Study of thermodynamic properties of Np-Al alloys in molten LiCl-KCl eutectic

Pyrochemical methods are investigated worldwide within the framework of Partitioning and Transmutation concepts for spent nuclear fuel reprocessing. Electroseparation techniques in a molten LiCl-KCl are being developed in ITU to recover all actinides from a mixture with fission products. During the process, actinides are selectively electrochemically reduced on a solid aluminium cathode, forming solid actinide-aluminium alloys. This work is focused on the thermodynamic properties of Np-Al alloys in a temperature range of 400-550 °C and on the characterisation of the structure and chemical composition of deposits obtained by electrodeposition of Np on solid Al electrodes in a LiCl-KCl-NpCl₃ melt. Cyclic voltammetry and open circuit chronopotentiometry have been used to examine the electrochemical behaviour of Np on inert W and reactive Al electrodes. Gibbs energies, enthalpy and entropy of formation and standard electrode potentials of Np-Al alloys were evaluated and compared with ab initio calculations. Galvanostatic electrolyses at 450 °C were carried out to recover Np onto Al plates and the solid surface deposits were characterised by XRD and SEM-EDX analyses. Stable and dense deposits consisting of NpAl₃ and NpAl₄ alloys were identified. In addition, the conversion of NpO₂ to NpCl₃ is described, using chlorination of the oxide in a molten salt media by pure chlorine gas.     

Electrochemical and thermodynamic properties of europium(III), samarium(III) and cerium(III) in 1-butyl-3-methylimidazolium chloride ionic liquid

Electrochemical behavior of trivalent lanthanides such as europium(III), samarium(III) and cerium(III) in 1-butyl-3-methylimidazolium chloride (bmimCl) was investigated by cyclic voltammetry and chronopotentiometry at platinum working electrode in the temperature range of 343-373 K. The cyclic voltammograms of these metal ions in bmimCl at Pt electrode consisted of quasi-reversible waves occurring at a cathodic peak potential of −0.769 V, −0.547 V and −0.558 V (vs. Pd, 373 K) were attributed to the reduction of Eu(III), Sm(III) and Ce(III) to their respective divalent states. A prewave observed in the cyclic voltammogram of Eu(III) represents the reduction of Eu(III) to Eu(II) adsorbed on Pt electrode. The diffusion coefficients of these trivalents in bmimCl were determined to be of the order of ∼10⁻⁸ cm² s⁻¹ by cyclic voltammetry and chronopotentiometry and the charge transfer rate constants (k_s) were determined to be of the order of ∼10⁻⁵ cm s⁻¹ by Nicholson method. The apparent standard potentials, E^0* (vs. Cl₂/Cl⁻), and some thermodynamic properties of reaction of LnCl₃ from LnCl₂ were determined and the results are reported in this paper.     

Electrochemical behaviour of gadolinium ion in molten LiCl-KCl eutectic

This work presents the electrochemical study of GdCl₃ in the molten LiCl-KCl eutectic in the temperature range 723-823 K. Transient electrochemical techniques such as cyclic voltammetry and chronopotentiometry, on an inert metallic tungsten working electrode, have been used in order to investigate the reduction mechanism and transport parameters. This study shows that Gd³⁺ ions are reduced to Gd metal by a single step mechanism with exchange of three electrons. Diffusion coefficient of GdCl₃ ions was determined at various temperatures, at 723 K the value is D = 0.88 10⁻⁵ cm² s⁻¹. Apparent standard reduction potential of the redox couple Gd³⁺/Gd has been determined by the open-circuit chronopotentiometry technique at several temperatures. Also the Gibbs free energy of GdCl₃ formation was determined and compared with thermodynamic data for pure compounds in the supercooled state in order to estimate the activity coefficient of Gd³⁺ in the molten LiCl-KCl eutectic.     

Vaporisation thermodynamics of caesium iodide and caesium chromate

Vaporisation studies over solid and liquid CsI have been carried out by Knudsen effusion (753 – 897 K) transpiration (862 – 1125 K) and boiling temperature (977 – 1430 K) methods. The Knudsen mass loss and transpiration data over CsI(s) are consistent with the presence of monomeric species upto 873 K and the vapour pressure values can be represented by the equation: log₁₀P(kPa) = (7.59 ± 0.12) ? 0.00113 T(K) ? (10301 ± 95)/T(K) ? 21088/T²(K) + 1.13 log₁₀T(K) Transpiration and boiling temperature data were used to calculate partial pressures of monomeric (p₁°) and dimeric (p₂°) species over CsI(1) and the values obtained can be represented by the following equations: log₁₀P₁°(kPa) = (19.67 ± 0.38) ? (9628 ± 344)T(K) ? 3.65 log₁₀T(K)log₁₀P₂°(kPa) = (28.42 ± 0.41) ? (9986 ± 415)T(K) ? 6.56 log₁₀T(K) Vapour pressure of Cs₂CrO₄(1) was measured in the temperature range 1225 to 1405 K by transpiration method and vapour pressure calculated assuming monomeric species can be represented by the equation: log₁₀p(kPa) = (8.32 ± 0.26) ? (13434 ± 344)T(K)     

Cerium and neodymium co-precipitation in molten chloride by wet argon sparging

Co-precipitations of cerium (III) and neodymium (III) at 10 wt.% in LiCl–CaCl₂ (30-70 mol%) molten salt at 705 °C have been achieved using an original way of precipitation, wet argon sparging. Several CeCl₃/NdCl₃ ratios have been studied, and the isolated powders were analyzed using different characterization methods including XRD investigations. The lanthanides precipitation yields have been determined around 99.9% using ICP-AES analysis. XRD demonstrated that the precipitates mainly contained mixed oxychloride (Ce_1?xNd_x)OCl and a small amount of the mixed oxide Ce_1?yNd_yO_2?0.5y. Calcination of these precipitates has resulted in the cerium and neodymium mixed oxides. For the precipitation with a Ce/Nd = 50/50 ratio, an hydroxychloride Ln(OH)₂Cl and the oxychloride Ce^IV(Nd_0.7Ce_0.3)^IIIO₃Cl have been identified as unexpected intermediate compounds.     

Electrochemical behavior and electronic absorption spectra of uranium trivalent ions in molten LiCl-CsCl mixtures

The redox reactions and coordination circumstances of uranium trivalent ions in molten LiCl-CsCl mixtures were investigated by cyclic voltammetry and spectrophotometry. The formal redox potential, E°′(U³⁺|U), in LiCl-CsCl mixtures with the CsCl mole fraction of 0.2 was more positive than that in LiCl melt. The CsCl system showed the most negative E°′(U³⁺|U). The electronic absorption spectra of U³⁺ in LiCl-CsCl mixtures showed that the intensities of absorption peaks decreased with the increase of CsCl mole fraction. The oscillator strength of the hypersensitive f-f transition, f, decreased with the increase of CsCl mole fraction.     

Decomposition of uranyl chloride and its interaction with uranium dioxide in molten NaCl−KCl

The thermal decomposition of uranyl chloride and its interaction with uranium dioxide in a chloride melt of NaC1– KC1 was investigated. It was demonstrated that the decomposition products of uranyl chloride and the products of its interaction with uranium dioxide are uranium octoxide and tetrachloride. An explanation was given for the nonstoiehiometry of the cathodic deposits of uranium dioxide produced in the electrolysis of chloride melts containing uranyl ions.Translated from Atomnaya Énergiya, Vol. 22, No. 1, pp. 30–33, January, 1967.     

Heat transfer and thermophysical properties of molten alkali metals

An investigation was undertaken of heat transfer to molten sodium during turbulent flow in a round copper or nickel tube. An interpolation formula was obtained. Experiments were conducted to determine thermal resistance between liquid sodium and a solid wall of copper, nickel, and stainless steel. Methods were developed for measurement of viscosity, temperature conductivity and density of molten metals. Results are given for measurements of these physical parameters for molten alkali metals (sodium, potassium, lithium and the eutectic solution of sodium and potassium) in a broad interval of temperatures.     

Effect of reprocessing on neutrons of a molten chloride salt fast reactor

Due to their unique features, such as the inherent safety, simplified fuel cycle, and continuous on-line reprocessing, molten salt reactors (MSRs) are regarded as one of the six reference reactors in the Generation IV International Forum (GEN-IV).Molten chloride salt fast reactors (MCFRs) are a type of MSR. Compared to molten fluoride salt reactors (MFSRs), MCFRs have a higher solubility of heavy metal atoms, a harder neutron spectrum, lower accumulation of fission products (FPs), and better bre...     

Electrochemical transient techniques for study of the electrochemistry and thermodynamics of nuclear materials in molten salts

Advantages and disadvantages of electrochemical transient techniques and potentiometry for the determination of formal standard potentials were discussed. It was shown that long potentiometric measurements inevitability led to strong interaction between oxide materials and melts involving actinide or lanthanide compounds. Reactions of disproportionation were observed in melts containing lower oxidation states due to the appearance of oxide ions from oxide materials, because actinides and lanthanides have a great affinity to oxygen. Only at the final stage of each experimental set, was the reference electrode immersed in the melt for a short time for the determination by electrochemical transient techniques and the melt being no longer used after these measurements. It was shown that electrochemical transient techniques in contradiction to potentiometry can be used for the determination of formal standard potentials for irreversible process and redox process accompanied by reaction of disproportionation.     

Thermal and physical properties of molten fluorides for nuclear applications

LiF-BeF₂-ThF₄ is a key system in molten salt reactor fuel studies. In this paper we give an overview of some important features of this ternary system. We discuss the phase behavior, vapor pressure, density and viscosity, based on what is known in the literature and on our own data from previous work on the thermodynamic assessment of LiF-BeF₂-ThF₄.     

High-temperature thermodynamic properties of hypo- and hyper-stoichiometric uranium carbides

Measurements of carbon activity over univariant and bivariant UC_x compositions, including the difficult region near stoichiometry, have been made in the temperature range 2155–2455°K. A flowing-gas technique was used with H₂-CH₄ mixtures in order to fix the carbon potential in the gas stream. The shape of the isotherms showed a sharp decrease in carbon activity near the stoichiometric composition as the composition of the condensed phase was reduced toward the lower phase boundary in the hypostoichiometric region. With argon as the carrier gas, a congruent vaporization composition of C/U ≈ 1.05 was obtained. Values for the free energy and heats of formation, relative partial molar enthalpies and entropies of solution, enthalpy of vaporization and uranium activity are presented.     

Phase relations and thermodynamic properties of the uranium-nitrogen system

The present state of knowledge of the phase relations and thermodynamic properties of the uranium-nitrogen system is given. Emphasis is placed on the nonstoichiometric and thermodynamic properties of uranium sesquinitride. The present paper consists of two parts. The first part is on the phase relations; an equilibrium phase diagram is proposed, and the relationship between structure and lattice parameter for the α-U₂N₃-UN₂ system is shown. The second part deals with thermodynamic properties of UN and α-U₂N₃; the data on decomposition pressures, specific heats, and heats and free energies of formation are summarized and evaluated.