Process Engineering Challenges for the Development of Electrolytic Reduction of Uranium Oxide in Molten LiCl-Li2O

Abstract

Electrolytic uranium oxide reduction has the potential to be a key process for recycling spent fuel from commercial light water reactors to advanced nuclear reactors—including molten salt reactors and metal fueled reactors. However, several problems with the process have been identified that need to be addressed to support efficient, cost-effective commercial implementation. Process optimization requires attention to metal corrosion, anode stability, cell efficiency, and cathode product purity. Generation of oxygen bubbles at the anode combined with high temperature (650oC) and molten chloride salt (LiCl + 1 wt% Li2O) create a highly oxidizing environment for metals needed for salt containment and shrouding of the anode. The current standard choice for the inert anode is platinum metal, which has fairly good stability in the system. But given the high cost of platinum, alternative anode materials need to be discovered. Efficient removal of oxygen bubbles requires use of a shroud around the anode. The easy fabricability of metals makes them a reasonable choice for the shroud material, but corrosion due to interaction with the salt and oxygen needs to be minimized. Cell efficiency is affected by hydroxide impurity in the salt. Methods for removing water and hydroxides from the salt will be reported that improve cell efficiency. Cell efficiency can also be affected by controlling the reduction mechanism. Uranium oxide can either be reduced chemically by reaction with electrochemically generated lithium atoms or electrolytically with direct reduction of UO2. The reduction mechanism also affects the buildup of entrained lithium oxide in the cathode product. Entrained lithium oxide is problematic as it gets carried on to subsequent processing steps such as electrorefining and molten salt oxidation.