The main wastes arising from reprocessing are the structural elements of the fuel assemblies and raffinate. The former are compacted and concreted into steel drums; the latter is vitrified as described below.

Vitrification

Raffinate concentrates 99.5% of the activity — fission products and minor actinides — into a small volume. This material is not reusable and must therefore be transformed into a solid waste. In countries using the PUREX process at industrial scale (France, United Kingdom and Japan), vitrification is the selected method. The de-nitrated concentrates are calcined after which borosilicate glass frit is added. The radionuclides are chemically incorporated into the molten glass, which is then poured into special stainless steel containers — canisters. This immobilizes the radionuclides in a durable glass matrix that is virtually insoluble in water and relatively immune to the action of natural physico-chemical agents. The content of fission product oxides in a canister is around 12% by weight while the content of actinides oxides is around

1%. Containers are stored in cooled or ventilated shafts or, for foreign spent fuel, returned to their country of origin (Fig. 16.18).

Most processes use a large volume ceramic smelter, heated using the Joule effect by means of electrodes. At La Hague, however, vitrification is performed with a small volume, induction-heated metal smelter. Several production lines in parallel are used to match the flow rates of the plant and the method uses cells that are suitably shielded and remotely operated (Fig. 16.19).

An evolution and improvement of this process is the cold crucible technology (Fig. 16.20), which eliminates some of the limitations of the hot crucible technology:

• The behaviour of the material of existing hot crucibles limits the temperature to 1100-1150 °C and, consequently, the solubility and level of incorporation of radionuclides into the glass matrix.

• The requirements for regular maintenance of the vitrification lines.

A cold crucible enables higher temperatures to be reached in the crucible and simplifies the process by removing the calciner.

Unlike the melting pot, which heats glass by thermal conductivity from the walls of the pot to the core of the glass bath, the principle of the cold crucible is to induce electric currents within the glass to raise its temperature without directly heating the crucible.

The cold crucible operates according to the direct induction principle and combines the following:

16.20 Cold crucible melter (Source: Etienne Rousset, Patrice Brun, Emilien Sauvage, Armand Bonnetier, Steven Daix, French Atomic Energy Commission (CEA) Nuclear energy division, Confinement research and engineering department. Flux Users Conference, Barcelona, Spain — October 9-10 2008).

16.21 Prototype of cold crucible melter (numerical modelling of a cold crucible for direct induction melting of a glass (Source: Etienne Rousset, Patrice Brun, Emilien Sauvage, Armand Bonnetier, Steven Daix, French Atomic Energy Commission (CEA) Nuclear energy division, Confinement research and engineering department. Flux Users Conference, Barcelona, Spain — October 9-10 2008).

• the wall of the cold crucible is cooled by a pressurized water circulation system

• the sectorized cylindrical structure is made of stainless steel

• a protective layer of condensed glass then forms, called a ‘self-crucible’. This protects the metal crucible from the effects of high temperatures and corrosion caused by the bath of molten glass

• a high thermal power is released to the melt

• a temperature higher than 1200 °C can be reached

• thermal homogenization is performed with a stirrer

• this enables high throughput capacities

• a cold cap on the crucible helps to confine the hotter, more volatile glass

It is thus possible to have a low wall temperature while inside the crucible the temperature is very high. Insulated by a thin layer of glass, the wall is protected from the melted glass (Fig. 16.21).

The intense radioactivity of the resulting high-level waste creates heat, which obliges it to be cooled during storage for several years. At the end of this period, the waste could be sent for deep disposal although it will be necessary to distribute the canisters over an area that is large enough to allow the heat to be dissipated without producing an unacceptable increase in temperature (Fig. 16.22).