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Magic-angle-spinning NMR has been used to establish the structural roles of various cations added to the borosilicate glass which is used for the vitrification of high-level nuclear waste (HLW). Representative surrogate oxides with nominal valencies of +1, +2 and +3 have been studied which span the range of oxides from modifier to intermediate and conditional glassformer. NMR has been carried out on those nuclei which are accessible and the species observed have been correlated with the physical and chemical behaviour. The controlling factor is the manner in which the alkali cations partition between the various network groups, changing the distribution of silicon Qn species and the boron N4 ratio. Identifiable superstructural units are also present in these glasses. The aqueous corrosion rate increases with Q3 content, as does the weight loss due to evaporation from the melt. The activation energy for DC conduction scales with N4. Values of N4 obtained for these glasses deviate significantly from those predicted by the currently accepted model (Dell and Bray) and are strongly affected by the modifier or intermediate nature of the surrogate oxide and also by its effect on the distribution of nonbridging oxygens between the silicate and borate polyhedra.

Glasses related to those used for the vitrification of high-level waste (HLW) have been produced from the sodium-lithium borosilicate system. Their thermal and structural characteristics have been measured for a wide range of boron oxide contents (NBS series) and for a range of alkali oxide to boron oxide ratios, R, for several fixed silica to boron oxide ratios, K (ABS series). The NBS series of glasses was seen to exhibit a maximum in the glass transition temperature, Tg, and a minimum in the fraction of 4-coordinated borons, N 4 , at 22 mol% and 29 mol% boron oxide, respectively. With variation of K and R, initial results indicate an increase in Tg to a maximum before a gradual decrease. Density measurements show a general increase before remaining within experimental error at larger R. Initial a-particle irradiation tests have been carried out on several base glass samples and show evidence of B(a,n) reactions, the occurrence of which could have important consequences for the future viability of wasteforms designed for reprocessing of higher bum-up fuel.