Assessing the impact of potential future glaciations on the long-term safety of radioactive waste repositories: relevant processes and examples from current studies

Abstract

The safety of repositories for high-level radioactive waste has to be assessed for very long time periods (e.g., 1 Ma by German regulations), which implies that the impact of potential future cold stages and glaciations on the geological barrier of a repository needs to be considered. The largest impact may occur if a repository site is transgressed by an ice sheet or at least in the immediate proximity of an ice margin. However, also repository sites outside the maximum ice-sheet extent may be affected. Examples of relevant processes with potentially huge impacts include glacigenic erosion and ice-load induced deformation of rocks. Erosion by glaciers and meltwater is capable of mobilising and redistributing substantial amounts of rock and sediment. Overdeepened basins and (tunnel) valleys may attain depths of more than 500 m, which is within the depth range considered for repositories. Subglacial overdeepenings form independently of any regional base level as their formation relates to pressurised meltwater that effectively removes sediment. Ice-load induced deformation of rocks includes glacial isostatic adjustment (GIA), deformation of salt structures and glacitectonics. The weight of an ice sheet triggers long-wavelength glacial isostatic adjustments, which strongly modify the regional stress field. Such changes of the stress field may cause the activation or shutdown of faults and affects areas beyond the ice margin. More locally, ice-sheet loading may trigger the deformation of salt structures due to the viscous behaviour of salt rocks. Glacitectonic deformation affects near surface rocks and includes the formation of glacitectonic shear zones, folds and faults. Movements of rock masses induced by ice loading, particularly along faults, have the potential of creating fluid pathways. Additionally, glacitectonic thrusting can relocate coherent rock masses from depths of up to 350 m. Key to assessing the potential impact of future glaciations is the thorough understanding of the processes during past glaciations, which serve as analogues for the future. The effects of past glaciations can be analysed to reconstruct processes and allow the quantification of past extreme scenarios (e.g., depth of erosion). Numerical models are another important tool and allow the quantification and evaluation of controlling factors and the testing of extreme values (e.g., thickness of ice sheets). Ideally, both approaches should be combined to assess the potential impact of a glaciation on the geological barrier of a repository. Examples will be presented from current projects incorporating reconstructions of Pleistocene processes into long-term safety assessments. The first case study is on the maximum depth of Pleistocene erosion in northern Germany (Breuer et al. 2023). Based on the mapped depth zones, the potential for erosion can be assessed. The second case study is a numerical simulation of the response of salt structures to ice-sheet loading (Lang & Hampel 2023), which provides new insights into the relevant controlling factors of ice-salt interactions.