Vitrification of historic and future high level nuclear wastes within alkali borosilicate glasses.

The disposal of highly radioactive and toxic wastes generated by the nuclear industry is one of the biggest challenges facing the world today. Currently, in the UK there is a large legacy waste holding which has been accumulating since nuclear energy was first harnessed during World War 2. Processing of this waste with a view to final disposal is a complex and difficult task. This work
investigates one aspect of that process, namely turning this waste into glass (or vitrification). This work uses multiple techniques including x-ray absorption spectroscopy, magic angle spinning nuclear magnetic resonance and molecular dynamic simulations, to investigate the structural role of Zr02 and U03 within the alkali borosilicate glass used in the UK for waste
immobilisation. The effect of these additions on the bulk glass structure and selected glass properties are also explored.
In waste glasses Zr occurs as a 6 co-ordinated Zr ion surrounded by Si, B, Na and Li. The effect of Zr02 additions on the bulk glass structure and properties is highly complex. The addition of Zr02 appears to be characterised by a non-linearity in the trends of certain physical and structural parameters. At low levels of Zr02 the level of leaching from the glasses and the
coordination of B increase. However, with higher Zr02 contents this trend is reversed. It is believed that the increase in B co-ordination destabilises the glass network and so increases leaching from the glass.
Work with Molecular Dynamic (MD) computer simulation of glasses showed the applicability of this approach to nuclear waste glasses and its ability to accurately reproduce changes in glass structure with changing compositions. The possibilities for solubility limit prediction using MD are also shown.
In waste glasses U03 is shown to exist as the uranyl (U02)2+ species bound to the glass network. Addition of UO3 to glasses decreases the co-ordination of B requiring 2 moles of alkali oxide to charge compensate its presence within the glass network. This indicates that U acts as an intermediate. Limited leach tests show that the presence of U03 within the glasses destabilises the
formation of a gel layer.