From An Introduction to Nuclear Waste Immobilisation

17.11. Glass Composites

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).

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Glass Ceramics
Glass ceramics are ceramics that can be fused and then molded, formed, ground, or machined using conventional glass fabrication techniques. After part fabrication, the glass ceramics' structure is transformed by precipitating a crystalline phase or phase within an amorphous, vitreous matrix.
Carbon, Graphite, and Diamond Materials
Carbon, graphite and diamond materials include vitreous, amorphous, or glassy carbon; hexagonal and pyrolytic graphite (PG); and naturally-occurring and synthetic diamonds, diamond-like carbon (DLC) materials, and diamond-like coatings.
Ceramic Matrix Composites
Ceramic matrix composites (CMC) consist of ceramic fibers reinforced with particulates, fibers, or cloth. Compared to conventional technical ceramics, which fracture easily under loads, CMCs have a greater crack resistance. Some matrices and combinations also provide greater strength, thermal shock stability, and erosion resistance at very high temperatures.
Glass Powders and Precursors
Glass powders and glass precursors typically consist of oxides with a random, liquid-like or non-crystalline molecular structure. Glass powders and glass precursors are used in a wide variety of applications, so they come in a range of particle sizes and particle distributions.
Nanomaterials
Nanomaterials have features or particle sizes in the range of 1 to 100 nm.

Topics of Interest

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...

17.9. Difficult Elements S, Cl and Mo are less glass-compatible elements. Sulphur can be incorporated in the borosilicate glass structure as up to 1 wt.% of SO 3 equivalent. Greater than 1% SO 3...

5 STRUCTURAL MODELS FOR BORATE GLASSES 5.1 Vitreous Boric Oxide The current model for the structure of vitreous boric oxide differs significantly from that for vitreous silica. Although boron occurs...

17.2. Immobilisation Mechanisms Vitrification involves melting of waste materials with glass-forming additives so that the final vitreous product incorporates the waste contaminants in its macro-and...

9 HALIDE GLASSES 9.1 Fluoroberyllates Beryllium fluoride melts to form a very viscous liquid. Crystalline BeF 2 contains beryllium-fluorine tetrahedra in quartz and cristobalite forms, i.e., the...