TABLE OF CONTENTS Borosilicate glasses are the first choice of material worldwide for immobilising both HLW and LILW. This selection is based on the flexibility of borosilicate glass with regards to waste loading and the ability to incorporate many different kinds of waste elements, coupled with good glass-forming ability, chemical durability, mechanical integrity, and excellent thermal and radiation stability. Borosilicate glasses generally have SiO 2 as the major component, relatively high B 2O 3, CaO, MgO, Na 2O and Al 2O 3 content and minor amounts of many other oxides. SiO 2, B 2O 3 and A1 2O 3 are generally network formers because they form strong covalent bonds involving SiO 4, AlO 4 and BO 4 tetrahedra and BO 3 triangles.
Silicon is the main glass-forming element in a borosilicate waste glass and its basic elements are SiO 4 tetrahedra, which comprise bridging or cross-linking and non-bridging atoms of oxygen (NBO). In a silicate glass the SiO 4-tetra-hedra vertices connect these elements to each other through bridging oxygen atoms so that the network consists of chains of various lengths. The glass network is not regular as in the case of crystalline silica, for example the bond angle Si-O-Si can range from 120 to 180 while in quartz it is a constant 109 28', although the Si-O bond length remains constant (1.62 ). Alkali, alkaline earth ions, transition metals, and ions of high charge...
