 at 550 °C, and NaCl–KCl–SrCl2 and NaCl–KCl– CsCl–SrCl2 melts at 750 °C was investigated. Sodium phosphate was employed as the source of phosphate ions. The experiments were carried out under static conditions in an inert atmosphere. X-ray powder diffraction analysis was employed for identifying the compounds formed and particle size distribution curves were measured.

Electrochemical behavior of gallium was studied in 3LiCl–2KCl and 6NaCl–9KCl–5CsCl eutectic based melts at 450–650 °C using potentiometry, cyclic voltammetry and cathodic polarization measurements. Mechanism of Ga(III) ions reduction was assessed and electrode potentials of gallium determined.

Reduction of ytterbium, samarium and europium trichlorides to dichlorides was studied in NaCl–KCl–CsCl eutectic and NaCl–KCl equimolar mixture based melts at 823–1123 K employing high temperature spectroelectrochemistry. Electronic absorption spectra of solutions of Yb(II), Sm(II) and Eu(II) chloro species in these melts were recorded.

Cyclic voltammetry was applied for studying electrochemical behavior of iron, nickel and chromium in melts based on the ternary LiF–NaF–KF eutectic mixture. Solubility of iron(III), nickel(II) and chromium(III) fluorides in FLiNaK was determined.

Electronic absorption spectroscopy was used to assess palladium speciation in fused alkali chlorides. Electronic absorption spectra of palladium (II) chloro-ions were recorded in molten LiCl–KCl–CsCl eutectic mixture at 300–800 °C. Experimental spectra were resolved into individual bands for more accurate determination of the energy of electronic transitions. Increasing temperature resulted in decreasing absorption and ligand field splitting parameter Δ.

Closeness of electrochemical properties of uranium and zirconium makes separation of these metals in pyroelectrochemical reprocessing of spent nuclear fuels a challenging task. Varying electrode material can change metals’ deposition potentials. The study was aimed at assessing the effect of the cathode material on deposition potentials of zirconium and uranium from 3LiCl–2KCl based melts. Solid (tungsten) and liquid (gallium, zinc, Ga–Zn, Ga–Sn and Ga–In alloy) working electrodes were tested at 532–637◦C. Galvanostatic cathodic polarization was employed and the applied cathodic current varied from 0.0001 to 1 A. Gallium–zinc eutectic alloy demonstrated the largest difference of zirconium and uranium deposition potentials. Zirconium/uranium separation factors were experimentally determined in a “molten salt—liquid metal” system for gallium and Ga–Zn eutectic based alloys. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. The study was supported by the Russian Science Foundation, project no. 17-73-20156.

Electrochemical behavior of neodymium and uranium in a molten Nd(U)–(Ga–xAl)/3LiCl–2KCl system (x = 1.5; 5.0 and 20.0 wt% Al) between 723 and 823 K was studied. Temperature dependencies of apparent standard potentials of Nd–(Ga–Al) and U–(Ga–Al) alloys were determined versus Cl−/Cl2 reference electrode. Neodymium and uranium activity coefficients and Nd/U separation factor were calculated. Partial excess free Gibbs energy, partial enthalpy of mixing and partial excess entropy of Nd–(Ga–Al) and U–(Ga–Al) alloys were estimated.

The behavior and properties of lanthanum in a “molten salt – liquid metal” system in relation to pyrochemical reprocessing of spent nuclear fuels was considered. Temperature dependencies of lanthanum formal standard electrode potential E* La/La(III) and Gibbs free energy change of lanthanum trichloride formation in LiCl–KCl–CsCl eutectic based melts were obtained between 668–976 K. Also the behavior of lanthanum was studied in Ga–Al (1.6 wt. % Al) and Ga–Zn (3.64 wt. % Zn) eutectic based metallic alloys, and activity, solubility and activity coefficients of lanthanum determined.

Phosphate precipitation is one of the possible methods for removing rare earth fission product (FP) elements from the technological melts in pyrochemical processes of spent nuclear fuel (SNF) reprocessing. Chemical and phase composition of the phosphates depends on the cationic composition of the melt and, in some instances, on the precipitant employed. Technologically attractive melts include 3LiCl–2KCl and NaCl–2CsCl eutectic mixtures and NaCl–KCl equimolar mixture. Previous studies showed that the normal rare earth orthophosphates (REEPO4) are formed in the 3LiCl–2KCl based melts. In NaCl–KCl and NaCl–2CsCl melts the composition of the precipitate is influenced by the initial phosphate-to-rare earth element molar ratio, and normal as well as double alkali metal – rare earth phosphates (M3REE(PO4)2 or M3REE2(PO4)3) can be produced. After precipitating rare earth FPs, the precipitate needs to be separated from the melt to allow the electrolyte reuse. Solid phosphates can be removed by filtering or collected employing submergible centrifuges. Particle-size distribution (PSD) of the solid precipitate can greatly affect the efficiency of the collection device. After the phosphates are separated they can be further treated for the final disposal, for example, by incorporating them into a suitable glass matrix (a vitrification process). Here again the PSD of the solid material can be important in understanding the properties of the final waste form. Despite the phosphate precipitation process has been studied by various techniques, there is very little information concerning the particle size of the precipitates formed in the process. In the present work the effect of the initial phosphate precipitant-to-rare earth mole ratio on the particle size of the rare earth phosphates was investigated. The experiments were performed in NaCl–2CsCl and NaCl–KCl based melts at 650 and 750 oC, respectively. The REEs studied included La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, where the heavier lanthanides (which are not the fission products) were included for comparison. Phosphate precipitant was added in the amount required for 50 and 100 % precipitation of the REE. In addition, the melts containing the mixture of REEs imitating their content in a typical light water reactor SNF arriving for reprocessing were studied. Particle size of the precipitated phosphates was determined by laser diffraction method employing ANALYSETTE 22 NanoTec (Fritsch) instrument with a measuring range of 0.02–2100 μm. The measurements were performed both with and without using ultrasound to asses the effect of ultrasonic treatment on the particle deagglomeration. The obtained results showed that the particle size of the precipitated phosphates typically lies in the range of 0.1–100 μm. Fraction of the particles with the size below 1 μm is relatively small. The results of the measurements showed that the particles of the phosphates are agglomerated, maxima in the PSD curves shift towards lower values after the ultrasonic treatment of the samples. Heavier lanthanides tend to form larger particles. For example, maxima in the PSD curves of the phosphates produced at 100 % precipitation in NaCl–2CsCl melts shifted from ca. 6 μm for lanthanum to ca. 20 μm for ytterbium. The effect of the initial phosphate-to-REE molar ratio in the melt on the particle size is not straightforward. For the lighter lanthanides (from Ce to Sm) PSD curves for the precipitates obtained at 50 % degree of precipitation have two maxima – around 1 and 10–20 μm, and the average particle size is smaller than for the solids obtained at 100 % REE precipitation. For the heavier lanthanides the average size of the particles obtained at 50 % REE precipitation was greater than at complete precipitation. Cationic composition of the melt does not significantly affect the average size of the particles of REE phosphates, although influences the particle-size distribution. Examples of the results obtained for precipitation of the sum of REE from NaCl–2CsCl and NaCl–KCl based melts are shown in the Figure. Bulk of the particles has the size within 1–100 μm limits. Figure 1

Diffusion coefficients of the uranium(III) and (IV) ions in the eutectic melt of the lithium, potassium, and cesium chlorides in the temperature range of 573–1073 K have been determined using two independent methods: cyclic voltammetry and chronopotentiometry.

The behavior of uranium(III) and (IV) ions in the melts based on the eutectic mixture of lithium, potassium, and cesium chlorides has been studied using cyclic voltammetry in the range of 573–1073 K. The red-ox potentials of uranium have been determined and formal standard E U(III)/U(IV)* red-ox potentials have been calculated.

Alkali chloride melts are considered as prospective working media for non-aqueous pyrochemical reprocessing of spent nuclear fuels (SNFs). Separation of fissile materials from fission products in pyrochemical reprocessing can be achieved electrochemically and amongst all the fission products zirconium has the closest electrochemical properties to uranium. Uranium and plutonium fission produces several zirconium isotopes (from Zr-91 to Zr-97) and depending on the reactor neutron spectrum, nuclear fuel type, burnup and cooling time SNF arriving for reprocessing can contain ca. 5–13 kg of zirconium (as the fission product excluding cladding) per ton. Electrochemical separation of uranium and zirconium in fused salts is a challenging task. The present work was devoted to studying the electrochemical behavior of zirconium and uranium in 3LiCl–2KCl based melts using cyclic voltammetry and cathodic polarization. The experiments were performed in LiCl–KCl–ZrCl4, LiCl–KCl–UCl4, LiCl–KCl–UCl3 and LiCl–KCl–ZrCl4–UCl4 melts. On a solid tungsten electrode zirconium(IV) ions were reduced to Zr(0) in two stages and the metal deposition potentials were between –2.17 and –2.07 V (at 532–637 oC) vs. Cl–/Cl2 couple. Changing the solid electrode to liquid zinc, gallium or gallium–zinc eutectic alloy (3.64 wt. % Zn) resulted in significant shift of zirconium deposition potential in the positive direction. Examples of the polarization curves are shown in the Fig. Polarization measurements performed in LiCl–KCl–ZrCl4–UCl4 melt on the Ga–Zn electrode showed that reduction of U(IV) to U(III) and deposition of zirconium occurred at very similar potentials and deposition potential of uranium was significantly more negative. Separation factor for uranium/zirconium couple was also determined. Fig. Polarization of Ga–Zn eutectic alloy cathode in LiCl–KCl–UCl4 (524 oC) and LiCl–KCl–ZrCl4 (550 oC) melts. Figure 1

Alkali chloride based melts can be used as working electrolytes in prospective technologies of pyrochemical reprocessing of spent nuclear fuels. After extracting fissile materials the melts contain ions fission product elements having more negative electrode potentials than uranium and plutonium. These include rare earth, alkaline earth and alkali metals. One of possible methods for removing rare earths from alkali chloride based melts is phosphate precipitation. Alkaline earth metals also form sparingly phosphates and therefore phosphate precipitation can be potentially used for removing strontium and barium from molten electrolytes. The present work was aimed at studying the reaction of solutions of strontium or barium chloride in alkali chloride based melts with sodium orthophosphate. The experiments were conducted in LiCl–KCl and NaCl–KCl–CsCl eutectic based melts at 550 oC, and in NaCl–KCl equimolar mixture at 750 oC. To determine the conditions required for complete removal of strontium and barium, the initial phosphate-to-barium molar ratio in the melt was set at 0.5; 1; 2; 4; 6 and 8. Residual alkaline earth content in the melt was determined by chemical analysis. Precipitated strontium and barium phosphates were subjected to X-ray powder diffraction analysis. Selecting a possible method of separating solid precipitate from molten salt requires information on size of particles forming the solid phase. Particle size was determined by laser diffraction and examples of particle size distribution curves for precipitates formed in LiCl–KCl–BaCl2 melts are shown in Fig. The precipitates consisted of particles ranging from 0.5 to 100 microns. Increasing the initial PO4 3– : Ba2+ molar ratio resulted in increasing particle size but even at the molar ratio of six over 50 % particles were less than 10 μm. Fig. Particle size distribution curves for barium phosphate precipitated from LiCl–KCl eutectic based melts at 550 oC. Initial PO4 3– : Ba2+ mole ratio was 0.5 (line 1); 1.0 (2); 2.0 (3); 4.0 (4) and 6.0 (5). Figure 1

High purity rare earth metals are essential for research and use as high-performance materials. Even a minute amount of impurities can have significant influence on physical and chemical properties of metals, so some special properties of rare earth materials are exhibited only when they are in the state of high purity. Molten salts are promising reaction media for extractive metallurgy. In particular, molten chlorides are good electrolytes for selective dissolution or deposition of pure reagents; using molten salts provides a promising route for treatment of raw materials. In addition, molten salts proved to be suitable media for metal electrowinning and electrorefining. Available experience in high-temperature electrochemistry allows producing metals in the solid form. One of the advantages of molten salts is their variety. So, it is possible to find a solvent, which chemical and electrochemical characteristics are suitable to carry out a given process. The goal of this work was studying the mechanism of electrochemical reduction of dysprosium ions in molten LiCl–KCl eutectic for production of high purity dysprosium metal. The cathodic reduction of Dy(III) ions was studied on inert molybdenum electrode in the temperature range of 723–843 K under inert atmosphere. Cyclic voltammograms contained one cathodic peak at–3.19±0.11 V and the corresponding anodic peak at –2.95±0.11 V vs. chlorine reference electrode, Fig. The cathodic peak potential was not constant and shifted to the negative values with increasing scan rate. The cathodic peak current was directly proportional to the square root of the polarization rate. It was found that increasing scan rate led to an expected increase of irreversibility of the cathode process. The number of electrons (n) of the electrode reaction for reduction of Dy(III) ions was determined by square-wave voltammetry and the calculated value was equal to 2.93±0.05. According to the theory of linear sweep voltammetry the redox system Dy(III)/Dy(0) is irreversible and controlled by the rate of the charge transfer. So, cathodic reduction of dysprosium ions proceeded in one three-electron electrochemical step at the potential of –3.19 V versus the Cl–/Cl2 reference electrode: DyCl6 3– + 3 e– = Dy + 6 Cl–. Temperature dependence of Dy(III)/Dy couple apparent standard potential was determined by chronopotentiometry at the zero current. The experimental values are described by the linear equation: E*Dy/Dy(III) = –(3.401±0.009) + (6.2±0.1)∙10–4∙T. The apparent standard Gibbs energy change, enthalpy, and entropy of dysprosium trichloride formation from the elements in fused LiCl–KCl eutectic and activity coefficient of DyCl3 were calculated. The influence of different parameters on the composition of the cathode product was investigated. It was found that for fine purification of dysprosium from its impurities, the electrolysis should be carried out in two stages, in which the first stage is the purification electrolysis, and the second one is the base electrolysis. The reported study was funded by Russian Foundation for Basic Research according to the research project No. 20-03-00743. Figure. Typical cyclic voltammograms for the reduction of dysprosium trichloride on molybdenum electrode (S = 0.14 cm2) in fused LiCl–KCl eutectic at 723 K. m(DyCl3) = 4.1∙10–2 mol/kg. Scan rates, V s−1: 1 – 0.075; 2 – 0.2; 3 – 0.5. Figure 1

Alkali chloride melts have numerous potential applications in nuclear fuel cycle including pyrochemical reprocessing of spent nuclear fuels, uranium electrowinning and electrorefining. Oxygen is a common technological impurity that can affect uranium speciation and behavior in fused salts. The present work was devoted to studying the reactions of oxygen with solutions of uranium tetrachloride in molten alkali chlorides. The experiments were performed in LiCl–KCl, NaCl–KCl–CsCl and NaCl–CsCl eutectic based melts at 450–750 oC. Pure oxygen and argon–oxygen mixtures (containing ca. 1 and 10 % O2) were used. Amount of oxygen passed through the melt varied from less than one to over 100 moles per mole of uranium present. Effect of moisture (0.4–2.5 % H2O) presence in oxygen or Ar–O2 mixtures was also investigated. The course of the reaction was followed by in situ electronic absorption spectroscopy measurements with the spectra recorded at the certain time intervals. Depending on temperature, cationic melt composition and oxygen-to-uranium molar ration the reaction resulted in oxidation of uranium(IV) to soluble uranyl chloride and/or precipitation of uranium dioxide. Analysis of the spectra provided the information on kinetics of U(IV) concentration change. Increasing temperature, O2 : U(IV) molar ratio or decreasing mean radius of alkali cations of the solvent melt resulted in faster decrease of U(IV) concentration in the melt. Under certain conditions U(IV) can be oxidized to UO2Cl4 2– without precipitation of UO2. Therefore sparging the melt with oxygen can be used as a way of separating uranium from certain fission products, for example rare earth elements. Rare earth chlorides react with oxygen yielding oxychlorides or oxides insoluble in alkali chloride melts. Interaction of oxygen with melts containing a mixture of uranium and rare earth chlorides was therefore also investigated and an example of the spectra recorded in LiCl–KCl–UCl4–NdCl3 melt is shown in Fig. Fig. Spectra recorded in the course of reaction of O2 with LiCl–KCl–UCl4–NdCl3 melt at 550 oC. Arrows show the direction spectra changed. Figure 1

Melts based on mixtures of alkali and/or alkaline earth halides are considered as prospective media for electrowinning rare earth metals as well as for pyrochemical reprocessing spent nuclear fuels. Fluoride or mixed fluoride-chloride baths can be operated at high temperatures yielding molten rare earth metals (REMs) thus simplifying separation of the metal and salt. One of the problems in using fluoride melts is possible formation of fluorine at the anode. This can be avoided by adding a rare earth oxide to the melt both as the source of REM and oxide ions. The latter will be oxidized to oxygen producing carbon mono- or dioxide at the anode. The limiting factor for feeding the electrolysis bath with REM oxides is their solubility in the halide melt. The aim of the present study was determining the effect of temperature and melt composition on solubility of REM oxides in fused halides. The experiments were performed in CaCl2–CaF2 mixtures containing 20 or 75 mol. % CaF2; BaCl2–BaF2 mixtures containing 15 or 73 mol. % BaF2; equimolar CaF2–BaF2 mixture and NaCl–NaF eutectic mixture (34 mol. % NaF). Solubility of REM oxides was determined by the method of isothermal saturation. Time required for reaching the equilibrium between solid REM oxides and fused salts was determined in a preliminary set of experiments. To compare the behavior of 4f- and 5f-elements, solubility of uranium dioxide was also measured. The measurements were performed at the temperatures up to 1400 oC under argon atmosphere. The lower limit of the temperature range varied from 700 to 1100 oC depending on the melting temperatures of the salt mixtures used. Oxides of yttrium, lanthanum, cerium, praseodymium, neodymium and samarium were selected for the study. To assess a possible mutual influence of rare earth elements on solubility of their oxides in fused salts the solubility of a mixture of REM oxides was determined in a separate series of experiments and concentrations of individual REMs in the melt was determined. Solubility of REM oxides increased with increasing temperature and an example of effect of temperature on solubility of neodymium oxide in various melts is shown in Fig. Fig. Concentration of neodymium in alkali and alkaline halide based melts saturated with Nd2O3. Melt: CaCl2–CaF2 20 mol. % (1); CaCl2–CaF2 75 mol. % (2); BaCl2–BaF2 73 mol. % (3); BaCl2–BaF2 15 mol. % (4); CaF2–BaF2 50 mol. % (5; and NaCl–NaF 34 mol. % (6). Figure 1