More than 50 countries worldwide are currently exploring options for radioactive waste (RW) disposal, considering the link between geoscience fundamentals and the safety of RW disposal sites. The presentation will focus on the lessons learned from representative RW disposal projects worldwide (such as those conducted in Sweden, Finland, France, Spain, Switzerland, Japan, and the USA), summarizing the existing deep geological repository (DGR) concepts, including potential DGR site selection and characterization, as well as long-time modeling predictions. A comparative assessment of models of coupled thermo-hydro-mechanical-chemical (THMC) processes has been performed within the scope of the international project DECOVALEX, which has helped advance the understanding of THMC processes in geological systems. The experimental and modeling studies of the DGR concepts are ultimately linked to assessing the safety of RW disposal. The presentation will provide critical references and case studies related to the representative national disposal programs, which would interest geoscientists, engineers, and decision-makers working on national RW disposal programs. We will summarize and compare challenging geological problems and experiences in siting nuclear waste repositories in different host rocks, such as hard rock (crystalline and sediments), clayey, and salt formations. The availability of rock in a country limits the choice of rock type for the DGR. The interplay of geological conditions with technical feasibility, an engineering design for different rock types and operational and post-closure safety is critical in technical evaluating potential sites. We will also present summaries of the progress in international cooperation studies and testing of the design of buffer and backfill materials, the development of the concept of RW disposal in deep boreholes, the R&D research in Underground Research Laboratories (URL), and multi-national RW repository initiatives. 

Acknowledgments: LBNL work was supported under Contract Number DE-AC02-05CH11231 with the US DOE. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

How to cite: Faybishenko, B., Birkholzer, J., Zheng, L., Sassani, D., and Stein, E.: International Concepts for the Assessment of Deep Geological Disposal of Radioactive Waste, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13377, https://doi.org/10.5194/egusphere-egu24-13377, 2024.

In Germany disposal of low- and intermediate level radioactive waste is intended and took place in deep geological repositories. Two repositories in very different stages of their life cycle exist in Germany, both are disused mines. For the ERAM repository (Endlager für radioaktive Abfälle Morsleben) decommissioning and closure is expected to start in 2029. Radioactive waste was last stored in 1998. For the KONRAD repository (Schachtanlage Konrad) commissioning and storage of radioactive waste is anticipated in 2030. The respective permits regulate, how the final repository is designed, what protective measures must be taken, which waste may be stored in what form and how the final repository is to be closed. ERAM and KONRAD were approved in 1986 and 2002, respectively; both approval processes took place in previous decades. Since then, both repositories have been continuously adapted to the current state of the art in science and technology. But what is the “current state of the art in science and technology”, who defines it and what does it mean for the approval of deep geological repositories? In this contribution we will show examples of how the state of the art in science and technology finds its way into supervision and licensing activities and which actors are involved. Finally, we discuss how we could optimize these processes and what we might learn for the permission of a repository for high radioactive waste.

How to cite: Schneider, S., Kause-Berg, H., and Baasch, F.: State of the art in science and technology for unique and lengthy administrative procedures - How does it work?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14424, https://doi.org/10.5194/egusphere-egu24-14424, 2024.

The site selection procedure for the deep geological storage of high-level radioactive waste in Germany started with a “white map”, i.e., with the entire federal territory, and considers all of three potential host rocks: rock salt, clay rock, and crystalline rock. In a first phase of the site selection procedure, Bundesgesellschaft für Endlagerung (BGE, i.e., Federal Company for Radioactive Waste Disposal) is determining regions for surface exploration based on the previously identified sub-areas that meet the minimum requirements defined by law. As the sub-areas cover approximately 54% of Germany's area, the site selection methodology must not only include safety assessments but must also lead to a significant safety-oriented reduction of the considered area towards the best-suited regions for surface exploration.

Several challenges arise for the determination of the regions for surface exploration. As legal regulations provide only a framework for the procedure but largely lack explicit assessment criteria, a methodology must be developed that is in accordance with the law and that also supplies the tools to evaluate and narrow down the remaining areas. In addition, the safety-oriented evaluation and area reduction must be done legally on the basis of available data only, which is sparsely available and mostly not specific to the needs for site evaluation. Only in the next phase of the site selection procedure can BGE collect data during surface exploration for a more detailed site characterization. Finally, the developed methodology, as well as decisions based on its application to the sub-areas, must be transparent and comprehensible to permit discussions with stakeholders and the public. Data availability and associated uncertainties provide a specific challenge in ensuring this transparency during the decision-making.

To address the challenges, BGE is developing a methodology that concretizes the legal requirements and translates them into a plausible evaluation system with well-defined criteria. These host-rock-specific evaluation criteria derived from the legal requirements allow a reproducible, systematic and thus transparent evaluation of safety-relevant attributes. The criteria are applied successively in a series of work steps, such that the level of detail and the strictness of requirements increase throughout the selection process. Thus, less-suitable areas are identified early on in the procedure with limited effort, while areas that are more promising are analyzed in greater detail – providing also the basis for a comparison between areas – and must pass successively higher requirements. The level of knowledge, which increases throughout the selection process, is taken into account when deciding on the evaluation criteria for a specific work step, such that heterogeneous data availability influences the decision-making as little as possible. However, poor data availability and verifiability of geologic realities are fundamental problems for the decision-making process that need to be addressed and discussed openly.

How to cite: Rempe, M., Fink, R., Panitz, F., and Romer, R.: Selection of regions for surface exploration in the search for a repository for high-level radioactive waste in Germany: Decision-making in a safety-oriented site selection procedure with sparsely available data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14693, https://doi.org/10.5194/egusphere-egu24-14693, 2024.

Here we provide an overview of an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. The creation of the international DECOVALEX Initiative, now running for more than 30 years, was motivated by the recognition that prediction of these coupled effects is an essential part of demonstrating the performance and safety of radioactive waste disposal and other subsurface engineering applications. DECOVALEX emphasizes joint analysis and comparative modeling of state-of-the-art field and laboratory experiments, across a range of host rock options and repository designs. Participating research teams are from radioactive waste management and other organizations, national research institutes, regulatory agencies, universities, as well as industry and consulting groups, providing a wide range of perspectives and solutions to these complex problems. The most recent phase of the initiative, referred to as DECOVALEX-2023, started in 2020 and ended in December 2023. Modeling teams from 17 international partner organizations participated in the comparative evaluation of seven modeling tasks involving complex experimental and modeling challenges. The presentation provides examples of research contributions made within DECOVALEX-2023 and illustrates how these have helped building confidence in long-term performance predictions. These examples range from the modeling of large-scale in situ heater tests representing mock-ups of nuclear waste disposal tunnels, the analysis of gas transport tests in clay-based materials, the prediction thermal and gas pressure rock fracturing, to the comparison of performance assessment models.

The main characteristic of DECOVALEX is the close collaboration on analyzing and simulating state-of-the-art field and laboratory experiments, which provides a wide range of experiences, perspectives and solutions to these complex problems and allows for detailed comparison of analysis and modeling results. Much insight can be gained through this cooperative comparison of results from different research teams using different model approaches, not only on the effects of complex THMC processes, but also on the strengths, weaknesses, and adequacies of the various approaches and predictive models used by these research teams. We make the case in this presentation that DECOVALEX has contributed, and continues to contribute, to enhancing confidence in the technical adequacy of radioactive waste disposal, by improving our collective understanding of complex subsurface perturbations and coupled processes, by developing and comparing predictive models for these processes, by evaluating their uncertainties, by recognizing areas for additional research, and by emphasizing means of learning from each other and knowledge sharing. The insight and scientific knowledge gained would not have been possible if individual groups had studied these data alone rather than within a truly collaborative setting.

How to cite: Birkholzer, J. and Bond, A.: Overview of the DECOVALEX Initiative - Building Confidence Via Model Comparison, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3647, https://doi.org/10.5194/egusphere-egu24-3647, 2024.

The identification of appropriate locations for secure nuclear waste disposal, a crucial aspect of Germany's nuclear phase-out strategy, remains a significant scientific, technical, and political challenge worldwide. The selection and safety assessment of sites demand extensive employment of numerical methods. The OpenWorkFlow project, initiated by Bundesgesellschaft für Endlagerung (BGE), is developing a new, open synthesis platform to virtualise repository systems. The simulation platform will evaluate far-field and near-field processes, aiding the site selection process first and geotechnical design of repository systems later on. Automated simulation and analysis pipelines developed in this project ensure the verifiability and reproducibility of all simulation results and the modularity of workflows brings flexibility, continuity and maintainability.

This project defines high scientific benchmarks and standards for numerical models and software development for the repository search in Germany. OpenWorkFlow as an efficient and holistic platform for numerical modelling will contribute to a science-based, transparent, and precise execution of the necessary safety assessments. This project includes all theoretical, numerical and computational methods and tools, including a virtual reality framework. OpenWorkFlow will be continuously developed by the core project team, but will also actively involve the community through its open concept (e.g. through the interactive benchmarking platform). This will create a win-win situation in the long run.

This talk is based on the paper Christoph Lehmann, Lars Bilke, Jörg Buchwald, Nico Graebling, Norbert Grunwald, Julian Heinze, Tobias Meisel, Renchao Lu, Dmitri Naumov, Karsten Rink, Ozan Özgür Sen, Philipp Selzer, Haibing Shao, Wenqing Wang, Florian Zill, Thomas Nagel, and Olaf Kolditz. Openworkflow - development of an open-source synthesis-platform for safety investigations in the site selection process. Grundwasser, 2024. in print.

How to cite: Lehmann, C., Nagel, T., and Kolditz, O.: OpenWorkFlow - Development of an open-source synthesis-platform for safety investigations in the site selection process, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14633, https://doi.org/10.5194/egusphere-egu24-14633, 2024.

Safe deep geological disposal of heat-generating nuclear waste requires an accurate assessment of barrier integrity. Therefore, the evaluation of the coupled mechanical, hydraulic, thermal and chemical processes, occurring in the host rock due to the nuclear waste storage, excavation among others is needed. For this purpose, numerical modeling is an essential and powerful tool. This contribution focuses on a generic repository system that relies on a Containment Providing Rock Zone (CRZ) in a crystalline rock, which acts as the principal containment barrier according to German law. Using the open-source finite element code OpenGeoSys version 6, a closer look at the CRZ regarding the influence of fractures on the local hydraulics and potentially available rock zone volume for repository emplacement is shown.

Since fractures and other type of discontinuities usually characterize crystalline rock, they are expected to influence the hydraulic behavior of the system. Hence, their representation in numerical models becomes non-trivial. Here a comparison between different numerical fracture representations and their impact on the hydraulic regime surrounding the CRZ, is presented.

Due to the presence of fractures, it cannot be assumed that a sufficiently large area in which to emplace the waste will be found. As a consequence, multiple smaller CRZs [1], each providing undisturbed rock, have to be defined. Typically, it is only possible to characterize fracture networks statistically, which leads to the use of stochastically generated discrete fracture networks. Using a geometrical characterization of the potentially undisturbed CRZs based on a stochastically generated discrete fracture network, a methodology is proposed to evaluate the feasibility of the multiple CRZs concept.

References

[1] Thiedau, J., et al.: CHRISTA-II - Analysen zur Integrität von geologischen Barrieren von Endlagersystemen im Kristallin. Ergebnisbericht, Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, 2021

How to cite: Guevara Morel, C., Thiedau, J., and Maßmann, J.: Advances in the numerical modeling strategy (concept) of a generic nuclear waste repository in crystalline rock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9854, https://doi.org/10.5194/egusphere-egu24-9854, 2024.

Pore pressure evolution and hydraulic flow are two important physical features to consider in barrier integrity and radionuclide transport applications. Existing literature suggests a potentially non-negligible impact of thermo-osmosis on pore-pressure evolution due to thermal gradients in clay rocks without arriving at a consensus. Numerical experiments based on models are a widely used method in the geosciences to test the relevance of physical features under specified conditions. Mismatch between the observations and the simulation can be partially attributed to the assumptions and simplifications made when developing those models. Their impact on the results and conclusions drawn from the numerical experiments can be significant and thus needs to be explored. Such exploration can proceed by jointly investigating the uncertainty associated with modelling choices, process selection and calibration. This study applies a hypothesis-testing method based on an assisted-history-matching workflow to integrate uncertainty evaluation of model selection, process representation and parameter identification. Three models were compared, representing, respectively, the correct, approximate and wrong hypotheses with respect to a synthetic data set resembling the ATLAS experiment, an in-situ, full-scale heating experiment performed at the HADES underground laboratory in Mol, Belgium. We show that the approach can recover the correct modelling hypothesis in the presence of parameter uncertainty despite competing hypothesis with the same amount of parametric degrees of freedom.

How to cite: Kiszkurno, F., Buchwald, J., Kolditz, O., and Nagel, T.: Hypothesis-testing and assisted-history-matching applied to evaluate uncertainty of model selection and parameter values: a case study of the impact of thermo-osmosis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6012, https://doi.org/10.5194/egusphere-egu24-6012, 2024.

This study explores the numerical simulation of gas transport in low-permeable rocks, specifically focusing on clay rock. Utilizing the finite-element method, we examine the transition from single-phase to two-phase flow conditions. Our approach diverges from traditional methods by avoiding persistent primary variables or variable switching. We validate our methodology through two benchmark tests: the first simulates gas injection relevant to radioactive waste disposal, while the second models a core drilling experiment that induces mechanical unloading.

Our findings are significant for understanding gas behavior in geological formations, particularly in the context of nuclear waste disposal and CO2 storage. We offer a novel perspective on managing phase transitions in non-isothermal environments, bolstered by an extensive analysis of secondary variables. The outcomes of this research contribute to the improved modeling of large-scale repository systems, highlighting the intricacies and complexities involved in gas transport within clay rock.

This paper not only provides insights into the physical processes underpinning gas movement in these environments but also proposes a scalable and adaptable framework for future research in similar geological contexts.

How to cite: Grunwald, N., Nagel, T., Pitz, M., and Kolditz, O.: Finite-Element Analysis of Phase Transitions in Gas Migration within Clay Rock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15328, https://doi.org/10.5194/egusphere-egu24-15328, 2024.

Continental-scale glaciations lead to deformation, geopotential, rotation, and stress changes of the Earth. Especially glacially induced stress changes in the lithosphere can significantly impact potential nuclear waste repository sites, planned for depths a few hundred meters below ground. Ensuring the long-term stability of these sites warrants analyzing such additional stresses in the crust that might be imposed by future ice sheets during forthcoming glaciations. We focus on North America by investigating the past to quantify such stresses.

Utilizing a refined, high-resolution North American ice history spanning 122,000 years from the University of Toronto Glacial Systems Model, alongside various one- and three-dimensional earth structures, we investigate the dynamic nature of glacially induced stresses at Canada’s candidate sites South Bruce and Ignace. Both sites are situated in Ontario but 1000 km apart. Additionally, we examine the corresponding deformation and strain changes.

We find that both sites undergo strong variations in glacially induced stresses and strains over a glacial cycle. Especially the horizontal components can change from tensional to compressive within a few thousand years due to fluctuations in ice cover. Surprisingly, despite South Bruce's location farther from the ice sheet center and its temporary position in the forebulge of the ice sheet, stress and strain magnitudes resemble those of Ignace. Moreover, there's potential at South Bruce for altering the direction of pre-existing maximum horizontal stress. Interestingly, the choice of earth structure in the modelling affects strain more than stress.

Although the sites are 1000 km apart, our results do not indicate a superior repository site. Instead, they emphasize the need to integrate our findings into the site selection process and barrier integrity assessment. We recommend further studies focusing on a stress analysis with more detailed earth models, considering faults and lineaments near the candidate sites.

This research was funded by the Nuclear Waste Management Organization, Canada.

How to cite: Steffen, H. and Steffen, R.: Impact of glacially induced stresses and strains at Canadian candidate sites for nuclear waste disposal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2049, https://doi.org/10.5194/egusphere-egu24-2049, 2024.

The site selection procedure in Germany is searching for a site with the best possible safety for a repository for high-level waste (HLW). The integrity of the overburden is crucial in the assessment of the best possible safety (StandAG §23 (5), para.3). Beyond the safe inclusion in the selected host rock for a HLW repository, a sufficient stability of the overlaying overburden horizons must be ensured. Erosion by glaciation induced processes are but one of the potential threats to the stability of these overburden horizons. The research project presented here is funded by BASE under the grant number FKZ 4721F10401.
Within the research project “Evaluation of methods and models to predict the protective function of the overburden in Germany over the period of 1 Ma” (MeMoDeck) a framework to assess the risks introduced by glacial erosion that are to be expected within 1 million years was developed. As an indication of this, the erosion processes, prevailing during the Pleistocene, have been analysed with respect to the extent of the erosion depth introduced by them. The largest depths are attained to incisions by subglacial meltwater transport in the form of tunnel valleys. 
The authors employed a combined modelling approach, which includes a three-dimensional deterministic numerical modelling of the tunnel valley genesis with FLAC3D and a multivariate probabilistic modelling with GoldSim to account for the remarkable uncertainties over 1 Ma. The uncertainties result from the lack of data with respect to hydraulic conditions during the melting of the ice sheet and the variability of the erosion resistance of the geological horizons of the overburden, which overlies the respective host rock. 
The authors determine the depth of a prospective tunnel valley by a simulation at well-constrained geological and melt water conditions by the deterministic FLAC3D model.
The results from the deterministic modelling with Flac3D are the basis of the simulation within the probabilistic model with GoldSim, which relaxes the modelling constraints towards melt water and geological properties using statistical distribution functions for these quantities.
The results of the modelling will be presented and the most influential parameters towards the final depths of the overburden incisions will be discussed.

How to cite: Brandt, M., Carl-Dworschak, A., Jockel, A., and Kahnt, R.: Modelling of glacial melt water erosion of the overburden of a HLW over 1 million years, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16099, https://doi.org/10.5194/egusphere-egu24-16099, 2024.

In the safety assessment of a nuclear waste repository, it is crucial to identify and isolate possible pathways for radionuclides from the waste canister to the biosphere and vice versa. Despite the potential self-sealing properties depending on the host rock and the decrease of the permeability over time, the migration depends on the presence of a fracture network, the connectivity of the fractures, and their connectivity to tunnels or drifts. To describe the initial state of said pathways, we demonstrate the inversion of transient pressure measurements to delineate the structural and pneumatic properties of an excavation-induced fracture network at the Meuse/Haute-Marne Underground Research Laboratory (URL) operated by the French radioactive waste management agency ANDRA.

The tomographic tests were carried out as sequential constant-rate injections of nitrogen and the resulting pressure perturbations were recorded in nearby borehole intervals. In total, nine boreholes with two injection/monitoring intervals each are available on a volume of approximately 3m times 3m times 5m. Therefore, the joint inversion of more than 300 signals allows unique insights into the excavation-induced fracture network.

A discrete fracture network (DFN) model is applied for the forward and inverse modeling. Thereby, the strong heterogeneity of the distribution of hydraulic or pneumatic properties caused by the fracture network can be described. A numerical model is used to simulate the transient pressure diffusion in the DFN. The structural properties and the permeability of the DFN model are characterized by solving the inverse problem. The inversion relies on a stochastic model of the DFN parameters based on the Bayesian equation. The posterior distribution, i.e., the distribution of the DFN parameters given the measured data, is the product of likelihood and prior distribution. The likelihood function compares the error between the measured data and the simulated outcome of the tomography experiments for a given DFN model. The prior distribution includes information about the fracture properties obtained in previous studies. The posterior distribution is characterized by generating samples from the posterior with Markov chain Monte Carlo (MCMC) methods. Due to the unknown number of fractures, the insertion and deletion of fractures are possible according to the reversible jump MCMC algorithm.

The inversion approach results in several DFN realizations that are approximately equally likely which is illustrated in a fracture probability map and a map of the permeability distribution. Thereby, preferential flow paths can be characterized.

How to cite: Ringel, L. M., de la Vaissière, R., Brauchler, R., and Bayer, P.: Three-dimensional characterization of an excavation-induced fracture network with gas tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4976, https://doi.org/10.5194/egusphere-egu24-4976, 2024.

Compacted bentonite is the primary engineered barrier in a deep geological repository for high-level radioactive waste. The bentonite must exhibit a sufficiently high swelling capacity and low hydraulic conductivity to seal the waste and to hinder radionuclide migration (Sellin & Leupin, 2013). The erosion of the barrier due to the flow of groundwater, which promotes clay swelling and expansion through fractures in the crystalline host rock, would entail a loss of mass in the bentonite, which could compromise the effectiveness of the barrier.

Bentonite erosion is usually experimentally studied at laboratory scale, by analyzing the role of different parameters such as the type of clay, water chemistry or the physical conditions of the fracture in a controlled manner. The extrapolation of laboratory results to a real repository requires compulsory evaluating the role of those aspects more sensitive to scale effects. This study investigated whether the amount of clay emplaced or the clay surface in contact with fractures play a role in erosion process.

For this, an experimental set-up to simulate an artificial fracture was used (Alonso et al., 2019). In this setup, a compacted bentonite sample is placed between two methacrylate plates with a known aperture. All experiments were performed at a dry density of 1.4 g/cm³ with a previously sodium equilibrated clay (NANOCOR^®). Fractures are placed in horizontal position and filled with a low saline solution (10^-3 M NaCl), to monitor the expansion distances of the clay in the fracture by periodic photographs. The clay is allowed to expand during 30 days and the amount of clay eroded is quantified post-mortem. Two sets of experiments were carried out, the first set evaluated the impact of clay area exposed to hydration in the fracture, fixing the amount of clay mass, with samples of ring geometry. For the second set, the clay exposed surface contacting the fracture was kept constant, but the amount of clay installed was varied with cylindrical compacted samples of variable height.

Results showed that clay expansion can only occur in water conductive fractures, and no expansion was observed in the inner zone of the ring geometry tests, which were not water filled. Additionally, no large differences in the expansion distance were observed in the tests with a larger exposed surface area. For the second set of experiments with cylindrical geometry and the same exposed area, it was observed that the expansion and mass loss did not vary significantly with different clay mass. The indication is that in a genuine repository featuring substantially greater volumes of clay, the impact of this factor on its expansion should not be considerable.

REFERENCES

Alonso, U., Missana, T., García-Gutiérrez, M., Morejón, J., Mingarro, M., & Fernández, A. M. (2019). CIEMAT studies within POSKBAR project Bentonite expansion, sedimentation and erosion in artificial fractures (Technical Report TR-19-08). SKB.

Sellin, P., & Leupin, O. X. (2013). The Use of Clay as an Engineered Barrier in Radioactive-Waste Management – A Review. Clays and Clay Minerals, 61(6), 477–498. https://doi.org/10.1346/CCMN.2013.0610601

How to cite: Dieguez, M., Morejon, J., Mingarro, M., García-Gutiérrez, M., Missana, T., Sellin, P., and Alonso, Ú.: Bentonite erosion by expansion in fractures: effect of exposed surface and clay mass, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16507, https://doi.org/10.5194/egusphere-egu24-16507, 2024.

Assessing barrier integrity under varying environmental conditions is crucial in the context of radioactive waste disposal. This study, originally  conducted at the Mont Terri Rock Laboratory in Switzerland within the Cyclic Deformation (CD-A) experiment, gains new understanding in this context. The laboratory, located within an Opalinus Clay formation, has been also instrumented to observe the triggers of desiccation cracking in open and closed excavations. Seasonal changes play an important role especially during winter where the relative air humidity reduces and drives desiccation cracking. To model this dynamic, we employed a hydro-mechanical model incorporating macroscopic poromechanics, the Richards equation for partial saturation and the phase-field modeling approach for cracking. Key to our study was the evaluation of unsaturated hydro-mechanical responses using field-acquired parameters such as the measured crack apertures, supplemented by existing literature. We established a good correlation between the observed and calculated crack formation by using the measured seasonal changes in relative air humidity as boundary condition as well as using  homogeneously and randomly distributed material parameters [1]. Furthermore, our study delves into the implications of enhanced permeability due to cracking on barrier integrity. Our findings offer insights into the dynamics of crack development and their implications, thereby making an  incremental contribution to the broader goal of ensuring safe and effective management of radioactive waste disposal.

[1] T. Cajuhi, G. Ziefle, J. Maßmann, T. Nagel and K. Yoshioka, Modeling desiccation cracks in Opalinus Clay at field scale with the phase-field approach. InterPore Journal (in press).

How to cite: Cajuhi, T., Ziefle, G., Maßmann, J., Nagel, T., and Yoshioka, K.: Modeling in-situ desiccation cracks in a ventilated niche using homogeneously and randomly distributed material parameters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7469, https://doi.org/10.5194/egusphere-egu24-7469, 2024.

The United States initiated the Spent Fuel and Waste Storage and Transport (SFWST) Campaign over ten years to evaluate various generic geological repositories for the disposal of spent nuclear fuel. Most previous international work describes Engineered Barrier Systems (EBS) for repositories focused on low temperature conditions. Our hydrothermal experiments on EBS materials were conducted to characterize high temperature interactions of bentonite clay and argillite wall rock +/- cement with candidate waste container steels (304SS, 316SS, low-C steel).

Over eight years of hydrothermal experiments were performed using Dickson reaction cells at temperatures ranging from 150 to 300°C and pressures of 15 – 16 MPa, respectively, for five to eight weeks. Wyoming bentonite was saturated with a 1,900 ppm K-Ca-Na-Cl solution in combination with stainless and low-C steel coupons to replicate EBS conditions in deep geological disposition of nuclear spent fuel. The solid-reaction products and steel coupons were characterized post experiment via XRD, XRF, SEM, and EMP.

Preliminary mineralogic phase transformations for the experiments are as follows: Smectite clays did not transition to illite,. Clinoptilolite, appears to have formed from the remnant glass which was present in the original bentonite. The Si/Al ratios for the clinoptilolite are dominantly between 4 and 6. The Na/(Na+Ca) values range from 0.55 to 0.75. Calcite and gypsum were also observed as minor reaction products. Aqueous SiO₂ remains saturated with respect to quartz throughout the experiments.

Our experiments focused on bentonite-cement interactions, including 1) Baseline bentonite stability in argillite, 2) reactions with Uncured OPC powder, 3) Cured OPC chip, and 4) Low-pH cement chips. In the Opalinus Clay, Wyoming bentonite + OPC powder experiments (#2), spherical, calcium aluminum silicate hydrate (CASH) phases formed within the fine-grained clay matrix. Based on the composition of this mineral, the C(A)SH phases are likely a hydrated calcium silicate, such as Al-tobermorite (Ca_4.3Si_5.5Al _0.5O₁₆(OH)₂•4(H₂O)). Very Ca-rich hydrous minerals, such as Al-tobermorite, have been observed in experiments involving bentonite and cement with highly alkaline bulk chemistries and pH > ~10 (Savage et al., 2007).

The formation of CASH minerals contrasts with the products of previous experiments with Wyoming Bentonite ± Opalinus Clay host rock (#1), in which zeolites (analcime–wairakite solid solution) formed that have similar morphologies and textural contexts.

The experiments with cured OPC chips (#3) resulted in Portlandite dissolution during the early elevated pH values, but pH values stabilize at near-neutral values by equilibrium. Montmorillonite was stable, and zeolite formation was observed throughout the groundmass of the reaction products but was not abundant at the bentonite-cement interface. Reactions between the groundwater solution, bentonite, and the low-pH cement (#4) resulted in a lower steady-state silica concentration than reactions between groundwater solution, bentonite, low-pH cement, and Opalinus Clay wall rock. This suggests a controlling input on silica concentrations is affected by Opalinus Clay wall rock that is not advanced by the bentonite or low-pH cement alone.

How to cite: Caporuscio, F., Rock, M., and Zandanel, A.:  Argillite Host Rock and Cement - Engineered Barrier System Experiments: Mineralogical Evolution at Repository Pressures and Temperatures., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3238, https://doi.org/10.5194/egusphere-egu24-3238, 2024.

With nuclear energy being one of the trending topics in the world regarding energy supply, at the same time the question for safe disposal of the high-level radioactive waste becomes increasingly popular in the public. 

The TU Bergakademie Freiberg wants to provide answers to this challenging task by practical big-scale in-situ underground tests and new backfill material development. The underground mining chair commenced the GESAV (Gefügestabilisierter Salzgrusversatz; engl.: Matrix-stabilized salt grit backfill) project series back in 2013 with the development of a matrix-stabilized salt backfill which solidifies in an early stage by the development of internal polyhalite crystals. The innovative material combines the benefits of a salt backfill material with a stability compared to conventional materials, such as cement like building materials.

The GESAV project series has been finished in 2021 with the successful proof of the big-scale practical applicability of the matrix-stabilized backfill by building several backfill bodies underground with different methods from the conventional underground mining industry.

The goal for the currently going-on SAVER (Entwicklung eines salzgrusbasierten Versatzkonzeptes unter der Option Rückholbarkeit; engl.: Development of a salt grit based backfilling concept with regards to retrievability) project is to build and compare a GESAV material backfill body to a conventional salt backfill body, since in the GESAV II project only the in-situ applicability of the GESAV material in general has been researched. Both of the test drifts in the Sondershausen rock salt underground mine have the same dimensions and are instrumented exactly the same. Moreover, a radioactive waste casket dummy has been developed within the project and placed into the backfill bodies in order to simulate a real-life backfilling process. So far, both backfill bodies have been built with a slinger-vibration-backfill method that already highlighted very high in-situ densities in the GESAV II project (2017-2021). Not only did this validate the applicability of the method on GESAV material but also proved that it is transferable to other salt backfill materials.

During the building process of both bodies, a big sampling campaign was carried out in order to gain an insight into actual possible built-in in-situ densities and other parameters that are relevant for further optimization and understanding of real-life backfilling in a mining scale. Once analyzed, the samples, in combination with the other sensors, can provide a deep insight and further understanding of the geochemical and mechanical properties of both backfill bodies. The sensor data shows that GESAV material highlights better properties regarding settlement than conventional salt grit as well as pressure distribution within the backfill body. The measurement systems will continue to run as long as the project is taking place in order to continuously gain an insight into ongoing processes within both backfill bodies. With the chosen sensor setup as well as the sampling regime it is expected to gain more valuable insights into actual underground in-situ behavior of those materials in the near future. Those insights are very likely to contribute to the ongoing question of developing a feasible concept for high-radioactive waste disposal in rock salt formations.

How to cite: Schaarschmidt, L. and Mischo, H.: Big Scale In-Situ Application of Matrix-stabilized vs. Conventional Salt Grit Backfill with Use of Improved Backfilling Method, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3566, https://doi.org/10.5194/egusphere-egu24-3566, 2024.

In many European countries (e.g., France, Switzerland), thick steel casks are foreseen for the containment of high-level radioactive waste in deep geological repositories. In contact with pore-water, steel corrodes forming mixed iron oxides, mainly magnetite (Fe₃O₄). After tens of thousands of years, the casks may breach allowing for leaching of the radionuclides (e.g., Tc and Pu) into pore-water. The radionuclides can be retarded by corrosion products either by adsorption or via structural incorporation [1,2]. The molecular scale mechanisms of these phenomena are investigated by ab initio simulations and X-ray absorption spectroscopy (XAS).

The dominant low-index surfaces of magnetite particles and their termination at the relevant conditions were identified based on Kohn-Sham density functional theory (DFT), using the open-source CP2K code [3]. The DFT+U method was employed for the strongly correlated 3d and 5f electrons of Fe and Pu, respectively. After benchmarking the model setup, the surface energies of the (111) facet with different surface terminations and water coverage were analyzed as a function of redox conditions and pH. The Eh and pH predominance diagram could be predicted for the most stable surfaces under real repository conditions [4]. Further, we confirmed these findings for nanocrystals with approximately 2 nm size. Subsequently, ab initio molecular dynamics (MD) were applied to simulate sorption structures of radionuclides on the expected magnetite (111) surfaces based on experimental findings [2,5].

[1] R. Kirsch et al. Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite. Environmental Science & Technology, 2011, 45(17), 7267.

[2] E. Yalçintaş et al. Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite. Dalton Transactions, 2016, 45(44), 17874.

[3] T. D. Kühne et al. CP2K: An Electronic Structure and Molecular Dynamics Software Package - Quickstep: Efficient and Accurate Electronic Structure Calculations. The Journal of Chemical Physics, 2020, 152(19), 194103.

[4] A.S. Katheras et al. Stability and Speciation of Hydrated Magnetite {111} Surfaces from Ab Initio Simulations with Relevance for Geochemical Redox Processes. Environmental Science & Technology, 2024, 58(1), 935.

[5] T. Dumas et al. Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption. ACS Earth & Space Chemistry, 2019, 3(10), 2197.

How to cite: Katheras, A. S., Karalis, K., Krack, M., Scheinost, A. C., and Churakov, S. V.: Ab initio modelling of magnetite surfaces for radionuclide retention, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8817, https://doi.org/10.5194/egusphere-egu24-8817, 2024.

Low-pH concrete (LPC) is used as one of the main construction components of deep geological repositories (DGR) for radioactive waste. The mechanical and chemical stability of this material is crucial for the long term DGR sustainability. However, the presence of microorganisms in the underground environment and backfill matrices could significantly affect its stability and in turn the safety of DGRs. The buffer and backfill matrix (bentonite) and underground water are potential long-term sources of bacteria in DGR. The WP MAGIC as a part of the European joint program EURAD focuses on studying the chemo-mechanical behavior of concrete under varied conditions. The project compares the effect of three different conditions – air, water and bentonite - on aged SURAO LPC samples. The 2-year experiment is disposed in Czech Underground Research Facility (URF) Bukov. Our aim was to investigate microbial activity in LPC discs and describe the capability of present microorganisms to affect the mechanical properties of LPC in time. The microbiological analysis comprised of both cultivation dependent and independent approach. The microbial activity was also confirmed using epifluorescent microscopy and scanning electron microscopy. Our findings reveal unique microorganisms in each of in-situ condition, highlighting their potential effect on mechanical properties of LPC.

How to cite: Hlavackova, V., Le, T. D., Riha, J., Vecernik, P., Cernousek, T., Hausmannova, L., Vasicek, R., and Sevcu, A.: Microbiological Analysis of SURAO Low-pH Concrete (LPC) in repository-like conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20815, https://doi.org/10.5194/egusphere-egu24-20815, 2024.