Advances in the Use of Carbon Anodes for Pyrochemical Electroreduction of Used Nuclear Fuel

Pyrochemical processing of used nuclear fuel is a molten salt process performed at elevated temperatures and is part of a series of processes resulting in the separation of actinides from fission products. Used metal oxide fuel (MOX) is reduced electrochemically to generate metals prior to electrorefining, typically performed at 650^oC in LiCl-Li₂O. The reduction of kilogram lab scale quantities of UO₂ have been successfully performed as surrogates to actual fuel using platinum anodes^1-2, but progression toward engineering or pilot scale-up 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 result from this change in anode materials in both the salt and off gas streams. Instead of forming oxygen at a platinum anode, CO/CO₂ becomes the byproduct gas. Although initially insoluble, CO/CO₂ residence time within the LiCl-Li₂O salt is thought to be problematic because of the formation of reactive carbon species, with respect to the metal, that are soluble and could lead to undesirable side reactions in the system.

This report will discuss an advanced test cell for evaluating the use of carbon as an anode material for the electroreduction of UO₂ to uranium metal in molten LiCl-Li₂O. We will present design challenges and report on the implementation of advances for the accommodation of process changes with respect the new anode material. Finally, preliminary results on the efficiency and chemical effects of carbon anodes will be discussed.

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.