The site selection procedure for a high-level radioactive waste repository in Germany is based on the Repository Site Selection Act (StandAG, 2017) and comprises three phases. Commissioned by the Bundesgesellschaft für Endlagerung mbH (BGE), the BGR contributes to this procedure with the projects „GeoMePS“ and „ZuBeMErk“, which aim to develop recommendations for phase 2 of the site selection procedure – the surface exploration of siting regions. For this purpose, the BGR has developed a systematic approach that includes (1) deducing exploration targets, (2) compilation of geoscientific and geophysical exploration methods in a database structure, and (3) evaluation of case studies of national and international exploration programs. The deduction of exploration targets was based on the criteria and requirements as defined by the StandAG. Subsequently, the identified exploration targets together with a large number of geoscientific (e.g., geological mapping) and geophysical exploration methods (e.g., reflection seismics) were integrated and linked within the database – the so-called “GeM-DB”. All methods were evaluated according to their suitability and applicability for (a) the three defined host rocks (crystalline rock, claystone, rock salt) and (b) the previously defined exploration targets. In step (3) the BGR evaluates national and international exploration programs, for example the site selection processes for deep geological repositories in Switzerland or Sweden, to infer additional exploration directives.The entire systematic approach aims to develop recommendations for a non-destructive and minimal invasive surface exploration program of siting regions in Germany, regarding the lithological, structural, mechanical, and hydrogeological characterization of the different host rock formations.

Seit 2013 hat in Deutschland die Standortsuche für ein Endlager für hochradioaktive Abfälle mit dem ersten Standortauswahlgesetzes (StandAG) neu begonnen. Viele Jahrzehnte lang wurde in Deutschland die Entwicklung von Endlagerkonzepten und Sicherheitsanalysen für ein Endlager in einem Salzstock priorisiert, während Endlagerkonzepte für Ton- und Kristallingestein erst seit zwei Jahrzehnten berücksichtigt werden. Zielstellung eines vergleichenden Standortauswahlverfahrens ist, bis 2031 einen Standort zu finden, der für 1 Million Jahre die bestmögliche Sicherheit für den Einschluss hochradioaktiver Abfälle bietet.

Sicherheitstechnischer Grundgedanke ist dabei der Einschluss der Schadstoffe im sogenannten einschlusswirksamen Gebirgsbereich (ewG). Im Mittelpunkt eines Langzeitsicherheitsnachweises steht folgerichtig der systematische Nachweis des langfristig sicheren Einschlusses der endgelagerten Abfälle durch den Nachweis der Integrität des einschlusswirksamen Gebirgsbereichs, als wesentliche geologische Barriere, sowie der geotechnischen Barrieren.

Aufgrund ihrer einzigartigen Eigenschaften, u.a. Dichtheit und Kriecheigenschaften, werden Salzgesteine in Deutschland und weltweit seit Jahrzehnten bergbaulich und insbesondere für die Energiespeicherung genutzt.

Während Steinsalz in Salzstöcken (Typ: steile Lagerung) vor allem in Nordwestdeutschland verbreitet ist, dominieren in Mitteldeutschland flach gelagerte Salzgesteine (Typ: flache Lagerung) sowie in Teilen Nordostdeutschlands Salzkissen (Typ: Salzkissen). Beide Typen der „flachen Lagerung“ (bedded salt) spiegeln in ihrer lateralen Erstreckung – abgesehen von diagenetisch bedingten Veränderungen – weitgehend sedimentationsbedingte Lagerungsverhältnisse wider.

Ausgehend von der Darstellung der wirtsgesteinsspezifischen Randbedingungen der verschiedenen Salzgesteine fokussiert dieser Beitrag auf die Vorgehensweisen für den Integritätsnachweis des geologischen Barrieregesteins Salz auf Basis vorliegender Erfahrungen und entsprechender Referenzstudien sowie Analogbeispielen. Im Ergebnis wird nachgewiesen, dass Endlagerkonzepte in der flachen Lagerung in der Konstellation Salzkissen aufgrund eines standortspezifischen Multibarrierensystems erhebliche sicherheitstechnische Vorteile bieten.

Claystones in the deep subsurface have various barrier-effective properties for solute transport. Changes in the mineralogical composition (incl. fluids), the structure of claystones in the strict sense, as well for claystone formations in a broader sense, and their regionally different geological development cause a great variety of petrophysical, mineralogical and geochemical properties of claystone sequences. Therefore, sound knowledge of the characteristics of claystone formations is required for the different usage options of the subsurface in order to provide technical support for economic, political and scientific investigations or decisions and to ensure the safety of underground geosystems.

In our study MECHTON existing data will be used and evaluated for petrophysical and further regional characterisation of the lithological expression of claystone formations. For this purpose, statistical methods are used for the interpretation of geophysical borehole measurements in order to record changes in the regional lithological characteristics and the spatial-petrophysical characteristics of a potentially repository-relevant claystone formation, the Opalinus Clay.

The objectives of the study will be presented and methods for characterising the lithological and petrophysical properties of claystone on basis of logs will be discussed. The focus is: How can regionally characteristic properties of claystone be recorded from logs and transferred to a larger study area?

The rock mass around man-made underground structures inevitably experiences major changes in hydraulic and mechanical properties, commonly referred to as excavation damage. In host rock formations for underground nuclear waste storage, such an excavation damaged zone (EDZ) is potentially critical and therefore requires reliable field data sets for safety assessment and the estimation of the long-term behavior. In this study, different on-site measurements were carried out in the EZ-B niche of the Mont Terri Rock Laboratory in order to characterize the EDZ with regard to its changed hydraulic effectiveness and its mechanical and geophysical properties.

The discrete fracture network (DFN) around the EZ-B niche was investigated using a transient airflow permeameter as well as combined microscopic imaging with automatic evaluation. The DFN exposed in the non-lined tunnel with measured hydraulic fracture apertures of 84 ± 23 μm basically represents a network for advective fluid transport. Due to their formation mechanism, we encountered noticeable differences between the newly-formed unloading fractures and the reactivated tectonic discontinuities. Geomechanical and geophysical characterization of the Opalinus Clay was conducted based on needle penetrometer testing at the rock surface. This proved to be a valuable tool for accurately determining the anisotropic uniaxial compressive strength of the claystone, but also led to a general underestimation of other physico-mechanical parameters in bedding-parallel direction, presumably due to unperceived microcracks.

For decades, claystone is investigated as a likely host rock material for the deep geological disposal of highly radioactive waste. To ensure safe operation, knowledge about the long-term (thermo-hydro-) mechanical behavior of clay rock is of significant relevance: How will the excavation induced differential stresses gradually dissipate while causing convergence?

Laboratory triaxial tests on the mechanical behavior allow for the controlling of major and minor principal stress (differential stress), pore pressure (effective stress), saturation, temperature and loading history. Long-term deformation tests running at low differential stresses reveal gradually declining deformation rates (consolidation + primary creep). With time, the deformation process asymptotically approaches an apparently constant deformation rate at constant volume (secondary creep). At higher differential stresses, accelerated tertiary creep occurs, associated with dilatant deformation and forming of new microcracks, leading to final failure of the specimen. In claystones the primary consolidation process is slow and comparatively well understood: due to low hydraulic conductivity pore fluid is slowly squeezed out, at the same time rearrangement of clay particles and micro grains takes place leading to changes in the microstructure. Moreover, rearrangements in the grain skeleton with a variable deformation rate can occur at constant effective pressure. This creep process is hard to determine and still poorly understood. Only very long creep tests give the possibility to detect secondary creep and to distinguish it from primary consolidation processes.

Creep tests run over several months/years on clays from Mont Terri give examples of the settlement mentioned and different types of creep.

In Claystones, the storage concept for spent nuclear fuel mainly consisting of uranium is based among others on the isolation of the radionuclides within the effective containment zone due to the high retention capacity of the host rock. The transport properties are typically determined in laboratory experiments representing the host rock and for defined geochemical conditions. However, previous numerical studies have shown for the Swiss Opalinus Clay, that sorption of uranium and with that the migration are highly affected by variations in the composition of the porewater resulting from mineralogical heterogeneities. The hydro-geological system of the Opalinus Clay is characterized by a 210 m thick, low permeable section embedded between aquifers. The porewater components of the low permeable section show an asymmetric geochemical gradient towards the aquifers due to diffusive exchange over several million years between them. By modelling the past history of the porewater based on multi-component diffusion simulations, we quantify uranium migration for one million years considering the changing geochemical conditions as a result of the hydro-geological system. Our results show, that uranium is retained within the theoretical effective containment zone and adjacent aquifers are not reached. Consequently, a clay rock with a thickness > 100 m in a geochemical and mineralogical framework like the Opalinus Clay would be a suitable host rock for the storage of uranium from spent fuel.