Conclusion
We investigated the vapour hydration of the SON68 glass under different conditions of temperature (35‒125 °C) and relative humidity (92‒99.9%) relevant to high-level waste geological disposal in France. The glass hydration increased with increasing time, temperature and relative humidity. Measurements of hydration thicknesses using 3 different techniques, SEM, FTIR and TOF-SIMS, allowed calculating a glass hydration activation between 34 (±4) and 68 kJ mol-1 , consistent with a diffusion and chemical reaction processes. The water diffusion coefficient at 90 °C and 95% RH, 7.5 10-20 m2 s -1 , is 350 times higher than that reported by Ferrand et al. [22] (2 10-22 m2 s -1 ) for the SON68 glass altered at 90 °C in a Sisaturated (240 ppm) synthetic solution.
In the presence of D2O 18 (20 %), the incorporation of 18O in the alteration layer revealed two mechanisms responsible for the glass alteration. The formation of phyllosilicates occurs by a dissolution/ precipitation process from the supersaturation of the bulk solution while the gel is formed by a succession of very local hydrolysis/condensation reactions (away from the bulk solution). The apparent diffusion coefficient of D2O 18 is about 2.3 10-19 m2 s -1 . The alteration in COx water of pre-hydrated glass samples showed the instantaneous release of glass tracer elements, much faster than for the non-hydrated glass sample. The phenomenon is due to an instantaneous dissolution of fast-dissolving phases (e.g. salts) incorporated in the hydrated layer.
Finally, in the context of geological disposal of HLW it is important to evaluate the water under-saturation period to better assess the radionuclides release in the groundwater contacting the hydrated glass.
