Electroanalytical measurements of binary-analyte mixtures in molten LiCl-KCl eutectic: Uranium(III)- and Magnesium(II)-Chloride

Abstract

The electrochemical behavior of MgCl 2 in molten LiCl-KCl eutectic was investigated to evaluate its suitability as a surrogate for PuCl 3 in studies related to the eletrorefining of used nuclear fuel. The reduction of Mg 2+ was found to be electrochemically reversible up to 300 mV s-1 at 773 K. The diffusion coefficient for Mg 2+ was calculated to be 1.74 and 2.17 × 10-5 cm 2 s-1 with and without U 3+ present, respectively, at 773 K using cyclic voltammetry (CV). Upon comparison to literature data, the diffusion coefficient of Mg 2+ differs by only 8.8% (with U 3+ present) from that of Pu 3+ and the difference in peak potentials was only 79 mV. Binary-analyte mixtures of UCl 3 and MgCl 2 in eutectic LiCl-KCl were further investigated using CV, normal pulse voltammetry (NPV), chronoamperometry (CA) and open-circuit potential (OCP) measurements for the purpose of comparing each technique's accuracy in measuring U 3+ and Mg 2+ concentrations. Of all the techniques tested, NPV resulted in the lowest error which was, on average, 11.4% and 9.81% for U 3+ and Mg 2+ , respectively.

Application of the rotating cylinder electrode in molten LiCl-KCl eutectic containing uranium(III)- and magnesium(II)-chloride

“…The possible causes for the large variation in mass transfer coefficients in Table 2 have already been discussed in Section 4.2. The potential of the RE may also have shifted between mixtures, as has been observed in static electrochemical measurements [11]. Between each mixture, the RE was cooled and then reheated with the melt.…”

“…In this work, apparent standard reduction potential or formal potential, , was used in place of and was set to one. values determined elsewhere [11] for U 3+ /U and Mg 2+ /Mg redox couple were used in this analysis. Since both ions undergo single step reduction to the metallic state [11], the valance states of 3 and 2 were used as values for U and Mg, respectively.…”

“…values determined elsewhere [11] for U 3+ /U and Mg 2+ /Mg redox couple were used in this analysis. Since both ions undergo single step reduction to the metallic state [11], the valance states of 3 and 2 were used as values for U and Mg, respectively. This leaves , , and as crucial properties to be determined using an RCE.…”

“…Under these assumptions, Re was calculated to be 7 to 77 using rotational rates from 50 to 500 RPM, a dynamic viscosity of 2.2×10 -3 Pa s [20,21], and a density of 1.62 g cm -3 [22]. Sc was calculated to be 1250 for U 3+ and 780 for Mg 2+ using diffusion coefficients of 1.09 and 1.74 × 10 -5 cm 2 s -1 for U 3+ and Mg 2+ , respectively [11]. Lastly, Sh was calculated using the following equation: [25] are also plotted in Figure 4.…”

“…UCl 3 and MgCl 2 were used as analytes to simulate a UNF electrorefiner salt, in which U 3+ and Pu 3+ are primary ions of interest both of which could potentially deposit at the cathode. MgCl 2 is a good surrogate for PuCl 3 due to the proximity of their standard apparent reduction potentials, as previously demonstrated at stationary electrodes [11]. Four LiCl-KCl-UCl 3 -MgCl 2 mixtures were analyzed using the…”