Contribution of zeolite-seeded experiments to the understanding of resumption of glass alteration

Understanding the origin and the consequences of glass alteration regimes is necessary for the prediction of nuclear glass durability. The so-called stage 3 or resumption of alteration regime of glasses used to sequester nuclear waste by vitrification, is characterized by a sudden acceleration of glass alteration rate arising from the precipitation of secondary minerals, mainly zeolites. To study this process, a promising approach is developed, based on seeding by synthesized zeolite seeds. This study quantitatively links the alteration of a six-oxide reference borosilicate glass (ISG) and the precipitation of zeolites that affects concentrations of key species-in particular aluminum-and thus the glass dissolution rate. The characterization of stage 3-easier at alkaline pH-can now be extended to pH conditions more representative of those found in a geological repository thanks to seeding that reduces, or even eliminates, the latency period preceding a resumption of glass alteration. The resumption occurrence and glass dissolution rate are related with temperature and pH. This study shows that the detrimental effect of zeolite precipitation decreases with decreasing pH and temperature, until it is no longer detectable at a pH around 9 imposed by the dissolution of the ISG glass. Even for both high temperature and high pH, the resumption rate is lower than the initial alteration rate, which remains the fastest kinetic regime. Nuclear glass: planting a zeolitic seedAccelerating nuclear glass dissolution allows for its study under geochemically relevant conditions at laboratory time scales. Waste resulting from the recycling of nuclear fuel is typically packaged into borosilicate glass and stored in a geological repository. These glasses however dissolve upon contact with water, and as a result of mineral precipitation, leaching nuclear waste. Understanding how the geochemical environment influences glass dissolution is crucial, but the lengthy process makes studies at the laboratory level unfeasible. A team led by Maxime Fournier at the CEA, Marcoule, introduce synthetic zeolite seeds into the equation, encouraging mineral precipitation and hence enhancing the rate of glass alteration. With faster kinetics in hand, they are able to examine glass behavior at moderate pH and temperature ranges, gaining important insight under conditions relevant to nuclear waste disposal environments.