PLENTIFUL ENERGY

The electrorefining process brings actinides to the liquid cadmium cathode. At
the start, they go into solution. But only up to a certain point, as each has a very
well defined solubility limit. At concentrations in the range of 2 percent of the
cadmium amount, their solubility limits are reached. There the mode of deposition
changes dramatically, and it affects both the ratio of plutonium to uranium and the
product amount.
When neither uranium nor plutonium is saturated, the actinide contents are low
enough that their activities, their driving force to react, can be taken to be directly
dependent on concentration. This can be expected to always be at least a little bit
―wrong,‖ but in these circumstances not significantly so.
The ratios of cathode actinide metal and electrolyte actinide chloride activities
will satisfy the equilibrium constant expression and continue to do so as deposition
progresses. This goes on smoothly with no dramatic changes until the solution
becomes saturated with either uranium or plutonium.
When one or the other is saturated but not both, deposition changes markedly.
Plutonium is the important case. No solid phase PuCd 6 forms until the Pu
concentration reaches its solubility limit. When this limit is reached, no more of the
saturating Pu metal enters in solution; it deposits preferentially as a solid phase,
PuCd 6 . When both U and PuCd 6 are saturated, they accumulate at the same ratio as
ratios of activities PuCl 3 and UCl 3 in the salt and the ratio does not change until all
the cadmium or actinide chloride is used up. These are the different possible
operating regimes, but as mentioned above, the last regime is not very important as
inconvenient things happen to the deposit in practice.
The principle here is that free energy change remains opposite in direction to the
reaction we would like, but by loading up the ratio of PuCl 3 /UCl 3 heavily in PuCl 3
an equilibrium is achieved that allows us a useful ratio of Pu/U in the cathode. The
exact value of the equilibrium coefficient isn‘t important for purposes of
understanding the process; the fact that it is an important criterion is. However, our
calculated value of 4.1 is probably adequate, certainly for insight. Off equilibrium
in actinide chloride ratios for saturation, the system tries to achieve saturation of
both elements in cadmium and get back to the corresponding equilibrium ratio of
actinide chlorides.
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. Both saturate at the
same time. All are nicely in equilibrium. The Pu/U ratio will be about 1.55. But let
us vary the actinide chloride ratio and observe what happens. A consistent way of
thinking about all this is provided by the equilibrium expression.
197