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Reactive transport processes occurring during nuclear glass alteration in presence of magnetite

Rébiscoul, D., Tormos, V., Godon, N., Mestre, J.-P., Cabie, M., Amiard, G., Foy, E., Frugier, P., Gin, S.
Applied geochemistry 2015 v.58 pp. 26-37
Raman spectroscopy, X-radiation, clay, corrosion, energy-dispersive X-ray analysis, fuels, gels, geochemistry, glass, groundwater, iron, magnetite, mixing, powders, scanning electron microscopy, silica, sorption, transmission electron microscopy
In this study, we have investigated and clarified the processes occurring during the alteration of SON68 glass – the reference nuclear glass for the waste arising from reprocessing of spent fuel from light water reactors – at 50°C in Callovo-Oxfordian clay groundwater in presence of magnetite. Magnetite is known to be one of the iron corrosion products expected to be present in the vicinity of glass in geological disposal conditions. The effects of the amount of magnetite relative to the glass surface and the transport of aqueous species during glass alteration were studied. A first series of experiments was focused on the effect of various magnetite amounts by mixing and altering glass and magnetite powders. In a second series of experiments, magnetite was separated from the glass by a diffusive barrier in order to slow down the transport of aqueous species. Glass alteration kinetics were analyzed and solids were characterized by a multiscale approach using Raman Spectroscopy, Scanning and Transmission Electron Microscopy, Energy-Dispersive X-ray and Scanning Transmission X-ray Microscopy coupled with Fe L2,3-edge and C K-edge NEXAFS.It appears that glass alteration increases with the amount of magnetite and that the transport of aqueous species is a key parameter. Several processes have been identified such as (i) the silica sorption on the magnetite surface, (ii) the precipitation of Fe-silicates in the vicinity of the glass (iii) the precipitation of SiO2 on the magnetite surface, (iv) the incorporation of Fe within the alteration layer. Process (iv) was not frequently observed, suggesting local variations in geochemical conditions. Moreover, this process is strongly influenced by the transport of aqueous species as indicated by the morphology and composition of the alteration layers. Indeed, when glass and magnetite are homogeneously mixed, the glass alteration layer consists of a gel enriched in Fe having the same Fe(II)/Fe(III) ratio as in magnetite. When both materials are separated by a diffusive barrier, the glass alteration layer consists of a porous gel (not enriched in iron) in presence of a mixture of Fe-silicates with Fe having the same valence as in magnetite, rare-earth precipitates and phyllosilicates. These results suggest that Fe incorporation within the alteration layer changes depending on the distance and the time required for dissolved Fe originating from the magnetite to reach the glass.