Shaft seals are part of the multi-barrier system of a deep geological repository for high-level radioactive waste. They are designed to limit the fluid exchange between the repository and the overlying rock mass and biosphere. A particular component of German shaft seal designs is the Sandwich sealing system which can improve the sealing performance of bentonite seals by combining them with intermediate equipotential segments of mineral materials with a higher hydraulic conductivity. This leads to an even distribution of intruding fluids via the equipotential segments across the seal and a homogeneous hydration and swelling of the bentonite (Nüesch et al. 2002, Schuhmann et al. 2009).

The international Sandwich project investigates the behavior of the Sandwich sealing system by experimental testing on multiple scales and hydro-mechanical simulation, see (Emmerich et al. 2019, Wieczorek et al. 2024) and the synthesis (Hinze et al. 2025). The laboratory experiments range from swelling pressure measurements and MiniSandwich experiments to semi-technical scale tests. Moreover, a large-scale experiment has been conducted at the Mont Terri rock laboratory since July 2019. The in-situ test consists of two experimental shafts with Sandwich sealing systems installed in each. Both are hydrated from the bottom with artificial pore water and closely monitored, alongside the surrounding rock. The simulation work in the project comprises the calibration of swelling models describing the complex hydro-mechanical behavior of bentonite and interpretative modeling of the Sandwich sealing system itself including the interaction with the surrounding host rock.

This contribution is particularly related to the in-situ test and its numerical simulation. It provides and interprets recent measurement data (comprising water content (acquired by electrical resistivity tomography and time-domain reflectometry), relative humidity and temperature, pore pressure, axial and radial stress, and displacements) and relates them to simulation results for models of the experimental shafts and the surrounding rock mass.

References

Emmerich, K. et al.: Joint project: Vertical hydraulic sealing system based on the sandwich principle – preproject (Sandwich-VP). Final report, Karlsruhe Institute of Technology (KIT) & Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, https://doi.org/10.2314/KXP:1692488228, 2019.

Hinze, M. et al.: Multi-scale testing and simulation of a vertical hydraulic Sandwich sealing system, Waste Management 2025 Conference, Phoenix, Arizona, USA, March 9 - 13, 2025. 

Nüesch, R., Brandelik, A., Hübner, C. & Schuhmann, R.: Verschlussstopfen und Verfahren zum Verschließen von untertätigen Hohlräumen, German Patent DE 10149972 C1, 2002.

Schuhmann, R., Emmerich, K., Kemper, G. & Königer, F.: Verschlusssystem mit Äquipotenzialsegmenten für die untertägige Entsorgung (UTD und ELA) gefährlicher Abfälle zur Sicherherstellung der homogenen Befeuchtung der Dichtelemente und zur Verbesserung der Langzeitstabilität: Schlussbericht, 117 S., https://doi.org/10.2314/GBV:637752392, 2009.

Wieczorek, K. et al.: Sandwich-HP – Vertical hydraulic Sandwich sealing system, Final Report, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH & Karlsruhe Institute of Technology (KIT) & TU Bergakademie Freiberg (TUBAF), Technical Report GRS – 745, https://www.grs.de/de/aktuelles/publikationen/grs-745, 2024.

How to cite: Hinze, M., Emmerich, K., Hesser, J., Furche, M., Jaeggi, D., Schefer, S., Königer, F., and Friedenberg, L.: Testing and simulation of a large-scale Sandwich shaft sealing system, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-22, https://doi.org/10.5194/safend2025-22, 2025.

Geomechanical-numerical modeling aims to provide a comprehensive characterization of the stress tensor within rock volumes by leveraging localized stress magnitude data for model calibration. This calibration involves optimizing boundary conditions to achieve the closest alignment with in-situ stress measurements from boreholes, which provides magnitudes of the minimum and maximum horizontal stress. However, the high cost of acquiring stress magnitude data often results in sparse and incomplete datasets, potentially hindering meaningful calibration.

In this study, we use a comprehensive dataset of 45 stress magnitude data records acquired for the geomechanical characterization of the candidate siting region Zürich Nordost, a potential site for a deep geological repository in northern Switzerland. We demonstrate how the number of available stress magnitude data records influences the accuracy of 3D total stress tensor predictions. To achieve this, we introduce a novel statistical approach that enables the analytical estimation of a large number of model simulations, each calibrated using different numbers of stress magnitude data records. This approach evaluates how the availability of data influences stress predictions across formations with varying rock stiffness by rapidly assessing the stress states associated with numerous combinations of stress magnitude data records.

By comparing the resulting stress fields with an increasing number of data records, it is possible to estimate the minimum number of calibration points required to achieve a prediction range comparable to the range expected due to inherent data uncertainties. The results indicate that for the region Zürich Nordost, fewer than 15 data records are sufficient to achieve the same model precision and accuracy, suggesting that additional data would not significantly improve model accuracy.

In addition, detailed analysis of the dataset revealed an outlier with respect to our model, linked to a local stiffness anomaly. While this outlier represents a physically valid measurement, it significantly impacts stress predictions when calibration data are limited. However, as the calibration dataset size increases, the influence of the outlier diminishes. Our statistical approach also allows the objective identification of clear outliers within the calibration dataset, which in turn affects the minimum number of data points required for model calibration.

These results highlight the importance of dataset size and composition in reducing uncertainties, and providing a framework for optimizing calibration strategies. This study provides valuable insights for subsurface projects, such as energy storage, CO₂ sequestration, deep geological repositories, and geothermal energy, where precise stress predictions are critical.

How to cite: Laruelle, L., Ziegler, M., Reiter, K., Heidbach, O., Desroches, J., Giger, S., Degen, D., and Cotton, F.: Minimum number of stress magnitude data records for model calibration, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-19, https://doi.org/10.5194/safend2025-19, 2025.

In the concept for a high-level nuclear waste (HLW) repository in rock salt, crushed salt fulfils the crucial role as long-term barrier together with the surrounding geological barrier rock salt. Crushed salt is a long-term stable material which guarantees a maximum of compatibility with the host rock, and which is easily available (mined-off material). The sealing effect of crushed salt evolves by porosity/permeability reduction due to compaction driven by the convergence of the surrounding rock salt. The compaction process of crushed salt is influenced by environmental conditions (e.g., temperature, stress state, convergence rate) and internal properties (e.g., grain size/grain size distribution, moisture content), therefore, comprises thermal-hydraulic-mechanical (THM) coupled processes.

Comprehensive knowledge of the sealing effect evolution as required for the long-term safety analysis for a HLW repository, includes full process understanding of the porosity/permeability reduction during ongoing compaction and a reliable prediction of the compaction process in long-term.

The project MEASURES is initiated by an international group of organizations with wide ranges of experience in the field of repository research and crushed salt compaction (Friedenberg et al., 2025). The aim of the project is to be able to predict real behavior based on the entire process chain from laboratory tests, the microstructural analysis of these tests, and material models that take these test and analysed results into account, and also to be able to demonstrate this in-situ.

The MEASURES project is based on a strong interaction between experimental and microstructural studies, and numerical methods. The following main areas are addressed:

• Sensitivity of long-term compaction to mean stress, water content and initial porosity;
• Quantification of contributions from individual microstructural mechanisms;
• Permeability measurements over a range of relevant porosities;
• Calibration of constitutive models against generated experimental data;
• Quantification of uncertainties both in laboratory and numerical simulations;
• Addressing sample-to-sample variability and lab-to-lab variability.

The outcomes of the MEASURES project will improve the process understanding of crushed salt and help to reduce uncertainties in the prediction of the evolution of crushed salt’s barrier properties. Thus, the project helps to strengthen the long-term safety case for a HLW repository in rock salt.

 Acknowledgements

Thanks go to the MEASURES family for the fruitful collaboration and constant support.

The project partner GRS, BGE-TEC, IfG and TUC greatly acknowledge the project funding received by the German Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV), represented by the Project Management Agency Karlsruhe (PTKA) under the contract FKZ 02 E 12214 A-D.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

References

Friedenberg, L., Bartol, J., Beese, S., Coulibaly, J. B., Düsterloh, U., Gartzke, A.‑K., Hangx, S., Jantschik, K., Kirby, M., Laurich, B., Lerch, C., Lerche, S., Li, L., Lüdeling, C., Mills, M. M., Naumann, D., Norris, S., Oosterhout, B. van, Rahmig, M., . . . Zemke, K. (2025). Multi-Scale Experimental and Numerical Analysis of Crushed Salt Used as Engineered Backfill in a Rock Salt Repository - 25062. In I. N. WM SYMPOSIA (Chair), Waste Management Symposia 2025, Phoenix.

How to cite: Friedenberg, L. and the MEASURES project family: A multi-scale approach for the investigation of crushed salt as long-term barrier in a rock salt repository (MEASURES), Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-23, https://doi.org/10.5194/safend2025-23, 2025.

Given that in-situ data of the stress field are limited, geomechanical models are used to receive a continuous prediction of the 3-D stress in the subsurface. For the numerical solution of these models the Finite Element (FE) method is often used as it allows to discretize complex structures using unstructured meshes. Therefore, the results strongly depend on the FE-mesh resolution, the element types, and the element order used. In a comprehensive approach to optimizing a mesh, it is necessary to find a balance between a detailed high-resolution mesh and the resulting computing time or available computing capacity.

To investigate such a question an extensive model series that change the model geometry and its resolution is required. However, such test series can only be conducted using simplified models, as the effort involved in producing the FE-mesh would otherwise be too substantial. For this reason, the complexity of a real 3-D structure was reduced to a 2-D profile section. As a template for this approach, we use a geomechanical-numerical model of the siting region of Nördlich Lägern for a deep geological repository for radioactive waste in northern Switzerland. Geologically, it consists of the crystalline basement, south-dipping Mesozoic units, and a cover of Cenozoic deposits. The key purpose of our study is to investigate the impact of the FE-mesh on the predicted 3‑D stress field within the thin stratigraphic units of the Mesozoic. The mesh resolution, element type, element order, and solver-specific elements with reduced integration points are tested. All models are calibrated separately with the same set of in-situ stress magnitude data from a borehole to find the best-fit displacement boundary conditions. All results are also displayed along the same borehole trajectory, which is located exactly on the profile section, in comparison to the available in-situ stress magnitude data. The result shows that horizontally elongated hexahedrons are more suitable for thin layers in comparison to tetrahedron elements; higher order elements also offer little added value in such static case.

For our study of Nördlich Lägern we use three different model geometry realizations that resulted from the different stages of the exploration process during the past 15 years. To achieve better comparability, the mechanical properties were also harmonized as far as possible. If almost identical rock properties are used, only small differences in the predicted stress field are visible, mainly in the areas where the stratigraphic boundaries differ between the models. Differences become more significant when the original and deviating rock properties are used. This indicates that the rock properties have a large influence on the model estimates. However, in comparison to predicted bandwidth of the predicted stress field that is controlled by the probability distribution of the rock stiffness in each lithology, the changes to the different model realization are small.

How to cite: Reiter, K., Heidbach, O., Degen, D., Ziegler, M., and Henk, A.: How the mesh controls accuracy in geomechanical-numerical models, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-24, https://doi.org/10.5194/safend2025-24, 2025.

The initial in-situ stress state is a key parameter for the evaluation of siting regions for deep geological repositories. To achieve a 3D description of the stress field geomechanical-numerical models are used to extrapolate between a usually small number of in-situ stress measurements and finding a best fit in a model calibration process. However, if all in-situ data points cannot be fitted equally well by the model, the predictive value decreases. Furthermore, this approach neglects that the data records of in-situ stress magnitudes are not data points, but ranges. Each data record is a range of possible values or even a probability distribution of the physical value. As a consequence, the modelled stress field has to be a range as well. 

Considering the inherent uncertainty of in-situ stress magnitude data a single fit of a model to the observed stress state is usually not meaningful as it can only explain a subset of the data records used for the model calibration. To quantify these uncertainties a range of model results can be used that contain extreme and average cases resulting in a range of possible stress states. However, this range of results also includes extreme and therefore unlikely scenarios and probably overestimates the range of modelled stress states resulting in an unspecific prediction of the stress field. If the in-situ stress magnitude data is provided as a range, this modelled stress range can be refined. Data records that come as a range can be fitted in a model scenario that also agrees with other ranges of in-situ stress magnitude data. At the same time, extreme model scenarios can be identified since they only agree with very few data ranges. This allows to narrow down the range of modelled stress states.

The concept is exemplified and its applicability demonstrated using a case study of the siting region Zürich Nordost located in northern Switzerland. Here in the context of the site-selection process for a deep geological repository a 3D geomechanical models has been built and a unique data set of 45 in-situ stress magnitude data records is used for the model calibration. Our results show that using the measurement uncertainties of the in-situ stress magnitude data narrows the modelled stress state range compared to an approach where data records are used as data points only. Thus, we propose that using the ranges of the in-situ stress magnitude data instead of treating them as data points using e.g. their mean value, will increase the significance of 3D geomechanical models. 

How to cite: Ziegler, M., Heidbach, O., Laruelle, L., Reiter, K., Desroches, J., and Giger, S. B.: Embrace the uncertainty – Geomechanical example for the value of uncertainties, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-51, https://doi.org/10.5194/safend2025-51, 2025.

The present work deals with the fracture mechanics of crystalline rocks and in particular with the barrier integrity for the isolation of hazardous waste. The experimental database is derived from the GREAT cell, a rock mechanics facility at the University of Edinburgh. The GREAT cell is a unique experimental facility that allows the investigation of thermo-hydro-mechanical (THM) processes in fractured rocks in rotating stress fields. The main idea of this work is to define a systematic benchmark suite for the development and testing of hydro-mechanical (HM) fracture mechanics codes based on GREAT cell experiments. The benchmarks represent simplifications of the original experiments to facilitate code testing. Two numerical fracture mechanics methods were used to simulate the complete benchmark suite, namely the variational phase field (VPF) and the lower-order interface element (LIE) methods.  The numerical methods and Jupyter notebooks for pre- and post-processing are available in the open source platform OpenGeoSys, following the FAIR principles of open science. This work is part of the DECOVALEX 2027 project (Task SAFENET-2), an international effort to validate models and codes against experimental data.

Reference:

Mollaali et al. (2025): Numerical benchmarking of GREAT cell experiments: Poly-axial stress effects on fluid flow in fractured rock using smeared and discrete methods. Journal of Rock Mechanics and Geotechnical Engineering, submitted

How to cite: Kolditz, O., Mollaali, M., Wang, W., and McDermott, C.: Numerical benchmarking of GREAT cell experiments: Poly-axial stress effects on fluid flow in fractured rock, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-90, https://doi.org/10.5194/safend2025-90, 2025.

Since primary and especially secondary creep is the dominating deformation mechanism of rock salt, the latter one is used as direct indicator for evaluation of possible sites for a repository for heat generating nuclear waste in Germany. For determination of long-term stability of these repositories, the creep of salt over long periods have to be well known. Next to the mechanisms described in constitutive laws, corresponding parameters are to be determined. At higher differential stress Δσ = σ_max - σ_min, the dislocation creep (DC), e.g. Hunsche and Hampel (1999), is effective. A power law can describe it, and deformation rate dε/dt is easy measurable in laboratory. The rate dε/dt is proportional to Δσⁿ , with n=5-7. The exponent is depending on rock salt type and e.g. content of anhydrite and other minerals. Numerous measurements to calibrate the constitutive laws for description of the DC are available. For low differential stress, the pressure solution creep (PSC), e.g. Urai and Spiers (2007), is the dominating process. The deformation rate dε/dt is proportional to differential stress Δσ. Next to e.g. temperature, it additionally depends on grain size. As the deformation rate is very small, only a few measurements (e.g. Berest et al. 2019, 2023; Blanco-Martín et al., 2024) at extremely low differential stress are available to verify and calibrate the constitutive law.

At BGR various data is available for the analysis of possibilities and challenges during the determination of creep rates. On one hand, numerous measurements on e.g. salt from Waste Isolation Pilot Plant (WIPP) and the Morsleben repository site (ERAM) are available, which can be used for calibration of the DC. Although the salt from WIPP-site is bedded salt and from ERAM-site is domal salt, the measurement results do not differ significantly. On the other hand a few measurements on ERAM salt at differential stresses and low temperature (DΔσ = 5 MPa at T = 21°C & and 3 MPa at T= 25°C) are available. They had a duration of up to 40 months and thus give insights into the development of creep over a long period of time.

Processes at small differential stresses dominate the long-term behavior of a final repository. Further measurements at low stresses and simultaneous analyses, e.g. of moisture and microstructure, are therefore necessary as a basis for well-founded long-term safety analyses at different locations.

References

Bérest, P., H. Gharbi, L. Blanko-Martín, et al. 2023. Rock Mechanics and Rock Engineering.

Bérest, P., H. Gharbi, B. Brouard, et al. 2019. Rock mechanics and rock engineering

Blanko-Martín, L., A. Rouabhi, F. Hadj-Hassen, et al. 2024. Rock mechanics and rock engineering

Hunsche, U., and A. Hampel. 1999. Engineering geology

Urai, J. L., and C. J. Spiers. 2007. Paper read at 6th Conference on the Mechanical Behavior of Salt at Hannover.

How to cite: Mayr, S., Gräsle, W., and Zemke, K.: Secondary creep of rock salt: mechanisms and challenges in determination, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-116, https://doi.org/10.5194/safend2025-116, 2025.

The safe disposal of heat-generating radioactive waste in deep geological repositories is a major challenge that requires a thorough understanding of the host rocks integrity and its ability to contain radionuclides over extended time periods. Crystalline rock is one of the potential rocks under consideration in Germany for hosting heat-generating nuclear waste. Fractures and other types of discontinuities usually characterize crystalline rock. It is therefore expected, that the presence of fracture networks influences both the hydraulic and mechanical system behavior. This will affect transport mechanisms in the system and could potentially impact the rocks integrity and therefore its containment capacity. Thus numerical modeling approaches to assess the thermal, hydraulic and mechanical (THM) processes require an appropriate representation of the fracture networks.

This contribution aims to provide an overview of the recent work done by the BGR towards the development of a numerical strategy for the safety assessment of crystalline rock in the context of deep geological repositories [1, 2, 4]. The modeling strategy focuses on the assessment of host rock integrity and the description of flow and transport in fractured crystalline. This comprises the numerical analysis of the complex interactions due to the coupled THM processes triggered by the decay heat of the disposed nuclear waste. The dominant influence of fracture networks in particular on flow and transport processes is also included. Based on the statistical characterization of fracture networks their properties are incorporated in numerical simulations by a combination of upscaling to an equivalent porous medium and explicit representation on lower dimensional elements. The strategy has also been partly developed within the framework of the DECOVALEX-2023 Task F joint project [2]. The open-source finite element code OpenGeoSys version 6 [3] has been used for numerical solutions in both the coupled THM problem as well as in the transport from radionuclides. We will present the developed strategy and its application to generic nuclear repository systems in crystalline rock.

References

[1] Guevara Morel, C.; Thiedau, J. & Maßmann, J. (2024): Advances in the numerical modeling strategy (concept) of a generic nuclear waste repository in crystalline rock. EGU General Assembly 2024, 14–19 Apr 2024, Vienna, Austria. DOI: https://doi.org/10.5194/egusphere-egu24-9854.

[2] Leone, R. et al. (2025) Comparison of performance assessment models and methods in crystalline rock: TASK F1 DECOVALEX-2023. Geomechanics for Energy and the Enviroment 2025; 41: 100629. https://doi.org/10.1016/j.gete.2024.100629.

[3] Bilke et al. (2025): OpenGeoSys (OGS 6.5.4), January 2025, https://doi.org/10.5281/zenodo.14672997.

[4] Thiedau, J., Maßmann, J., Guevara Morel, C., Weihmann, S. & Alfarra, A. (2021). CHRISTA-II - Analysen zur Integrität von geologischen Barrieren von Endlagersystemen im Kristallin. BGR, Ergebnisbericht, B3.5/B50112-52/2021-0003/001: 118 S., Hannover.

How to cite: Guevara Morel, C., Maßmann, J., and Thiedau, J.: Development of a numerical modeling strategy used to assess safe radioactive waste disposal in crystalline rock, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-32, https://doi.org/10.5194/safend2025-32, 2025.

Fault reactivation and associated seismic events are of concern in the context of site selection for a repository for the long-term safe storage of high-level radioactive waste. The stress state acting on a fault is one major factor for the fault reactivation potential. However, the in-situ stress state is often only poorly constrained. Stress data are usually only available for certain areas and even where stress data are available, they usually only describe parts of the full stress tensor and are limited to certain depths.

The use of geomechanical-numerical models allows a spatially-continuous first-order estimate of the stress tensor even for areas with limited stress data. For Germany, a 3D geomechanical-numerical model by Ahlers et al. (2002) provides such an estimate of the stress state throughout the whole country. We use the modeled 3D stress field and resolve the shear and normal stress on 10.000 3D fault geometries provided by geological models of federal states and project regions to estimate the reactivation potential of faults in Germany. As a measure for the fault reactivation potential, we use the effective slip tendency (T_S), i.e. the ratio between the maximum resolved shear stress and effective normal stress. The results of the T_S analysis can be used to identify areas of comparatively higher or lower fault reactivation potential. One area that shows high T_S values is the Upper Rhine Graben where T_S reaches and exceeds values of 0.6. The lowest overall T_S distribution is found throughout the Molasse basin, where T_S values rarely exceed 0.3. Faults striking in NNE-SSW direction and NW-SE direction show the overall highest T_S values, whereas faults striking in ENE-WSW direction show the overall lowest T_S.

Fault reactivation is considered likely, when T_S exceeds the coefficient of static friction. For most faults, this property is unknown and is often assumed to be 0.6 based on laboratory observations. To better interpret the results of the T_S analysis, further information about the frictional properties of the considered faults is required. Furthermore, hydrostatic pore pressure is assumed for the calculation of the effective stresses as data regarding the pore pressure are not yet available for all areas covered by the T_S analysis. The consideration of local pore pressure variations could further improve the T_S analysis as the pore pressure critically influences T_S. Further local effects such as stress modifications at faults are likely to influence the fault reactivation potential. Such effects can be more accurately captured in prospective smaller-scale models for specific regions and sites. Lastly, 3D fault geometries or high-quality data for their generation are not available all throughout Germany resulting in areas where no T_S could be calculated. Filling these gaps is a further step towards an improved prediction of the fault reactivation potential throughout Germany. 

Ahlers, S., Henk, A., Hergert, T., Reiter, K., Müller, B., Röckel, L., Heidbach, O., Morawietz, S., Scheck-Wenderoth, M., Anikiev, D., 2022. The Crustal stress state of Germany - Results of a 3D geomechnical model v2. https://doi.org/10.48328/tudatalib-437.5

How to cite: Röckel, L., Ahlers, S., Kuznetsova, V., Velagala, L. S. A. R., Laruelle, L., Müller, B., Reiter, K., Heidbach, O., Hergert, T., Henk, A., and Schilling, F.: Using data from geomechanical modeling for a slip tendency analysis of 3D faults in Germany, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-57, https://doi.org/10.5194/safend2025-57, 2025.

As part of the ThORN project [1], an in-situ experiment to quantify thermo-osmotic (TO) flow in Callovo-Oxfordian clays will be carried out at the Bure Underground Research Laboratory (URL) in Meuse/Haute-Marne, France.

The phenomenon of thermo-osmotic flow (TO) can potentially induce so-called pore water pressure anomalies, i.e., deviations from the frequently assumed linear profiles between adjacent aquifers in aquitards. Neglecting these anomalies can lead to erroneous estimations of flow direction and magnitude [2]. In addition to far-field analyses of the geological barrier within the natural geothermal gradient, changes in the temperature field due to decay heat released from the storage packages have been shown to affect thermo-osmotic flow. These alterations are most evident in the near field, potentially influencing the resaturation of geotechnical barriers or the development of pore water pressure within the near-host rock.

While prior studies on TO have been conducted on reconstructed samples, our in-situ experiment will be complemented by the assessment of meticulously controlled laboratory experiments on intact field samples. The aim of this project is to quantitatively assess the importance and parameterization of TO flow in clay under thermal gradients induced by the heat of nuclear decay. Numerical simulations in OpenGeoSys using the coupled THM process with implemented thermo-osmosis will support the design and evaluation of all experiments. The resulting models will be used to analyze near and far field effects in a repository environment. The thermo-osmotic coefficient will be estimated based on laboratory and in-situ data.

This contribution presents highlights of the preliminary design phase. The Objectives, expectations, and potential challenges are outlined and discussed. Predictive simulations of different designs and assumptions used in the design phase are presented and compared. We show how numerical simulations can be used to explore the potential results of physical experiments before they are built, and how this can optimize the workflow of the experiment.

References

[1] ThORN " Experimental investigations on thermo-osmotic flow in argillaceous materials relevant to deep geological repositories for radioactive waste " The Federal Office for the Safety of Nuclear Waste Management (BASE); Funds FKZ 4723F00104

[2] J. Gonçalvès, J.-M. Matray und C. J. Yu. „Assessing relevant transport processes in Opalinus Clay at the Mont Terri rock laboratory using excess-pressure, concentration and temperature profiles“. In: Applied Clay Science 242.May (Sep. 2023), S. 107016. ISSN: 01691317.

How to cite: Kiszkurno, F., Magri, F., de la Vaissiere, R., Plua, C., and Nagel, T.: ThORN - In-situ experimental investigation of the relevance of thermo-osmotic flow in clay for radioactive waste disposal, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-70, https://doi.org/10.5194/safend2025-70, 2025.

In the safety assessment of high-level nuclear waste geological repositories, the stability of placement caverns and boreholes is a critical factor. However, rock masses are exposed to complicated mechanical, thermal, and hydraulic loading conditions, which may induce the propagation of pre-existing fractures or the initiation of new fracture networks. Fracture propagation not only compromises the structural integrity of boreholes but also provides pathways for radionuclide leakage. Excavation, thermal effects from nuclear waste emplacement, and backfill sealing may all induce fracture development. In many disposal concepts, granite may serve as host rock, whereas the canisters containing the nuclear waste are embedded in compacted bentonite.

Therefore, the thermal-hydro-mechanical behavior of the system host rock (granite), engineered barrier (compressed bentonite) and nuclear waste container must be described from the excavation during the emplacement of the nuclear waste canister until encapsulation. In this study, a procedure is presented, starting from experimental studies of the fracture behavior of the granite and compressed bentonite, leading to numerical modeling with the Finite-Discrete Element (FDEM) Code Irazu (Geomechanica) to simulate fracturing and deformation patterns under various in-situ stress fields and temperature conditions.

In the laboratory experiments, the Semi-Circular Bend Test (SCB) and Double-Edge Notched Brazilian Disk Test (DNBD) are conducted to measure the fracture toughness of bentonite and granite under ambient conditions and heat treatments. Optical microscopic observation is conducted on thin slices to evaluate the thermal effects on the microscopic structure of granite, and the thermal damage mechanism is further analyzed. High-speed cameras and digital image correlation (DIC) technique are used to trace the fracture process, and important fracture parameters, such as the fracture initiation, fracture process zone (FPZ) size and critical opening displacement, are measured by full-field displacement and strain measurement.

Based on FDEM, a numerical model corresponding to laboratory tests is developed to simulate the entire process of fracture initiation, propagation, and ultimate failure. The simulation results are compared with experimental observations obtained using the DIC technique. Model parameters are calibrated against experimental data to construct a robust numerical model, which is then employed to simulate fracture behavior during distinct phases of a nuclear waste repository: the excavation stage, the thermal effect stage induced by the placement of nuclear waste canisters, and the backfill sealing stage. This approach provides theoretical support for investigating fracture behavior under repository excavation, operation and closure scenarios. Also, hydraulic parameters are to be derived from the various scenarios, helping to reveal the evolution patterns and influencing factors of permeability coefficients and to optimize the cavern and borehole design.

Keywords: Deep geological repositories; Host rock and engineered barrier; Fracture pattern; FDEM; Thermal treatment

How to cite: Zhou, Q. and Thuro, K.: Combined experimental and numerical study on the fracture pattern of the system host rock and engineered barrier in deep geological repositories, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-98, https://doi.org/10.5194/safend2025-98, 2025.

Numerical simulations play an important role in predicting the long-term behavior (up to 1,000,000 years) of potential repositories for high level radioactive waste (HLW) in deep geological formations. To provide reliable and realistic predictions, the numerical simulators must incorporate multiple physical processes that may be strongly coupled with each other. Analytical solutions for the underlying differential equations often do not exist for such complex scenarios and therefore can only serve as limited verification tool for the simulators. However, comparative benchmarking of different simulators for the same problem statement can close this gap and ensure the correct implementation of the differential equations and their couplings.

The objective of the international project BenVaSim II is to verify the implementations of thermally-hydraulically-mechanically (TH²M) coupled processes, considering hydraulic two-phase flow in various simulators, i.e. TOUGH-FLAC, CODE_BRIGHT, COMSOL Multiphysics, OpenGeoSys 6, Oscar, FTK simulator, used by the participating institutions (TUC, GRS, ENSI, LBNL, BGR, BASE). The investigated scenarios range from one to three spatial dimensions, examine various (nonlinear) couplings between the individual processes, and study the influence of anisotropic thermal and mechanical material properties.

Here, we present results for selected scenarios such as the build-up of pore-gas pressure due to canister corrosion (1D), temperature induced fluid pressure changes as a result of time-dependent radiogenic heat production (2D and 3D) and the influence of anisotropic properties as expected for claystone as host rock (2D). In general, the results of the individual simulators demonstrate a high level of agreement. Nonetheless, they also reveal non-negligible differences. We analyze, in time and space, important process variables like temperature, solid matrix displacement, liquid saturation as well as liquid and gas pressure to explain the interaction between the coupled processes. Furthermore, we discuss, based on the analyzed variables, potential reasons for the observed deviations between the different simulators.

In conclusion, our study highlights the crucial role of inter-institutional and international collaborations in validating numerical simulators. These collaborations are particularly important when predicting the long-term behavior of potential repositories for HLW, a domain where accuracy and reliability are mandatory.

How to cite: Kruse, J. P., Feierabend, J., Laatigue, A., Sun-Kurczinski, J., Wolters-Zhao, R., Friedenberg, L., Hinze, M., Rutqvist, J., Guo, G., Sentis, M., Schädle, P., Bjørnarå, T. I., Kock, I., Rücker, C., Hotzel, S., Pitz, M., Maßmann, J., and Gerolymatou, E.: Benchmarking for verification and validation of TH2M simulators: Current results from the BenVaSim II project, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-99, https://doi.org/10.5194/safend2025-99, 2025.

For prediction of the short and long-term geomechanical behaviour of a deep geological repository for nuclear waste, the present-day 3D in-situ stress is a key parameter. However, in Germany the knowledge concerning the crustal stress state is still quite low. It is mainly based on data from the World Stress Map (WSM) project providing data of the orientation of the maximum horizontal stress (S_Hmax) and a new compilation of stress magnitude data providing magnitudes of S_Hmax and the minimum horizontal stress (S_hmin). However, these two databases still provide only unequally distributed data records and in particular horizontal stress magnitude data records are only reliable at a dozen locations all over Germany. Thus, for an in-situ stress field prediction geomechanical-numerical models - calibrated on available horizontal stress magnitudes – are used. They enable a continuum-mechanics based description of the 3D present-day stress state and can resolve lateral and especially vertical variations. Two 3D geomechanical-numerical models of Germany have been published during the initial phase of the SpannEnD project (2018-2022). In the follow-up project SpannEnD 2.0 a new model has been set-up based on a new geological model enabling new insights into present-day crustal stress field of Germany, in particular due to higher vertical resolution. We also use a significantly enlarged stress magnitude database for model calibration.

The new 3D geomechanical-numerical model combines information of 27 regional geological models and comprehensive additional data. It comprises 49 geological units parametrized with elastic rock properties (Young’s modulus and Poisson’s ratio) and rock densities. Linear elasticity is assumed and the finite element method (FEM) is used to solve the partial differential equations that describe the equilibrium of gravitational and surface forces. Overall, the model contains ~10 million hexahedral elements providing a lateral resolution of 4 km and a vertical resolution of up to 45 m in the uppermost 5 km. The model results show an overall good fit with stress magnitudes used for calibration indicated by a mean of the absolute stress differences of ~3 MPa for S_hmin and of ~5 MPa for S_Hmax.  Furthermore, the results agree well with additional data sets - not used for calibration - e.g., an absolute mean deviation of the orientation of S_Hmax with regard to WSM data of ~10°.

How to cite: Ahlers, S., Henk, A., Reiter, K., Hergert, T., Röckel, L., Morawietz, S., Heidbach, O., Ziegler, M., and Müller, B.: SpannEnD 2.0 – New insights into the present-day stress of Germany by a new 3D geomechanical-numerical model, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-138, https://doi.org/10.5194/safend2025-138, 2025.

The simulation of thermo-hydro-mechanical (THM) coupled processes, i.e., the prediction of interactions between temperature, fluids and mechanical properties within a repository system, is an essential prerequisite for performing and evaluating preliminary safety analysis in the site selection process.

The presented open-source python software library Oscar is developed at the Federal Office for the Safety of Nuclear Waste Management (BASE) and has been released with its initial revision. The development of an own software library allows for the continuous development of expertise within BASE regarding numerical modelling and the diversification of the testing capabilities regarding the preliminary safety investigations. The choice for an open-source software strategy is motivated by the transparency requirement in the German site selection act and the potential to the provision of appropriately quality-assured and documented simulation tools to the public.

The main goal of this in-house development is to provide a quality-assured, flexible and easy-to-use toolbox for the modelling of THM coupled processes using the finite element method with the following key features:

• Finite element analysis with the usual cell types for 1D, 2D and 3D modellings (edge, triangle, quadrilateral, tetrahedron, hexahedron).

• Generic shape function generator with predefined base-functions for linear, and quadratic elements.

• Script-based interface to the solution of the governing equations for the THM coupled processes in non-dimensional form in a symbolic manner with its strong or weak formulation.

• Predefined process solver classes for frequently used THM processes (e.g., heat conduction, fluid flow, mechanical deformation).

• Growing set of Jupyter notebooks of intensively documented tutorials to the finite element theory and to the usage of the presented software library.

• Growing set of Jupyter notebooks of documented examples and commonly known benchmarks for the modelling of THM coupled processes.

The presented poster shows some key features of the software library and outlines potential applications.

How to cite: Rücker, C.: Oscar: A software library for modelling THM coupled processes, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-153, https://doi.org/10.5194/safend2025-153, 2025.