Barrier integrity is a crucial aspect for the assessment of nuclear waste disposal. Numerical simulations, in conjunction with experimental studies are an integral part of safety and environmental-impact assessment. Reliable comparative analyses of potential technological options require coupled THMC models capturing the particularities of each rock type and associated repository concept. Structural as well as process complexity are met by data scarcity and variability, necessitating the treatment of uncertainties and variability. The session provides a platform for the exchange on the following topics:
- THMC characterization of materials in natural or engineered barriers in lab- or field-scale experiments
- Hydro-mechanical behaviour of materials with extreme hydraulic properties (e.g. low permeability, high suction) and ranging from ductile viscopolastic salt rocks to quasibrittle fractured rock masses
- Hydraulic and chemical behaviour of geologic and geotechnical barriers
- Computational methods, models and uncertainty quantification for barrier integrity assessment in multi-barrier systems
- Geotechnical aspects of repository construction, operation, and post-closure, e.g. monitoring methods, excavation and support, retrieval/recovery, etc.
- Minimally invasive characterization of geology and underground installations using geophysical and geohydrological methods
Contributions can include lab-scale experimentation, underground research laboratories, observation of natural analogues, physics- and data-driven modelling and code development.
Furthermore, the session invites contributions addressing regulatory challenges, public outreach programs, lessons learned from national and international DGR projects, the need for transparent communication to ensure public confidence, and the relevance of geoscientific fundamentals in ensuring the safety of nuclear waste disposal.
Understanding the migration of gas within engineered barrier systems remains essential for evaluating the long-term safety of deep geological repositories (DGRs). This study presents a fully coupled hydro-mechanical (HM) modelling framework developed to examine gas transport in saturated bentonite. A modified formulation for intrinsic permeability evolution is introduced to better represent the mechanical response of bentonite during gas-induced fracturing. The approach employs an exponential law to capture the progressive transition of permeability during stress redistribution. The formulation explicitly incorporates the differential stress variable (σ-pg), enabling assessment of the combined influence of total stress variation and gas pressure on fracture initiation. Heterogeneity in dry density is represented through a spatially random distribution concept. Allowing simulation of realistic mechanical variability within compacted bentonite blocks. Model predictions are validated against laboratory experiments conducted under different HM boundary conditions. The numerical results reproduce the observed evolution of total stress, pore pressure, and gas breakthrough behaviour. This demonstrates the model’s capability to capture key coupled processes associated with gas migration in bentonite-based barrier materials.
How to cite: Sunkpal, D. T. and Fall, M.: A Coupled Hydro-Mechanical Framework for Gas Migration in Bentonite Barrier System for Deep Geological Repositories, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1440, https://doi.org/10.5194/egusphere-egu26-1440, 2026.
A set of useful hydro-mechanical (HM) iso-thermal characteristics of bentonite such as swelling under saturation, low hydraulic conductivity and high sorption may significantly be impaired by the thermal (T) effects. Multiple lab studies demonstrate a sharp drop of the swelling pressure in a confined fully saturated bentonite sample once it is subjected to high temperatures, e.g., [1]. Lab-scale evidence of potential bentonite’s deteriorating “sealing capacity” under thermal loading should be a subject of further investigation, especially when addressing large scale engineered barrier systems for radioactive waste repositories.
In our studies, the HotBENT in-situ experiment – a joint undertaking of multiple international partners at the Grimsel Test Site (Switzerland) operated by NAGRA [2,3] – is a point of departure. Geometry-, material-, and THM-process-wise, the experiment is designed to replicate a potential high level waste repository environment: electric heaters embedded in a bentonite buffer heat the buffers up to 200 °C, and the buffer is subjected to hydration from surrounding fractured granite and artificial hydration pipes. This configuration induces complex interactions between bentonite’s swelling, drying, vapour transport, and re-saturation processes. Two types of granular bentonite – Wyoming MX-80 and Bentonite Cerny Vrch (BCV) – are used in two different sections of the experiment as buffer also with compacted bentonite blocks serving as pedestals for the heaters.
For the HotBENT numerical simulations, the OpenGeoSys [4] computational multi-physics platform is used. The overall modeling campaign is multi-step with increasing dimension- and process-complexity.
Initially, simulations are limited to 2D geometry (cross-section of a heater in radial direction) and TH (non-isothermal two-phase two-component flow) response of buffer [5,6]. The available experimental data on temperature, relative humidity, pore pressure fields/distribution within the buffer enable model calibration of liquid and gas hydraulic conductivities, vapour diffusivity, bentonite’s effective thermal conductivity and water retention behaviour. Efforts have also been made to accurately represent heaters and the related heating modes, as well as to properly describe “buffer – host-rock” hydraulic interaction.
In this talk, we present some results of the extended 3D and fully coupled THM analysis. With accurate parametrization of MX-80 and BCV bentonite models at hand, our focus is shifted to comparison of both short- and long-term performance of these bentonite types. This regards the evolution of saturation and pore pressure fields, as well as buffer deformation response.
References:
[1] Najser, J., Mašín, D., (2024). An experimental study on thermal relaxation of BCV bentonite, Applied Clay Science 254, 107374. https://doi.org/10.1016/j.clay.2024.107374
[2] Grimsel Test Site (GTS). HotBENT – High-temperature effects on bentonite buffers: Introduction. https://grimsel.com/gts-projects/hotbent-high-temperature-effects-on-bentonite-buffers/hotbent-introduction
[3] Kober et al. (2023). HotBENT Experiment: objectives, design, emplacement and early transient evolution, Geoenergy, 1.
[4] OpenGeoSys Community. OpenGeoSys – Open-source finite element software for coupled THMC processes. https://www.opengeosys.org/
[5] Gerasimov et al. (2023-2025). Thermal-hydraulic modeling of the HotBENT experiment using the OpenGeoSys, HotBENT Partner Meetings, internal presentations.
[6] Tatomir et al. (2025). Thermo-Hydraulic Modelling of the In-Situ HotBENT Experiment: Investigating Bentonite Barrier Behaviour at High Temperature and Hydration. EGU General Assembly 2025.
How to cite: Gerasimov, T., Simo, E., Burlaka, V., Polster, M., Tatomir, A., and Liebscher, A.: Comparative studies of sealing capacity of Wyoming MX-80 and BCV bentonites: the HotBENT in-situ experiment and thermo-hydro-mechanical simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9938, https://doi.org/10.5194/egusphere-egu26-9938, 2026.
This study investigates the coupled hydro-mechanical-chemical (HMC) behavior and multiphase flow properties of Opalinus Clay – a potential caprock candidate for geologic carbon storage. A comprehensive series of laboratory tests is conducted to support the CO2 Long-term Periodic Injection experiment (CO2LPIE) project at the Mont Terri Underground Rock Laboratory in Switzerland, providing essential parameters for caprock characterization. Facies-dependent poroviscoelastic and transport properties are quantified: the sandy facies exhibit higher drained and unjacketed bulk moduli and permeability than the shaly facies, yet both facies display favorable long-term sealing potential with intrinsic permeability on order of ~10-20 m2 and breakthrough pressure of 2-4 MPa. Particular attention is given to the transport properties of the sandy facies under different testing scenarios including the experimental duration, pore pressure difference, fluid types, and saturation history. Long-term tests highlight exponential permeability reduction driven by time-dependent compaction, which is effectively described by a poroviscoelastic model coupled with a power-law porosity-permeability relationship. In contrast, CO2-rich water injection yields relatively stable permeability with only minor irreversible changes likely controlled by fluid-rock interactions, fluid affinity, and electrokinetic effects. A hydro‐mechanical‐chemical coupling framework is employed to evaluate the time‐dependent response of fluid‐saturated rock subjected to CO2 exposure. Carbonate mineral dissolution appears to play a key role in altering poroviscoelastic properties at experimental time scales of 3 to 5 weeks, so the HMC model is calibrated with the experimental data on limestones. The model predicts CO2 injection‐induced porosity changes by accounting for the competing processes of chemical dissolution and time‐dependent compaction. Two-phase flow tests further reveal that CO2 displaces water more effectively in the sandy facies, while CO2 relative permeability is insensitive to lithological differences. Overall, these findings demonstrate that heterogeneous Opalinus Clay retains strong sealing integrity under coupled hydro-mechanical-chemical conditions and provide critical laboratory insights that complement ongoing in-situ monitoring within CO2LPIE.
How to cite: Makhnenko, R., Kim, H., and Vilarrasa, V.: Coupled hydro-mechanical-chemical behavior of shale caused by CO2 injection in lab and pilot-scale experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10855, https://doi.org/10.5194/egusphere-egu26-10855, 2026.
Opalinus clay exhibits extremely low permeability together with pronounced swelling and self-sealing behavior upon hydration, making it a highly promising host rock for deep geological repositories. This study presents a dual-porosity hydromechanical model to simulate the permeability evolution during swelling. The coupled hydromechanical behavior of clay rocks is described by unsaturated flow using Richards’ equation and by a linear, saturation-dependent swelling deformation. The hydromechanical coupling between swelling and flow is realized via a strain-dependent permeability formulation. The hierarchical pore structure of clay rock is incorporated into the model by distinguishing between macro and micro pore domains, to which different van Genuchten parameters are assigned. The mathematical model is implemented in the open-source software OpenGeoSys. Experimental validation of the model is provided by flow-through multi-load swelling experiments. In these experiments, our samples were flowed through by applying a differential water pressure between the inflow at a central bore of the cylindrical samples, and the outflow at the mantle. At the same time, axial swelling strain was measured during stepwise mechanical unloading. The permeability evolution was determined by the measured outflow rate and pressure difference between the inflow and outflow. The model was calibrated by a dual-objective NSGA-II (Non-dominated Sorting Genetic Algorithm II) optimization, which simultaneously calibrated the strain and permeability evolution. The model effectively reproduces the observed swelling strain development at the different load stages, as well as short-term, unloading-induced permeability changes and a long-term self-sealing trend. These results highlight the capability of the proposed model to predict long-term hydro-mechanical evolution in clay rocks.
How to cite: Tian, M., Taherdangkoo, R., and Butscher, C.: Dual-porosity hydromechanical modeling of swelling processes in Opalinus clay, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2848, https://doi.org/10.5194/egusphere-egu26-2848, 2026.
Clay rocks are prime candidates for host rocks as stable geological formations with self-sealing properties and can provide a permanent barrier for repositories for radioactive waste [Yardley et al., 2016; Zwingmann et al., 2024]. We here report dry and wet clay deformation experiments using the Silurian Rochester Shale [RS] [Brett 1983, Zwingmann et al., 2019] and the Jurassic Opalinus Clay [OC] [Zwingmann et al. 2017] from the Mt. Terri laboratory, CH, which were selected based on different clay mineralogies. The RS consists mainly of illite (60%) and quartz (23%) whereas OC samples are characterized by higher kaolinite (35%), chlorite (11%), lower illite (30%) and quartz (13%). Our study investigates the generally unknown impact of physical deformation (shearing and grinding) on clay mineral composition and isotopic signatures. Laboratory deformation experiments were conducted to assess how mechanical comminution influences the compositional and isotopic signatures of the RS and OC samples. The study utilized different comminution techniques, applying both ball mill [BM] and McCrone mill [MC] for periods of 5 to 60 minutes between room temperature and 300 ºC. This multi-variable approach provides a comprehensive view on how physical and thermal processes influence isotopic signatures (Ar, ẟD) of RS and OC. In addition to dry experiments, pilot wet deformation experiments (5-30 min) involving both mill types were conducted on RS and OC clays with Bern and Fiji water selected because of their different isotopic hydrogen compositions.
Radiogenic Ar loss in the RS was time- and processing-dependent, ranging between ~ 15–56% in BM and ~ 32–80% in MC mill experiments. For the OC, the impact was reversed: BM induced significant loss (9–48%), whereas MC milling had minimal effect (2–14%).
For the RS, ẟD ‰ values decrease with dry MC milling from -61 (5 min), to -72 ‰ (30 min). Wet experiments using Bern water yield similar values (~-61‰) for all three milling times. The values for RS wet experiments with Fiji water range from -57 to -61 ‰ (5-30 min). Regarding the BM RS experiments, ẟD values vary more significantly from -66 to -81 ‰ (5-30 min).
For the dry OC MC experiments, a similar ẟD trend is observed ranging from -71 to -79 ‰ with increasing dry milling time. OC MC wet milling with Bern water shows relatively homogenous ẟD values ranging from -67 to -65 ‰, when using Fiji water from -67 to -70 ‰. The OC BM wet milling experiments with Bern water yield lower values ranging from -72, -to -87 ‰ (5-30 min).
Milling experiment data suggests that the mechanical interaction between hard framework minerals (quartz and feldspar) and the clay fraction drives the variation in Ar loss and affect hydrogen isotope compositions. Under water saturated conditions, the chemical impact of shearing is dimmed, suggesting that these chemical fingerprints may have the potential to indicate the relevance of a fault zone detected within the emplacement drifts of a DGR.
How to cite: Rahn, M., Zwingmann, H., Todd, A., Mulch, A., and Berger, A.: Impact of fault activity on the chemistry of clay-rich rocks in deep geological repositories: Effects of mechanical comminution on Ar and ẟD isotope composition in clay minerals, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16416, https://doi.org/10.5194/egusphere-egu26-16416, 2026.
The Mont Terri Project (MTP) is a globally unique underground rock laboratory, run by the Swiss federal administration swisstopo, independent of any implementer or regulator either in the nuclear field or CO2 storage or geothermal communities (https://www.mont-terri.ch/en). For over three decades, researchers, scientists, engineers and technicians from currently 22 European, American and Asian organisations have participated in international cooperation. More than 170 experiments were performed, three quarters completed and published. Currently, 46(47) experiments are running (https://www.mont-terri.ch/en/experiment-portfolio).
All partners – from B/CAN/F/FRG/JAP/NL/Spain/Switz/UK/USA – are responsible for the research programme, and that with the same rights and obligations. An annual programme is prepared and, on the recommendation of an advisory committee (“Commission de suivi”) – particularly on work safety and risk issues –, must be approved by the owner of the underground, the Canton of Jura.
Apart of the generic aim – to investigate the characteristics of argillaceous formations – novel drilling, measurement and evaluation techniques have been developed in MT, e.g. on pore water content, coupled hydraulic-mechanical simulation or micro-seismic methods. Technologies like container storage, backfilling and sealing of storage tunnels are tested. The MT team not only operates the lab but contributes to conceptualise and execute experiments as well as publish them in scientific journals (e.g., latest: Mosler et al. 2026, Bonitz et al. 2025).
This success might come to an end as – within the so-called “relief program 27” to save 300 million Swiss francs (MCHF) in the federal administration – the Swiss government, in April 2025, mandated its ministry in charge that “swisstopo examine the transfer of responsible operation of the Mont Terri rock laboratory to a third party outside the federal administration” (Federal Council 2025, transl. tf). The Swiss investment of 1.5 MCHF (to be saved according to the government’s idea) very well pays off for Switzerland though, as the research share of the Swiss administration is 14 per cent. The partners, on their side, have invested 120 MCHF so far.
The loss of knowledge and collaboration might have negative repercussions on several levels and in several fields, for instance:
- Research: danger to discontinue experiments on long-term safety or feasibility regarding geological disposal (of nuclear waste) as more than half (25) of the running 47 experiments have the focus on these aspects (MTP 2024, p. 14);
- Knowledge exchange: partners (or other players) may not be as willing as before to share knowledge;
- Reputation: internationally and nationally all actors may lose trust if the federal administration cuts the budget for MT; nuclear waste policy (disposal) or climate policy (Carbon Capture and Storage, CCS, net-zero target) are likely to be negatively affected;
- Acceptance: underground labs considerably contribute to public acceptance in the nuclear waste policy field (Mayer et al. 2023) which should also be investigated in CCS (Federal Council 2022, p. 11).
At any rate, the existing special arrangement of the MPT should be preserved despite any short-sighted austerity measures in the public sector – for scientific, environmental policy and public interest reasons.
References: on slides to be provided
How to cite: Flüeler, T.: The Mont Terri underground lab – edifying on research, regulation, reputation … and shortsightedness, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8203, https://doi.org/10.5194/egusphere-egu26-8203, 2026.
Understanding how granular salt compacts is essential for estimating the timescales required to seal backfilled galleries and shafts in radioactive waste repositories hosted in rock salt. It is well established that the presence of brine, even as thin liquid films on grain surfaces, strongly accelerates compaction by enabling fluid-assisted grain boundary diffusion, or pressure solution. For this reason, the addition of liquid brine to salt backfill in radioactive waste repositories hosted in rock salt has been considered as a means to accelerate compaction and reduce sealing timescales. However, the presence of liquid brine also enhances corrosion of the waste canister and promotes gas generation, which has therefore been used as an argument against the intentional addition of brine.
Previous studies on salt-moisture interaction show that adsorbed fluid films retain liquid-like properties down to relative humidities of at least 40%, suggesting that pressure solution can operate well below full saturation. However, compaction behavior at relative humidities below the deliquescence point of salt (about 75% RH) remains poorly constrained.
We present compaction experiments on fine-grained sodium chloride conducted under controlled relative humidities between 53% and 73%. Under these conditions, pressure solution is the dominant compaction mechanism, although compaction rates are 2-4 orders of magnitude lower than when pore spaces are fully saturated with brine. Nevertheless, our results demonstrate that humid air alone significantly accelerates the compaction of granular salt compared to dry conditions, with pressure-solution creep rates increasing systematically with relative humidity. These findings suggest that, in repository backfill, moisture from the surrounding host rock or from ventilation systems may be sufficient to induce pressure-solution-controlled compaction, even in the absence of intentionally added liquid brine or brine inflow from the host rock. Interaction between granular salt backfill and humid air therefore plays a key role in backfill evolution, with important implications for compaction rates, sealing timescales and the long-term containment of radioactive waste.
How to cite: van Oosterhout, B., Hangx, S., and Spiers, C.: Humid air as a driver of compaction creep in granular salt backfill: How a little water makes a big difference, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11800, https://doi.org/10.5194/egusphere-egu26-11800, 2026.
In the context of the disposal of high‑level nuclear waste (HLW) in clay rock formations, crushed clay‑rock‑based materials can be used to backfill repository drifts and shafts and to construct sealing elements, as their expansive clay content provides favourable properties, such as the development of swelling pressure and the reduction of hydraulic conductivity. The installation of backfill and sealing elements serves to limit the propagation of the excavation damaged zone (EDZ) by stabilizing the surrounding rock formation and by inhibiting fluid transport between the emplacement units and the accessible biosphere. Once installed, these elements complement the host rock and contribute to ensuring the long‑term integrity of the repository.
The behavior of backfill and sealing elements - particularly their volume‑change and fluid transfer behavior - is significantly controlled by the initial (or as‑compacted) dry density. This parameter, in turn, depends on the material’s mineralogy, grain‑size distribution, initial (or as‑compacted) water content, and the applied compaction energy.
This study presents a simplified approach using the compressibility index to derive the initial dry density of crushed clay‑rock‑based materials from the applied axial stress, initial water content, and expansive clay content.
The approach is validated by static compaction experiments in a drained oedometer setup. Prior to compaction, crushed clay rock is mixed with sodium bentonite at three wet‑weight ratios, and each mixture is prepared to six target water contents. The results show that, for low expansive clay contents, the compressibility index exhibits a quadratic dependence on the initial water content, which transitions to a linear dependence as the expansive‑clay content increases. Overall, the findings underline the relevance of this simplified approach for the installation of backfill and sealing elements, particularly with respect to the selection of appropriate installation techniques.
How to cite: Middelhoff, M., Kaufhold, S., Laurich, B., and Jantschik, K.: Predicting the compaction behavior of crushed clay rock-based materials, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16838, https://doi.org/10.5194/egusphere-egu26-16838, 2026.
The verification and validation of complex models for the numerical simulation of nonlinear, coupled multiphysical processes plays a central role in the preparation of reliable safety analyses, e.g. for the deep geological disposal of radioactive waste, but also for the design of geotechnical facilities, e.g. for the use of geothermal energy systems or energy storage.
The DECOVALEX project (Birkholzer et al. 2025, Kolditz et al. 2025) has been dedicated to the validation of coupled process models for many years, in particular using experimental data from various underground laboratories worldwide. New model developments and the corresponding model validations also play an important role in the European partnership project EURAD (Churakow et al. 2024).
The Model Hub introduces a new concept for benchmarking according to the FAIR principles (Bilke et al. 2025). It is a web platform on which benchmarks are jointly developed, tested and made available using Jupyter Notebooks. The aim is to develop an interactive platform for benchmarking and to be able to test results ‘live’. Pre- and post-processing is supported by the Python library OGSTools. Benchmarking can be performed online via Binder, with quality assurance provided through automation. The Model Hub concept is being developed as part of the DigBen project (BMFTR funding grant 03G0927) in close cooperation between UFZ, TUBAF and Federal Institute for Geosciences and Natural Resources (BGR), and is already being used in the DECOVALEX and EURAD projects.
References
Bilke, L., Fischer, T., Naumov, D. et al. (2025): Reproducible HPC software deployments, simulations, and workflows – a case study for far-field deep geological repository assessment. Environ Earth Sci 84, 502. https://doi.org/10.1007/s12665-025-12501-z
Birkholzer, J.T., Graupner, B.J., Harrington and et al. (2025): DECOVALEX-2023: An international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. Geomech. Energy Environ. 42 , art. 100685 10.1016/j.gete.2025.100685
Churakov, S.V., Claret, F., Idiart, A. et al. (2024): Position paper on high fidelity simulations for coupled processes, multi-physics and chemistry in geological disposal of nuclear waste. Environ. Earth Sci. 83 (17), art. 521 10.1007/s12665-024-11832-7
Kolditz, O., McDermott, C., Yoon, J.S. et al. (2025): A systematic model- and experimental approach to hydro-mechanical and thermo-mechanical fracture processes in crystalline rocks Geomech. Energy Environ. 41 , art. 100616 10.1016/j.gete.2024.100616
Model-Hub (mock-up): https://www.opengeosys.org/stable/hub/
OGSTools: https://ogstools.opengeosys.org/stable/
How to cite: Kolditz, O., Bilke, L., and Nagel, T.: Model-Hub – Automated and Interactive Benchmarking for Multiphysics Processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14931, https://doi.org/10.5194/egusphere-egu26-14931, 2026.
Assessing safety in a deep geological repository for radioactive waste requires a thorough evaluation of coupled thermal, hydraulic and mechanical (THM) processes. This allows the analysis of a possible reduction of the containment capacity of the host rock. For this purpose, numerical modeling is regarded as a necessary and powerful tool. Moreover, a distinctive characteristic of crystalline rock is its fractured nature. These fracture networks are expected to influence both the hydraulic and mechanical behavior of the system. Therefore, for a safety concept in crystalline rock, the influence of fractures on the containment of the radionuclides has to be considered. Here, the repository concept in fractured crystalline rock presented in (2), in which multiple smaller containment rock zones are used for the disposal of the nuclear waste.
This contribution aims to extend the numerical analysis concept for host rock integrity proposed in (2) by including an assessment of the possible risk of fracture reactivation. To this end, the numerical results obtained in (2) are taken as starting point, in particular the temporal evolution of the stress field. The calculated stresses are used to evaluate the deformation behavior at fracture locations (1), specifically for a comparison between dilation and shear. In this context, the evaluation of the German integrity criterion, which focuses on the safety-relevant dilatant behavior of the containment providing rock zone, is extended by the consideration of discrete fractures. Results show that, due to the emplacement of the nuclear waste in the crystalline rock, a change in the potential fault reactivation can be expected. The intensity of this change depends, among others, on fracture orientation and the development of the temperature field and is therefore transient.
References
1. Ferrill, D. A., Smart, K. J. and Morris, A. P. (2020): Resolved stress analysis, failure mode, and fault-controlled fluid conduits. Solid Earth, 11, 899–908, https://doi.org/10.5194/se-11-899-2020.
2. Guevara Morel, C., Thiedau, J. and Maßmann, J. (2025): Numerical assessment of the barrier integrity for a generic nuclear waste repository in crystalline rock. International Journal of Rock Mechanics and Mining Sciences 197, 106326, https://doi.org/10.1016/j.ijrmms.2025.106326.
How to cite: Guevara Morel, C., Thiedau, J., and Jobst Maßmann, J.: Using THM modelling to evaluate the potential reactivation of faults in a generic nuclear waste repository in crystalline rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7657, https://doi.org/10.5194/egusphere-egu26-7657, 2026.
This contribution presents an improved explanation for the field data from the the Deep Borehole experiment in Mont Terri underground research laboratory (MTDB) presented by Gonçalvès et al. [1]. The formulation of the THM process with thermo-osmosis (TO) is introduced and discussed. Field data and analytical solution are used to validate the implementation of the thermo-hydro-mechanical (THM) model with TO in multiphysical simulator OpenGeoSys Bilke et al. [2]. Furthermore, the improved explanation of the observed data from the MTDB experiment is presented and discussed. The closer match between field data and numerical and analytical solutions was achieved by inclusion of the spatial variability of the geothermal gradient and intrinsic permeability, and applying SciPy’s Dual Annealing optimizer [3]. With those extensions of the model, the observed data (pressure and temperature) is reproduced more closely and uncertainties has been reduced. The results from the original and this studies are compared in Fig. 1.
Figure 1: Comparison of the results from the original study by Gonçalvès et al. [1] and presented by the authors [4] (paper submitted for publication to a journal).
Acknowledgments
This work has been funded by the German Federal Office for the Safety of Nuclear Waste Management (BASE), ThORN project: “Experimental investigations on thermo-osmotic flow in argillaceous materials relevant to deep geological repositories for radioactive waste” (Grant number: 4723F00104).
References
[1] Julio Gonçalvès, Jean-Michel Matray, and Catherine Ji Yu. Assessing relevant transport processes in Opalinus Clay at the Mont Terri rock laboratory using excess-pressure, concentration and temperature profiles. Applied Clay Science, 242:107016, September 2023. ISSN 0169-1317. doi: 10.1016/j.clay.2023.107016.
[2] Lars Bilke, Dmitri Naumov, Wenqing Wang, Thomas Fischer, Feliks K. Kiszkurno, Christoph Lehmann, Jäschke Max, Florian Zill, Jörg Buchwald, Norbert Grunwald, Kristof Kessler, Ludovic Aubry, Maximilian Dörnbrack, Thomas Nagel, Lion Ahrendt, Sonja Kaiser, and Tobias Meisel. OpenGeoSys. Zenodo, January 2025.
[3] Pauli Virtanen, Ralf Gommers, Travis E. Oliphant, Matt Haberland, Tyler Reddy, David Cournapeau, Evgeni Burovski, Pearu Peterson, Warren Weckesser, Jonathan Bright, Stéfan J. van der Walt, Matthew Brett, Joshua Wilson, K. Jarrod Millman, Nikolay Mayorov, Andrew R. J. Nelson, Eric Jones, Robert Kern, Eric Larson, C J Carey, İlhan Polat, Yu Feng, Eric W. Moore, Jake VanderPlas, Denis Laxalde, Josef Perktold, Robert Cimrman, Ian Henriksen, E. A. Quintero, Charles R. Harris, Anne M. Archibald, Antônio H. Ribeiro, Fabian Pedregosa, Paul van Mulbregt, and SciPy 1.0 Contributors. SciPy 1.0: Fundamental algorithms for scientific computing in python. Nature Methods, 17:261–272, 2020. doi: 10.1038/s41592-019-0686-2.
[4] Feliks Kuba Kiszkurno, Fabien Magri, and Thomas Nagel. Learning from data - validation and improvement of modeling thermo-osmosis effects in THM simulations based on the Mont Terri Deep Borehole experiment, January 2026.
How to cite: Kiszkurno, F., Magri, F., and Nagel, T.: Learning from data - validation and improvement of modeling thermo-osmosis effects in THM simulations based on the Mont Terri DeepBorehole experiment , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7821, https://doi.org/10.5194/egusphere-egu26-7821, 2026.
The predictive capability of numerical simulations of flow fields, such as those used to assess radionuclide migration, strongly depends on the accuracy of the underlying pore-network geometry. Experimental validation of such simulations is limited by the availability of suitable techniques. In recent years, positron emission tomography (PET) has become a powerful method for investigating transport processes in porous media1. The use of tailored radiotracers enables the analysis of advective transport and diffusive fluxes in complex pore systems, providing time-resolved visualization and statistical evaluation of transport-controlling parameters, offering additional insight into surface reactivity2, 3.
Using complex fractured and mineralized host rock types relevant to underground radioactive waste repositories, we investigate flow-field heterogeneity at the laboratory scale4. We demonstrate that specific structural and compositional features, together with their pore-size distributions and pore-network geometries, control transport behavior. We incorporate these characteristics into generalized pore-network models and transport simulations to determine effective diffusivities through a multiscale upscaling workflow. PET measurements demonstrate the strong influence of parameter variability over multiple spatial scales for complex transport in fractured and mineralized crystalline rocks, with surface structures ranging from nanometers to millimeters governing breakthrough-curve behavior.
1Bollermann, T.; Yuan, T.; Kulenkampff, J.; Stumpf, T.; Fischer, C., Pore network and solute flux pattern analysis towards improved predictability of diffusive transport in argillaceous host rocks. Chemical Geology 2022, 606, 120997.
2Schöngart, J.; Lindemann, M.; Klotzsche, M.; Franke, K.; Fischer, C., Quantitative tomography of contaminant phytomobilization: β+ emitters 83Sr and 86Y as tracers of fission-product analog mobility. Journal of Hazardous Materials Advances 2026, 21, 100952.
3Schöngart, J.; Kulenkampff, J.; Fischer, C., Positron emission tomography quantifies crystal surface reactivity during sorption reactions. Chemical Geology 2024, 665, 122305.
4Zhou, W.; Kulenkampff, J.; Zuna, M.; Jankovský, F.; Butscher, C.; Kammel, R.; Schäfer, T.; Fischer, C., Variability of effective diffusivity in fractured and mineralized metamorphic host rock from Bukov URF, Bohemian Massif (CZ). Applied Geochemistry 2025, 193, 106574.
How to cite: Fischer, C.: Improved migration predictability in host rocks for radioactive waste by a combination of numerical and tomographic analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9082, https://doi.org/10.5194/egusphere-egu26-9082, 2026.
Evaluating the suitability of geological sites for storage of nuclear waste requires long-term safety assessments in different setups. For these, numerical simulations encompassing groundwater flow and contaminant transport under varying conditions, such as varying salinity, temperatures and pressures are indispensable. As flow patterns are significantly impacted when permafrost soils form or melt, and permafrost conditions could occur at the prospected sites during their usage, the freeze-thaw process should be included in models assessing long-term site safety.
In this work the general approach to modeling water-ice phase transitions for groundwater flow, benchmarked by the Interfrost project [1], was extended to be applicable to saline soils. The model considers the effect of freezing point depression on the phase transition and includes density-driven flow depending on temperature and salinity. The model was implemented and tested based on the ug4 simulation toolbox using a collocated vertex-based finite volume discretization and the adaptive time stepping method LIMEX. For solving the fully coupled system of partial differential equations an efficient linear solver with a geometric multigrid preconditioner was applied.
In numerical experiments, the effect of salinity was studied, and an accelerated melting of ice at lower temperatures due to freezing point depression was observed.
[1] Grenier et. al. "Groundwater flow and heat transport for systems undergoing freeze-thaw: Intercomparison of numerical simulators for 2D test cases." Advances in water resources 114 (2018): 196-218.
How to cite: Wittum, R., Nägel, A., Logashenko, D., and Wittum, G.: Hydrological Model and Numerical Simulation of Freeze-Thaw Processes in Saline Groundwater Flow, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9222, https://doi.org/10.5194/egusphere-egu26-9222, 2026.
In the study of radionuclide transport through sparsely fractured rock, time domain random walk algorithms are often used due to their low numerical dispersion and reduced computational cost. However, a limitation of these algorithms is that steady flow and geochemical conditions are often assumed, or rough approximations are used to simulate transient conditions. In this work, we show how a time domain random walk method that simulates transport through a fracture-matrix system can be extended to account for transient flow magnitude and transient geochemistry. The method is based on approximating those transient conditions using piecewise constant conditions, with sudden stepwise changes. Then, the effect of these stepwise changes can be simulated by interrupting the algorithm at the time of each change and sampling the current location of the particle. The changes in the flow and geochemistry can then be applied, and the algorithm can be resumed. The method has been implemented in the code Migration Analysis of Radionuclides in the Far Field (MARFA) for the case of transport through a fracture system with diffusion into a rock matrix of infinite extent. A few tests are simulated, and the obtained breakthrough curves are compared against a semi-analytical solution that we derive, as well as against equivalent models simulated using the PFLOTRAN code. Results show that the new generalization is a reliable approach to simulate solute transport under a wide range of flow and geochemistry conditions.
How to cite: Sanglas Molist, J., Trinchero, P., Painter, S., Selroos, J.-O., and Poteri, A.: Generalizing time domain random walk algorithms to account for transient flow and geochemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12947, https://doi.org/10.5194/egusphere-egu26-12947, 2026.
Worldwide, three host rocks are considered for hosting high‑level radioactive waste repositories: salt, clay, and crystalline formations. In crystalline rocks, a reliable, site‑specific representation of the fracture network is critical for safety assessment because fracture characteristics control both mechanical integrity and hydraulic response. In this study we use fracture data from three 10‑m drillholes at the Bedretto Underground Laboratory, Switzerland. The wells are spaced 2m-apart giving access to the local fracture network.
We present an automated workflow to generate discrete fracture networks (DFNs) from field observations. The workflow has two main steps. Step 1: orientation clustering — we fit a mixture model to fracture orientation data to derive orientation sets, then validate the fitted parameters against the field observations using statistical tests. Step 2: abundance calibration — we perform Monte‑Carlo simulations using the best‑fit orientation sets and filter realizations by comparing simulated per‑meter fracture counts to in‑situ observations.
Our results show that mixture models reliably recover orientation parameters, but stochastic simulations are sensitive to the random seed; we therefore recommend ensemble simulations and sensitivity analysis. Combined with the proposed fracture‑count calibration, our approach produces robust, site‑specific DFN realizations suitable for numerical safety assessments and hydraulic or mechanical modelling of fractured rock mass.
How to cite: Kelka, U., Müller, C., Monnamitheen, A., and Herold, P.: Automated Generation of Site‑Specific Discrete Fracture Networks: Mixture‑Model Orientation Clustering and Fracture‑Count Calibration, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17621, https://doi.org/10.5194/egusphere-egu26-17621, 2026.
In Germany, the Federal Company for Radioactive Waste Disposal (BGE) is responsible for implementing and performing the Site Selection Procedure, which is regulated by law, for a repository for high-level radioactive waste. The procedure is organized in three phases. At the end of Step 1 of Phase I, ninety sub-areas with favorable geological conditions for safe disposal for one million years were identified. These sub-areas cover approximately 54 % of Germany and include three different host rocks: claystone, rock salt (halite), and crystalline rock.
In Step 2 of Phase I, the ninety sub-areas are currently reduced to a limited number of smaller areas that are suitable for exploration, so-called siting regions. Within this step, representative preliminary safety analyses (rvSU) are applied. Essentially, these evaluate whether the safe containment of radioactive waste can be achieved. As stipulated in the regulatory framework, a maximum limit of a fraction of 10-4 in total and a fraction of 10-9 annually of both the mass and number of atoms over 1 million years is allowed to be released outside the main barrier of the repository system. Quantitative assessment is based on a vertical 1D finite-differences code for modeling the transport of radionuclides in the subsurface. This method, however, is currently suitable for claystones only. Rock salt is considered impermeable, and the models do not apply with the data at hand. For crystalline rocks, the available data in Step 2 of Phase I is not sufficient for reliable transport modeling as well. As an alternative approach, a qualitative evaluation method has been developed. The EVENT method (Evaluation of developments in the rvSU) evaluates whether geogenic processes have an influence on the safety functions of the geological barriers (host rock and overburden) within the assessment period of one million years. Safety functions are defined within the preliminary safety concept. They include the geometry – for example, thickness – or hydraulic properties of the barriers. Geogenic processes include processes such as glacial processes, erosion, or volcanism. FEP (features, events, processes) catalogues are used to structure the interactions and dependencies of processes and components (barriers). To account for climate evolution, the one-million-year assessment period is subdivided into four periods: the container cooling, the remainder of the current interglacial, the first glacial, and the rest of the assessment period. Continuation of the glacial cycles as in the Pleistocene is expected.
For each period, the impact of each process on the safety functions is evaluated. Not all processes will take place during all periods. Processes may have a positive or negative effect on the safety functions. Positive effects are documented but not considered further. Negative impacts are classified as “negligible,” “significant,” or “very significant,” and justified. A very significant negative impact or a considerable number of significant negative impacts indicate safe containment is not ensured for the area.
Assessments are carried out first for each host rock and are specified and adjusted for each area. All assessments are documented and stored in a sophisticated in-house database.
How to cite: Rühaak, W., Messerschmidt, Y., Fahrenholz, C., Förster, B., Mayer, K.-M., Panitz, F., Rübel, A., Wengorsch, T., Kreye, P., Bartetzko, A., and Wolf, J.: A systematic approach to assess the impact of processes on the safety functions in a repository system for high-level radioactive waste, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7787, https://doi.org/10.5194/egusphere-egu26-7787, 2026.
In Germany, the search for a repository for high-level radioactive waste aims to identify the most suitable location for a deep geological repository in one of three types of host rock: salt, clay or crystalline rock. To this end, the Federal Company for Radioactive Waste Disposal (BGE) is conducting a series of increasingly refined safety assessments. In the upcoming Phase II of the site selection process, these assessments will be carried out at the level of several siting regions. Surface exploration will take place alongside the safety assessments.
A key aspect of these safety assessments is the numerical simulation of coupled thermal, hydraulic, mechanical and chemical (THMC) processes within the repository system. In the BGE-funded OpenWorkFlow project (Lehmann et al., 2024), we are developing automated simulation workflows to support this. These workflows will enable the efficient analysis of different siting regions and the easy variation of model parameters and geometries. For example, they will facilitate uncertainty analyses and modelling of different scenarios (features, events and processes, FEPs), as well as the quick adoption of data updates during the site selection process. Furthermore, automation ensures the reproducibility of analyses.
This contribution provides an overview of the current development status of the OpenWorkFlow platform. Among other things, we discuss the modularity of workflows. We demonstrate how various (partial) couplings of the THMC processes and the necessary parameterisations relevant to different scenario simulations and queries are implemented at workflow level. We present our approach to long-term workflow maintenance (Bilke et al., 2025). Finally, we discuss the traceability and verifiability of our workflows.
References
Bilke, L., Fischer, T., Naumov, D. et al. (2025): Reproducible HPC software deployments, simulations, and workflows – a case study for far-field deep geological repository assessment. Environ Earth Sci 84, 502. https://doi.org/10.1007/s12665-025-12501-z
Lehmann, C., Bilke, L., Buchwald, J. et al. (2024): OpenWorkFlow—Development of an open-source synthesis-platform for safety investigations in the site selection process. Grundwasser - Zeitschrift der Fachsektion Hydrogeologie 29, 31–47. https://doi.org/10.1007/s00767-024-00566-9
How to cite: Lehmann, C., Kolditz, O., Nagel, T., Behrens, C., Kreye, P., and Rühaak, W. and the OpenWorkFlow-Team: OpenWorkFlow – Automated Simulation Workflows for Safety Assessments of Nuclear Waste Repositories, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21089, https://doi.org/10.5194/egusphere-egu26-21089, 2026.
For a deep geological repository (DGR) for radioactive waste, the stress field is a key parameter for the assessment of potential siting regions, the DGR design, and the evaluation of its long-term safety. Therefore, a 3-D description of the stress state (orientation and magnitudes) is required not only for the repository host rock, but also for the under- and overlying formations that act as additional geological barriers. The main objectives of the project SpannEnD are to predict the spatial variability of the present-day stress state in Germany and to develop the tools that are required to set up and calibrate 3-D geomechanical models. Furthermore, we also developed a statistical framework that uses such models to investigate the number of stress magnitude data that are needed for a robust model calibration, and to test at which depth and in which lithologies microhydraulic fracturing and sleeve re-opening tests should be conducted that is used to derive the magnitudes of the minimum and maximum horizontal stress magnitudes Shmin and SHmax, respectively.
Another objective is to investigate all relevant aspects of the stress state for Germany on a supra-regional scale. To describe the 3-D stress state with geomechanical-numerical models, stress data for the model calibration are required in combination with the knowledge of the geological structures of the subsurface and the rock properties. The orientation of SHmax has been compiled for decades in the World Stress Map database using a wide range of stress indicator. As part of the SpannEnD project, we updated the dataset for Germany and its surroundings to 1573 data records. For the compilation of stress magnitude data we developed for the first time a quality-ranking for the stress magnitude data applied it to the new open access German database with 1330 stress magnitude data records. These datasets are used to calibrate a large-scale 3-D geomechanical model covering the entirety of Germany. The numerical model is based on a comprehensive geological model that integrates all the structural information from the federal geological surveys and other publicly available sources. This model is populated with the rock properties of the individual lithological units and calibrated using the compiled stress data. The resulting geomechanical model for Germany enables an initial assessment of the crustal stress state for the entire country and can be used to investigate the impact of supra-regional structures. Furthermore, the first-order stress predictions are used to define initial conditions for regional-scale models.
In addition to stress data compilation and geomechanical modelling, the project also compiled a fault geometry database for Germany which, in combination with the model results, allows the prediction the slip tendency for these faults and a distribution function for all potential fault orientations. Furthermore, the usage of sub-modelling techniques, the impact of faults on the stress state and uncertainties in the predicted stress state due to the material properties and the calibration data has been investigated. The presentation will provide an overview of the main achievements of the SpannEnD project over the past eight years.
How to cite: Reiter, K., Ahlers, S., Röckel, L., Kuznetsova, V., Laurelle, L., Velagala, L. S. A. R., Heidbach, O., Ziegler, M., Henk, A., Müller, B., Hergert, T., and Schilling, F.: Stress state from data and modelling in the site selection process for a deep geological repository for radioactive waste in Germany - The SpannEnD Project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13634, https://doi.org/10.5194/egusphere-egu26-13634, 2026.
In [1] indicators have been developed to assess confinement and isolation of radionuclides within the essential barriers of a deep geological disposal repository (containment-providing rock zone and geotechnical barriers respectively according to § 23 (4) of the German site selection act). In total, nine indicators have been proposed which are related to different physical reference quantities. Functionality and impact of the indicators were illustrated by means of transport and dispersion calculations for a generic disposal system in clay, based on simplifying assumptions. As a basis for the calculations (using the computer programs MARNIE and TOUGH2-GRS), the inventory of high-level radioactive waste in Germany derived within the VSG (Vorläufige Sicherheitsanalyse Gorleben) [2] and parameter bandwidths of the geoscientific weighting criteria of the site selection act were assumed. The afore mentioned research work [1] was important for the justification of § 4 (5) of the safety requirement ordinance [3].
In the work presented here the prognosed inventory from the VSG in 2011 [2] was updated with the adjusted amount of high-level waste due to the shutdown of all nuclear power plants in Germany and other smaller changes in the nuclear waste management chain. In addition, transparent selection criteria for the radionuclides taken into account within the calculation were proposed. Subsequently, the assumptions of [1] were analysed according to their inherent uncertainties. E.g. the assumption of a linear sorption term for the nuclides overestimates the sorption capacity of the containment-providing rock zone and the assumption of an instant release of the full inventory overestimates the amount of radionuclides being transported. It is one goal of the work to deepen the understanding of the consequences of the inherent uncertainties of such assumptions. With the open-source reactive transport code PFLOTRAN, further developed at BASE, selected calculations from [1] were carried out again to ensure the reproducibility of the results with a different modelling tool. From the analyses of the afore mentioned uncertainties new assumptions had been derived and updated calculations were carried out to demonstrate the applicability of the indicators. Based on the parameter bandwidth of the geoscientific weighting criteria of the site selection act, a systematic sensitivity and uncertainty analyses was carried out. Similar to the findings in [1], the presented results may contribute to a possible review and enhancement of the rules and standards for nuclear waste disposal.
References
| [1] | Navarro, M., Weyand, T., Eckel, J. & Fischer, H. (2019): Indikatoren zur Bewertung des Einschlusses und der Isolation mit exemplarischer Anwendung auf ein generisches Endlagersystem mit dem Wirtsgestein Tongestein, GRS-A-3985. |
| [2] | Peiffer et al. (2011): Abfallspezifikation und Mengengerüst, GRS-274, GRS-278. |
| [3] | Deutscher Bundestag (2020): Drucksache des deutschen Bundestages, 19/19291. |
How to cite: Eckel, J., Weyand, T., Frieling, G., and Navarro, M.: Evaluation of uncertainties related to isolation and containment of a containment-providing rock zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9534, https://doi.org/10.5194/egusphere-egu26-9534, 2026.
The KIMoDA (Artificial Intelligence for Modelling Diffusive/Advective Flow in Porous Media) project aims to explore the opportunities and limitations of applying AI-driven simulation methods for the methodological development and evaluation of modelling approaches relevant to long-term safety assessments of deep geological disposal of high-level waste in Germany. The project investigates transient advection and diffusion processes in porous and fractured media, across kilometer-scale domains and timeframes of up to 1,000,000 years, where the use of conventional numerical solvers is computationally challenging.
On the technical side, KIMoDA evaluates AI-based surrogates and hybrid models, including deep learning architectures and physics-informed neural networks (PINNs), and, for the same calculation case, compares the results of these models against the results obtained using PFLOTRAN, an open-source solute transport simulation code. To this end, standardised reference cases are developed for the three host rock types considered in the German site selection process: claystone, rock salt and crystalline rock. Performance is assessed using metrics such as RMSE, R², and maximum norm error.
Interwoven with the development and benchmarking of AI surrogates and hybrid models, KIMoDA assesses socio-technical and ethical risks and opportunities that may arise when different AI systems are considered or discussed by experts and officials embedded in organisational, societal, and political contexts. Taking key requirements of the German Site Selection Act (StandAG) as a reference framework—traceability, reproducibility, accountability and participation, and precaution under deep uncertainty – the analysis examines how model properties (e.g., data dependence, opacity, bias, non-determinism) may interact with data governance, validation practices and use-patterns to shape trustworthiness. Explainable AI, sensitivity analyses, and targeted visualisations are applied to strengthen auditability and communicability of results for safety-critical decision-making. By uniquely combining AI modelling with socio-technical and ethical perspectives, KIMoDA aims to contribute to the development of methodological approaches relevant to reproducible, explainable, and publicly credible AI-supported safety cases in nuclear waste management.
How to cite: Nardi, A., Gailhofer, P., Kampffmeyer, N., Pekala, M., Prasianakis, N., Iraola, A., Ambikakumari Sanalkumar, K., Mosquera, X., Trinchero, P., Kock, I., Schopmans, H., and Magri, F.: KIMoDA: Opportunities and Limitations of AI-Driven Simulation Methods for Supporting Long-Term Safety Assessment of Deep Geological Disposal, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13329, https://doi.org/10.5194/egusphere-egu26-13329, 2026.
A systematic understanding of microcracking behavior and fracture properties in granite is essential for rational design and long-term stability assessment in deep underground engineering applications, including high-level radioactive nuclear waste geological disposal and geothermal energy exploitation. Among the three fundamental fracture modes, Mode I fracture is the most prevalent. To investigate the influence of thermal treatment on the Mode I fracture behavior of brittle rocks, notched semi-circular bend (SCB) specimens of Flossenbürger granite are thermally treated at seven target temperatures (25–350 °C) and subjected to two cooling methods (air cooling and water cooling), followed by semi-circular bend tests. Acoustic emission (AE) counts and cumulative energy are adopted to track internal microcrack evolution, while full-field displacement/strain measurements from digital image correlation (DIC) are used to identify fracture initiation, fracture process zone (FPZ) development, and the critical crack opening displacement. The combined AE–DIC approach provides multi-source cross-validation and supports quantitative interpretation of damage evolution and fracture mechanisms. Microscopic observations of granite thin sections are further performed to assess thermally induced microcracks under different scenarios and to relate microscale damage to the macroscopic response.
Keywords: Thermal treatment; Digital image correlation; Acoustic emission; Mode I fracture characteristics.
How to cite: Zhou, Q. and Thuro, K.: Experimental study on mode I fracture characteristics of heated Flossenbürger granite with different cooling methods using DIC and AE , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6655, https://doi.org/10.5194/egusphere-egu26-6655, 2026.
The final disposal of high-level radioactive waste in crystalline host rock poses particularly high requirements on the integrity of engineered and geo-engineered barriers, especially on the waste canister. Therefore, it is essential to understand and be able to predict the corrosion behavior of waste canisters and the implications for the release of radionuclides to the host rock.
This work shortly summarizes various approaches to model the corrosion of canisters and highlights a modeling approach on the release of radionuclides. Details are given in (Bracke & Philipp 2025).
Numerous approaches with different complexity exist for the modelling of corrosion mechanisms for potential canister materials. Widely-used models are based on empirical approaches. Some models include mass balance or mass transport. Beyond that, very complex mechanistic models (such as reactive transport models) exist, which are crucial for the demonstration of process understanding within the safety case, justifying the simplifications made in larger numeric models.
When a potential failure of (geo-)engineered barriers occurs, radionuclides could be released into the host rock. This work considers that parts of the barriers are still present after a failure. These parts may retard or delay the release of radionuclides or contain a fraction. This would imply that an instant and complete release of all radionuclides to the geosphere does not take place after a failure of all barriers. The modeling approach uses hypothetical functions and hypothetical parameter values to quantify this retardation potential. As a first approximation e-functions were used to simulate some underlying processes for retardation of radionuclide in different technical barriers. These functions describe the onset time of failure and the time-dependent extent/size of barrier defects of zircaloy hulls and outer canisters. Sorption and diffusion processes in corrosion products of the inner canister and in bentonite were simulated using the Fick’s law with a realistic but hypothetical apparent diffusion coefficient.
The modeling results show as a proof of principle that the consideration of a retention potential of failed barriers, especially sorption on canister corrosion products and the buffer, can have a significant effect on overall radionuclide release to the host rock. Since the modeling used hypothetical functions and parameter values the real retardation potential of failed technical barriers and the subsequent radionuclide release still needs to be developed and evaluated in detail.
Bracke, G. & Philipp, T. (2025): Konzepte und Testrechnungen zur Vorhersage der Behälterintegrität und des Einschlussvermögens im Kristallingestein. Forschungsbericht 3 von 3 zum Projekt „Langzeitintegrität von Behäl-tern in Kristallingestein“ (LaKris). Forschungskennzahl 4722B10401, Bundesamt für die Sicherheit der nuklearen Entsorgung (BASE). BASE-Forschungsbericht, BASE-008/25: Berlin, August 2025.
How to cite: Bracke, G. and Philipp, T.: Modeling the release of radionuclides to the host rock considering retardation by failed (geo‑)engineered barriers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7073, https://doi.org/10.5194/egusphere-egu26-7073, 2026.
This contribution addresses the understanding of the observed time-dependent behavior of Opalinus Clay (OPA) due to excavation. During six years of the CD-A experiment at the Mont Terri Rock Laboratory (Switzerland), data from two parallel, unsupported niches (approximately 11 m long and 2.3 m in diameter) was collected. The twins were excavated in 2019, with one is exposed to seasonal climate fluctuations (leading to desaturation of the clay) andthe other sealed to maintain high humidity. Long-term measurements of deformation using extensometer and laser scans show a non-elastic convergence trend, especially in the open niche. This certainly cannot be explained by elastic models, but driven by a mechanical creep mechanism or by hydraulic-mechanically (HM) coupling. Comparisons of the behavior of the two niches provide useful insights here. Further measurements of hydraulic parameters, as well as geophysical surveys and geological characterization, support the investigation. Although many long-term measurements are available today, a clear statement cannot be made yet. One reason for this is the complex behavior of the OPA, especially in the unsaturated zone affected by excavation, where shrinkage effects are pronounced and damage and inhomogeneities occur. Coupled numerical HM modeling is an important tool for gaining further insights.
In addition to the initial outcomes (Ziefle et al., 2022), this contribution provides an update. The 3D HM model has been significantly improved, and now includes a more accurate representation of the equilibrium state prior to excavation, time-dependent excavation processes, and a physics-based description of the swelling behavior. Model validation against extensometer data establishes the model as a robust basis for further investigations, including parameter studies and testing of alternative constitutive models.
Ziefle, G. et al (2022): Multi-disciplinary investigation of the hydraulic-mechanically driven convergence behaviour: CD-A twin niches in the Mont Terri Rock Laboratory during the first year. Geomechanics for Energy and the Environment 31.
How to cite: Maßmann, J., Ziefle, G., Cajuhi, T., and Chen, S.: Time-Dependent Behavior of Opalinus Clay: Insights from six Years of CD-A Experiment at Mont Terri, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9656, https://doi.org/10.5194/egusphere-egu26-9656, 2026.
The assessment of potential repository sites for high-level radioactive waste requires a robust understanding of coupled thermo-hydro-mechanical (THM) processes, particularly in the near field of heat-emitting waste. In host rocks, near-field evolution is characterized by an initial desaturation phase caused not only by thermal loading and deformation, but also by repository construction processes such as excavation, ventilation, and the emplacement of initially unsaturated engineered barrier materials (e.g., bentonite). This phase is followed by long-term re-saturation governed by strongly coupled and nonlinear THM processes. These dynamics are critical for integrity-related criteria and are therefore highly relevant for safety assessment and repository design.
High-fidelity numerical simulations (see, e.g., [1]) provide detailed insights into near-field THM processes, but typically produce large datasets that are difficult to analyze systematically, particularly when uncertainties in material properties and process parameters are taken into account. As the number of uncertain inputs increases, the resulting parameter space becomes increasingly challenging to explore using conventional post-processing and sensitivity analysis workflows.
In this contribution, we present first results on interactive parameter-space exploration of near-field simulations under uncertainty. A surrogate model is constructed from a set of deterministic THM simulations, capturing the full temporal and spatial evolution of primary variables without prior data reduction. This surrogate enables rapid evaluation of simulation results for arbitrary combinations of uncertain input parameters within predefined physically plausible ranges [2].
Building on the surrogate model, an interactive dashboard is developed using the plotting ecosystem Makie.jl [3]. The dashboard allows users to explore the influence of thermal, hydraulic, and mechanical parameter uncertainties on near-field evolution in real time. It supports both qualitative and quantitative assessment of desaturation and re-saturation dynamics, as well as their implications for integrity-related criteria, across the full physical and temporal domain.
The presented dashboard emphasizes transparency and comprehensibility. It provides a systematic and reproducible framework for investigating sensitivities, parameter interactions, and plausible ranges of near-field system behavior, thereby supporting safety assessments and the development of robust repository concepts.
[1] Kolditz, O., et al. (2012). OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media. Environmental Earth Sciences 67 (2012): 589-599.
[2] Bittens, M. (2024). OpenGeoSysUncertaintyQuantification.jl: a Julia library implementing an uncertainty quantification toolbox for OpenGeoSys. Journal of Open Source Software, 9(98), 6725.
[3] Danisch, S., & Krumbiegel, J. (2021). Makie.jl: Flexible high-performance data visualization for Julia. Journal of Open Source Software, 6(65), 3349.
How to cite: Bittens, M. and Thiedau, J.: Interactive Exploration of Thermo-Hydro-Mechanical Near-Field Processes for Repository Site Selection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10498, https://doi.org/10.5194/egusphere-egu26-10498, 2026.
As part of the ThORN project, 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.
This project aims to fill an existing gap in the literature regarding the impact, importance and parameterization of TO in the context of nuclear waste storage in clay rocks. An in-situ experiment is being developed in order to address this gap. The design and evaluation of all stages of the experiment will be supported by numerical simulations in the open source-simulator OpenGeoSys [1]. Parallel to OpenGeoSys, COMSOL Multiphysics was used for verification. The results obtained from both were compared and iterated on. This allowed to catch and correct errors and ensure the assumptions and parameterization are consistent between the institutions participating in the ThORN project. The resulting models will be used to analyze near and far field effects in a repository environment.
This contribution delivers an overview of the highlights and conclusions from the experiment design phase. The parameters and assumptions on which the design is based are presented and discussed. Predictive simulations of the design are presented with focus on the pressure development and water flows. We show how the numerical simulations can aid in exploring the potential experiments of the
physical experiments before they are build.
Acknowledgments
This work has been funded by the German Federal Office for the Safety of Nuclear Waste Management
(BASE), ThORN project: “Experimental investigations on thermo-osmotic flow in argillaceous
materials relevant to deep geological repositories for radioactive waste” (Grant number: 4723F00104).
Reference
[1] Lars Bilke, Dmitri Naumov, Wenqing Wang, Thomas Fischer, Feliks K. Kiszkurno, Christoph
Lehmann, Jäschke Max, Florian Zill, Jörg Buchwald, Norbert Grunwald, Kristof Kessler, Ludovic
Aubry, Maximilian Dörnbrack, Thomas Nagel, Lion Ahrendt, Sonja Kaiser, and Tobias
Meisel. OpenGeoSys. Zenodo, January 2025.
How to cite: Kiszkurno, F., Magri, F., de La Vaissière, R., and Plua, C.: ThORN – in-situ experiment design for the investigation of the relevance of thermo-osmotic flow in clay for radioactive waste disposal, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10945, https://doi.org/10.5194/egusphere-egu26-10945, 2026.
Computational analysis of host rock integrity is an important tool for assessing the long-term safety of potential final repository sites during site selection process. To ensure the safe operation of a repository and the integrity of the barrier host rock, the temperature increase in the host rock relevant to the containment must be limited. Therefore, knowledge of the current temperature distribution in the subsurface is a prerequisite for designing the final repository.
The focus of this work is on stochastic FE computations for typical geological conditions in sedimentary host rocks in Northern Germany. Using generic models of varying complexity, comparative computational analyses have been carried out to investigate the effect of uncertainties in thermal model parameters and boundary conditions on the transient temperature field. Another focus is set on the thermal effects of permafrost conditions on the temperature field during potential future cold phases, e.g., the penetration depth of permafrost. To evaluate these effects, a temperature-time profile of past climate has been used as the upper boundary condition of the model. This profile was calculated based on a global proxy of a Pliocene–Pleistocene stack of benthic δ18O isotope records. For the lower boundary condition, a heat flow density has been imposed along the base of the model.
The effect of thermal model parameter uncertainty has been quantified and analysed within three applications considerung different host rocks: [1] 1D-vertical model with parameter variations covering different rock types, [2] 2D-vertical model with claystone in the Lower Cretaceous as host rock and [3] 2D-rotationally-symmetric model of a salt diapir with rock salt as host rock. In [2] and [3] the heat power generated by radioactive waste is accounted for in the simulations via a source term.
For the thermal parameters, which reflect a representative state of knowledge for Northern Germany, uniform distributions are assumed. During the numerical analysis of the generic repository systems [2,3], numerous simulation runs are performed to capture all possible manifestations of the uncertain thermal parameters and their combinations. By statistical evaluation, the impacts of uncertainties on the calculated results can be quantified. These results may also help to assess the robustness of a final repository system.
References
Liu, W., Völkner, E., Minkley, W. & T. Popp (2017): Zusammenstellung der Materialparameter für THM-Modellberechnungen (Ergebnisse aus dem Vorhaben KOSINA) – Ergebnisbericht; Hannover (BGR).
Lisiecki, L.E. & M.E. Raymo (2007). Plio–Pleistocene climate evolution: trends and transitions in glacial cycle dynamics. Quaternary Science Reviews, 26, (1–2): 56-69. https://doi.org/10.1016/j.quascirev.2006.09.005.
Maßmann, J., Thiedau, J., Bittens, M., Kumar, V., Tran, Tuong Vi, Guevara Morel, C.,Kneuker, T. & S. Schumacher (2022): ANSICHT-II – Methode und Berechnungen zur Integritätsanalyse der geologischen Barriere für ein generisches Endlagersystem im Tongestein – Ergebnisbericht; Hannover (BGR). https://download.bgr.de/zsn/201997/201997_01.pdf.
How to cite: Noack, V., Bittens, M., Maßmann, J., and Frenzel, B.: Effects of long-term climate variation in numerical thermal models under uncertainties – analyses for the site selection process, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11386, https://doi.org/10.5194/egusphere-egu26-11386, 2026.
Reliable safety assessments of deep geological repositories require transparent, reproducible exploration of a wide range of processes, design options, and site conditions. To address this need, we developed a fully automated simulation workflow for coupled thermo-hydro-mechanical (and chemical) processes using the open-source code OpenGeoSys (OGS), within the scope of the OpenWorkFlow research project.
The workflow integrates newly developed tools within the OGSTools package and builds on OGS’s transparent modelling framework to support reproducible simulations: automated generation of simulation-ready meshes from CAD geometries with symbolic identifiers for repository components; consistent assignment of thermo-hydro-mechanical-chemical material properties from a dedicated database into OGS input files; and standardized post-processing routines for multiphysics result evaluation. This approach enables script-based model creation and simulation with minimal manual effort.
We focus on near-field analyses of vertical sections through generic repository prototypes, allowing evaluation at arbitrary positions. Multiple process couplings (HM, THM, TH2M, etc.) are supported, and large ensembles of model variants can be generated, facilitating systematic variation of design parameters, material data, or boundary conditions, and enabling both sensitivity and uncertainty analyses.
The workflow has been applied to repository configurations in the Northern Lägern site region. Automated simulations reproduced system behaviour reported in previous studies and showed consistency with results from other modelling groups, demonstrating both the validity of the workflow and its applicability to safety-relevant assessments.
Overall, this study illustrates the potential of open-source, automated multiphysics workflows to enhance transparency, reproducibility, and efficiency in repository performance evaluation, and to provide a versatile platform for comparing design alternatives across host rocks and site conditions.
How to cite: Grunwald, N., Wang, W., Naumov, D., Lehmann, C., Buchwald, J., and Kolditz, O.: OpenWorkFlow - Reproducible Near-Field Multiphysics Simulations of Radioactive Waste Repositories, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12021, https://doi.org/10.5194/egusphere-egu26-12021, 2026.
Geomechanical-numerical modeling aims to provide a continuous description of the stress tensor within rock volumes by leveraging localized stress magnitude data for model calibration. This calibration process involves optimizing displacement boundary conditions to achieve the best possible alignment with in-situ stress measurements obtained from microhydraulic fracturing (MHF) and sleeve reopening (SR) test in boreholes providing magnitudes of the minimum and maximum horizontal stresses. However, the high cost associated with acquiring stress magnitude data often results in sparse and incomplete datasets, which can potentially hinder a robust model calibration.
We investigate the relationship between calibration dataset size and stress field prediction quality by leveraging an exceptionally high-quality set of 45 in-situ stress magnitude data from MHF and SR tests performed in two boreholes in the Zürich Nordost region, a potential deep geological repository location in northern Switzerland. We present a statistical framework that analytically evaluates extensive model ensembles, where each realization uses a distinct subset of the entire stress magnitude dataset for the model calibration. This methodology quantifies the effect of data quantity on modeled stress distributions in lithological units with different elastic properties by efficiently computing stress states for multiple data combinations.
By systematically comparing modeled stress magnitudes derived from incrementally larger calibration datasets, we identify the minimum number of stress magnitude data records needed to achieve a modeled stress range narrower than the predicted stress range resulting from rock stiffness variability. For Zürich Nordost, approximately 15 stress magnitude data records are sufficient to reach this threshold, beyond which additional data provides only minimal improvement to model predictions.
Examination of the dataset revealed one measurement that deviated from the expected model, due to localized variations in the mechanical properties of the rock. Although this observation is geologically valid, it substantially distorts predictions when there are few calibration points available. However, its distorting effect weakens progressively as the size of the calibration dataset increases. Our analytical framework also enables the systematic detection of such anomalous records, which in turn affects the minimum number of data points required for model calibration.
Our findings emphasize the importance of dataset size and composition in reducing uncertainties and help establishing a practical methodology for efficient data acquisition planning. These insights are particularly valuable for subsurface engineering applications that require reliable stress estimates, such as geothermal systems, carbon storage facilities, radioactive waste repositories and underground energy infrastructure.
How to cite: Laruelle, L., Ziegler, M. O., Reiter, K., Heidbach, O., Desroches, J., Giger, S. B., and Degen, D.: How much stress data do we need for a robust geomechanical integrity assessment?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12723, https://doi.org/10.5194/egusphere-egu26-12723, 2026.
Safety assessments of deep geological repositories for high level radioactive waste require a robust quantitative and qualitative characterization of the future repository environment over time horizons up to one million years. For instance, in Germany this time-frame spans approximately ten glacial and interglacial cycles, during which ice sheet advance, permafrost formation, and thawing can repeatedly modify the thermal and hydraulic regime of the overburden from near surface to intermediate depths. For assessing radionuclide-transport during glaciation the necessary parameters for process based numerical predictions are required and have to be obtained from laboratory measurements under controlled conditions.
In this study, an improved rock volumetric deformation testing system is used to measure freezing induced deformation and hydraulic response. System and coolant effects were calibrated using an stainless steel standard. Freeze thaw tests were performed on consolidated and unconsolidated materials to obtain temperature dependent deformation behavior, water migration characteristics during freezing, and the evolution of permeability with freeze thaw cycle number.
These experimentally derived relations were then used to constrain a coupled thermos-hydraulic numerical framework implemented via a FEFLOW plug in. The model resolves liquid water and ice phase change in saturated porous media, incorporates nonlinear temperature dependent hydraulic conductivity and effective thermal properties, and represents lateral groundwater flow driven by hydraulic gradients. Permeability levels and their freeze thaw induced evolution were parameterized using the laboratory results, and the simulated stage wise redistribution of water during cooling was evaluated against the measured inlet and outlet flux signatures. The calibrated framework was applied to quantify how advective heat transport associated with lateral flow controls the timing, persistence, and spatial heterogeneity of the freezing front.
How to cite: Sheng, H., Schedel, M., Pham, H., Schüth, C., Sass, I., and Rühaak, W.: Freezing induced deformation and hydraulic response of saturated rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14579, https://doi.org/10.5194/egusphere-egu26-14579, 2026.
The APaRat-project investigates radionuclide transport through geological barriers over the one-million-year timescale, with the goal of supporting long-term safety assessments for deep geological repositories (DGRs). It aims to define conservative yet realistic scenarios in transport of uranium and iodine on the host-rock scale, defining benchmarks to be computed for safety analysis of geological barriers. To this end, specific models are set up for clay-, crystalline- and salt-rock, exhibiting unfavorable geological conditions and evolutions, which reflect possible long-term developments of the repository system and are used to quantify the transport of radionuclides as well as their potential release from the containment-providing rock zone. Focusing on the host rock and on processes induced by the repository itself, relevant physical couplings affecting radionuclide transport as well as the key geochemical parameters affecting radionuclide mobility should be identified. The approach emphasizes the importance of material parameter variations, model sensitivity, conceptual simplifications, dimensionality reduction, and the influence of fractures and disturbance zones. A central aspect is the application of containment indicators according to the German regulation, which are used to assess potential radionuclide release through the geological barrier. Numerical simulations are implemented using OpenGeoSys supported by automated software workflows. Preliminary results from benchmark studies and model setup workflows will be presented.
Acknowledgment
This work has been funded by the German Federal Office for the Safety of Nuclear Waste Management. Project APaRat: „Auswirkungen von Parametervariationen auf den Radionuklidtransport“ (“Effects of Parameter Variations on Radionuclide Transport”) (BASE research contract 4724F10301)
How to cite: Selzer, P., Shao, H., Grabow, L., Vehling, F., Bergmann, P., Magri, F., and Kolditz, O.: Radionuclide Transport on the Host-Rock Scale: Coupled Physics and Safety Assessment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14748, https://doi.org/10.5194/egusphere-egu26-14748, 2026.
The present-day crustal stress state is a key parameter for the assessment of a potential siting region for a deep geological repository (DGR) of radioactive waste. It is also crucial for the DGR design and the evaluation of its long-term safety. Therefore, a three-dimensional description of the stress state, including both orientations and magnitudes, is required not only for the repository host rock but also for the underlying and overlying formations that act as additional geological barriers.
For the site selection process in Germany, we present results from an updated large-scale 3D geomechanical-numerical model developed within the SpannEnD project. For the model calibration, we employ a new compilation of data records of the orientation of the maximum horizontal stress SHmax as well as stress magnitude data of SHmax and the minimum horizontal stress Shmin. The model geometry is based on a new geological model of Germany and comprises 50 individually parameterized units. We assume linear elasticity and assume that the stress state is a superposition of gravitational volume forces and surface forces related to plate tectonics. The resulting partial differential equations describing the force equilibrium are solved numerically using the finite element method. The model consists of approximately 107 hexahedral finite elements allowing a vertical resolution of ~50 m within the uppermost 5 km of the model.
To avoid overrepresentation of data clusters, we compare our modeled stress orientations with estimates of the mean SHmax orientation on a regular grid using the data records from the new WSM release 2025. The model results show good agreement with the mean SHmax orientation with a mean of the absolute differences of ~10°. Furthermore, our model results indicate an improved prediction of Shmin in comparison to previous models with a mean of the absolute stress differences with regard to the calibration data records of 2.5 MPa. The modeled SHmax magnitudes exhibit larger deviations from the calibration data with a mean of the absolute differences of 7.5 MPa. These discrepancies are probably attributed to uncertainties associated with the common derivation of SHmax magnitudes from the Shmin data. To further improve the reliability of the model results, additional reliable data records of the SHmax magnitude are required as they currently represent the largest source of uncertainty in the model results.
How to cite: Ahlers, S., Henk, A., Reiter, K., Hergert, T., Röckel, L., Heidbach, O., and Müller, B.: Prediction of the present-day stress field of Germany by a new 3D geomechanical-numerical model , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15076, https://doi.org/10.5194/egusphere-egu26-15076, 2026.
In a geological disposal facility, the geochemical evolution of a component in the near field (waste form, backfills, structural components and the host rock) is, amongst other factors, influenced by the geochemical properties of the other factors. One approach to assess the evolution of a so-called disposal cell is by coupled reactive transport modeling. Under the assumptions made in the model, this approach allows to evaluate the long-term interactions of the different components of the disposal cell, providing valuable insights into dominating processes across different time-scales, and critical parameters. The present study considers a specific disposal cell configuration with a bituminized waste form, a cementitious engineered barrier system and a clayey host rock. After resaturation of the disposal facility, the hygroscopic salts present in bituminized waste will drive the uptake of water by the waste form. NaNO3 and CaSO4 salts in the waste form will dissolve and diffuse into the cementitious materials. Eventually, these salts may reach the host rock, thereby altering properties such as for example the occupancy on the cation exchange complex. At the same time, there is the alkaline plume effect on the host rock, and the leaching of cement materials. The objectives of this study are (i) to implement a two-dimensional coupled reactive transport model at the disposal cell scale (engineered barriers and near host rock), (ii) to evaluate the geochemical evolution in the cementitious materials and the host rock as a consequence of leaching of soluble salts from the bituminized waste over a time-scale of a few thousands of years, and (iii) to identify the most critical parameters and processes. The results show that NaNO3 will alter the cement by transforming AFm phases into mononitroaluminate, while CaSO4 will first convert to ettringite and later also gypsum. Specifically, the latter will influence porosity and tortuosity of the cement materials. For the assumptions made in the model and considered time scales, the host rock geochemistry is only limitedly perturbed by these salts. In fact, geochemical changes in the host rock close to the interface with the engineered barrier system are mainly caused by cement leaching.
How to cite: Jacques, D., Liu, S., Bleyen, N., and Valcke, E.: Assessment long-term geochemical evolution of disposal cell – NaNO3-CaSO4 interactions with cement and host rock from Eurobitum bituminized waste , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16436, https://doi.org/10.5194/egusphere-egu26-16436, 2026.
The site selection procedure in Germany attempts to find the most suited location for a deep geological repository (DGR) for high-level nuclear waste in either salt-, clay-, or crystalline-rock. For safety assessment, the modeling of coupled thermal-hydraulic-mechanical-chemical (THMC) processes in the far-field of potential repositories plays an important role, e.g. for quantification of potential radionuclide release through the geological barrier or for modeling large scale influences on the engineered repository hypothetically affecting its integrity.
In the BGE-financed project OpenWorkFlow (Lehmann et al., 2024), automated simulation workflows are developed, which shall support the safety assessments in the far-field, among others. These workflows
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take into account the specifics of the different host rocks,
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allow for selection of different THMC-process couplings for plausibility checks during development and for troubleshooting in the modeling workflow, and
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enable the modeling in 2D or 3D with meshes of varying geometric complexity.
Going beyond previous works (Silbermann et al., 2025; Zill et al., 2024), we realized a uni-directional coupling of reactive transport (C) to existing THM-coupled FEM models. For this purpose, we use the Darcy-velocity and other parameters from the THM-simulations to feed them into radionuclide transport simulations. This is valid, as long as there are no significant feedbacks from reactive transport on the THM-formulation via porosity- or fluid-density-alterations, among others, which is often the case.
A uni-directional coupling has the advantage that models for THM-processes and reactive transport can be set up and simulated independently from each other, while meshes are ideally identical or subsets of each other. This allows for separate modeling approaches for THM-processes and reactive transport, which can be optimized numerically for both models independently, which improves the accuracy and robustness of the individual simulations.
The developed workflows are applied to THM far-field models, where the model domain spans from bedrock to the ground surface. Based on this, radionuclide transport is computed on a subset of the model domain which focuses on the host rock to assess, if the potential radionuclide release through the geological barrier stays below the legal limits.
References
Lehmann, C., Bilke, L., Buchwald, J. et al. (2024): OpenWorkFlow—Development of an open-source synthesis-platform for safety investigations in the site selection process. Grundwasser - Zeitschrift der Fachsektion Hydrogeologie 29, 31–47. https://doi.org/10.1007/s00767-024-00566-9
Silbermann, C.B., Zill, F., Meisel, T. et al. (2025). Automated thermo-hydro-mechanical simulations capturing glacial cycle effects on nuclear waste repositories in clay rock. Geomechanics and Geophysics for Geo-Energy and Geo-Resources. 11, 58. https://doi.org/10.1007/s40948-025-00960-4
Zill, Florian, Silbermann, Christian B., Meisel, Tobias, et al. (2024). Far-field modelling of THM processes in rock salt formations. Open Geomechanics, Volume 5, article no. 3, 16 p. https://doi.org/10.5802/ogeo.20
How to cite: Zill, F., Selzer, P., Shao, H., Meisel, T., Walter, L., Lehmann, C., Nagel, T., and Kolditz, O.: Automated THMC-Workflows for Safety Assessments of Deep Geological Repositories in the Far-Field, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17127, https://doi.org/10.5194/egusphere-egu26-17127, 2026.
Brittle failure phenomena such as spalling occur around deep underground excavations in crystalline rock, leading to the formation of excavation damage zones (EDZ). The EDZ is characterized by stress-induced micro- and macrocracks that significantly increase the permeability of the rock mass surrounding the underground opening. In the context of deep geological repositories, such permeability increases pose a safety concern by creating preferential pathways for radionuclide migration around the underground excavation. The PRECODE project (“Progressive Excavation Disturbance Zone Evolution during and Post Mine-by Tunneling”) at the Bedretto Underground Laboratory in Switzerland investigates the temporal and spatial evolution of the EDZ in the Rotondo Granite at ~1300 m overburden depth. The tunnel section investigated in this study was excavated using a non-explosive mechanical excavation method to avoid blast-induced damage in the surrounding rock. Continuous pore pressure monitoring before, during, and after excavation, combined with repeated hydraulic and pneumatic testing using modular multiple packer systems (MMPS) in dedicated boreholes, contributes to a better understanding of transient hydro-mechanical processes in the near-field rock. Analysis of pressure responses from hydraulic and pneumatic testing reveals the impact of stress redistribution on the adjacent rock mass. The spatial extent of the EDZ varies locally between 0.6 and 1.1 m from the tunnel’s sidewall into the rock mass, and the hydraulic conductivity of the borehole intervals within the EDZ increased progressively by up to five orders of magnitude with time after excavation. The evolving EDZ interacts with pre-existing fractures, developing combined natural and induced fracture-controlled flow paths. These results offer new insights into the short-term evolution and hydraulic behavior of EDZs in crystalline rocks, supporting improved assessment and modeling of repository stability and long-term safety.
How to cite: Dickmann, J., Hamdi, P., Kruszewski, M., Achtziger-Zupančič, P., Wiemer, S., and Amann, F.: Short-term Permeability Evolution Within the Excavation Damage Zone Around a Deep Tunnel in the Rotondo Granite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17620, https://doi.org/10.5194/egusphere-egu26-17620, 2026.
Due to their advantageous properties (i.e. swelling and sorption), bentonites are used as geotechnical barrier materials in the engineered barrier system (EBS) for radioactive waste disposal in crystalline host rocks [1]. To reduce the spatial footprint of the repository, these systems are being revised for thermal optimization [2]. A key aspect is determining the maximum thermal load at the canister-bentonite and bentonite-host rock interfaces under which bentonite maintains its functionality. Thermal loading affects both swelling [3] and possibly erosion behaviour, potentially compromising long-term integrity of the barrier.
Three samples were analysed to evaluate prolonged elevated thermal load effects on the erosion of bentonite: MX-80 Wyoming-type bentonite (Block 18) from the LOT S2, and MX-80 (Block 8) and FEBEX bentonite (Block 25) from the ABM 45:5 experiment, all conducted at Äspö HRL. Erosion experiments on untreated raw material from the same batches served as references.
The LOT S2 experiment operated at 500 W over a 20-year period, resulting in a temperature gradient ranging from ~95 °C at the heater-bentonite interface to 60 °C at the bentonite-host rock interface [4]. The ABM 45:5 experiment applied a higher thermal load of 1800 W during the last year of its five-year runtime, producing a temperature gradient from ~250 °C at the heater interface to 170 °C at the interface with the host rock [5].
Plugs (13 mm diameter, 5 mm height) were placed in a 1 mm aperture artificial fracture within a PMMA flow cell to assess erosion [6]. Groundwater from the Grimsel Test Site, representing low-ionic-strength meteoric/glacial meltwater (0.2 mM), was used to simulate future repository scenarios. Swelling pressure, nanoparticle concentration and gel expansion were monitored to calculate mass loss and erosion rates. Erosion rates were determined to be 2.8 ± 1.2 kg/m²/a for raw MX-80 (runtime: 100 d) and 4.6 ± 1.7 kg/m²/a for thermally loaded MX-80 (runtime: 174 d) from the LOT experiment.
The results show increased erosion in thermally loaded bentonites, with higher erosion rates at the heater interface than at the host rock interface. Although erosion rates increase following thermal exposure, all values remain below the threshold of 6.5 kg/m2/a for 1 mm fractures deemed safe by Posiva [6]. Mineralogical analyses reveal no alterations in the 0.2-2 µm size fraction. However, distinct alterations were identified in the < 50 nm fraction via ATR-IR and XRD. These changes might have a high impact on the erosion behavior of the bentonites despite being not the major mass fraction. Based on these results, further mechanistic investigation is required.
[1] Wersin, P. et al. (2007) Phys. Chem. Earth 32:780–788
[2] Kim, J.-S. et al. (2019) J Korean Tunn. Undergr. Space 21(5): 587-609
[3] Kašpar, V. et al. (2021) Minerals 11(9):965
[4] Sandén, T. ; Nilsson, U. (2020) SKB TR-20-11
[5] Svensson, D. et al. (2023) SKB TR-23-25
[6] Schäfer, T., et al. (2024) NTB 23-10
[7] Hedström, M. et al. (2023) Appl. Clay Sci. 239: 106929
How to cite: Van Overloop, L., Brandt, F., Klinkenberg, M., and Schäfer, T.: Impact of a long-term thermal pulse on bentonite erosion rates in lab scale experiments: Initial results from samples of the LOT/ABM experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18801, https://doi.org/10.5194/egusphere-egu26-18801, 2026.
Petrophysical properties of claystones, including hydraulic, geomechanical, and pore-network characteristics, and their modification by diagenesis, are critical for the integrity and performance of geological barrier systems and thus relevant to the long-term safety of high-level radioactive waste deposits. Claystones are regarded as suitable rocks for nuclear waste storage due to their low permeability, self-sealing-, and radionuclide sorption capacity. All these properties are strongly dependent on mineral composition and evolve with diagenesis and burial history.
This contribution presents the MATURITY project, which targets the Pliensbachian (Lower Jurassic) Amalthean Clay in the Hils and Sack synclines (Lower Saxony, Germany). Eight 100-m boreholes were drilled at five sites along a SE–NW thermal maturity axis (0.48–1.45% vitrinite reflectance), corresponding to maximum burial depths of roughly 1400–3300 m as derived from basin modeling (Castro-Vera et al., 2024).
The project aims to (i) quantify compositional trends, (ii) characterize the evolution of geomechanical and pore-network properties, and (iii) assess the influence of thermal maturity on repository-relevant properties, with implications for fluid transport under repository conditions.
Compositional data (XRD, XRF, CEC, TOC) combined with multivariate inferential statistics suggest that the Amalthean Clay is compositionally homogeneous irrespective of burial history. Detailed clay mineral analyses show that mixed-layered illite/smectite—with an illite fraction of ~80% independent of burial—dominates and that the clay content is generally high (>0.55 g/g). Pore-network descriptors (e.g., pore-size mode, total pore volume, porosity, water suction), geomechanical (e.g., uniaxial compressive strength, friction angle and cohesion), and hydraulic parameters, such as permeability, display a consistent, method-independent trend: properties follow a trend up to intermediate thermal maturity and exhibit a rebound (>10%) at the highest maturity, a feature previously noted by Gaus et al. (2022) for the Amalthean Clay. Though micropore-controlled parameters (BET area and micropore volume) do not exhibit this rebound.
The results indicate that the Amalthean Clay in the Hils syncline constitutes an ideal natural laboratory for deriving transfer functions between thermal maturity (or maximum burial depth) and repository-relevant rock properties. These findings provide essential insight into the coupling of transport processes, hydro-mechanical behavior, and pore-network characteristics in clay-rich rocks as a function of burial history, forming a foundation for assessing the transferability of key parameters to potential host formations and guiding site evaluation efforts.
References
Castro-Vera, Leidy, Sebastian Amberg, Garri Gaus, Katharina Leu, and Ralf Littke. 3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and implications for petrophysical properties of potential Mesozoic shale host rocks for nuclear waste storage. International Journal of Earth Sciences 113, no. 8 (2024): 2131-2162.
Gaus, G., Hoyer, E.M., Seemann, T., Fink, R., Amann, F., Littke, R. (2022). Laboratory investigation of permeability, pore space and unconfined compressive strength of uplifted Jurassic mudstones: The role of burial depth and thermal maturation. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 173 (3), 469-489.
How to cite: Seemann, T., Winhausen, L., Gaus, G., Mbui, B. M., Burchartz, R., Jalali, M., Littke, R., and Amann, F.: Exploring the Relationship of Burial History, Mineral Composition, Geomechanics and Petrophysical Properties of Pliensbachian Claystones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19238, https://doi.org/10.5194/egusphere-egu26-19238, 2026.
The long-term safety of deep geological repositories relies on the integrity and performance of engineered and geological barrier systems under coupled thermo–hydro–mechanical–chemical (THMC) processes. Among these barriers, engineered waste packages play a critical role in ensuring the isolation and protection of radioactive waste during the early phases of repository evolution. Reliable experimental methodologies to detect their mechanical behaviour, damage mechanisms, and early-stage degradation processes are therefore essential for safety assessment and performance prediction.
Within the framework of the PALLAS experimental project, this study investigates the use of distributed fiber optic sensing (DFOS) for real-time monitoring of deformation and damage in a full-scale mock-up of a mortar radioactive waste package. Fiber optic sensors were embedded directly within the package to continuously measure strain variations during controlled mechanical loading and chemical degradation experiments, including accelerated sulfate attack representative of aggressive geochemical conditions. The experimental program aimed to reproduce relevant repository-induced stress and chemically driven alteration processes affecting engineered barriers.
The results demonstrate that DFOS enables continuous, high-resolution monitoring of deformation, crack initiation, propagation, and localization within the waste package. During chemical degradation, the technique allows for identification and spatial mapping of fracture development, as well as the estimation of crack width and length. Under mechanical loading, strain evolution not only captures fracture onset and growth but also reveals early strain anomalies that act as precursors to damage, enabling the identification of zones prone to future cracking prior to any visible manifestation. This predictive capability represents a significant step toward anticipating barrier degradation.
Overall, these findings illustrate the interest of distributed fiber optic sensing technologies for the monitoring of engineered barrier systems. The results demonstrate the capability of the approach to measure strain evolution and to identify deformation signatures associated with different coupled processes under controlled experimental conditions. While the experimental configuration relies on non-representative waste packages and requires sensor installation within the structure, the study provides valuable insights into the applicability and limitations of such monitoring techniques for future developments aimed at waste package and concrete structure monitoring.
How to cite: Simon, N., Dick, P., and Dauzères, A.: From monitoring to prediction: fiber optic–based detection of damage precursors in radioactive waste packages, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21863, https://doi.org/10.5194/egusphere-egu26-21863, 2026.
