Reprocessing is a large-scale industrial enterprise that entails handling spent nuclear fuel in quantities of the order of 1000 tonnes/year. While it is highly desirable to use a continuous process, this is not simple because of a specific nuclear industry constraint: the danger of criticality within the dissolver given that it contains both fissile material and water (a neutron moderator). To avoid this there are three main restrictions:

• the handled mass must be limited; unfortunately this is too severe a constraint for an industrial operation involving plutonium

• the volumetric concentration of fissile material must be limited; this is impossible to achieve in the course of dissolution

• the capacity of the dissolver must be limited and must use a favourable geometry

An example of an appropriate solution is illustrated by the continuous rotary dissolver used by the AREVA reprocessing plants in La Hague. This device allows four actions to be performed simultaneously: (1) the loading of the pieces of fuel to be dissolved, (2) dissolution itself, (3) draining of the solution and (3) the unloading of the empty pieces of cladding (hulls). The dissolver includes a wheel with buckets rotating step by step in a flat tank containing the hot nitric acid.

Figure 16.4 illustrates the process: the hulls fall into the dissolver filled with hot concentrated nitric acid. The nuclear material is dissolved and separated from other components. The structural elements of the spent fuel (hulls, end-pieces) are removed for storage and compaction.

Gaseous effluents are treated, washed and filtered while the acid solution is clarified. Centrifugation (Fig. 16.5) separates the small particles (shearing fines) and insoluble fission products from the solution of uranium, plutonium and fission products in nitric acid.

After clarification, the solution contains:

• 200 g/l uranium

• 2.5 g/l plutonium

• 3.5 mol/l nitric acid

• between 6 and 7 g/l fission products

The fines are stored for later vitrification and the clarified solution is transferred to the extraction and concentration plants where the nuclear materials are separated.