Analysis of the hydrated zone in nuclear waste glass forms by electron microprobe, Raman spectroscopy and diffusion models

Metal oxides containing borosilicate glasses were investigated for structur e, composition and static leaching. Besides leachate analysis, glasses were analysed by electron microprobe at varying primary electron beam energies to determine compositional trends with sample depth. The glass structure wa s determined by Raman spectroscopy. Structure, composition, Si-O bridging R aman band intensities and diffusion models were empirically correlated to e lucidate elemental mobility. The diffusion of water to varying sample depth s is apparent from the optical thickness of the hydrated zone. The composit ions of the leached glasses approach the unleached levels towards the inter ior. Na2O and MgO form the disilicates, metaborates and tetraborates. ZrO2 unbridges the network to a metasilicate type. ZrO2 also couples with MgO an d Na2O and deprives additional depolymerisation. Consequently, durability i ncreases in the direction of increasing ZrO2 and bridging/nonbridging compo nents. Durability also increases towards the glass interior from pristine s urface and elemental release is in the order. Si, O, Mg, Na, B. Silicon is depleted least and is released within ten microns from the surface. Boron i s depleted most and continues to be released from deeper levels. Zirconium and aluminum are leach resistant and enrich near the glass surface. Incomme nsurable leaching between neutron absorber boron and actinide surrogate zir conium necessitates toughening of pristine surface to safeguard high level waste glasses from nuclear criticality.