12For the first time, the influence of glass composition on the vapor hydration kinetics of the French AVM 13 nuclear waste glass simulants was investigated. Three complex borosilicate glasses (> 20 oxides) along 14 with three simplified Na/(Ca, Na)/(Mg, Na)-alumino-borosilicate glasses with four or five oxides were 15 altered at 50°C in water vapor (95% RH) for up to 557 days. The solid characterization of the altered 16 samples (by SEM, TEM, XRD, ToF-SIMS, SAXS) revealed that the rate-controlling vapor hydration 17 mechanism is composition dependent. The vapor hydration rate of the more durable glasses, whose 18 molar ratio of Al 2 O 3 /(CaO or MgO) is ≥1, seems to be limited by network-hydrolysis, whereas the overall 19 glass alteration rate of the less durable glasses, whose Al 2 O 3 /MgO ratio is <1, seems to be driven by the 20 precipitation of Mg-rich secondary phases. All the vapor hydrated glasses show the presence of a 21 homogeneous gel layer of a few tens of nm thickness. The more durable glasses have a smaller quantity 22 of secondary phases precipitated on their surface. The less durable glasses have a relatively significant 23 quantity of secondary phases precipitated above the gel layer and irregularly shaped, scattered and 24 highly porous altered zones formed beneath the gel layer. The overall alteration rate of the less durable 25 glasses was 10-20 times faster than the more durable glasses. For all glasses, FTIR spectroscopy indicated 26 an inflexion in the vapor hydration rate after 120-200 days of alteration, likely due to a passivating effect 27 of the altered layer formed under unsaturated conditions. The average pore size of the altered layer in 28 50°C [34]. The vapor hydration of SON68 (French inactive reference nuclear waste glass) at high 58 temperatures (90-200°C, activation energy (E a )=43-47 KJ/mol) [28] and lower temperatures (35-90°C, 59 by studying the behavior of three AVM (Atelier de Vitrification de Marcoule) glasses in contact with water 87 vapor. The AVM glasses are complex borosilicate glasses containing more than 20 oxides including fission 88 products issued from the UNGG (Uranium Naturel Graphite-Gaz) reactor at Marcoule. The range in the 89 AVM glasses composition is extensive due to the variation in the composition of fission products 90The synthesis of the AVM glasses has already been described by Thien [46]. The glass samples for this 113 study were retrieved from the same batch. Q, QCa and QMg were prepared using the oxide precursors 114

While glass alteration in liquid water has been widely studied for decades, glass alteration in unsaturated atmosphere (relative humidity, RH < 100%) has been far less examined. However, the understanding of the mechanisms involved in the reactions between glass and water in vapor state is fundamental to several fields such as glass industry, conservation of glasses of the cultural heritage and long-term assessment of nuclear waste glasses. This paper outlines the issues raised by the atmospheric alteration of glass in these fields and attempts to summarize the scientific approaches and findings of the three communities. This short review reveals that atmospheric alteration should not be confused with liquid alteration at high S/V (S = exposed surface of glass and V = volume of solution), because the kinetics and the nature of the alteration products are distinct. Notably, alkalies and non-bridging oxygens may be significantly retained in glass hydrated in unsaturated atmosphere, depending on the glass composition. Future lines of research are drawn to progress in the understanding of the specificities of atmospheric glass alteration.

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