Structural evolution of glass surface during alteration: Application to nuclear waste glasses
Résumé
The structural modification of the surface of the inactive SON 68 nuclear glass has been followed during the first stages of alteration at 90 degrees C, using X-ray Absorption Spectroscopy. The structure of the amorphous interface layer depends on the alteration conditions. In undersaturated alteration conditions, Zr changes from 6- to higher coordination, with an increasing contribution of a hydrous zirconium oxide (HZO). The fast change of Fe3+ from 4- to 6-coordination after 3 h of alteration corresponds to the formation of hydrous Fe-oxyhydroxides (HFO). HZO and HFO are not linked to a silicate framework and do not have a protective role. The appearance of a Q3 local structure in the gel may be a precursor of the further formation of clay minerals. By contrast, in near-saturated conditions. Zr and Si do not show any change on their local structure. Zr retains Ca as a charge compensator and provides a protective role. There is indication of a disorder that may hinder gel densification by a closure of its porosity, which explains why, on the long term, Zr does not play a protective role. Under the same conditions, Fe3+ changes from 4- to 6-coordination, indicating the formation of non-protective HFO at the glass surface. The molecular-scale origin of the glass-to-gel transformation shows how repository conditions play a major role in the long-term stability of nuclear waste glasse