PLENTIFUL ENERGY

At equilibrium, with a concentration ratio of PuCl 3 to UCl 3 of 4.1 in the
electrolyte, the actinide ratio Pu/U in the cadmium is about 1.55. But if the ratio of
actinide chlorides in the salt differs substantially from 4.1, the ratio of the metals in
cadmium will differ greatly from this. With one actinide saturated—plutonium,
say—the plutonium to uranium ratio will be greater in the cadmium at a
concentration ratio of PuCl 3 to UCl 3 greater than 4.1, and less at a lesser ratio. If
both are saturated, the PuCl 3 /UCl 3 equilibrium ratio can only be 4.1. The increase or
decrease of the Pu/U ratio in the cathode toward equilibrium value is slow if both
are unsaturated, rapid if just one is saturated. But by designing to saturate
plutonium only, the ratio of the amounts of plutonium to uranium can be increased
to achieve adequate plutonium enrichments at reasonable actinide chloride ratios in
the electrolyte.
A related effect is how well the fission products are separated from the actinide
product. The degree of separation of the waste from the product—the higher the
better—is quantified by defining a ―separation factor‖ which quantifies the success
of this portion of the process. Stating matters as simply as we can, for this is
important, the separation factor is how much of the waste element—cesium, say—
is in chloride form divided by how much is in metal form, divided by the same ratio
for uranium. The separation factors tell us how clean the separations are likely to
be. Their basis, once again, is the difference in free energies of formation of the
chlorides of the various elements relative to that of uranium.
Measured values of the separation factors are listed in Appendix A, taken from
Ackerman and Johnson. [3] The values vary from separation factors of 43.1 to
1.6x10 9 for fission products, and from 1.88 to 3.52 for plutonium and the higher
actinides. These are perfectly adequate fission product separations for IFR fuel, and
for the actinides, small separations between plutonium and both uranium and the
actinide above plutonium, assuring the desirable (for safeguarding purposes)
imperfect actinide separations.
9.5 Actinide Saturation in Liquid Cadmium: Adequate Plutonium
Depositions
In the operation of the liquid cadmium cathode, saturation effects in the liquid
cadmium play a role. With saturation comes the ability to control, within limits, the
possible composition and amount of the actinide product. [4] The concentrations of
plutonium and uranium going into the cathode change markedly from the state
when neither element is saturated, uranium or plutonium, to the state when one
element, but not both, is saturated. The third state, when both are saturated, has still
different characteristics, but practical difficulties generally rule out operation in that
regime.
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