Development of Carbon Anodes for Use in Electrolytic Reduction of Uranium(IV) Oxide

Abstract

Recycling of used nuclear fuel using pyrochemical methods involves a series of molten salt processes performed at elevated temperatures and results in the separation of actinides from fission products. The first step in the pyrochemical treatment of light water reactor (LWR) oxide fuel is electrolytic reduction. This electrochemical operation converts used fuel oxides to metal before electrochemical separations can be performed by electrorefining. Conversion of oxides to metal is commonly performed in molten LiCl-Li2O at 650℃. Electrolytic reduction of UO2, the primary component of used nuclear fuel, has been successfully demonstrated at the kilogram-scale using platinum anodes1-2, but progression toward engineering scale presents different anode material requirements. Carbon, specifically graphite, is commonly used for ore refining in commercial molten salt processes because it is cost effective, readily available, and easily fabricated to meet scale-up requirements. Use of carbon anodes in LiCl-Li2O results in different salt chemistries, with respect to using platinum or other oxygen evolving electrode materials. In particular, the evolved gaseous byproduct of carbon anodes is CO and CO2, which can react with Li2O to form Li2CO3. If the carbonate concentration is allowed to increase, the desired reaction, namely the transport of oxide from the cathode to the anode via Li2O, is replaced by the undesirable transport of carbon from the anode to the cathode via Li2CO3. This report will discuss use of carbon as an anode material for the electrolytic reduction of UO2 to uranium metal in molten LiCl-Li2O at 650℃. A discussion of the salt chemistry, minimizing Li2CO3 formation, method developments, cell operation, and reduction efficiency will be presented.