TABLE OF CONTENTS Glass composite materials (GCMs) are used to immobilise glass-immiscible waste components such as sulphates, chlorides, molybdates and refractory materials requiring unacceptably high melting temperatures. GCMs comprise both vitreous and crystalline components. Depending on the intended application, the major component may be a crystalline phase with a vitreous phase acting as a bonding agent, or, alternatively, the vitreous phase may be the major component, with particles of a crystalline phase dispersed in the glass matrix. GCMs may be produced by dispersing both melted materials and fine crystalline particles in a glass melt and may be used to immobilise long-lived radionuclides (such as actinide species) by incorporating them into the more durable crystalline phases, whereas the short-lived radionuclides may be accommodated in the less durable vitreous phase. Synroc-glass is an example of GCM with Synroc crystalline phases in a vitreous matrix (Fig. 17.8a). Synroc-glass has been developed for sodium-rich wastes such as those at the Hanford site in the USA. Crystalline phases such as zirconolite and perovskite are the hosts for actinides and waste loadings of 50 70% by weight have been demonstrated in such composites with a high durability. The French have developed a U-Mo GCM to immobilise Mo-rich HLW. Another example is the GCM developed to immobilise sulphur-enriched waste streams in Russia (Fig. 17.8b) containing conventional borosilicate glass vitreous phase with uniformly distributed particles comprising up to 15% by volume of yellow phase. The durability of this GCM is similar to that of conventional waste-form glasses (Table 17.12).
