Design and manufacture of the fuel have to take account of the way it is to be reprocessed after discharge from the reactor (assuming it is not to be consigned to long-term storage as waste material). Oxide fuel is usually reprocessed by the “Purex” process, which involves dissolving it in nitric acid. However (U, Pu)O2 containing more than about 40% plutonium does not dissolve readily. It is therefore necessary to ensure that the fuel is mixed uniformly during manufacture, because particles of fuel with a high plutonium content might not dissolve and would cause difficulty at some later stage in the process. It is not difficult to ensure uniformity of co-precipitated fuel but if pellets are formed from mixed UO2 and PuO2 powders care has to be taken that the grains are very small and are mixed thoroughly. In vipac fuel all the plutonium may be in the larger granules so their plutonium concentration has to be greater than the average for the fuel as a whole, and if after irradiation (when the plutonium concentration is still higher) it exceeds 40% difficulty will be experienced in dissolution.

Purex is a solvent extraction process that separates first uranium and plutonium from the fission products, and then uranium and plutonium from each other. The separation depends on differences between solubility in water and in an organic solvent, and these differ­ences depend on acidity. The feed for the process is the aqueous stream of fuel dissolved in nitric acid, containing the uranium, plutonium and fission products as nitrates, which is brought into contact with a stream of the organic solvent tributyl phosphate (TBP). TBP and water are not mutually soluble. Under acid conditions uranium and plutonium are more soluble in TBP whereas the fission products are more soluble in water, so when the aqueous and organic phases are agitated together the heavy metals are transferred to the TBP while the fission products remain in the water. The two phases are then separated and the TBP stream is brought into contact with water of neutral acidity where the heavy metals go back into the water. The process can be repeated in a second cycle to remove any residual fission products. With two cycles a decontamination factor (the ratio of the fission-product concentration in the final heavy metal product to that in the initial feed) of 10-6 can be achieved.

Plutonium can be separated from uranium by making use of the different valency states it can take. Tri-valent plutonium in the form of Pu(NO3)3 is not soluble in TBP, so by adding a reagent that reduces Pu(IV) to Pu(III), uranium can be taken into the organic phase, leaving plutonium in the aqueous. Separation factors of 10-4 can be achieved.

These solvent extraction processes take place in contactors that agitate the immiscible aqueous and organic streams together in such a way as to make the area of the interface between them as large as possible. The contactors may be horizontal “mixer-settler” tanks fitted with alternating agitated and quiescent compartments, but more usually are vertical packed columns. In a “pulsed column” the effect of the packing or perforated plates in the column as they break up the streams of TBP (flowing upwards) and water (flowing down) is enhanced by pulsing the feed flow.

Whatever the form of the contactors they have to be designed to avoid criticality. This can be done by means of the geometry (for example by minimising the diameter of a pulsed column) and by incor­porating structural materials that contain thermal neutron absorbers such as boron or gadolinium.