Conference Agenda

The development of competences and continuos training and education of early-career researchers are of paramount importance for century-long projects such as the final disposal of high-level nuclear waste. The TRANSFER graduate school is dedicated to this task, offering PhD students the opportunity to investigate geological disposal in claystones through mutually complementary projects. It integrates various disciplines, qualifying the next generation of scientists for roles in academia, industry, regulatory authorities, and policy-making within radioactive waste disposal and beyond.

The core focus is evaluating the suitability of claystone formations as host rocks, paying particular attention to spatial and temporal heterogeneities and their impact on radionuclide retention. Initially, Opalinus Clay serves as a reference material to transfer findings from previous Swiss studies to occurrences in southern Germany, contextualizing them within a broader geological framework. Subsequently, the acquired insights will be applied to other clay formations from the North German Basin.

A central objective is developing simulation methods capable of representing heterogeneities within repository systems, founded on data from laboratory tests and in situ experiments in underground research laboratories. Based on this process-oriented research, a more precise selection protocol for claystones is being developed, benefiting both the German and international scientific communities. Ultimately, a structured workflow—utilizing the process knowledge generated across the PhD projects to optimize parameter requirements—will be presented to serve as a foundation for site investigations and future site selection processes.

Subrosion, the dissolution of salt by circulating groundwater, is an important process in the long-term safety assessment of geological repositories in salt formations. Although salt dissolution can occur rapidly in contact with undersaturated water, insoluble residues contained within the salt, mainly anhydrite, accumulate during dissolution and progressively form a caprock. This caprock acts as a barrier that limits fluid exchange between freshwater aquifers and the underlying salt, thereby reducing subrosion rates.

This study investigates the distribution of lithologies within Zechstein salt diapirs in the Netherlands and evaluates the influence of caprock thickness and lithology on subrosion processes. A comprehensive geological and hydrological repository was developed from seismic, well, and lithological data and integrated into a TemisFlow™ simulator. The resulting inventory highlights significant spatial variability in salt purity, non-salt intercalations, and caprock thickness at diapir crests.

4D hydrological models were used to quantify the influence of groundwater flow, salinity, caprock thickness, and porosity on salt dissolution. Results indicate that subrosion rates are primarily controlled by caprock properties and remain low where an effective caprock is present. Compared with uncapped scenarios, caprock development reduces predicted dissolution rates by up to two orders of magnitude, demonstrating the importance of caprock characterization for evaluations of salt formations considered for geological disposal and other subsurface storage applications.

The study demonstrates the importance of integrating geological inventories with regional hydrological modelling to constrain long-term salt stability and support the evaluation of Zechstein diapirs as potential host formations for nuclear waste disposal and other underground storage projects.

The long-term disposal of high-level radioactive waste remains a major scientific and societal challenge. In Germany, the Federal Company for Radioactive Waste Disposal (BGE) identified several potential repository sub-areas during the first phase of the site selection process, including seven regions within the crystalline basement. However, many of these areas are still poorly characterized regarding their mineralogical, geochemical, petrophysical, thermal, and mechanical properties, limiting the assessment of their suitability as potential host rocks.

To improve the current knowledge base, a comprehensive reference database for crystalline basement rocks in Germany was developed and continuously expanded. Existing information from more than 40 published studies was compiled and standardized before being supplemented by extensive new sampling and laboratory campaigns in Bavaria, Baden-Württemberg, and Hessen. More than 1,500 cylindrical specimens from drill cores, quarries, and outcrops were systematically investigated to capture the lithological diversity of magmatic and metamorphic basement rocks. Additional triaxial and thermal conductivity experiments were conducted at elevated temperatures (up to 275 °C) and pressures (up to 30 MPa) to better approximate in situ conditions.

The resulting database currently comprises more than 35,000 data points covering up to 34 rock properties derived from over 8,300 samples across eight federal states. The results reveal considerable variability in rock properties reflecting the geological heterogeneity of Germany’s crystalline basement. Lithology-dependent trends in density, magnetic susceptibility, and thermal conductivity were identified, while high-temperature and high-pressure experiments demonstrate significant changes in permeability and thermal behavior, emphasizing the importance of site-specific characterization.

Characterizing the 3D crustal stress is essential for the safety and stability of subsurface engineering, particularly deep geological repositories (DGRs) for radioactive waste. The Shazaoyuan granitic block in Beishan area is a key candidate site for China's planned DGR. To assess the in-situ stress, 119 microhydraulic fracturing (MHF) tests, conducted in five boreholes, provide information about the S_hmin magnitudes. However, because MHF tests sample stresses only at the meter scale, their representativeness for the larger rock mass remains uncertain, motivating the construction of a 3D geomechanical numerical model calibrated against these data. Also, the S_Hmax magnitudes were calculated using the measured S_hmin magnitudes.

We use this model to investigate two key factors: (1) the effect of subsurface variability in elastic properties on predicted stress magnitudes, and (2) the spatial influence of faults on the far-field stress field. We show that the rock stiffness, represented by Young's modulus (E), is the primary control on horizontal stress magnitudes for the region of interest. Given the statistical variation of E within a geological unit, stress predictions should reflect this variability through a distribution scaled to the stiffness distribution. We also account for the change in E with depth, without which it is impossible to have a good model stress prediction, which fits the measured data. Moreover, the fault effects are spatially limited to less than 1 km and are significantly smaller than the uncertainty introduced by stiffness variability, indicating that faults can be omitted from site-scale models without meaningful loss of predictive accuracy.

A comprehensive understanding of time-dependent behavior of rocks is essential for engineering geological applications ranging from anthropogenic decades up to 1 Ma. In particular, the long-term stability of geological systems represents a fundamental challenge in the context of nuclear waste repositories, where system integrity must be ensured over extended timescales. Reliable prediction of the temporal evolution of rock mechanical response to stress requires a detailed understanding of deformation and damage processes in potential host rocks. In Germany, the crystalline basement is currently under consideration as a potential host rock.

In this study, granite was investigated under uniaxial compression at varying strain rates from 10^-7 to 10^-3 s^-1. Post-failure specimens were saturated with blue-colored epoxy resin followed by preparation of thin sections perpendicular to the main fracture to enable high-resolution 2D microstructural analysis.

The mechanical results show that uniaxial compressive strength increases with increasing strain rate. At the same time, crack density decreases significantly. The resulting crack pattern shows main fractures and fine inter- and intragranular cracks. Both, tensile and shear cracks were identified, while tensile cracks were dominant in all thin sections. Microfractures preferentially develop along pre-existing zones of weakness, such as grain boundaries or cleavage planes of biotite minerals.

These findings provide critical insights into time-dependent micromechanical damage processes in granitic systems subjected to varying strain rates. They contribute to a more robust evaluation of mechanical integrity in potential host rocks and establish a foundation for advanced numerical modelling approaches aimed at predicting long-term failure mechanisms over geological timescales.