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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.

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.

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.

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.

Pr-Ga-Al alloys were prepared from individual praseodymium, gallium and aluminum purified from oxide films in an inert atmosphere of the glove box. The Ga-Al phase diagram was used to choose the eutectic composition (2.1 mol. % Al). The metals were mixed in a certain proportion in crucibles (BeO) and annealed at 832.4±1.7 °C and 366.2±6.3 °C for 19 hours and 53 hours, respectively. Three ingots were obtained with different praseodymium content. The alloys were investigated by X-ray phase analysis, scanning electron microscopy and electron backscatter diffraction technique. A general chemical analysis was also performed. Depending on praseodymium content the alloys consisted of PrGa6 intermetallic compound phase; a mixture of PrGa6 and ϵ-PrGa2 IMCs; and ϵ-PrGa2 phase. Ternary intermetallic compounds of praseodymium were not detected. A small addition of aluminum does not affect the phase characteristics of intermetallic compounds of praseodymium in alloys. © 2019 Author(s).

Thermodynamic properties of uranium were studied in (U + Ga), (U + Sn) and (U + Ga + Sn) systems. Activity and activity coefficients of uranium were determined in alloys with tin and (gallium + tin) eutectic (0.135 Sn mass fraction) between T = (573 and 1073) K. Solubility of uranium in (Ga + Sn) eutectic and in pure Ga was measured between T = (298 and 1073) K. Activity coefficients, partial and excess thermodynamic functions of uranium in alloys with Ga, Sn and (Ga + Sn) eutectic were calculated.

Partitioning of uranium and neodymium was studied in a ‘molten chloride salt - liquid Ga-X (X = In or Sn) alloy’ system. Chloride melts were based on the low-melting ternary LiCl-KCl-CsCl eutectic. Nd/U separation factors were calculated from the thermodynamic data as well as determined experimentally. Separation of uranium and neodymium was studied using reductive extraction with neodymium acting as a reducing agent. Efficient partitioning of lanthanides (Nd) and actinides (U), simulating fission products and fissile materials in irradiated nuclear fuels, was achieved in a single stage process. The experimentally observed Nd/U separation factor valued up to 106, depending on the conditions.

Activity, activity coefficients and solubility of uranium was determined in gallium-aluminium alloys containing 1.6 (eutectic), 5 and 20 wt.% aluminium. Additionally, activity of uranium was determined in aluminium and Ga–Al alloys containing 0.014–20 wt.% Al. Experiments were performed up to 1073 K. Intermetallic compounds formed in the alloys were characterized by X-ray diffraction. Partial and excess thermodynamic functions of U in the studied alloys were calculated.

Activity, activity coefficients and solubility of uranium was determined in gallium, indium and gallium–indium alloys containing 21.8 (eutectic), 40 and 70 wt.% In. Activity was measured at 573–1073 K employing the electromotive force method, and solubility between room temperature (or the alloy melting point) and 1073 K employing direct physical measurements. Activity coefficients were obtained from the difference of experimentally determined temperature dependencies of uranium activity and solubility. Intermetallic compounds formed in the respective alloys were characterized using X-ray diffraction. Partial and excess thermodynamic functions of uranium in the studied alloys were calculated. Liquidus lines in U–Ga and U–In phase diagrams from the side rich in gallium or indium are proposed.

The praseodymium and neodymium solubilities in the gallium–tin eutectic melt were measured for the first time in a temperature range of 423–1073 K using a high-temperature sampling technique. The data on the activity and solubility are used to calculate the activity coefficients, the excessive thermodynamic characteristics of α-praseodymium and α-neodymium in the Ga–Sn eutectic melts, and the separation factor for the Pr/Nd pair on gallium–tin electrodes in chloride melts.

Solubility of lanthanum was determined in gallium-aluminum alloys containing 5 and 20 wt. % aluminum at 428–1005 K employing sedimentation and high temperature filtration methods. Composition of the intermetallic compounds formed was determined.

Thermodynamic properties of La, Nd, and Y were studied in pseudo-quaternary systems La–Nd–(Ga–In)eut and La–Y–(Ga–In)eut. Temperature dependences of lanthanum, neodymium, and yttrium activity in the alloys were determined at 300–800 °C employing the e.m.f. method. Partial thermodynamic functions of lanthanum in the alloys studied were calculated on the basis of the experimental data.

Thermodynamic properties of gadolinium in Ga–Sn and Ga–Zn eutectic based alloys were studied. Temperature dependences of gadolinium activity in the studied alloys were determined at 573–1073 K employing the EMF method. Solubility of gadolinium in the Ga–Sn and Ga–Zn alloys was measured at 462–1073 K using IMCs sedimentation method. Activity coefficients as well as partial and excess thermodynamic functions of gadolinium in the studied alloys were calculated on the basis of the obtained experimental data.

Thermodynamic properties of lanthanum were determined in gallium–zinc alloys of the eutectic and over-eutectic compositions. The electromotive force measurements were used to determine thermodynamic activity and sedimentation technique to measure solubility of lanthanum in liquid metal alloys. Temperature dependencies of lanthanum activity, solubility and activity coefficients in alloys with Ga–Zn mixtures containing 3.64, 15 and 50 wt. % zinc were obtained.

Thermodynamics of separation of praseodymium and neodymium employing liquid low-melting metals and alloys is considered. Pr/Nd separation factors on liquid Ga, In, Bi, and Ga-In eutectic were calculated from the thermodynamic data. The obtained results are compared with the literature data on experimentally determined separation factors on “traditional” liquid metals: Bi, Zn, and Cd. It is shown that one-stage selective extraction of praseodymium or neodymium from the mixture using any of the mentioned metals is not possible. In the separation process of lanthanides from minor actinides using a chloride molten salt–liquid metal system, cerium group lanthanide fission products will be accumulated in the salt phase.

Pyrochemical processes employing high temperature molten salts can be used for treating spent nuclear fuels (SNF), especially with a high burn-up and after low cooling time. Before returning the fissile materials to the fuel cycle, fission products must be separated and the required separation factor depends on the nuclear reactor type. Alkali metal chlorides are normally considered as the working media for the pyrochemical reprocessing. Metallic or nitride SNFs can be anodically dissolved in the melt, whereas oxide fuels can be reduced to the metallic state and then subjected to the anodic dissolution. SNF arising from molten salt reactors can be directly used for the reprocessing. Rare earth elements (yttrium, lanthanum and lanthanides) represent an important group of fission products with the electrode potentials more negative than those of uranium and plutonium. During SNF dissolution these elements will also dissolve. Liquid metals can be effectively employed for the subsequent separation of the fissile materials from the fission products. The separation factor Θ depends on the electrode potentials (E*) of separating elements in the salt phase and their activity coefficients (γ) in the metallic alloy. If the separating metals form in the salt melt the ions in the same oxidation state (like actinides and lanthanides), the separation factor can be expressed as lgΘMe1/Me2 = [n·F / (2.303·R·T)]·(E*Me2 – E*Me1) + (lgγMe1 – lg γMe2). In the present study the effect of temperature, cationic composition of the alkali chloride based melts, and composition of the liquid metal were considered, and separation of uranium from neodymium was studied as an example. Gallium, indium, tin and their mixtures of various compositions were chosen as low melting metals. Thermodynamic and electrochemical properties of uranium and neodymium were determined between 573–1073 K thus allowing to model separation processes. Cationic composition of the salt phase has little effect on the separation factor, whilst the effect of the metallic phase nature is more pronounced. Efficiency of separation (Θ value) increases with lowering temperature. An example of thermodynamically possible Nd/U separation factors for various liquid metals and LiCl–KCl–CsCl eutectic melt is presented in the Fig. Here experimentally determined values of E* and γ were used for the calculations. To confirm the results of calculations, experiments on separating Nd and U were conducted. The experiments were performed under various conditions (e.g., static or with stirring the phases) and the experimentally achieved separation factors compared with the theoretical expectations. Figure 1