Aqueous alteration of Silicate - Glass Case of Nuclear Glasses

In France, as in many other countries, glass is the standard solution for the encapsulation of long-lived radioactive waste. Indeed, it is an amorphous material capable of incorporating a wide variety of chemical elements. It is therefore well-suited for encapsulating fission products resulting from the processing of spent nuclear fuel. Furthermore, glass is a relatively easy material to produce: the vitrification processes for solid or liquid waste do not require a large number of steps or material transfers; the glass can be poured directly into a storage container. It is the only solidification process compatible with high levels of radioactivity that has been successfully implemented on an industrial scale, whereas in the 1960s, various solidification processes were considered. Finally, glass is a durable material, resistant to both the effects of irradiation and water-induced weathering. On this last point in particular, the need to quantify the very long-term evolution of the glass matrix’s ability to contain fission products has led to the development of a solid understanding of the mechanisms of glass weathering, despite the inherent difficulty in measuring very slow rates of weathering.

After reviewing some basic characteristics of silicate glasses—and in particular those of alumino-borosilicate glasses, whose composition is suited for the disposal of nuclear waste—this article presents the general phenomenology observed during the aqueous weathering of these materials, then details the mechanisms involved, highlighting the influence of various parameters (temperature, pH, and the composition of the glasses and weathering solutions). Finally, it explains how competition among the various weathering mechanisms leads to weathering kinetics that, depending on the conditions, can vary by several orders of magnitude. The article also presents the experimental methods that can be used to characterize the mechanisms and quantify the weathering kinetics of the glasses, as well as the main models.