A process developed to separate and recover fissionable plutonium from irradiated nuclear fuel. Originally developed to extract plutonium for nuclear weapons, nuclear reprocessing is used to recover plutonium and uranium for reuse (p. 259) as fuel in commercial nuclear power stations. Spent uranium fuel contains both fissionable uranium-235 and plutonium-239, and has substantially higher fuel content than natural uranium. It is necessary to separate the uranium and plutonium from the neutron-absorbing fission products, followed by separation of the plutonium from the uranium, and can include enrichment of the uranium. The reprocessing involves a solvent extraction process and based on the fact that both uranium and plutonium nitrate form complexes with tri-butyl phosphate (TBP). The process involves chopping up the spent fuel, which is then charged to dissolvers containing nitric acid. The uranium and plutonium nitrates are extracted from the solution by a solution of TBP in kerosene. Other fission products are retained in the aqueous phase, and then concentrated by evaporation. The uranium and plutonium complexes are backwashed from the kerosene phase into the nitric acid solution. The plutonium is reduced from a valency of 4 to 3 with ferrous sulphate since the valent state of 3 does not form complexes with TBP and can therefore be separated from the uranium. The plutonium is then oxidized back to the 4 state.

There are various variations on this separation. The purex solvent extraction process is currently used to reprocess spent nuclear fuel to separate plutonium, uranium, and fission products.