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Influence of composition on vapor hydration of AVM nuclear glasses

  • Narayanasamy, Sathya
Publication Date
Nov 20, 2019
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The vapor hydration of inactive surrogates of AVM (Atelier de Vitrification de Marcoule) glasses (simple and complex) has been studied in this thesis work with a special focus on the influence of glass composition. In the first part of the thesis, multiple glass samples were altered at 50°C and 95% RH and the altered samples were characterized using SEM, TEM, XRD, ToFSIMS and SAXS to study the altered (gel) layer. The results show that aluminum plays a key role in glass durability under the given conditions, especially in relative proportions to alkaline-earth elements. When the molar ratio of Al2O3/MgO<1, the overall vapor hydration rate was accelerated by 10-20 times due to the formation of Mg-rich smectites. In other cases, network-hydrolysis was identified as the rate controlling vapor hydration mechanism for the first six months. Complementary studies on the effect of temperature and relative humidity gave further insights into secondary phase precipitation, behavior of elements in the gel layer and altered layer morphology. These studies show that the predominant vapor hydration mechanism varies with temperature and glass composition as well. In the second part of the thesis, aqueous alteration experiments are discussed in two contexts. First, vapor hydrated glasses immersed in pure water showed that the gel layer did not have a passivating effect against aqueous alteration and that some of the secondary phases formed during vapor hydration are readily soluble. Secondly, comparative structural study of ¹⁷O enriched gel layers that were formed during vapor hydration (at 90°C) and aqueous alteration at a very high SA/V ratio using NMR spectroscopy showed for the first time, evidence of recondensation of boron with oxygen from the vapor phase. The results suggest that glass alteration in vapor phase is not equivalent to alteration in aqueous medium at a very high SA/Vratio.

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