Electrochemistry of Uranium in Molten LiCl-KCl Eutectic

Abstract

The Institute for Transuranium Elements ͑ITU͒ is building up an accurate database of actinide behavior in chloride melts in support of its nuclear fuel reprocessing development program. The electrochemical properties of uranium, dissolved in LiCl-KCl eutectic melt, were investigated by transient electrochemical techniques, such as cyclic voltammetry ͑CV͒ and chronopotentiometry on an inert tungsten electrode. It was shown that U 4+ is reduced to U 0 by a two-step mechanism corresponding to U 4+ /U 3+ and U 3+ /U 0 transitions. In the 400-550°C ͑673-823 K͒ range, the diffusion coefficients of U 3+ and U 4+ were similar and equal to: D͑U 3+ ͒ = 13.7 ϫ 10 −4 exp͕−24.2 ϫ 10 3 /RT͑K͖͒ and D͑U 4+ ͒ = 5.25 ϫ 10 −4 exp͕−19.8 ϫ 10 3 /RT͑K͖͒ cm 2 s −1 . The apparent standard potentials of U 4+ /U 3+ and U 3+ /U 0 redox systems were E o* ͑U 4+ /U 3+ ͒͑V͒ = −1.902 + 0.0006104T ͑K͒ vs. Cl 2 /Cl − and E o* ͑U 3+ /U 0 ͒͑V͒ = −3.099 + 0.0007689T ͑K͒ vs. Cl 2 /Cl − , respectively. Some thermochemical properties ͑⌬G * ,␥͒ of uranium solutions were also derived from the electrochemical measurements. The Gibbs free energies of dilute solution of UCl 3 and UCl 4 in the LiCl-KCl were determined to be: ⌬G * = −897.09 + 0.226T ͑K͒ and −183.53 + 0.0589T ͑K͒ in kJ mol −1 , respectively. In the 400-550°C ͑673-823 K͒ range, the activity coefficients ␥ of UCl 3 and UCl 4 range between 0.3 − 4.5 ϫ 10 −3 and 17.1 − 12.7 ϫ 10 −3 , respectively.Pyrochemical reprocessing is a promising method for recycling spent nuclear fuel. Uranium is the major element in most nuclear fuel cycles, thus the assessment of accurate thermochemical data for this element in molten salts is of high importance. Uranium behavior has been studied in many chloride-based melts: for example, MgCl 2 -NaCl-KCl, 1 NaCl-KCl, 2,3 LiCl-NaCl-CaCl 2 -BaCl 2 , 4 and LiCl-KCl eutectic. [5][6][7][8][9][10][11][12][13][14][15] In the literature, the first electrochemical studies on uranium behavior in molten chlorides appeared in 1959. 1,7,8 It was established that the reduction of UCl 4 to uranium metal proceeds via a two-step mechanism, i.e., U 4+ is reduced into U 3+ before forming uranium metal. In a MgCl 2 -NaCl-KCl mixture, Hill et al., 1 who used a platinum needle microelectrode, reported a threeelectron system for the U 3+ /U 0 transition. The apparent standard potential was close to −2.467 V vs. Cl 2 /Cl − at 450°C ͑723 K͒. In the NaCl-KCl eutectic, by electromotive force ͑emf͒ measurements, Flengas 2 measured E o* ͑U 3+ /U 0 ͒͑V͒ = 0.000592T ͑K͒ − 2.973522 vs. Cl 2 /Cl − and E o* ͑U 4+ /U 3+ ͒͑V͒ = 0.000492T ͑K͒ − 1.808311 vs. Cl 2 /Cl − in the 670-850°C ͑943-1123 K͒ range. In the NaCl-KCl eutectic, Serrano et al. 3 showed by cyclic voltammetry ͑CV͒ that the reduction U 3+ to uranium metal is reversible and proceeds through a three-electron mechanism. The diffusion coefficient of U 3+ was determined equal to 5.5 ϫ 10 −5 cm 2 s −1 at 715°C ͑983 K͒. Recently, in LiCl-NaCl-CaCl 2 -BaCl 2 baths, Poa et al. 4 studied the electrochemical reduction of uranium U 3+ to uranium metal b...

On the formation of U–Al alloys in the molten LiCl–KCl eutectic

“…The experimental set-up for the electrochemical measurements has been described in a previous paper [11]. Several electrolytic baths consisting of a mixture of 30 to 40 g of anhydrous LiCl-KCl eutectic (Aldrich 99.99 %) containing U 3+ (1.00-1.60 mass.%) were prepared by oxidising uranium metal.…”

“…Precise X-ray powder diffraction analysis was performed on a Siemens D8 On the tungsten electrode, U 3+ ions are reduced to U metal in one step involving the transfer of three electrons [14,15]. At 460°C, this reduction peak (Ep Ic ) is observed at a potential of -1.52 V vs. AgCl/Ag, and associated with a sharp reoxidation peak (Ep Ia ) at -1.20 V vs. AgCl/Ag, corresponding to the dissolution of the deposited U metal during the cathodic sweep.…”

“…At 460°C, this reduction peak (Ep Ic ) is observed at a potential of -1.52 V vs. AgCl/Ag, and associated with a sharp reoxidation peak (Ep Ia ) at -1.20 V vs. AgCl/Ag, corresponding to the dissolution of the deposited U metal during the cathodic sweep. At potentials more positive than the U 3+ /U 0 redox couple, a wave with a shape characteristic of a soluble-soluble transition is observed; it corresponds to the U 4+ /U 3+ redox couple [14,15].…”

Electrochemistry of thorium in LiCl–KCl eutectic melts

“…This behaviour is rather unexpected, since most of the other actinides have shown a reversible behaviour at low scan range, typically below 200 mV.s -1 [1][2][3].…”

“…Previous work performed in ITU concerned the study of the electrochemical properties of U, Pu, Np and Am cations in a LiCl-KCl eutectic molten salt [1][2][3]. Transient electrochemical techniques were used to determine the diffusion coefficient of the dissolved actinide ions, as well as their redox potentials.…”