Volume change due to thawing/freezing processes in the context of nuclear waste repository safety assessment

Safety assessments of high-level radioactive waste repositories require the long-term performance assessment of the repository system over a 1-million-year evaluation period. This timeframe encompasses approximately ten glacial-interglacial cycles in northern lattitudes, during which processes such as glaciation, permafrost formation, and thawing may impose significant mechanical and thermal stresses on the geological barrier. These environmental dynamics will alter the parameters of the overburden rock. Although it is not expected that this reduces the safety of the repository, it is still required to have a good understanding of the respective processes to be able to obtain an integrated safety assessment.

To investigate the effects of freeze-thaw cycles on geological and engineered barriers, in this study both consolidated materials (e.g., sandstone, granite, and claystone) and unconsolidated materials (e.g., clay and sand) will be tested. For the measurements an enhanced triaxial experimental system is used. This system allows to observe volume changes induced by freeze-thaw cycles under controlled confining pressure conditions. The experimental design captures the relationships between freeze-thaw-induced volume changes and key factors, including material properties, temperature variations, the number of cycles, and saturation.

The correlation models obtained from the experiments will be used for the validation and refinement of three-dimensional finite element models, and the experimental results will be reproduced and to extend the findings to broader spatial scales through numerical simulations.

At the hydrogeological scale, a comprehensive catchment model will be applied to evaluate the interactions between glacial cycles, regional groundwater flow mechanisms, and their cumulative impact on the safety assessment of high-level radioactive waste repositories. This study provides critical insights into the long-term stability of geological barriers under cyclical freeze-thaw conditions and offers a robust foundation for advanced repository safety assessments in glacial-interglacial scenarios.