Once the fission products have been removed, the uranium and plutonium (both now in the organic solvent) must be separated from each other. This separation takes place in a third pulsed column (the 1BX column).
The separation relies on a key chemical principle:
- Pu(IV) is readily extracted into TBP/OK (high distribution factor).
- Pu(III) is not soluble in the organic phase (very low distribution factor).
Therefore, by reducing Pu(IV) to Pu(III), the plutonium will transfer back into the aqueous phase while the uranium remains in the organic phase.
The reduction reaction (used in THORP and UP3):
The reductant is uranium in the 4+ oxidation state, U(IV). A stabiliser (hydrazine) is also added to mop up excess nitric acid, which would otherwise re-oxidise Pu(III) back to Pu(IV). The advantage of using U(IV) as the reductant is that it combines with the product uranium stream and adds nothing to the waste — the reaction is salt-free.
Result:
- Plutonium (now as Pu(III)) transfers to the aqueous phase and falls in the column.
- Uranium remains in the organic solvent and rises to the top.
The aqueous plutonium stream then undergoes further scrubbing in additional pulsed columns to remove any remaining uranium.