Graphite Anodes for Electroreduction of Uranium Oxide

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 anodes^1-2, 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.

References:

1. Laszlo Redey and Karthick Gourishankar, “Direct Electrochemical Reduction of Metal-Oxides” US Patent US6540902 B1, Granted April 1, 2003
2. S. D. Herrmann, S. X. Li, M. F. Simpson and S. Phongikaroon, Separation Science and Technology 2006, 41, 1965-1983.

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