Electrochemical Reduction of (U, Pu)O2 in Molten LiCl and CaCl2 Electrolytes

Abstract

The electrochemical reduction of UO2-PuO2 mixed oxides (MOX) was performed in molten LiCl at 923 K and CaCl2 at 1,123 K to evaluate the behavior of the plutonium quantitatively and to define the optimum conditions for the electrochemical reduction of those materials. In LiCl, excess deposition of lithium metal can be avoided and the MOX was smoothly reduced at −0.65 V vs. Bi-35 mol% Li reference electrode. The reduction ratio calculated from the mass change of the samples taken during the electrochemical reduction and the ratio evaluated by gas-burette method were in good agreement. The cathodic current efficiency remained 30–50% mainly due to the deoxidation of tantalum cathode basket. Although dissolution of plutonium and americium into the electrolyte was found by the chemical analysis, the dissolved amount was negligible and had no immediate influence on the feasibility of the electrochemical reduction process. In CaCl2, reduction of the MOX occurred in whole range of the tested cathode potential (−0.15 V to −0.40 V vs. Ca-Pb reference electrode). The cathodic current efficiency was around 30%. Although the MOX was completely reduced at −0.25 V, the reduction was interrupted by formation of the surface barrier made of the reduced material and the vacancy between the reduced and the non-reduced areas at −0:30 V. Plutonium and americium dissolved also into the CaCl2 electrolyte to slightly higher concentrations than those observed in LiCl electrolyte. The analyses for the reduction products showed that the amount of those actinides lost from the cathode was much larger than that found in the electrolyte, probably due to the formation of mixed oxide precipitate.