Fig. 2.13. ZWILAG plasma oven (image courtesy of ZWILAG).

2.5.2. Conditioning

To reduce the potential hazard of waste for its safe storage and disposal, it is considered to be good practice to convert it into a stable solid form that will prevent any dispersion to the surrounding environment. Various methods exist and the commonly applied commercial processes are:

2.5.2.1. Cementation

Cementation is a versatile conditioning method that can, via the use of specially formulated grouts, permit the solidification of most wastes, even if they are chemically challenging or are subject to expansion, e.g., ion-exchange resins. Generally, two techniques exist: either a fully formed grout is used to condition solid wastes, e.g., miscellaneous mixed wastes, ashes, resins etc., or the waste contains sufficient water and is directly mixed with cement powder and any necessary process additives, e.g., sludges and flocs.

2.5.2.2. Bituminisation

Bituminisation is generally used for the treatment of liquid wastes/sludges and involves drying of the waste before it is combined with the bitumen to form a conditioned product. Bituminisation, due to the flammable nature of the constituent materials, results in a conditioning process that must be strictly controlled (cf. the fire in the Tokai Works, Japan; Sasaki et al., 1998) but, if correctly implemented, these controls will result in safe operations.

2.5.2.3. Resin

Resin can be used to condition radioactive wastes such as ion-exchange resins that may be subject to swelling on contact with water. This process is no different to any other commercial process, with resin being mixed with styrene and then accelerator, inhibitor and catalyst being added as required.

2.5.2.4. Vitrification

Vitrification (Fig. 2.14) is the method most commonly used for conditioning of the highly radioactive liquors left over from reprocessing of SF. However, new techniques are currently being introduced for intermediateand low-level wastes, e.g., cold crucible techniques.

The high-level waste liquid is dried by a calcination process to produce a solid, which is then mixed with borosilicate glass and vitrified. The resulting glass is then cast into