PLENTIFUL ENERGY

1. At an actinide chloride ratio of precisely 4.1, the uranium and the PuCd 6
activities will maintain the same ratio right from the beginning, to the extent
that their activity coefficients are given by the ―ideal approximation‖—the
saturation value multiplied by the ratio of the concentration to the saturation
concentration. Both will saturate at the same time. All are nicely
in equilibrium.
2. At an actinide chloride ratio of two, the activity ratio is half the saturated
ratio; the uranium will saturate first while the PuC 6 is only half way to
saturation. The UCl 3 concentration has to be decreased to get to the
equilibrium value of 4.1, so uranium will preferentially deposit, trying to
increase the actinide chloride ratio toward 4.1. In this regime the cathode is
acting pretty much like a solid cathode and the resulting overall deposit will
be largely U.
3. At the other extreme, at a ratio of eight, say, the opposite effects take place;
the activity ratio is twice the saturated ratio, the PuCd 6 will saturate, and
PuCd 6 will try to deposit as the system tries to evolve to decrease the PuCl 3
concentration and move the actinide chloride ratio toward 4.1. In this case
the deposit will be plutonium rich, above the 1.5 Pu/U value for the bothsaturated
case 1, above.
Off equilibrium in actinide chloride ratios for saturation, the system tries to
achieve saturation and get back to the saturated cadmium equilibrium ratio of
actinide chlorides. Practical everyday considerations limit what can be done.
Criticality considerations are important. The amount of fuel that the anode baskets
can contain is to some degree limited. The cathode too almost certainly is limited in
capacity. The actinide content of the salt is high relative to the amounts in the
anode and cathode, making changes in its composition tedious. The composition of
the anode feed may make higher PuCl 3 /UCl 3 ratios difficult to achieve; they are still
possible, but perhaps only after several batches have been processed. What is
possible in practice is limited by other practical effects such as deposit growing out
of the cathode.
Finally, it is of interest to note that with everything at equilibrium, the ratio of
plutonium to uranium in the product of 1.55 is a factor of 2.65 less than the 4.1 ratio
of plutonium chloride to uranium chloride in the electrolyte. This gives an
indication of the difficulty of a pure product—whatever the PuCl 3 /UCl 3 ratios are in
the salt, the Pu/U product ratio will be degraded from this by a considerable factor.
9.6 Effect of Saturation on Chemical Activity
And now we come to the key point in the effect of saturation. The chemical
activity of a solute in a solution is independent of the amount of solute once the
solution is saturated in that solute. This is basic to understanding liquid cadmium
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