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ELECTROCHEMISTRY OF LiCI-Li2O-H20 MOLTEN SALT SYSTEMS
Natalie Gese and Batric Pesic2
1Idaho National Laboratory, Separations Department,
Nuclear Science and Technology Directorate;
P.O. Box 1625-6180; Idaho Falls, ID 83415, USA
2University of Idaho, Chemical and Materials Engineering Department;
Moscow, ID 83844-3024, USA
Molten Salt, Electrochemistry, Electrolytic Reduction, Electrorefining, Spent Fuel
Abstract
Uranium can be recovered from uranium oxide (UO2) spent fuel through the combination of the
oxide reduction and electrorefining processes. During oxide reduction, the spent fuel is
introduced to molten LiCl-Li20 salt at 650 C and the U02 is reduced to uranium metal via two
routes: (1) electrochemically, and (2) chemically by lithium metal (Li0) that is produced
electrochemically. However, the hygroscopic nature of both LiCl and Li20 leads to the
formation of LiOH, contributing hydroxyl anions (OH-), the reduction of which interferes with
the Li' generation required for the chemical reduction of U02. In order for the oxide reduction
process to be an effective method for the treatment of uranium oxide fuel, the role of moisture in
the LiCl-Li20 system must be understood. The behavior of moisture in the LiCl-Li2O molten
salt system was studied using cyclic voltammetry, chronopotentiometry and chronoamperometry,
while reduction to hydrogen was confirmed with gas chromatography.
1. Introduction
Oxide reduction is the key process used to convert spent oxide fuel to metal suitable for
treatment in an electrorefiner [1, 2, 3]. During the reduction process, oxide fuel is converted to
metal electrolytically and chemically in a vessel containing electrolyte comprised of molten
LiCl-Li20 at 650 C [4, 5]. Conversion efficiencies of 99.7% (for the conversion of oxide to
metal) and enhanced reduction rates (due to the fast kinetics of lithium reduction from Li2O)
have been reported [6, 7]. In the electrorefining process, remaining fission products, transuranics
and minor actinides are separated to yield a high purity uranium metal product (>99.99%) [6, 7].
One of the major causes of lower current efficiencies in the oxide reduction process is attributed
to the recombination of lithium with oxygen in the cell and the decrease in oxide ion
concentration from Li20 degradation [15]. The results here will provide a fundamental
understanding of the impact of moisture, and the mechanism and need to purify the molten salt,
in order to achieve higher throughput for the oxide reduction process. It has been determined that
moisture reacts with Li20 to produce LiOH and, as a result, the reduction of OH~ ions produces
hydrogen gas on the working electrode preferentially to lithium.
Electrochemical stability, reaction kinetics, and high ionic conductivities are among the
favorable properties of LiCl based melts [8, 9, 10]. However, lithium chloride as well as other
