Organic solvents suffer significantly from radiolysis. Therefore, before reprocessing they require sizeable cooling times of the spent fuels. Typical cooling times, such as those used at the COGEMA The Hague plant, are of the order of 5 years although cooling times as short as 1 year have been considered [138]. Shorter cooling times lead to a shorter lifetime of the expensive solvent, higher costs and larger amounts of wastes. When it is very important to minimize the cooling time, the fragility of organic solvents becomes a very serious drawback. This is especially the case in two occurrences: fuel reprocessing of breeders and molten salt reactors. In the first case the cooling time reflects directly on the doubling time. In the second case, online reprocessing involves treatment of an extremely active fuel. Dry inorganic processes appear to be much less sensitive to radiation effects and have been proposed and developed for the two aforementioned cases. In the breeder case, reprocessing of fast reactor fuels by chlorination was proposed, for example in the frame of the ‘integral fast reactor’ by the Argonne National Laboratory [151]. For molten salt reactors the reference is the fuel which was proposed in the MSBR project [50], which consisted of a mixture of fluorides.[49] Because of the subject of this book we shall restrict our considerations to the processing of fluoride salts rather than chloride salts. However, with the exception of vaporization of fluorides the possibili­ties offered by fluorination and chlorination are rather similar.

Elemental separation processes from a mixture of fluoride salts use one of the following techniques:

• vaporization of volatile fluorides

• gas purge

• liquid-liquid exchange

• selective precipitation

• electrolysis.

Efficiencies of these processes depend strongly on the relative amounts of fluor ions in the salt mixture. Large amounts of fluor ions lead to oxidizing conditions and shift the valence states of metals to high values. Oxidizing conditions often accelerate reaction rates and may be sought. However, the composition of the salts is often determined by other considerations such as fusion temperatures or corrosion reduction. Optimization with respect to such properties led the proponents of the molten salt breeder reactor to choose a mixture of the 7LiF, BeF2, ThF4 and UF4 fluorides in proportions (71.7: 16:12:0.3mol%) [154]. This mixture serves as a reference for our discussion. During irradiation 232Th is transmuted into 233Pa which can capture neutrons or decay to 233U. It is efficient to minimize the sterile captures in 233Pa, and therefore to extract it from the salt, and let it decay into U outside the neutron flux. Subsequently, the U can be re-injected into the salt or stored for future use. Similarly the quantity of fission products in the salt should be limited as much as possible. This is especially true for rare earths which have large neutron capture cross-sections. Reprocessing of the MSBR fuel, thus, consisted of extraction of 233Pa and of as many fission products as possible, using the different techniques mentioned above.