9.1 Basics of reprocessing

Fuel reprocessing was born at the same time as nuclear energy. It was developed within the Manhattan project in order to recover the plutonium needed for the fabrication of the Nagasaki atomic bomb. An excellent account of the pioneering techniques used by Seaborg and his collaborators can be found in reference [138]. Here, a wealth of different methods which have been used for recovering uranium and plutonium can also be found. Aside from the military needs to recover plutonium, with the associated proliferation aspects, the attractive potential offered by fast neutron breeders led to the building of industrial plants to recover both plutonium and uranium from spent nuclear fuels. The largest of these plants that are opera­tional today are La Hague and Sellafield. Because the breeding programmes have been stopped, these large plants have been converted to MOx fuel production for commercial light water reactors, a high recovery efficiency of plutonium and uranium being required. These plants use organic solvents for highly efficient recovery of these two elements. The reference molecule which has become a standard is tributyl phosphate (TBP) from which the so-called Purex (plutonium extraction) process was developed. The high efficiency allows quasi-complete plutonium extraction (99.9%), so that this element could be practically absent from the wastes. Of course, this requires complete incineration of plutonium in standard or dedicated reactors. Logically the minimization of the waste radiotoxicity requires that ameri­cium and curium should also be separated and incinerated. This led to the development of enhanced separation methods, based on the same principle as Purex, such as Diamex and Truex.

Although the wet process, Purex (which implies dissolving the fuel elements in an aqueous acid solution), is by far the most used and the only one which has reached industrial status, other processes which do not require aqueous dissolution have been explored. This has been done in two main instances:

1. The molten salt reactor programme initiated at Oak Ridge National Laboratory [49, 50]. Here the problem was, essentially, to recover thorium and uranium from a mixture of lithium, beryllium and fission product fluorides.

2. The fast breeder programme. The interest in the anhydric recovery of plutonium and uranium stems from the high plutonium enrichment of fast breeder fuels which leads to increased risks of criticality, especially with hydric processes, and from the interest of metallic fuels which lead to more energetic neutron spectra, with higher breeding capability. Indeed, the metallic fuels lend themselves very easily to fluorization.