PLENTIFUL ENERGY

This is the significant number. The concentration ratio of PuCl 3 to UCl 3 for
equilibrium when both the uranium and plutonium cadmium intermetallic are
saturated in the cadmium and in contact with the salt is 4.1. And this number is the
criterion for assessing the effect on the ratio of plutonium to uranium in the product
of any particular ratio of plutonium chloride to uranium chloride in the salt.
A.10.2 The Reaction as Plutonium Atoms
In this case, the reaction is written as Pu + UCl 3 U + PuCl 3.
G o f becomes G o f (PuCl 3 ) - G o f (UCl 3 ) = 21.5 kcal/mole, and we know from
previous sections that the equilibrium constant K eq for the free energy difference of
this magnitude becomes very large, 1.2x10 6 in fact.
This is the equilibrium constant for the case without PuCd 6 formation, as it is for
a solid-rod cathode, for example. From this it is obvious why metallic plutonium, as
such, can‘t exist at any reasonable PuCl 3 /UCl 3 ratio—in fact, not until the ratio
reaches a million or so.
For the liquid cadmium cathode, the plutonium activity corresponding to the
plutonium in PuCd 6 , 4x10 -6 , is the appropriate value. The uranium activity is unity
and the ratio of the activity coefficients of the actinide chlorides is 1.14, as before,
so the ratio of the concentrations of the actinide chlorides at equilibrium is,
PuCl3 / UCl3 = (1.2x10 6 ) x (4x10 -6 )/1.14 = 4.8/1.14 = 4.2.
Thus, within rounding error, the two formulations are identical. The very large
equilibrium constant, 1.2x10 6 , which leads to an extremely high PuCl 3 to UCl 3
equilibrium ratio in the salt, is counterbalanced by the tiny activity of plutonium
atoms in the intermetallic PuCd 6 , once again giving the reasonable ratio of
concentrations of PuCl 3 to UCl 3 of about four for equilibrium.
At equilibrium, with U and Pu both saturated in cadmium and in contact with a
salt containing actinide chlorides, the PuCl 3 /UCl 3 ratio in the salt is 4.1.
Whether plutonium is regarded as present as PuCd 6 molecules in the unsaturated
cadmium solution or as plutonium atoms does not matter, if consistency in
calculation in maintained, as we have done. Two calculations, same result. PuCd 6
with activity 1 and Pu with activity 4x10 -6 are equivalent if we base our calculation
on the correct chemical equation and corresponding equilibrium constant.
So we now know that the equilibrium ratio of PuCl 3 to UCl 3 when both U and
PuCd 6 are at saturation concentrations is about four. At saturation, a U /a Pu is
371