Conference Agenda

Aim of the project is the collection of data sets of solid material and groundwater physical and chemical properties for suitable host rocks such as claystone and various crystalline rocks. The data sets are stored in a database that compiles measured and already established and technically substantiated reference values from different data sources (databases, scientific publications) worldwide. Statistical variance, localisation and assignment to a certain geological unit are included. However, the main focus is on host rocks that have been investigated in Germany and surrounding countries like Belgium, Switzerland and the Scandinavian Countries.

The derived reference data sets will be used in Step 2, Phase I of the site selection procedure due to legislation of the StandAG to evaluate the geological units of an area of interest. A lack of measurement data is to be expected for many of the areas to be assessed by the BGE, so that the reference data sets are of great relevance for the representative preliminary safety investigations (rvSU). In the case that for an area-specific evaluation no or too few specific measured values are available parameter models have to be developed in order to approximate the site-specific properties. The aim of the rvSU is to be able to differentiate between subunits within a host rock type and within an investigation area. Thus, it should not only be possible to derive generalised reference data sets for the host rock type but also to provide differentiated value ranges based on the geological context.

The combination of TEM, based on focused ion beam (FIB) sample preparation, and high-resolution SEM allows the investigation of grain and phase boundary networks from nanometer to centimeter scale, i.e., over about 8 orders of magnitude. Recent studies show that the boundaries of various minerals in different metamorphic and magmatic rocks are lastingly open on the nanometer scale, due to the elastic response of crystals to temperature and pressure decrease during exhumation of rocks, and can be partly to totally filled with secondary minerals.

SEM measurements on square centimeter large areas in granite indicate that the boundaries between feldspars, quartz and biotite are nearly continuously and up to several hundred nanometer open and partly filled with secondary minerals. It is most likely that the boundaries form networks in even larger parts of the granite, which allow fluid flow. The occurrence of newly grown biotite indicates that open grain and phase boundaries are not just a phenomenon in rocks at uppermost crustal levels but can occur at depths of at least 10-15 km. Open and partly filled boundaries do not only control various physical properties of crystalline material and govern its behavior during different natural and technical conditions as well as in experiment. Such boundaries potentially affect the migration of materials even over larger distances in rocks, for example of radionuclides released from nuclear waste in deep geological repositories. Alternatively, fillings of boundaries by secondary minerals increase the absorptive capacity and, consequently, the bedrock's retention capability of fluid-carried materials.

Ensuring the safety for deep geological repositories for nuclear waste in crystlline host rock necessitates a comprehensive understanding of the far field and it's potential for radionuclide retention. In case of a repository leakage, radionuclides may get mobile and migrate through pathways in rock and aquifers. To asses the uncertainties in forcasting the migration of radionuclides it is essential to incorporate naturally occurring heterogeneities in rock composition and geological structures into the models, e.g. heterogeneities occurring near intrusion margins, tectonically influenced granitic bodies, or metamorphic formations like gneisses. This complexity significantly impacts the modeled radionuclide retention potential compared to simplistic isotropic granite models.
The SANGUR project (Systematic Sensitivity Analysis for Mechanistic Geochemical Models using Field Data from Crystalline Rock) aims to identify crucial parameters and their uncertainties essential for modeling radionuclide retention in crystalline rock. Our study presents a comprehensive workflow modeling how petrological variations in both granitic and metamorphic crystalline host rocks influence radionuclide retention. Utilizing Multinary Random Fields geostatistics, we simulate crystalline rocks based on analyzed spatial rock data to quantify uncertaintiesand to determine the appropriate model scale. The petrological variance is then considered for the chemical modeling through software such as PHREEQC or Geochemist's Workbench©: Surface Complexation Models (SCM) in chemical modeling software calculate partition coefficients (Kd values) for radionuclides, such as uranium, in diverse mineral environments in combination with varying aqueous phases. To enhance and simplify models, global sensitivity anlsysis is applied to determine critical features for radionuclide retention.

A robust prediction of the recent crustal stress field has a crucial role for forecasting the short- and long-term safety of a high-level radioactive waste repository. However, no reliable and comprehensive prediction of the complete stress tensor for Germany is possible with the amount of stress data records available. The only comprehensive data set is the World Stress Map, which, however, only provides the orientation of the maximum horizontal stress. Stress magnitude data records of sufficiently reliable quality are only available from a few boreholes. However, 3D geomechanical-numerical models, which represent the geometry of the subsurface and its mechanical properties and are calibrated with stress magnitudes, allow a continuum-mechanics based prediction of the complete stress tensor and its lateral and vertical variability.

A new geomechanical-numerical model – developed within the SpannEnD 2.0 (Spannungsmodell Endlagerung Deutschland) project - provides new insights into the recent crustal stress field of Germany. A new model, by combining ~25 existing 3D geological models and a five time higher vertical resolution of ~45 m allow a better mechanical representation of individual units and mechanical inhomogeneities. In addition new stress magnitude data records are compiled and used for calibration.

The results provide a comprehensive prediction of the complete stress tensor for Germany and can be used for a wide range of scientific questions and applications. Examples are the prediction of the fracture potential, the slip tendency of faults or as boundary conditions for small-scale models.

The Site Selection Act (StandAG) for Germany´s nuclear waste repository states that the presence of post-Eocene (< 34 Ma old) faults is an exclusion criterion that makes any potential site unsuitable. Identifying such faults is problematic in the German uplands (Mittelgebirge), where large areas have no Cenozoic deposits and exposure quality is generally low. Radiometric age dates of faults are still very sparse and unlikely to become widely available in the near future. We have used different methods to identify potential post-Eocene faults. These include GIS-based analysis of the spatial relationships of faults with post-Eocene units on existing maps as well as automated lineament extraction and visual interpretation of possibly fault-related topographic features from high-resolution DEMs. Fault and lineament density maps were created. All results based on geological maps are affected by their uneven quality and inconsistency. Much of the variation is caused by different mapping concepts, particularly for the 1:25.000 sheets. The fault networks have been analyzed for fault length distribution and topology to identify interpretation problems. Fault lengths and connectivity are underestimated. A Python code has been developed to automatically extract stratigraphic throws of faults from digital geological maps with an aim being to make better predictions about fault trace lengths. A concept for safety distances around faults has been developed that considers not only wall damage along faults, but also damage at fault tips, bends, steps and interaction zones. Proof of post-Eocene faulting will require additional analyses for each suspected case.