TABLE OF CONTENTS Almost any melted substance, if cooled sufficiently fast, can be made as a glass. Glasses are amorphous materials with an internal structure made of a three-dimensional network of interconnected structural blocks, which in silicate glasses are SiO 4 tetrahedra. Upon heating, glasses continuously change most of their properties to those of a liquid-like state in contrast to crystals where such changes occur abruptly at a fixed temperature (the melting point). The solid-like behaviour of glass at low temperatures is separated from liquid-like behaviour at high temperatures by the glass transition temperature, T g. The glass transition is a kinetically controlled, fairly sharp change in derivative properties such as thermal expansion coefficient or specific heat. T g in fact depends on the rate of cooling, although empirically it can be assessed from the relation:
| (17.1) |  |
where T L is the liquidus temperature at which a phase diagram shows a crystal-free melt. Below T g glasses have solid-like behaviour. A higher T L provides a higher T g, but high processing temperatures are not acceptable for an efficient waste immobilisation process. The most important parameters of nuclear waste glasses are: normalised radionuclide leaching rates NR i(g/cm 2 day), mechanical strength (MPa), density ?(g/cm 3), thermal expansion coefficient ?(K -1), specific heat C P (J/kgK) and thermal conductivity ?(W/mK). Important glass processing parameters are melting temperature T m, viscosity ?(Pa s) and...
