Effects of Containment Materials on the Dissolution Behaviour of Vitrified Wasteforms

Vitrification is used in many countries to immobilise high activity waste liquors and is increasingly considered for low and intermediate activity waste streams. Radioactive elements are chemically incorporated in the glass structure and, therefore, will be released as the glass dissolves. As such, investigating the impact of the localised environment on durability and long-term behaviour is vital to the development of the safety case for disposal. Many studies conducted focus on the effects of environmental factors (e.g., pH, temperature and groundwater chemistry), however, within the disposal environment, a series of natural and engineered barriers will also be in place to isolate wasteforms from groundwater. These barriers will influence groundwater compositions prior to contact with wasteforms.<br/><br/>Iron is known to influence the rate of dissolution of silicate glasses [1]–[3]. The impact of steel corrosion in close contact to nuclear waste glasses has been shown to influence glass corrosion and must be considered in the safety case although the type and steel to be used is specific to a nations disposal policy. United Kingdom (UK) High-Level Waste (HLW) is to be disposed of in canisters comprised of 309 stainless steel, with potential overpack material of S355 steel. Intermediate-Level Waste (ILW) within the UK is to be disposed of within 304L stainless steel, which is also under consideration for the containment of ILW. At the US Hanford site, mild steel grades are also under consideration for wastes disposed of at the Integrated Disposal Facility (IDF).<br/><br/>Here preliminary studies have been performed on the influence of canister and overpack materials using conditions relevant to the safety case for the disposal of HLW in the UK, using MW25, 309 stainless steel and a saline groundwater currently under investigation. Alongside this data, long term tests set up, through the GLAD (Glass Leaching Assessment for Disposability) project, will explore the influence of steel corrosion on the dissolution rates of Low-Activity Waste glasses destined for disposal at the IDF. Container materials and simulant wasteforms are corroded with an average gap of 200 um between materials to determine the localised effect of steel on glass alteration, in repository relevant conditions.<br/><br/>[1] T. De Echave, M. Tribet, S. Gin, and C. Jégou, “Influence of iron on the alteration of the SON68 nuclear glass in the Callovo-Oxfordian groundwater,” <i>Appl. Geochemistry</i>, vol. 100, no. February 2018, pp. 268–278, 2019, doi: 10.1016/j.apgeochem.2018.12.007.<br/>[2] H. Aréna <i>et al.</i>, “Impact of iron and magnesium on glass alteration: Characterization of the secondary phases and determination of their solubility constants,” <i>Appl. Geochemistry</i>, vol. 82, pp. 119–133, 2017, doi: 10.1016/j.apgeochem.2017.04.010.<br/>[3] E. Burger, D. Rebiscoul, F. Bruguier, M. Jublot, J. E. Lartigue, and S. Gin, “Impact of iron on nuclear glass alteration in geological repository conditions: A multiscale approach,” <i>Appl. Geochemistry</i>, vol. 31, pp. 159–170, 2013, doi: 10.1016/j.apgeochem.2012.12.016.