Molten Salts as Sealing Materials for a Repository in Rock Salt – The Feasibility Study SalVE

A critical aspect of ensuring the long-term safety of a high-level radioactive waste repository in rock salt is the sealing of the underground workings. While there are several proven sealing materials, the SalVE study investigated the feasibility of using low-temperature molten salts to increase the level of safety of the geotechnical barrier system. While general advantages such as low porosity, low permeability, low water content and thus pure dry diffusion were expected for molten salts with solidification, none of the candidates with a melting temperature between 100°C and 200°C demonstrated feasibility for sealing tasks due to inherent limitations.

Based on the initial requirements arising from the long-term disposal of nuclear waste, a comprehensive literature review excluded the majority of molten salts in this temperature range on the basis of defined exclusion criteria and identified the NaCl-AlCl₃ system as the most promising candidate. For this material, the study developed a concept for production, underground handling and application as sealing elements within shafts or main tunnels. However, this process revealed critical operational safety and construction challenges, as well as a notable lack of essential geotechnical and geochemical data. Subsequent laboratory experiments were conducted to evaluate the properties of the selected molten salt, including its melting behaviour, viscosity, strength, permeability and interaction with surrounding materials.

While the experiments confirmed the formation of a strong bond between the solidified molten salt and the rock salt, they also highlighted significant drawbacks. A critical finding was the significant volume changes that occur during solidification, leading to cracking and increased porosity. However, with the potential for reaction with moisture under volume increase by hydration, the main concern regarding the long-term stability of the sealing system is the geochemical instability due to interaction with potential cementitious sealing materials.

In summary, this investigation demonstrates that low-temperature molten salts have inherent limitations. The research underscores the importance of experimental validation and comprehensive geotechnical and geochemical evaluations in the development of effective repository sealing strategies.