18.1. New Approaches

The main immobilisation technologies that are available commercially and have been demonstrated to be viable are cementation, bituminisation and vitrification. The highest degree of volume reduction and safety is achieved through vitrification although this is the most complex and expensive method requiring a relatively high initial capital investment. Current immobilising technologies ensure a satisfactory level of safety although they have limitations as discussed in Chapters 15 17. Furthermore, difficult legacy waste streams are known for which current technology is inadequate, so that new approaches must be developed. These comprise development of new waste-forms such as crystalline (mineral-like) and composite radionuclide hosts as well as of new immobilising technologies such as thermochemical and in situ methods. New approaches aim also to create geochemically stable materials in equilibrium with the disposal environment to ensure a safer nuclear waste disposal scenario.

Wastes may be difficult for a number of reasons. They may be highly radiotoxic emitting high-energy radiation, have very long half-lives, be highly mobile, easily assimilated and have a long biological half-life. Furthermore, many legacy wastes are ill-characterised. Elements which are inherently difficult to immobilise include Tc, halogens ( ³⁶Cl, ¹²⁹I) and ¹⁴C. In the UK ⁹⁹Tc has, for many years, been discharged into the sea but a new process using tetraphenylphosphonium bromide separates the ⁹⁹Tc from liquid wastes as pertechnetate, . No waste-form is currently available for the . Halogens are ubiquitous in nuclear wastes; e.g. chlorides arise from HCl dissolution of defence...