Graphite Anodes for Electroreduction of Uranium Oxide

Abstract

Recycling of used nuclear oxide fuel is under investigation using pyrochemical methods, which involve a series of molten salt processes performed at elevated temperatures and result in the separation of actinides from fission products. During this process, used fuel oxides are electrochemically reduced to metals prior to electrochemical separation of the actinides from fission products in a subsequent process step. This electroreduction is typically performed at 650°C in LiCl-Li2O. 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 larger scales presents different requirements for anode materials. Carbon (graphite) is commonly used in commercial molten salt processes within the ore refining industry. It is cost effective, readily available, and a logical choice for scale-up needs. However, different chemistries in molten LiCl-Li2O result from this change of anode material in both the salt and off-gas streams. Instead of forming oxygen at a platinum anode, the byproduct gas becomes CO and CO2. Although slightly soluble, these species can react with Li2O to form Li2CO3 as a result of residence time within the salt. The carbonate concentration can build up in the salt during the reduction process, which is problematic because of potential reaction between actinides and carbonate ions to form undesired byproducts in the reduced product. This report will discuss use of carbon as an anode material for the electroreduction of UO2 to uranium metal in molten LiCl-Li2O at 650°C. We will present design challenges and a discussion of salt chemistry with respect to carbonate formation in the system, along with results on reduction efficiency.