Sun-Seok Hong

CHEMICAL BEHAVIOR OF FISSION PRODUCTS IN THE PYROCHEMICAL PROCESS

Abstract The chemical behavior of lanthanide oxides has been studied both for the electrolytic reduction process and the electrorefining process. At high concentration of Li2O in LiCl, lanthanide oxides reacted with Li2O to form mixed oxides, LiLnO2 (Ln = lanthanides), which decomposed to the starting materials at relatively low Li2O concentration. The chemical behavior of lanthanide oxides under the condition of electrorefining process was investigated by optical fiber spectrophotometry and X-ray diffraction. Lanthanide oxides reacted with U3+ to produce Ln3+ and UO2. The solubility of lanthanide oxides was measured under the electrolytic reduction and the electrorefining condition. All of the lanthanide oxides except Eu2O3 had relatively low solubility values in LiCl-KCl eutectic mixture at 450°C. Electrochemical behavior of Br-, I-, and Se2- in LiCl was also investigated by cyclic voltammetry and by X-ray diffraction. All of the anions reacted with platinum anode and gave platinum compounds.

Electrochemical reduction of UO2 to U in a LiCl–KCl-Li2O molten salt

An electrochemical reduction of UO2 to U in a LiCl–KCl-Li2O molten salt has been investigated in this study. A diagram showing equilibrium potentials (relative to Cl2/Cl−) plotted versus the negative logarithms of oxide-ion activity (pO2−) was constructed. The crushed UO2 pellets in the cathode basket of an electrolytic reducer were successfully reduced to U. The reduction of UO2 is proved to proceed mainly through chemical reaction with in situ generated Li and K at the cathode. The control of cathode potential is essential to prevent the deposition and subsequent vaporization of K metal at the cathode for the applications of a LiCl–KCl-Li2O molten salt as an electrolyte for the metal production from its oxide sources.

Electrochemical properties of noble metal anodes for electrolytic reduction of uranium oxide

Noble metals (Rh, Pd, Ir, and Au) were investigated as O2-evolving anodes for electrolytic reduction of UO2 in LiCl–Li2O molten salt to replace Pt anodes, which are gradually consumed owing to Li2PtO3 layer formation. Anodic behaviors of these metals were examined by cyclic voltammetry. Au only showed O2 evolution in a moderate potential range without side reactions, suggesting better electrochemical stability relative to Pt anodes. With Au anodes, UO2 was electrochemically reduced to metallic U. No oxide layer was observed on the surface after the reduction. However, local dissolution remains a potential issue from a stability viewpoint.