Vitrification is the process of incorporating radioactive waste with glass to form a vitreous product.  This is accomplished though temperatures high enough to melt the glass such that the radioactive waste is either dissolved in the melt or encapsulated by the glass.  Chemically, a vitreous product is a non-crystalline amorphous solid.  This means it does not have a definite geometric or crystalline shape and there is no long-term order in the positions of the atoms.  This makes the chemical bonds between the individual molecules extremely strong with the important consequence that the resultant product is extremely stable and leach resistant.