PLENTIFUL ENERGY

evolution at the anode and prevented chlorine evolution at the cathode. As
operation began, there was an initial gradual drop in anode potential. After four
hours there was a sharp drop in the cathode potential of 300 to 500 mV, observed in
all experiments, as it shifted from direct reduction of UO 2 to the lithium reduction
potential.
The combined effects of a metallic surface layer building up on the fuel
particles, the surface area of the oxide particles decreasing, and the buildup of
oxygen ions in the immediate area of the cathode, causes the voltage to drop down
to the lithium deposition potential when constant current is maintained. In this
circumstance, lithium metal is generated, and its deposition contributes significantly
to the cell current and to the reduction mechanism itself. The balance between the
two processes, direct reduction and lithium chemical reduction, depends on factors
such as electrical isolation of particles and on the stage of the process. The current
must be adjusted to avoid generation of excess lithium at the cathode while
maximizing the reduction rates. Lithium generation should therefore match its
consumption by the chemical reaction. Designing to allow stirring in the area of the
cathode may well be necessary.
The product retained its shape generally, some fines were generated, and a
significant amount of electrolyte was entrained. Reduction proceeded from the
outer surface inward and a metallic layer of uranium formed on the particle surface,
decreasing the process rate. Particle size, it was concluded, will be important.
There are three important factors in operation. Anode potential has to be
maintained below the level where chlorine gas is evolved, the cathode potential
must avoid lithium droplets and films, and the oxide ion concentrations have
opposing important effects on both anode and cathode reactions and must be
optimized. As they can be controlled, such optimization is possible.
A second set of experiments with actual spent fuel, representative of later stages
in the development, were performed at Idaho National Laboratory. [5] Here again
the electrolyte was 650 o C LiCl with 1w% Li 2 O. A series of ten experiments were
done with LWR spent fuel, long out of the reactor. The fuel was from the Belgian
reactor BR-3, irradiated in 1979. The fuel was crushed into particles, with batch
sizes of about 50 grams. Two power sources were used, with the cathode lead in the
center of the cathode fuel mass acing as the negative electrode for both. The voltage
of the platinum anode in the primary circuit was kept below the value which would
cause platinum to dissolve, and the primary current was controlled to maintain the
center lead voltage below lithium formation potential. The secondary circuit anode
was the basket wall, and this circuit was activated when the potential on the basket
wall indicated lithium formation. In effect, the metallic lithium was pumped from
the wall to the center of the fuel mass, where it aids in reduction, and cannot cause
other troubles.
216