Contribution to the Knowledge of the Electrochemistry of Uranium in Molten LiCl‐KCl Eutectic: II. Chronopotentiometric study of the reaction U(IV) + e⇆U(III) and measurement of the diffusion coefficient

Abstract

The reaction U(IV)+e⇆U(III) in molten LiCl‐KCl eutectic has been investigated by means of chronopotentiometry. The diffusion coefficient of U(IV) passes from 4.46 . 10−6 to 12.15 . 10−6 cm2/sec. when the temperature increases from 400°C to 500°C. The diffusion coefficient of U(III) passes from 4.14 . 10−6 to 13.62 . 10−6 cm2/sec. when the temperature increases from 400°C to 550°C. The mechanism of diffusion is discussed and the hypothesis is put forward that for each ion there is a temperature at which the diffusion takes place according to the viscous flow mechanism. Some aspects of the kinetics of the reaction are also clarified.

The Application of Low-Melting LiCl-KCl-CsCl Eutectic to Electrodeposit Uranium Metal

“…Moreover, in Figure 2b, the typical chronopotentiogram recorded in the LiCl-KCl-CsCl-1.78 wt% UCl 3 melt at 670 K also presents two platforms (τ 1 and τ 2 ) at about −0.6 and −1.5 V, corresponding to reductions of U 4+ and U 3+ , respectively. Thus, based on the description mentioned above and previously published literature on the chloride molten salts, 6,8,16,18,21,[24][25][26][27][28][29][30]39 the electrochemical reactions of U cations in the LiCl-KCl-CsCl-1.78 wt% UCl 3 melts can be expressed as follows:…”

“…16 This conclusion has been extensively verified in this system in the literatures. 8,[17][18][19][20][21][22][23][24][25][26][27][28][29] The diffusion coefficients of U 3+ and U 4+ are important kinetic values in the electrochemical process, and various investigations have been conducted on the diffusion coefficients of U 3+ and U 4+ in LiCl-KCl, 18,28,30 NaCl-AlCl 3 13 and LiCl-KCl-CsCl 25,31 melts. Moreover, the electrodeposition of uranium metal and corresponding nucleation mechanism detection, however, were primarily conducted in LiCl-KCl, [32][33][34][35][36] and NaCl-KCl 37 at higher temperatures at/above 773 K. In particular, in the ternary eutectic LiCl-KCl-CsCl, Maltsev et al determined the diffusion coefficients of U 3+ and U 4+ as well as the apparent standard potentials of U 3+ /U 0 and U 4+ /U 3+ redox couples from 573 K to 1073 K. 6,25,31 However, to our knowledge, there are no research reports on the nucleation mechanism and electrodeposition of uranium metal in the low melting LiCl-KCl-CsCl eutectic at present.…”

Electrochemical Behavior and Some Thermodynamic Properties of UCl[sub 4] and UCl[sub 3] Dissolved in a LiCl-KCl Eutectic Melt

“…This interaction may certainly influence the determination of kinetic parameters and distort experimental results on diffusion coefficients in studies. 1,19,20…”

“…The diffusion coefficients of U͑IV͒, U͑III͒, in the LiCl-KCl eutectic were obtained by Martinot et al by chronopotentiometry method. 19,20 They reported D U͑IV͒ values of 4.5•10 −6 and 12.2•10 −6 cm 2 s −1 at the temperatures 673 and 773 K, respectively. The diffusion coefficients of U͑III͒ were less than those of U͑IV͒ and they changed from 4.1•10 −6 up to 13.6•10 −6 cm 2 s −1 in the temperature range 673-823 K. In these studies it was found that the activation energy for the diffusion of U͑IV͒ and U͑III͒ decreases with increasing temperature.…”

Review—Fundamental Uranium Electrochemistry and Spectroscopy in Molten Salt Systems

“…The essentially equivalent coordination environments [UCl 6 ] 3− /[UCl 6 ] 2− are consistent with the facile nature of the U(III)/ U(IV) redox chemistry reported during the same decade. 21,[24][25][26][27][28][29][30][31][32][33] By today's measures of the literature on [UCl 6 ] 3− and [UCl 6 ] 2− redox speciation, there is little doubt these two species are predominant in fused salt eutectics-a mixture of salts in which the melting point of the whole is lowered. 34 Although widely (but not unanimously) accepted, in 2016, following some six decades of research on uranium redox behavior and speciation in fused salt eutectics, Rappleye et al reopened the issue about the extent of U (III) chlorocomplexation.…”