PLENTIFUL ENERGY

For the other actinides, and plutonium in particular, there is an added constraint.
Although the action of lithium metal on the actinide oxides like PuO 2 has free
energies of formation of Li 2 O of similar magnitude to UO 2 (and also negative), and
thus would be expected to reduce completely to their metals, they may not do so. If
the concentration of Li 2 O dissolved in the electrolyte is too high, an intermediate
oxide compound will form, Pu 2 O 3 , instead of the reduction going right on to metal.
This compound has a positive ΔG for Li 2 O formation (about +4.5 kcal/mol), and
thus the action of lithium metal on it will not reduce it further. The way around this
problem is to avoid forming the compound in the first place. By maintaining the
Li 2 O concentration in the LiCl salt to less than 3.3 w% throughout the reduction
process, the intermediate compound is avoided and the reduction goes directly to
the metal, as desired.
It may be noted that the oxides of several of the rare earth fission products have
limits on the Li 2 O as well, some well below the concentrations necessary for the
actinides. To reduce rare earths, the Li 2 O concentration needs to be lowered into the
0.1% range. The decomposition potential of Li 2 O rises as the concentration is
lowered, and these concentrations bring the Li 2 O decomposition potential close to
those necessary for LiCl decomposition, which could result in undesirable chlorine
gas evolution. Where these fission product oxides are not reduced, they remain as
oxides in the waste. Fortunately, most of the rare earth fission products are formed
as metal in the UO 2 matrix, and therefore do not need to be reduced.
10.2.4. The Equipment
The principle of electrolytic reduction equipment is schematically illustrated in
Figure 10-1. At the anode oxygen gas is swept from the cell. The cathode process
yields metallic product that will then be electrorefined. In fact, the cathode basket in
the reduction step can be directly transferred to the electrorefiner as the anode
basket for the next step.
Engineering-scale reduction cells were designed and fabricated, and they are
currently used to understand the parameters for process scaleup. In particular the
effect of cathode bed thickness and oxygen removal/handling on the extent and rate
of reduction are being examined. These cells are operated at a one-kilogram scale
using depleted UO 2 feed. The engineering-scale tests will provide electrochemical
engineering data needed for design and modeling of pilot-scale cells. So far, the
tests have shown significant improvements, ~40% to ~65% in current efficiency,
while producing a high quality metallic product. Although most of the cell studies
have used a platinum anode, several inert ceramic materials are being evaluated as
alternatives to costly platinum as well.
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