Effect of Electric Field on I - V Relation at Liquid Cadmium Cathode in LiCl-KCl-UCl3 Molten Salt

Need to reduce CO₂ emissions calls for the growth of nuclear energy expected to meet the growing energy demand with a carbon-free source. However, the problem of accumulating used nuclear fuel considered as waste to be disposed in permanent repositories should be addressed. Pyroprocessing, based on high-temperature molten salt electrolysis, is one of the key technologies for reducing the amount of used nuclear fuel and destroying toxic waste products, such as the long-life fission products. Since group actinide is recovered using a liquid cadmium cathode (LCC) where the activity coefficients are lowered, pyroprocessing enhances the proliferation resistance significantly. In this study, we investigated the current (I)-potential (V) relations at LCC for two different electrode configurations of A-C-A and A-C, where A and C represent anode and cathode, respectively. A lab-scale electrolytic apparatus was installed in a glove box. A cathode comprising a liquid Cd contained in alumina crucible located on a ring connected with the bottom of anode columns. This structure allows using symmetric and asymmetric positioned anode, A-C-A and A-C, respectively. All the measurements were conducted under high purity Ar(g) enviroment (O₂ < 2 ppm, H₂O < 2 ppm). Graphite rods were packed with SiC shroud into each anode column. A silver-silver chloride (1 mol % AgCl in LiCl-KCl) electrode incorporated in a thin Pyrex tube was used as a reference electrode. No significant effect of mass transfer on the I-V relations was observed for the salt (LiCl-KCl-4wt% UCl₃) stirring rate of 50 ~ 150 rpm. In both of the configurations (A-C-A and A-C), the LCC potential value linearly increased with log scale of current density, indicating it follows the Butler-Volmer model. At low current density, the I-V relation for A-C was consistent with that for A-C-A. However, it started to deviate from the relation above the current density of about 0.1 A/cm² and the LCC potential at A-C configuration was lower than that of A-C-A. It may be due to the surface deformation of liquid Cd in electric field. Previous studies reported that the destabilization of a flat surface of liquid metal by a normal DC electric field [Melcher, Continuum Electromechanics, MIT Press, Cambridge, MA, 1981; Néron de Surgy, et al., App. Surf. Sci. 87/88 (1995) 91]. When the applied electric field exceeded a critical value, the interface of liquid/liquid metal would be unstable and then the liquid metal was fluctuated with a characteristic wavelength [Lin, et al., J. Chem. Phys. 114 (2001) 2377; Lin, et al., Macromolecules 35 (2002) 3971]. The formation of peak patterns on the liquid metal surface makes the enlargement of electroactive area. Thus, it can be explained that the LCC surface area at A-C configuration is increased due to the surface fluctuation formed by electric field at high current density (> 0.1 A/cm²), resulting in the decrease of LCC potential. The specific condition may be caused by asymmetric arrangement of electrode (A-C). The effect of electric field on the surface instability of liquid Cd was also influenced by the anode-to-cathode distance. When the anode-to-cathode distance was shortened to 1 mm, this behavior appeared at lower current density (0.075 A/cm²) (data not shown). This work was supported by the Nuclear Research & Development Program of the National Research Foundation (NRF), in a grant funded by the Korean Government.