Spent fuel, after decay storage, is transported to the reprocessing plant using dedicated casks (Fig. 16.3(a)) and transferred to storage pools (Fig. 16.3(b)) where the fuel may be allowed to cool for a further 3 to 5 years. The consequent decrease in activity facilitates subsequent processing. The spent fuel is covered by about 4 m of water, which provides protection to staff. The demineralized water in the pool is constantly cooled and filtered.

When a fuel element has cooled sufficiently, it is removed from the pool. The first step is to break the fuel cladding in order to expose the fuel to a nitric acid solution. This is usually achieved by mechanical cutting (or shearing) of the fuel element. The result is a mixture of fuel rod sections of a few centimetres length and of debris largely consisting of broken fuel pellets. A trough enables the fragments to fall into a dissolver containing hot concentrated nitric acid.

The choice of nitric acid is underpinned by following criteria:

• very low corrosion of stainless steel, from which much of the equipment is made

• volatility of the acid that allows it to be removed by evaporation

• the high solubility of most nitrate salts

As concentrated nitric acid is an oxidant, most of the elements when dissolved are

at their maximum degree of oxidation (uranium is at level of valence 6). Plutonium,

however, is at level 4.

Most of the elements within the fuel are dissolved except:

• Gaseous fission products (krypton, xenon, iodine, some C-14), which emanate as off-gas from the dissolver along with steam and nitrogen oxides (NO and NO2), which that are formed by the reaction of nitric acid with the fuel. These gases are routed to the ‘head gases’ treatment unit where nitrogen oxides are recombined into nitric acid and returned to the process.

• Metallic fuel element components — hulls, end-pieces, grid debris, etc. — that remain undissolved owing to their composition (stainless steel alloys, zircaloy) are separated from the solution and are rinsed. They can be then compacted and conditioned as wastes.

• Insoluble fission products or fines remaining from dissolution. These mainly consist of ruthenium, molybdenum, rhodium, palladium and technetium and cladding particles produced by the shearing (shearing fines). These are usually separated by centrifugation, a process known as clarification. This separation prevents the fines from affecting the extraction operations.