Morphological evolution of alteration layers formed during nuclear glass alteration: new evidence of a gel as a diffusive barrier

This paper presents X-ray reflectometry and nitrogen adsorption results on alteration films formed at the surface of the French high level radioactive waste reference glass: SON68 glass (R7T7-type). For the first time, in situ (underwater) X-ray reflectometry has been used to investigate the alteration of this nuclear glass. A morphological evolution of alteration films is proposed and related to different steps of alteration kinetics. Initially a de-alkalinized glass layer is obtained, followed by the formation of a gel having an open porosity. Subsequently the gel densifies as the glass alteration rate decreases and finally the pore size increases upon formation of a dense and thin zone within the film. The latter dense zone within the gel could constitute a diffusive barrier. This hypothesis is also discussed for simplified glass gels.