A Study of Graphite as Anode in the Electro-Deoxidation of Solid UO2 in LiCl-Li2O Melt

Abstract

Electrochemical behavior of graphite in LiCl and LiCl melts containing up to 1 wt% Li2O was studied by using cyclic voltammetry and electrolysis, in the context of its probable use as anode in the electrochemical reduction of solid UO2 in the molten salt at 650°C. The lithium deposition and chlorine evolution potentials of LiCl melt were determined by CV of tantalum (cathodic polarization) and graphite (anodic polarization) working electrodes as −2.19 V and +1.21 V vs Ni/NiO reference electrode respectively. In LiCl-Li2O melts, carbon dioxide was formed on the graphite WE from > +0.75 V onwards and its current and potential range of formation increased with increase in the Li2O content of the melt. Cathodic polarization of a UO2 disc WE in LiCl-1 wt% Li2O melt showed that the reduction of UO2 to U occurs at −2.18 V and the lithium metal deposition on it at −2.36 V. Electro-reduction experiments carried out with a dense UO2 pellet cathode and graphite anode in both LiCl and LiCl-Li2O melts proved that the surface of the pellet was reduced to U metal at ≥ −2.2 V. The reduction became increasingly difficult with increase in the Li2O content of the melt.