ERE3.3 | Geological Repositories - Geosciences in the assessment of radionuclide migration and long-term evolution of the geosphere
Geological Repositories - Geosciences in the assessment of radionuclide migration and long-term evolution of the geosphere
Convener: Vanessa Montoya | Co-conveners: Axel Liebscher, Thomas Nagel, Vaclava Havlova, Koen Beerten, Alwina Hoving, Theresa HennigECSECS
Orals
| Tue, 25 Apr, 08:30–12:30 (CEST), 14:00–15:45 (CEST)
 
Room G2
Posters on site
| Attendance Tue, 25 Apr, 16:15–18:00 (CEST)
 
Hall X4
Posters virtual
| Attendance Tue, 25 Apr, 16:15–18:00 (CEST)
 
vHall ERE
Orals |
Tue, 08:30
Tue, 16:15
Tue, 16:15
Geoscience knowledge is essential to investigate safety requirements that are established by national agencies to construct a geological disposal facility for high-level and/or long-lived radioactive waste in a specific selected site. Safety requirements include i) isolation of the nuclear waste from humans and the accessible biosphere, ii) containment e. g. by retention and retardation of contaminants, iii) limited water flow to the geo-engineered facility and iv) long-term geological stability of the site. Experiences in many countries have shown that acceptable conditions for selecting a disposalconstruction site can be found in diverse rock types as granites, metamorphic basement rocks, plastic clays, indurated claystones, evaporites, porous volcanic tuffs and highly compacted volcanic tuffs.
This session is a forum for discussing challenging issues faced by geoscientists including

Thermo-hydro-mechanical-chemical (THMC) modellingprocesses with implications onf radionuclide migration transport and chemical effects on barrier performance

Studies related to the migration of radionuclides through the multibarrier system and radionuclide-rock interaction

Thermodynamics databases and geochemical properties of repository -relevant materials in different host -rocks

Water-rock interactions and flow and transport modellingincluding effects of porewater chemistry and drilling and borehole fluids in hydrogeological site characterization

Characterization of natural and repository-induced bio-geo-chemical effects in repositories

Linking hydrosphere, geosphere and biosphere in long-term evolution studies, including internal and external geodynamic processes

Data digitalization/management and parameter collection

Contributions on the above topics can include all aspects covering lab-scale experimentation, large-scale experiments in underground research laboratories, observation of natural analogues, physics- and data-driven modelling and code development. Contributions dealing with low-level waste and surface disposal are also welcome.

Orals: Tue, 25 Apr | Room G2

Chairpersons: Vanessa Montoya, Thomas Nagel, Fabien Magri
08:30–08:35
National Programme Management & Site Evaluation
08:35–08:45
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EGU23-9968
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ERE3.3
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On-site presentation
Status of Canada’s Geoscientific Site Evaluations for a Deep Geological Repository for Used Nuclear Fuel
(withdrawn)
Sarah Hirschorn, Alec Blyth, Geoff Crann, Andy Parmenter, and Rebekah Wilson
08:45–08:55
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EGU23-16679
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ERE3.3
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Highlight
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On-site presentation
Bo Strömberg, Elena Calota, Michael Egan, and Jinsong Liu

In January 2022, the Swedish government approved the development of a final repository for spent nuclear fuel at the Forsmark site. Leading up to this decision, and after completion of the main review phase in 2018, the Swedish Radiation Safety Authority (SSM) was asked by the Government to comment on new reported experiments related to degradation mechanisms for both the canister’s copper shell and cast iron insert. In particular, decision makers were concerned to ensure that such studies did not raise new questions regarding the suitability of the canister design.

In this paper, we provide an overview and assessment of the relevant degradation mechanisms:

  • Localised sulphide corrosion of copper: Although previously ruled out based on the condition that available sulphide would be consumed by general corrosion, for high sulphide loads passive films may form. Localised corrosion could then be induced by tensile stresses (SCC) or other factors (pitting corrosion).
  • Anoxic corrosion of copper: Although contradictory to conventional thermodynamic understanding, the detection of hydrogen evolution in certain laboratory experiments and an alleged higher than expected corrosion during a 20 year field experiment (LOT) have been proposed as evidence for this process.
  • Radiation induced effects: The initial radiation field is expected to have a limited effect on rates of corrosion and to some extent affect the properties of canister materials.
  • Strain hardening of cast iron: The implications for the canister’s mechanical integrity of deformation events that do not directly lead to canister failure (e.g. large earthquakes) have been proposed as being important in safety assessment.

It has been postulated that these processes represent a significant detriment for canisters, with suggestions that containment lifetimes could be reduced by orders of magnitude. Such claims have, however, typically been made in the absence of relevant data relating to repository conditions, and it has thus not been straightforward for decision makers to assess their accuracy.

SSM´s reviews generally found the first item to be most significant since experiments indeed reveal that passivating films could form under certain conditions. Even so, differences in overall repository performance were assessed to be limited, primarily because any appreciable effect would be restricted to a small number of deposition holes in which it was theoretically possible to sustain combined relatively large mass-transfer rates and high sulphide concentrations. The second item was judged to be of negligible significance, with no plausible connection to the LOT experiments, where detected corrosion is considered to be associated with oxidising conditions. For the third and fourth items, limited effects on canister integrity analysis can be expected and SSM looks for work to be conducted to ensure safety margins are maintained with respect to mechanical performance of canisters.

Although the fruitful Swedish canister debate did not result in a need to modify regulatory conclusions regarding compliance with regulatory criteria, experiences show that new scientific information can always emerge which must be considered in a site specific context regarding e.g. detailed canister design and production as well as safety assessment.

How to cite: Strömberg, B., Calota, E., Egan, M., and Liu, J.: Assessment of canister degradation for the encapsulation of spent nuclear fuel: Key research issues encountered in recent regulatory reviews and government decision making in Sweden, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16679, https://doi.org/10.5194/egusphere-egu23-16679, 2023.

08:55–09:05
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EGU23-2247
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ERE3.3
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Highlight
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On-site presentation
Ute Maurer-Rurack, Andreas Poller, Susie Hardie, Joachim Poppei, Marie Voss, and Paul Smith

Title:  Disposal of HLW and ILW/LLW at the same site: challenges in an international context

Authors:  Maurer-Rurack, U.*, Poller, A., Hardie, S., Poppei, J., Voss, M. **, Smith, P. ***

* Federal Office for the Safety of Nuclear Waste Management (BASE), Berlin, Germany

** CSD Ingenieure AG, Switzerland

*** Safety Assessment Management GmbH, Switzerland

 

Abstract:

Germany is looking for the site with best possible safety for an HLW (high level waste) repository. In the site selection procedure of Germany three host rocks: rock salt, clay and crystalline rock are considered. In accordance with §1 (6) of the StandAG (German Site Selection Act), an additional disposal of ILM/LLW (intermediate level/low level waste) might be allowed at the same site, provided that its characteristics satisfy the best possible safety criteria for the sole disposal of HLW waste. This point will be addressed within the framework of preliminary safety investigations pursuant to §27 (5) of the StandAG in the individual phases of site selection process for the HLW repository, according to the current state of science and technology.

The GemEnd research project (2022, in press) investigates the possible mutual influence of these different types of repositories at the same site and the resulting safety-relevant consequences for the HLW repository system in Germany.

This contribution provides an overview of the currently existing international disposal programs of non-heat-generating waste (ILW/LLW) and heat-generating waste (HAW) at the same site in the following countries: Belgium, France, Finland, Germany, Japan, Sweden, Switzerland and the United Kingdom.

The IAEA (International Atomic Energy Agency) also considers co-disposal of HLW and ILW/LLW as an option that has advantages over separate repositories. But particular critical interactions caused by gas formation and gas transport as well as interactions of cementitious materials with other engineered components and the host rocks should not be forgotten (IAEA 2009, 2018).

On this basis, existing international disposal programs for HLW and ILW/LLW at the same site are compared with each other. Important similarities and differences for the transferability of the findings to the site selection procedure in Germany are presented. Finally, the approach of other countries can contribute to a better assessment of the safety implications in Germany.

 

How to cite: Maurer-Rurack, U., Poller, A., Hardie, S., Poppei, J., Voss, M., and Smith, P.: Disposal of HLW and ILW/LLW at the same site: challenges in an international context, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2247, https://doi.org/10.5194/egusphere-egu23-2247, 2023.

09:05–09:15
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EGU23-16757
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ERE3.3
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Highlight
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On-site presentation
Florian Krob, Judith Krohn, Veronika Ustohalova, Stefan Wittek, and Dimitri Bratzel

In a science-based site selection process (StandAV), the Federal Republic of Germany searches for the site with the best possible safety for a repository of high-level waste (HLW) over a period of one million years. For this purpose, the geological subsurface of the German federal territory must be investigated and evaluated.

Challenges include the large area under investigation, that encompasses the entire federal territory of Germany with its large variability in geology, as well as the verification period which must be met to ensure the best possible long-term safety. Furthermore, an enormous amount of heterogenous geodata will have to be processed.

The application of AI-based methods in geosciences promises high potentials when dealing with large heterogenous data sets and cost- and time-consuming model calculations of complex and coupled processes. Accordingly, research on the application of AI has increased significantly in the geosciences over the last few years.

In our recent study “The use of artificial intelligence (AI) in the site selection process for a deep geological repository”, we succeeded developing an interdisciplinary assessment tool to evaluate the applicability of AI methods in geosciences in general and especially regarding their use for geoscientific issues in the StandAV. Here, we focus on potentials and challenges of applying AI in geosciences with respect to geological key activities in the StandAV. Thus, we emphasize on limitations that may arise from the use of AI regarding key activities in StandAV and propose necessary conditions for its applicability in the future.

Our results show that AI methods are superior to conventional methods, especially when it comes to data management and dealing with large geological data sets and model calculations of complex long-term and coupled geological processes. However, AI methods are generally only transferable to the geoscientific issues in the StandAV with methodological and subject-specific adaptations. Nevertheless, sufficient data, both in quality and quantity, is a prerequisite for the use of AI. Our study also shows that AI should only be used in a supportive way to tackle geological issues in the key activities and must not have any decision-making power when used in the StandAV.

High demands must be placed on the traceability of the applied AI methods. AI methods that do not meet the transparency requirements of the StandAV bear considerable risks of jeopardizing the trust of the population in the participation process. This could increase the general suspicion and scepticism towards AI in the public perception. Therefore, we strongly recommend to always evaluate and validate iteratively all methods and providing results to the public when applied to the key activities of the StandAV.

Title of study: “The use of artificial intelligence (AI) in the site selection process for a deep geological repository”,
a project on behalf of the Federal Office for the Safety of Nuclear Waste Management (BASE-FKZ 4721E03210)

How to cite: Krob, F., Krohn, J., Ustohalova, V., Wittek, S., and Bratzel, D.: Potentials and challenges of applying artificial intelligence (AI) in geosciences for the search of a repository of high-level waste in Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16757, https://doi.org/10.5194/egusphere-egu23-16757, 2023.

09:15–09:25
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EGU23-11590
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ERE3.3
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ECS
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Virtual presentation
Marco Brysch and Monika Sester

The characterization of pores in materials such as Opalinus clay is crucial for understanding the physical properties, including permeability and strength, which are important for the safe disposal of radioactive waste. Scanning electron microscopy (SEM) is a technique that allows high-resolution imaging of these pores at the nanoscale. However, the analysis of SEM images can be challenging due to the resolution limits of nanoscale pores and their manual segmentation. In the development of automatic segmentation methods, approaches of supervised or unsupervised machine learning (ML) and deep learning (DL) methods are increasingly applied. The main advantage of these methods is to achieve fast and more consistent results that do not rely on user input.

An essential component in DL is the so-called backbones, which can learn object features that are necessary for object recognition. In image processing, objects are recognized through groups of specific features that allow an unambiguous identification. Pre-trained backbones, which have been trained on large datasets such as ImageNet containing millions of everyday images, possess a wide range of features that are useful during image processing tasks. However, specialized applications, such as the automatic analysis of microscope images using DL may require features that differ from those of pre-trained backbones. The limited availability of SEM images makes it difficult to effectively train DL models, as these models typically require a large amount of data to learn new features. In these cases, ML methods may perform better due to their ability to use carefully selected, expert-defined features [Maitre et al., 2019].

In this study, the training behavior of eight different DL backbones was examined using a dataset of 2000 SEM images showing both the background and pores of an Opalinus clay sample. The backbones studied included VGG16, VGG19, ResNet50, Desenet, Xception, and Mobilenet. To train these models with the relatively small amount of training data available, a transfer learning technique was applied. We analyzed gradient-weighted class activation mappings (grad-CAM) [Selvaraju et al.,2019] during the learning process to obtain a general sense of the behavior of the different backbones. Through analysis of the model's adaptation efforts, the present study demonstrates which pre-trained backbones show good training behavior on SEM images and provides an estimation of the amount of data needed for effective training.

 

References

[Maitre et al., 2019] Maitre, J., Bouchard, K., and Bédard, L. P. (2019). Mineral grains recognition using computer vision and machine learning. Computers & Geosciences, 130:84–93.

[Selvaraju et al., 2019] Selvaraju, R. R., Cogswell, M., Das, A., Vedantam, R., Parikh, D., and Batra, D. (2019). Grad-cam: Visual explanations from deep networks via gradient-based localization.

How to cite: Brysch, M. and Sester, M.: Evaluating the Capabilities of Backbones for Scanning Electron Microscopy Images of Opalinus Clay, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11590, https://doi.org/10.5194/egusphere-egu23-11590, 2023.

09:25–09:35
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EGU23-17099
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ERE3.3
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Highlight
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On-site presentation
Emiliano Stopelli, Niklaus Waber, Michael Heidinger, Gesine Lorenz, Herwig Müller, and Dani Traber

The Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) carried out an underground investigation program for a comparative evaluation of possible sites for a deep geological repository (DGR) for nuclear waste. Between 2019 and 2022, one inclined and eight vertical multi-purpose exploratory boreholes were drilled in three siting regions in northern Switzerland.

In the frame of these works, accurate groundwater sampling and analytical data interpretation has been fundamental to derive solid information on groundwater composition, evolution, and residence times in aquifers limiting the low permeability sequence of the host rock (Opalinus Clay) and the confining units.

Eighteen deep groundwater samples were successfully collected from the Malm, Keuper and Muschelkalk aquifers at depths between approx. 350 m and 1150 m below ground level. The following selected highlights and lessons learned regarding hydrochemical groundwater characterisation will be presented:

  • Procedures to correct analytical results for the contamination of groundwater with different types of drilling fluids, using artificial and environmental tracer data and geochemical modelling approaches;
  • Krypton-81 (T1/2 229 ka) in the groundwaters was successfully analysed and, combined with hydrochemical and isotopic data, helped to constrain groundwater mixtures and model residence times far beyond the classical carbon-14 method;
  • Tools to cope with technical challenges at a drill site resp. in deep exploratory boreholes (i.e. time constraints, low groundwater flow rates).

We demonstrate that a high level of quality for hydrochemical and isotope data of groundwaters can be reached even under challenging operational conditions.

The hydrochemical data of groundwater and the profiles of natural tracers in pore water are relevant datasets to constrain the site model used in the site selection process and in the long-term performance assessment for a DGR.

How to cite: Stopelli, E., Waber, N., Heidinger, M., Lorenz, G., Müller, H., and Traber, D.: Highlights and lessons learned from groundwater investigations in the framework of Nagra’s exploratory boreholes for a deep geological repository, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17099, https://doi.org/10.5194/egusphere-egu23-17099, 2023.

09:35–09:45
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EGU23-16333
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ERE3.3
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Highlight
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On-site presentation
Thomas Gimmi, Lukas Aschwanden, H. Niklaus Waber, Eric C. Gaucher, Jin Ma, and Daniel Traber

Clay-rich rocks have very low hydraulic permeability and they also have good chemical retention properties for cationic contaminants. This makes them ideal as host rocks for the underground disposal of radioactive waste. In Switzerland, the Opalinus Clay Formation, Jurassic sediments deposited ~174 Ma ago, is envisaged as potential host rock. A large drilling campaign has recently been run in three potential siting regions in northern Switzerland. Drill core samples from a ~400 m thick Mesozoic low permeability zone were obtained at high spatial resolution in one slanted and eight vertical boreholes. Data including various natural tracers were obtained from these core samples. Here we report on the profiles of the stable isotopes δ18O and δ2H in porewater and groundwater in the eight vertical boreholes. The distribution of the tracers results from hydrogeological and transport processes acting in the past, and the profiles can be interpreted as results of ‘experiments performed by nature’. Hydrogeochemical investigations of groundwater and veins mineralisations help to constrain the temporal evolution of the system and to assess the system’s large-scale transport properties. The comparably large number of boreholes allows us comparing the observed depth distributions of the two tracers not only vertically, but also laterally in the three regions (~15-20 kilometers apart), and in the 2 to 4 boreholes in each region (a few kilometers apart). The isotope profiles from the different boreholes show many similarities, but also distinct features that are mainly related to the lateral variation in aquifer properties. The regional aquifers in the Malm (one locality Hauptrogenstein) and Muschelkalk typically build the upper and lower boundary, respectively, of the δ18O and δ2H profiles. In some, but not in all tracer profiles, there are indications of a local Keuper aquifer in the lower part. The variability reflects the lithological heterogeneity of this rock unit in the lateral dimension. The maximum isotope values plot to the right of the GMWL, are often similar and are found in the central part of the profiles in the Opalinus Clay Formation. Towards the Keuper aquifer (if present), the values decrease and approach the GMWL, and often some gentle decrease is also observed towards the upper aquifer, without reaching the GMWL. Towards the Muschelkalk aquifer, the values decrease sharply and reach the GMWL. The shapes of the profiles hint to the importance of diffusive transport processes over large spatial and geologic time scales. With transport simulations, we try to narrow down the timing of any changes in the aquifer signatures in the more recent past (10 ka to few Ma ago), as well as to assess the importance of various transport mechanisms in the development of the profiles. The interpretation of such tracer profiles is a key element with respect to the assessment of the large-scale transport properties of a host rock.

How to cite: Gimmi, T., Aschwanden, L., Waber, H. N., Gaucher, E. C., Ma, J., and Traber, D.: Profiles of δ18O and δ2H in porewater of a Mesozoic rock sequence: Regional variability and relation to large-scale transport regimes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16333, https://doi.org/10.5194/egusphere-egu23-16333, 2023.

Safety Assessment
09:45–09:55
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EGU23-6380
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ERE3.3
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ECS
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Highlight
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On-site presentation
Adam Paxton, Jiejie Wu, David Applegate, Andrew Price, Tim Hicks, and Callum Eldridge

Nuclear Waste Services (NWS) goal is to ensure radioactive waste produced in the UK is managed safely and securely. This includes being responsible for the siting and construction of the UK’s Geological Disposal Facility (GDF) infrastructure for higher-activity radioactive waste. This waste includes large quantities of fissile nuclides, therefore a safe GDF design must account for the risk of a subsurface criticality event (i.e. a chain fission reaction). Fissile nuclides will be dispersed in a GDF over post-closure timescales governed by uncertain processes such as corrosion, radioactive decay, diffusion and reactive transport. To account for the uncertainty in the future evolution of a GDF, safety assessments apply probabilistic modelling and compute thousands of radionuclide migration scenarios. Previous assessments using existing models have used expert human judgement to manually estimate the worst-case scenario of radionuclide migration and then used a criticality code to calculate the effective neutron multiplication factor (i.e. reactivity). However, this manual approach may fail to select the most reactive scenario. Therefore, a new codebase has been developed by Jacobs for NWS in order to automate the safety assessment process, considering all scenarios and identifying trends in reactivity. The new codebase, GoldSim Monte Carlo Interfacing Tool (GMIT), comprises a set of Python modules and scripts which utilise parallel processing to allow the user to couple thousands of scenarios from a reactive transport model of a GDF with a choice of criticality code (MONK or MCNP). The tool is developed with user customisability in mind, and includes optimisation features to minimise space requirements of the GDF design while remaining safely below a prescribed reactivity threshold. This talk will discuss the main features of GMIT and will present some preliminary results that have been obtained through its use in support of the UK GDF criticality safety programme.

How to cite: Paxton, A., Wu, J., Applegate, D., Price, A., Hicks, T., and Eldridge, C.: Automated criticality calculations for radionuclide migration scenarios in the geological disposal of radioactive waste., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6380, https://doi.org/10.5194/egusphere-egu23-6380, 2023.

09:55–10:05
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EGU23-7559
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ERE3.3
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Highlight
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On-site presentation
Jobst Maßmann, Jan Thiedau, Maximilian Bittens, Vinay Kumar, and Tuong Vi Tran

Within the ANSICHT project, BGE Technology, BGR, and the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) developed a safety assessment methodology for a high-level radioactive waste repository in clay formations based on German legal requirements. This included the quantification of integrity criteria and their exemplary numerical evaluation by THM-modeling. In the project's second phase, the modeling concept was extended by a detailed near-field model and initial attempts were made to quantify uncertainty. For the safety assessment of a repository system, detailed knowledge of the processes in the nearfield, the processes in the host rock, and the evolution of the whole system up to the surface is equally important. Concerning the prediction period of 1 million years and the complex coupled processes triggered by the construction and operation of an underground repository, the computational effort for numerical modeling is enormous. To cope with this effort, the proposed concept consists of models with different resolutions, dimensions, considered processes, and local coverages: a detailed 3D model of the near-field, a 2D and a 3D model of the whole domain, with the 2D model allowing a much higher resolution. The associated mathematical models are solved using OpenGeoSys 6, a finite-element-simulator allowing fully implicit process couplings. A statement about the integrity criteria and the general THM behavior is made integrally by all three models. Furthermore, a simplified 2D model is used to quantify the effect of variations in material parameters on the uncertainty of the results with manageable effort. Various geological setups have been used to investigate the influence of large inhomogeneities, such as sand lenses or layers in meter- to kilometer-scale, on the integrity of the host rock.

In this contribution, the updated modeling concept is presented and applied illustratively to a generic but typical geological situation in northern Germany.

 

How to cite: Maßmann, J., Thiedau, J., Bittens, M., Kumar, V., and Tran, T. V.: Concept for Numerical THM Integrity Analysis of a Generic Repository in Clayey Rock in Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7559, https://doi.org/10.5194/egusphere-egu23-7559, 2023.

10:05–10:15
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EGU23-15492
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ERE3.3
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ECS
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Virtual presentation
Carlos Guevara Morel, Jobst Maßmann, and Jan Thiedau

Crystalline rock is one of the potential rocks under consideration in Germany for hosting a heat-generating nuclear waste repository. The focus of this contribution is a repository system, which relies on the Containment Providing Rock Zone (CRZ) in a crystalline rock as the principal containment barrier. In compliance with the German safety requirements, the integrity of the CRZ is required, i.e. to keep its containment capabilities for a period of 1 million years. This implies that the formation of new pathways must be excluded, temperature development must not significantly impair the barrier effect and anticipated stresses and fluid pressures should not exceed the dilatancy strength and the fluid pressure capacity, respectively. To meet this requirement a safety-oriented assessment of mechanical, hydraulic, thermal and chemical processes, as well as their couplings, occurring in the host rock due to the storage of heat-generating nuclear waste, excavation and/or gas production among others, is needed and numerical modelling is an essential and powerful tool for it.

Fractures and other types of discontinuities, which usually characterize crystalline rock, are expected to influence the hydraulic behavior of the system. Thus, an adequate representation of the fracture network is required in order to capture its relevant properties, which will ultimately define the hydraulic boundary conditions surrounding the CRZs. Typically it is only possible to characterize fracture networks statistically. This requires a systematic investigation to quantify the influence of multiple realizations on the repository system.

The decay-heat produced by the heat-generating nuclear waste leads to an increase of the local temperature through which the host rock as well as the fluid will expand. Consequently, a change in the effective stresses is also expected. With the aim of obtaining a better representation of the temperature-induced fluid pressure increase a bi-directional coupling of the hydraulic-mechanical (HM) processes is being tested.

Previous work done by [1] based on a one-directional coupling between the HM processes for the estimation of the fluid pressure and one realization of a statistically generated generic geological model. This contribution aims to further develop the work from [1] by using a bi-directional coupling between the HM processes, implemented in the open-source finite element code OpenGeoSys version 6, to calculate the thermal induced fluid pressure increase and quantify the impact on the integrity criteria. Moreover, preliminary results for an approach used to evaluate the influence of multiple statistically equivalent fracture networks on the repository will be presented.

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., Maßmann, J., and Thiedau, J.: Further development of a numerical modeling concept of the coupled THM behavior within a generic nuclear waste repository in crystalline rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15492, https://doi.org/10.5194/egusphere-egu23-15492, 2023.

Coffee break
Chairpersons: Koen Beerten, Jobst Maßmann
10:45–10:50
Thermal-Hydraulic-Mechanical (THM) Modeling
10:50–11:00
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EGU23-8791
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ERE3.3
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On-site presentation
Kyle Mosley, Lee Hartley, Robert Turnbull, and Neal Josephson

Many nations are electing to deal with high-level radioactive waste by developing and licensing deep geological repositories (DGRs). A key objective of DGR safety cases is to demonstrate that the engineered and natural barrier systems will remain resilient and functional far into the future, ensuring that no harmful contaminant levels ever reach the biosphere. DGR safety cases are typically supported by numerical modelling of the repository system and its environment, simulating the thermal, hydraulic, mechanical and chemical (THMC) coupled processes that influence the barrier system evolution over such timescales.

Numerical modelling of DGRs often calls for representation of complex geologies and closure engineering with varying scales of structures and physical processes, requiring unstructured meshes to balance accuracy and efficiency. Crystalline host rocks present particular challenges because simulation of THMC processes often involves both 3D volumetric meshes to model porous media such as engineered barriers, and discrete fracture network (DFN) meshes to represent the fractured host rock. Furthermore, the required mesh type may differ depending on the physics to be simulated. For example, thermal and mechanical processes, which are primarily transmitted via the bulk rock mass, are best simulated on volumetric meshes. Modelling of hydraulic processes and their associated chemical interactions, by contrast, may be undertaken on either a DFN or volumetric mesh, depending on the medium and the level of geometric detail required.

Here, we present examples of near-field THMC simulations enabled by new meshing workflows in the FracMan® modelling software. The workflows facilitate creation of a flexible set of medium representations, including so-called ‘hybrid’ meshes consisting of both volumetric and DFN representations in different regions of a single model.

In the case studies, illustrative DGR components are meshed using a coupling to the LaGriT gridding library, which supports the creation of unstructured meshes with deformable tetrahedral elements that accurately capture curved and complex geometries. Fractured rock sections of the model are represented either by a DFN mesh of 2D triangular elements or by a volumetric mesh, depending on the physics. In both cases, targeted mesh refinement is applied to regions of greater geometric complexity and/or steep numerical gradients, facilitating simulation of variable-scale processes. Iterative smoothing algorithms are employed to create a high-quality Delaunay mesh, with quality metrics included to assess the likely performance in simulations. In the final step, the mesh is converted to its Voronoi dual; a discretisation that is optimal for finite volume simulators that employ a two-point flux approximation. It also provides a flexible mesh connectivity topology, which allows for high-fidelity representation of material property contrasts and heterogeneity.

The generated meshes are applied in example THMC simulations, undertaken in the subsurface flow simulator PFLOTRAN together with FracMan geomechanics codes. FracMan post-processing calculations are used to simulate particle flow paths, providing input to performance assessments. The results demonstrate how the developed workflows are an innovative combination of methods that provide accurate and efficient solutions to some specific challenges of DGR systems – namely, strong contrasts in the dimensions and properties of individual barrier system components.

How to cite: Mosley, K., Hartley, L., Turnbull, R., and Josephson, N.: Workflows for generation of high quality meshes for simulating THMC processes in porous and fractured media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8791, https://doi.org/10.5194/egusphere-egu23-8791, 2023.

11:00–11:10
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EGU23-13593
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ERE3.3
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On-site presentation
Christian Silbermann, Florian Zill, Tobias Meisel, Friederike Tiedtke, Dominik Kern, Anton Carl, Andreas Jockel, Thomas Nagel, Olaf Kolditz, Heinz Konietzky, and René Kahnt

To properly assess the present and future conditions of potential nuclear waste repository sites, understanding their evolution in the past is mandatory. Here, glaciation cycles strongly affect the long-term thermo-hydro-mechanical (THM) evolution of the geosystem.
The AREHS project studies the effects of time-dependent boundary conditions on the evolution of large-scale hydrogeological systems. The focus is on numerical long-term modeling taking into account THM couplings. On the basis of generic geological models for different host rock formations, complex 3D THM simulations are performed. The long-term evolution during glacial cycles is simulated using the open-source multi-field finite element code OpenGeoSys, as well as multiple pre- and postprocessing tools integrated into an automatized workflow. This workflow facilitates testing/benchmarking and improves reproducibility as well as overall software quality in a sense of modularization. The impact of the glacial THM loading and atmospheric temperature evolution is taken into account using appropriate time-dependent THM boundary conditions. The simulation results are analyzed with respect to potential safety-critical parameters, such as maximum temperature, hydraulic pressure, subsidence, equivalent effective stress and strain. Some general conclusions will be drawn for the host rock clay in Germany.

Funding:

This research is funded by the Federal Office for the Safety of Nuclear Waste Management under Grant No. 4719F10402 (AREHS project).

How to cite: Silbermann, C., Zill, F., Meisel, T., Tiedtke, F., Kern, D., Carl, A., Jockel, A., Nagel, T., Kolditz, O., Konietzky, H., and Kahnt, R.: Automatized large-scale 3D THM simulations capturing glacial cycle effects on German nuclear waste repositories in clay rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13593, https://doi.org/10.5194/egusphere-egu23-13593, 2023.

11:10–11:20
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EGU23-8787
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ERE3.3
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On-site presentation
Anton A. Popov, Tobias S. Baumann, Boris J.P. Kaus, Maximilian O. Kottwitz, and Janos L. Urai

Various salt structures, such as diapirs and salt walls, are currently considered as potential locations for the long-term storage of radioactive waste. Yet many scientific questions need to be addressed before safety and integrity of such repositories can be ensured. In particular, the effect of cyclic glacial loading on the long-term stability of a potential nuclear waste repository needs to be investigated.

In the first part of this study, we perform a series of simplified 2D thermo-hydro-mechanical model scenarios selected to cover the basic geometrical features of salt diapirs and walls. Our models include internal heterogeneities in the salt and faults in the salt and overburden. We consider various repository design schemes and investigate the effect of the radiogenic heat production on the deformation of the surrounding salt in a thermo-mechanically coupled manner. We explore the sensitivity of the results to variations in the physical rock properties and evaluate the potentially significant effect of the different dominant microphysical deformation mechanisms such as dislocation creep, dynamic recrystallization and pressure solution creep in salt.

The second part of this study focuses on the parameterization of the relevant salt rheology that controls the viscoelastic response of the salt structure. We quantify the rock salt rheology in a probabilistic sense by implementing a statistical creep model. Here, we incorporate a priori constraints and observations from the microstructure and determine representative creep law parameters and associated uncertainties.

Following this integrated approach, we aim to identify key model parameters and find characteristics of generic salt structures to assist the selection process of suitable salt structures.

How to cite: Popov, A. A., Baumann, T. S., Kaus, B. J. P., Kottwitz, M. O., and Urai, J. L.: Radioactive waste storage in salt structures under cyclic glacial loading, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8787, https://doi.org/10.5194/egusphere-egu23-8787, 2023.

11:20–11:30
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EGU23-16935
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ERE3.3
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Highlight
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On-site presentation
Yuxin Wu, Jiannan Wang, Sebastian Uhlemann, Shawn Otto, Brian Dozier, and Kristopher Kuhlman

Salt is an ideal medium for permanent isolation of heat-generating radioactive waste due to its tightness regarding fluid flow, high thermal conductivity, and ability to creep close fractures. Understanding the thermal-hydrological-mechanical (THM) processes, including the resulting brine migration, is a key part of the scientific basis for safe radioactive waste disposal in salt formations. Underground at the Waste Isolation Pilot Plant (WIPP), to study brine migration near an excavation during active heating, we conducted joint in-situ geophysical monitoring experiments using electrical resistivity tomography (ERT) and high-resolution fiber optic-based distributed temperature sensing (DTS), during a controlled heating experiment. ERT electrode arrays and fiber optic sensors were cemented into parallel horizontal boreholes. In addition, DEM (Discrete Element Model) based numerical simulations were conducted to simulate the THM processes during heating to understand better the mechanisms that led to changes in these geophysical measurements. During heating, resistivity changes near the heater can be explained well with a simple temperature effect. Yet, at more distant regions that were cooler, the resistivity decreased much more than is predicted from temperature effects alone. DEM simulations indicate that brine migration, driven by a pore pressure gradient, is likely the primary reason for the significant resistivity decrease beyond temperature effects. Comparison between the predicted ERT responses and observations is much improved when considering the DEM simulated brine migration effects. In support of our understanding of salt for radioactive waste disposal, this geophysical and simulation evidence provided mechanistic insights into field-relevant THM processes. 

How to cite: Wu, Y., Wang, J., Uhlemann, S., Otto, S., Dozier, B., and Kuhlman, K.: Joint geophysical and numerical insights of the coupled thermal-hydro-mechanical processes during heating in salt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16935, https://doi.org/10.5194/egusphere-egu23-16935, 2023.

11:30–11:40
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EGU23-8051
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ERE3.3
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ECS
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On-site presentation
Adriana Quacquarelli and Frédéric Collin

In the context of nuclear energy production, the storage of radioactive waste deep underground in geological disposal facilities (GDF) is a promising solution to ensure the protection of people and the environment in the short and long term.

Due to the tunneling and storage operations, the thermodynamic equilibrium of the site is disturbed, triggering some damage and generating an interconnected fracture network around the gallery where the hydraulic conductivity can increase by several orders of magnitude [1] and facilitating the potential release and migration of radionuclides into the soil. However, once the gallery is closed and after the emplacement of waste, fractures can be sealed through resaturation by water coming from the claystone as a function of its self-sealing potential, i.e., the capacity of the rock to restore its hydraulic permeability leading to the hydraulic closure of the fracture [2]. During this self-sealing process, no structural changes are observed, meaning that this is a purely hydraulic process inducing any mechanical strengthening or bond at the interface. Due to their low permeability and their swelling capacity that favors the recovery of their hydraulic properties, clay materials prove to be eligible for nuclear waste storage. In France, Andra (Agence Nationale pour la gestion des Déchets Radioactifs) has selected the Callovo Oxfordian claystone (Cox), while the Boom Clay is studied as the host rock in the Belgian context and the Opalinus Clay in MontTerri in Switzerland. Over the last two decades, many experimental studies have been carried out to better understand the self-sealing process of such clay formations. In all cases, the final permeability is very close to the original one, which is not actually really achieved. Moreover, plastic clays such as the Boom Clay and the Cox show that the resaturation induces some secondary micro-cracks around the main fractures where the material can swell quickly favoring the sealing process.  

Based on these experimental observations, the objective of the following work is to describe the hydro-mechanical behavior of the fracture taking into account the self-sealing capacity through an appropriate constitutive equation that is implemented in a 2D finite element model. This model will then be validated through comparison with experimental results on Boom clay and Cox claystone samples tested in the laboratory and will allow further insight into the physical phenomena.

REFERENCES

[1]      G. Armand et al., “Geometry and properties of the excavation-induced fractures at the meuse/haute-marne URL drifts,” Rock Mech. Rock Eng., vol. 47, no. 1, pp. 21–41, Jan. 2014, doi: 10.1007/s00603-012-0339-6.

[2]      W. Bastiaens, F. Bernier, and X. L. Li, “SELFRAC: Experiments and conclusions on fracturing, self-healing and self-sealing processes in clays,” Phys. Chem. Earth, vol. 32, no. 8–14, pp. 600–615, 2007, doi: 10.1016/j.pce.2006.04.026.

How to cite: Quacquarelli, A. and Collin, F.: An interface constitutive model for fractured clayey formations in the context of nuclear waste disposal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8051, https://doi.org/10.5194/egusphere-egu23-8051, 2023.

11:40–11:50
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EGU23-1462
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ERE3.3
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ECS
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Highlight
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On-site presentation
Ajmal Gafoor, Christian Müller, and Philipp Herold

Rock salt, claystone and crystalline rock are considered as potential host rocks for the disposal of high-level radioactive waste in Germany. Rock types, properties, and composition determine how these potential geologic barriers contribute to the containment, isolation, and retardation of the radioactive waste. Crystalline rocks are anisotropic and heterogeneous due to the presence of discontinuities. Therefore, the consideration of discontinuities in numerical models is crucial with regard to the performance assessment of radioactive waste repository systems. The project aims at developing a modelling strategy that enables effective modelling of fractures in crystalline rock. The continuum modelling approach using a Ubiqitous-Joint model is adopted to model the rock fractures. Its applicabilty for large-scale anyalses of anisotropic rock masses is being investigated by developing the numerical models based on predefined structural-geologic information. The influence of fracture orientations on the modelling of the rock masses will be studied, and their impact on performance assessments for the safe disposal of high-level radioactive waste in crystalline rock will be assessed.

How to cite: Gafoor, A., Müller, C., and Herold, P.: Application of Ubiquituous-Joint Model for Modelling Fractures in Crystalline Rock Formations: Preliminary Studies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1462, https://doi.org/10.5194/egusphere-egu23-1462, 2023.

11:50–12:00
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EGU23-11317
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ERE3.3
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On-site presentation
Michael Kröhn and Klaus-Peter Kröhn

Visual observation of flow and transport processes in fractures requires transparent replicas. Quite easily realized are parallel plate models, which pose only a quite rough approximation, though, and require certain geometric conditions. A better representation can be gained by impressions from real fractures, either by forming the free space, a common technique but rather effortful, or by epoxi imprints of the fracture surfaces. Accurate surface measurements and determination of contact pressures indicate, though, that several imprints of the same locations may show significant differences.

A rather new class of transparent physical models has been made possible with the introduction of reasonably accurate 3D-printers in combination with transparent resins.  Hydraulic tests with principle models of single fractures as well as DFNs have been established quite early. Realistic single fracture replicas still pose a problem, though.

Three steps are required to produce a fracture replica by this method:
1. 3D-scanning of the fracture surfaces
2. Preparation of a printable digital model of the fracture
3. 3D-printing of the digital model

This procedure has a lot of appeal as repeated printing of the same physical fracture model allows for parallel as well as repeated destructive tests.  Additionally, it rather elegantly avoids the problem of air enclosure and bubble evolution between resin and fracture surface which is a common problem with resin imprints. Moreover, it is possible to add features to the digital model that facilitate hydraulic tests such as connectors to inflow and outflow tubes. Since it was intended to cover the whole production process of this method, a 3D-scanner as well as a 3D-printer have been acquired accordingly.

However, new challenges appear also at all three stages of production. One obvious point is the accuracy. The coordinates of a fracture surface can of course only be sampled at a limited number of scanning points. On the same scale, also the dimensional accuracy of the 3D-Printer is restricted. Less evident is the problem of alignment of the two fracture surfaces. Snapping points of opposing fracture surfaces at a distance of less than one millimetre have been found in printed replicas of 7 x 10 cm size, suggesting a potentially serious impact on the aperture distribution by misalignment. Another point concerns the general ability of plastics to take up water, which affects the resin material to a considerable extent in that weight and size change with time. Details and solutions to these problems are addressed.

In closing, the repeatability of an actual tracer test in a printed fracture replica is investigated. The experimental setup consists of an upper and a lower part. Transport of a colored solution in the fully water-filled replica has been observed with an industrial camera and repeated three times. By post-processing the probability of the presence of the tracer at each pixel was evaluated. The resulting video reveals a reasonable degree of repeatability.

How to cite: Kröhn, M. and Kröhn, K.-P.: Manufacturing of physical models by 3D-scanning and -printing for fracture flow tests, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11317, https://doi.org/10.5194/egusphere-egu23-11317, 2023.

12:00–12:10
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EGU23-4206
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ERE3.3
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ECS
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On-site presentation
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Tan-Minh Le, Xuan-Xinh Nguyen, and Jia-Jyun Dong

A thorough understanding of the hydro-mechanical behavior of rock joints is essential since joints typically serve as weak planes and fluid channels within rock masses. Joint roughness, among other factors, dominates the mechanical and hydraulic behavior of rock joints directly. Since 1970s, the joint roughness coefficient (JRC) has been used extensively to quantify joint roughness. As a result of using this variable in empirical equations, we can now anticipate the fluid flow characteristics under stresses. However, the validity of utilizing a single parameter to express the complicated geometry of joint surfaces should be testified. We used three-dimensional (3D) printers to produce a large number of joint samples with similar JRC (2 - 4) but varying surface geometry, including matched and mismatched joint surfaces. The mechanical and hydraulic apertures (E and e) of the printed joint samples were determined utilizing high confining pressure, permeability/porosity measurement equipment (YOKO2). The relationship between E and e can thus be quantified. The results indicate that the stress-dependent mechanical apertures of matched and mismatched joints with similar JRC is significantly different and result in the difference in e-E relationship. However, the measured e-E relationship is quite similar for matched or mismatched smooth joints regardless of joint surface geometry. To conclude, JRC is an appropriate index of smooth joint roughness for representing the stress-dependent e and E and accompanying e-E relationship. 

 

 

How to cite: Le, T.-M., Nguyen, X.-X., and Dong, J.-J.: Mechanical/Hydraulic Aperture of 3D-Printed Joints in Relation to JRC and Surface Geometry , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4206, https://doi.org/10.5194/egusphere-egu23-4206, 2023.

12:10–12:20
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EGU23-9636
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ERE3.3
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On-site presentation
Jörg Buchwald, Sonja Kaiser, Wenqing Wang, Olaf Kolditz, and Thomas Nagel

For the evaluation of the performance of nuclear waste repositories, a detailed investigation of the sensitivity and uncertainty of the underlying processes is essential. The verification and validation of the numerical tools under realistic conditions using experimental data from underground research laboratories is all the more crucial because a comprehensive experimental analysis or investigation of the final repository site is impractical for a variety of reasons. In our contribution, we apply design-of-experiment-based history matching to reduce and examine parameter uncertainties with pressure and temperature data from the FE experiment. The FE experiment is a full-scale multiple heater test in the Opalinus clay at the URL site in Mount Terri in Switzerland. We use the open-source program OpenGeoSys for the deterministic thermo-hydro-mechanical modeling. An initial parameter screening to identify heavy hitters was conducted prior to proxy building using a Gaussian Proxy model. The proxy model is then used for experiment-matching, employing Monte-Carlo sampling over the entire remaining parameter space. The obtained parameter boundaries were applied to a global sensitivity analysis based on the proxy model to quantify uncertainty and show how parameter sensitivities affect results.

How to cite: Buchwald, J., Kaiser, S., Wang, W., Kolditz, O., and Nagel, T.: Uncertainty reduction by DoE-based history matching of the FE-experiment at Mt. Terri, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9636, https://doi.org/10.5194/egusphere-egu23-9636, 2023.

12:20–12:30
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EGU23-1992
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ERE3.3
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ECS
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Highlight
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On-site presentation
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Moritz Ziegler and Oliver Heidbach

The undisturbed stress state of the subsurface is a key parameter for the assessment of geological barrier integrity in deep geological repositories. A robust knowledge of the stress state is required for forward simulations that predict the evolution of the deep geological repository during building, canister emplacement, and post-closure phase. However, stress magnitude data will always be sparse and point-wise and thus not sufficient to allow a continuous characterization of the stress field. 3D geomechanical-numerical models are incorporated to mitigate this shortcoming. The significance of such a model is tied to the stress magnitude data records whose availability is generally low and subject to measurement errors. In turn, the uncertainties of the modelled 3D stress state are large which lowers the predictive values of the ensuing forward models. Here, we present a novel approach to reduce the uncertainties of a modelled stress state (Ziegler and Heidbach, 2023).

In a first step we estimate all possible and realistic stress states with respect to the available stress magnitude data and their uncertainties. This provides an overall quantification of the range of stress states that are supported by data. In a second step we limit this usually very large range of stress states using additional indirect stress information to determine upper and lower limits. Formation integrity tests (FITs) are used as a lower limit for the stress state. Observed borehole wall failures (drilling induced tensile fractures and borehole breakouts) provide an estimate of the quality of a modelled stress state in relation with estimations of the rock strength. Observed seismicity and observed seismological quiescence in connection with a failure criterion is used as an upper constraint of the stress state. These additional data allow to identify the probability of a modelled stress state based on its agreement with observations. A weight is estimated for each modelled stress state in a formalized Bayesian approach. This allows an improvement in the significant interpretation of the initial stress state in terms of its robustness and transparency.

Results from an explanatory model and a case study in the Bavarian Molasse Basin show an improved significance in terms of clearly reduced model uncertainties. The amount of uncertainty reduction, however, depends significantly on the quality, suitability and assessment of the additional information.

 

Reference:

Ziegler, M. O., & Heidbach, O. (2023). Bayesian quantification and reduction of uncertainties in 3D geomechanical-numerical models. Journal of Geophysical Research: Solid Earth, 128, e2022JB024855. https://doi.org/10.1029/2022JB024855

How to cite: Ziegler, M. and Heidbach, O.: Uncertainties in geomechanical models – A novel concept to data-driven stress modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1992, https://doi.org/10.5194/egusphere-egu23-1992, 2023.

Lunch break
Chairpersons: Alwina Hoving, Theresa Hennig
Engineered Barrier System & Excavation Damage Zones
14:00–14:10
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EGU23-16189
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ERE3.3
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Virtual presentation
Ursula Alonso, Tiziana Missana, Miguel Garcia-Gutierrez, and Patrik Sellin

Smectites are selected as barriers in deep geological repositories for high-level radioactive waste repository due to their high swelling capacity and contaminant retention ability. Erosion of the clay barrier can affect repository safety and eroded particles may facilitate radionuclide migration within the host rock fractures. These processes depend on the physicochemical characteristics of the clay, on its structural properties and are affected by the chemical equilibrium established with the groundwater from the geological formation.

In order to assess the relationship between clay colloidal properties and the erosion and sedimentation behaviour in fractures, this study correlates the erosion behaviour of a compacted bentonite barrier, under repository conditions, simulating expansion and sedimentation in artificial fractures with physicochemical macroscopic properties (viscosity, turbidity,...) of different smectite suspensions. Studies are carried out under different geochemical conditions.

Results showed that sodium smectites had the highest viscosities, all showing similar behaviour: at low ionic strengths the viscosity remained constant and around 10 mM increased significantly. In spite of forming smaller particles, their expansion and sedimentation is hindered by their high viscosity. In contrast, clays with dominant bi-trivalent cations in their structure, has lower viscosity values, independent on the water ionic strength. Due to their higher particle size and lower viscosity, higher sedimentation in rock fractures is expected.

Turbidity measurements over time were done to assess sedimentation behaviour. It was observed that sodium smectites remained stable over time, even at high ionic strengths, as observed in viscosity studies. With calcium clays, turbidity decreased rapidly, indicating fast sedimentation occurred.

The study contributes to predict the erosion behaviour of the clay barrier in the fractures of the geological formation of a deep geological repository, starting from the colloidal properties of each clay.

This work was partially supported by the Swedish Nuclear Fuel and Waste Management Co, SKB (Sweden) and by the Spanish Ministry of Innovation and Science (PID2019-106398GB-I00, ARNO Project).

How to cite: Alonso, U., Missana, T., Garcia-Gutierrez, M., and Sellin, P.: Colloidal properties of clays related to the erosion and sedimentation behaviour of bentonite in fractures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16189, https://doi.org/10.5194/egusphere-egu23-16189, 2023.

14:10–14:20
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EGU23-3224
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ERE3.3
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On-site presentation
Klaus Wieczorek, Katja Emmerich, Rainer Schuhmann, Jürgen Hesser, Markus Furche, Hua Shao, David Jaeggi, Senecio Schefer, José Luis García-Siñeriz, Franz Königer, Juan Carlos Mayor, Simon Norris, and Chang Seok Kim

Safety of deep underground repositories for high-level radioactive waste is ensured by a multi-barrier system consisting of the geological barrier (host rock) and the engineered barrier system (EBS). Shaft seals are a major part of the EBS. The German regulator demands that the interplay of barriers has to be optimized in diverse redundancy to increase reliability and robustness of the barrier system. Shaft seals should also be constructed of diverse redundant components. The design of shaft seals for generic site models of Germany includes hydraulic sealing elements that are to be realized as Sandwich sealing systems. In contrast to conventional hydraulic seals of monolithic bentonite the Sandwich sealing system consists of sealing segments (DS) of bentonites and hydraulically conductive equipotential segments (ES) (Nüesch et al., 2002). Formation water, that is penetrating the hydraulic seal via preferential flow paths is contained in the ES and evenly distributed over the cross section of the seal. Thus, a more homogeneous hydration and swelling of the DS is obtained.

In July 2019, after a two-years planning phase (Emmerich et al., 2019), a large-scale experiment was launched at the Mont Terri rock laboratory (MTRL) to demonstrate the feasibility of installation, to investigate the saturation process, to qualify monitoring techniques, and to assess the sealing effectiveness. The in-situ experiment is supported by laboratory experiments at different scales and by numerical simulation.

The in-situ experiment consists of two experimental shafts of 1.18 m diameter and 10 ‑ 12.6 m depth located in the sandy facies of the Opalinus Clay. The DS in both shafts are constructed of German Ca-bentonites, while the ES consist of fine-grained sand. Both Sandwich sealing systems are hydrated with Pearson water type A3. Both shafts were drilled in 2020 and Shaft 1 was installed subsequently. Hydration of Shaft 1 started in May 2021. Shaft2 was installed about two years later so that experience from Shaft 1 operation could be included. Additionally, an excavation damaged zone had the chance to develop close to the shaft wall.

The presentation will focus on the operation phase of the two shafts. Measurement systems and monitoring results, in terms of the evolution of water content, stress, pore pressure, and relative humidity in the sealing systems will be presented.

Acknowledgment

The Sandwich pre-project and the Sandwich in-situ experiment were/are funded by the German Federal Ministry for Economic Affairs and Energy under contracts 02E11587 and 02E11799.

How to cite: Wieczorek, K., Emmerich, K., Schuhmann, R., Hesser, J., Furche, M., Shao, H., Jaeggi, D., Schefer, S., García-Siñeriz, J. L., Königer, F., Mayor, J. C., Norris, S., and Kim, C. S.: The Sandwich seal systems: A large-scale shaft sealing experiment at the Mont Terri rock laboratory – hydration and monitoring, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3224, https://doi.org/10.5194/egusphere-egu23-3224, 2023.

14:20–14:30
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EGU23-1504
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ERE3.3
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On-site presentation
Katja Emmerich, Eleanor Bakker, Christopher Rölke, Franz Königer, Martin Hoffmann, David Jaeggi, Jürgen Hesser, Uwe Glaubach, Maria Victoria Villar, Ralf Diedel, Rainer Schuhmann, and Klaus Wieczorek

Shaft seals are part of the engineered barrier system (EBS) that belongs to the multi-barrier system for deep underground repositories of high-level radioactive waste. The German regulator demands that with respect to the reliability of containment, the interplay of barriers has to be optimized in diverse redundancy and shaft seals should also be constructed of diverse redundant components. The design of shaft seals for generic site models of Germany contains hydraulic sealing elements that should be realized as Sandwich sealing systems. In contrast to conventional hydraulic seals of monolithic bentonite the Sandwich sealing system consists of sealing segments (DS) of bentonite and hydraulically conductive equipotential segments (ES) (Nüesch et al., 2002). Formation water that is penetrating the hydraulic seal via preferential flow paths is contained in the ES and evenly distributed over the cross section of the seal. Thus, a more homogeneous hydration and swelling of the DS is obtained.

In July 2019 a large-scale experiment was launched at the Mont Terri rock laboratory (MTRL) to demonstrate the feasibility of installation, to investigate the saturation process and to assess the sealing effectiveness. The in-situ experiment consists of two experimental shafts of 1.18 m diameter and 10 ‑ 12.6 m depth in the sandy facies of the Opalinus Clay. The DS in both shafts are constructed of German Ca-bentonites from Bavaria (Calcigel) and Westerwald region (Secursol), respectively. Both Sandwich sealing systems are hydrated with Pearson water type A3 and are intensely monitored, together with the surrounding rock. The in-situ experiment is accompanied by extensive laboratory work and numerical modelling. The laboratory work comprises mineralogical analyses, Oedometer tests, MiniSandwich experiments and semi-technical scale experiments. The variety of experiments on different scales and with different model geometries allow us to recognize and understand different scale-dependent and nonlinear effects on the system behaviour.

The presentation will focus on the differences of the two German bentonites with respect to HMC properties and the installation of the Sandwich sealing systems in both shafts at MTRL. The bentonite from Westerwald is characterized by a high smectite content and had to be blended with non-swellable material to adapt expected maximum swelling pressure to the specific site conditions of the in-situ experiment. Both bentonites were then installed as binary mixtures of bentonite pillows and bentonite granular material with similar EMDD, water content and dry density.

Acknowledgment

The Sandwich pre-project and the Sandwich in-situ experiment were/are funded by the German Federal Ministry for Economic Affairs and Energy under contracts 02E11587 and 02E11799.

How to cite: Emmerich, K., Bakker, E., Rölke, C., Königer, F., Hoffmann, M., Jaeggi, D., Hesser, J., Glaubach, U., Villar, M. V., Diedel, R., Schuhmann, R., and Wieczorek, K.: The Sandwich seal systems: A large-scale shaft sealing experiment at the Mont Terri rock laboratory – Installation and materials, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1504, https://doi.org/10.5194/egusphere-egu23-1504, 2023.

14:30–14:40
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EGU23-1181
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ERE3.3
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ECS
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On-site presentation
Eleanor Bakker, Franz Königer, Matthias Gruner, Martin Hofmann, Rainer Schuhmann, and Katja Emmerich

Disposal concepts for high-level radioactive waste involve underground repositories and involving an engineer barrier system (EBS) to ensure safe containment of radionuclides. Shaft and drift seals form part of the EBS and typical sealing concepts involve bulk bentonite. However, bulk bentonite is susceptible to the formation of preferential fluid flow pathways, which impact bentonite hydration and barrier performance. The Sandwich sealing system (CMM, Nüesch et al., 2002) is an alternative solution, and consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES) which allow for homogeneous saturation of the bentonite.

Semi-technical scale experiments (d = 80 cm, h = 180 cm) were required to probe the impact of different segment configurations, pore fluids, and German Ca-Mg-bentonites on the performance of the Sandwich sealing system. Columns were saturated with various artificial fluids mimicking either pore fluid of Opalinus Clay or rock salt. Experiments were terminated when fluid was detected in the final DS, typically a few hundred days duration. Experimental columns were dismantled with up to 20 sampling points on up to 20 sampling levels through the column to provide a 3D snapshot of the system via chemical and physical characterisation of samples.

Water content decreased through bentonite sampling levels from 30-40% adjacent to the fluid inflow to <15% in the final level. Saturation was relatively consistent in DS segments, despite the presence of artificial hydraulic defects. Stark differences were observed between neighbouring ES and DS. Regardless of pore fluid and segment configuration, highest ion concentrations coincided with higher water content and were higher in DS sampling levels directly adjacent to ES. Position of a sampling level in the column was indicative of Ca-Mg- to Na-bentonite transformations as longer exposure time to the pore fluid increased the level of soluble Na+ for interlayer Ca2+ and Mg2+. Concentrations of soluble ions with DS and ES showed little variation. Presence of artificial hydraulic defects only impacted soluble ion concentration in the immediate vicinity of the defect, otherwise segments had relatively uniform ion concentrations. Despite the long duration of the experiments, Ca2+ and Mg2+ persist in the interlayer of DS smectite after > 400 days indicating ongoing cation exchange contribution of interlayer cations to pore fluid composition. Bentonite soluble phases also contribute to pore fluid composition and indicate the systems snapshotted here are far from equilibrium.

Bentonite swelling in DS near fluid inflow caused low final fluid flow rates, indicating the Sandwich sealing system behaved as expected and reduced hydraulic conductivity. Results show a robust system suitable for installation using a variety of different Ca-Mg-bentonites, and in different locations with exposure to pore fluids with low or high salt concentrations, without affecting long-term performance. The Sandwich sealing system is also highly effective at minimising the formation and impact of preferential flow pathways. The authors thank the German Federal Ministry for Economic Affairs and Energy for funding the Sandwich-HP (FKZ 02E11799 A+B+C).

How to cite: Bakker, E., Königer, F., Gruner, M., Hofmann, M., Schuhmann, R., and Emmerich, K.: Impact of seal configuration and pore fluid type on ion transport and cation exchange in bentonite in semi-technical scale Sandwich sealing experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1181, https://doi.org/10.5194/egusphere-egu23-1181, 2023.

14:40–14:50
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EGU23-14788
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ERE3.3
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On-site presentation
Rebecca Lunn, Arianna Pagano, and Grainne El Mountassir

Over the past decades, researchers and industry have developed multi-barrier approaches to the sealing of deposition holes, tunnels, shafts and boreholes. Research has comprehensively described the hydromechanical behaviour of typical barrier materials, such as cement and compacted bentonite over a wide range of hydromechanical conditions. Where challenges remain for post-closure sealing is in controlling the hydromechanical behaviour at interfaces between materials within the repository. These interfaces include between the host rock and the barrier materials, as well as at the interfaces with materials required for safe construction and operation, such as shotcrete and steel.

 

This study explores the potential for colloidal silica grout as a secondary grouting material for repair of degraded cementitious construction materials and wasteforms and for sealing the interfaces between barriers materials and the host rock. Colloidal silica is an aqueous suspension of silica (SiO2) nanoparticles, with average particle size <100 nm.  The creation of siloxane bonds (Si – O – Si), typically triggered by the addition of an electrolyte accelerator, leads to the formation of a solid-like network of silica nanoparticles in the form of a hydrogel. Previous work at Strathclyde on colloidal silica gel has proved its potential to form low-permeability hydraulic barriers against fluid migration, and to inhibit the diffusion of radionuclides through the gel, making it a promising material for use in retrieval operations.

 

Here we present research to determine the potential for colloidal silica to be used in a range of geological disposal applications. We show that due to its excellent penetrability and controllable gel time, colloidal silica has the potential for repairing fine-aperture cracks within the cementitious materials, at the cement/steel interface, or at the interface between barrier materials and the host rock. We inject colloidal silica injected into fractured cement cores (0.2 and 0.5 mm fracture aperture) and expose them to varying pressure and temperature conditions. Fracture permeability upon water injection is assessed pre- and post-treatment. We find that permeability values after treatment are reduced by three orders of magnitude, thus confirming the potential of colloidal silica for repairing fine-aperture cracks. Further, our experiments show that mechanical strength of the cement is recovered, suggesting that additional C-S-H is produced during grouting. We then discuss the wider potential for colloidal silica as part of a multi-barrier approach to long-term sealing of geological disposal facilities.

How to cite: Lunn, R., Pagano, A., and El Mountassir, G.: The potential for colloidal silica grout in multi-barrier systems for geological disposal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14788, https://doi.org/10.5194/egusphere-egu23-14788, 2023.

14:50–15:00
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EGU23-11582
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ERE3.3
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ECS
|
On-site presentation
Vera Lay, Prathik Prabhakara, Frank Mielentz, Sergej Johann, Tobias Fritsch, Michael Stamm, Hans-Carsten Kühne, and Ernst Niederleithinger

Within the project SealWasteSafe, we advance construction materials and non-destructive monitoring concepts of sealing structures applied for underground disposal of nuclear waste. As these engineered barriers have high demands regarding structural integrity, an innovative alkali-activated material (AAM) that is highly suitable for the application in salt as a host rock is improved and tested on two laboratory scales. This AAM has a low heat evolution due to the reaction mechanism in comparison to common salt concretes based on Portland cement or magnesium oxychloride binders. Hence, crack formation due to thermally induced stress during the hardening process is reduced.

After successful laboratory tests with small specimens (height ~5 cm), comparably manufactured large cubic (edge length 70 cm) and cylindrical specimens (height 120 cm, diameter 40 cm) are equipped with sensing technologies to demonstrate the sensors´ technical capabilities.  A comprehensive multi-sensory monitoring scheme is developed and investigated to characterize and compare the different material behaviour during the setting and hardening process of two materials: (1) the newly developed AAM-based mortars with salt aggregate, and (2) a blended Portland cement-based salt concrete as reference. The analysed parameters include temperature and humidity of the material, acoustic emissions, and strain variations recorded by fiber optic cables. Passive sensor systems based on radiofrequency identification technology (RFID) embedded in the concrete provide an interface for the wireless readout of various sensors. In parallel to the embedded RFID sensors, conventional cabled systems to read out the temperature and humidity measurements are installed for comparison. Additionally, a detailed inspection of the two large cubic specimens after a monitoring period of more than six months has been undertaken. Active thermography and ultrasonic echo measurements are used to reveal potentially occurring inner cracks from the surface. To verify the non-invasive results, a core sample (diameter 2 cm) was extracted from each of the investigated cubic specimens and analysed in detail with X-ray computed tomography.

Furthermore, ultrasonic methods are used for quality assurance to detect obstacles, cracks, and delamination at in-situ scale sealing structures. Experimental layout and applied imaging techniques are optimised to enhance the image quality for measurements from the front side of the engineered barrier. To characterize the inside of the test sealing structure and to improve the detection of potentially existing cracks, an ultrasonic borehole probe using the phased array technique is developed. First analyses at a half-spherical specimen coincide with modelling results and prove the reliability of the directional response caused by the phased array technique of the newly constructed ultrasonic borehole probe. Overall, the project SealWasteSafe helps to characterize construction materials and improves multi-sensory monitoring concepts and ultrasonic equipment for the sake of quality assurance. Particularly for salt as a host rock, this will help to design safe sealing structures for nuclear waste disposal.

How to cite: Lay, V., Prabhakara, P., Mielentz, F., Johann, S., Fritsch, T., Stamm, M., Kühne, H.-C., and Niederleithinger, E.: Material research and multi-sensory monitoring for concrete sealing structures in rock salt underground repositories, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11582, https://doi.org/10.5194/egusphere-egu23-11582, 2023.

15:00–15:10
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EGU23-11604
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ERE3.3
|
Virtual presentation
Gesa Ziefle, Tuanny Cajuhi, Stephan Costabel, Markus Furche, Herbert Kunz, and Jobst Maßmann

Coupled hydro-mechanical effects in Opalinus Clay (OPA) are of significant interest regarding the  stability and integrity of a potential storage facility for high-level radioactive waste (HLW). In the Mont Terri Rock Laboratory, the CD-A twin niches differ only in their ventilation and enable the investigation of near field effects like desaturation and development of the excavation disturbed zone (EDZ). Amongst others, the evolution of water content and deformation are observed and provide a comprehensive data set as basis for a pragmatic approach to capture effects in the EDZ in safety assessment. The measuring campaign includes electric resistivity tomography (ERT), providing daily measurements of the electric resistivity distribution up to a few meters into the rock formation, as well as the nuclear magnetic resonance method (NMR) aiming on the evolution of water content near the surface of the niches. Due to the corresponding effort in handling the relatively new single-sided application, the NMR measurements are only conducted once per season. Nevertheless, the combined interpretation of NMR and ERT provides valuable data of water content and hydraulic conductivity evolution enabling a meaningful optimization of numerical modelling approaches. 
The interpretation of the water content evolution in combination with the geologic characterization demonstrates the high heterogeneity of the sandy facies of OPA, a significant impact of the ventilation, and a correlation of high resistivity regions with open fractures, indicating potential preferential flow paths that are relevant for transport processes. As a matter of fact, the results illustrate a need to discuss the set-up of numerical modelling, especially concerning the boundary conditions, the heterogeneities, and the parametrization and set-up of the EDZ. Further measuring focus on the displacements around the niches. While laser scans represent the convergence behavior near the surface, extensometers generate information about the displacements a few meters around the twin niches. Summarizing these data, the convergence behavior near the surface as well as a few meters around the twin niches depends significantly on the ventilation of the niches, indicating seasonal effects and differences in the long-term convergence behavior. Future work aims on an increased system understanding considering long-term effects due to climatic conditions, geotechnical boundary conditions, heterogeneities amongst others and their realistic representation in safety assessment.

How to cite: Ziefle, G., Cajuhi, T., Costabel, S., Furche, M., Kunz, H., and Maßmann, J.: Multi-disciplinary investigation of water content and displacement evolution around the Mont Terri twin niches in Opalinus Clay, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11604, https://doi.org/10.5194/egusphere-egu23-11604, 2023.

Radionuclide migration
15:10–15:20
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EGU23-17145
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ERE3.3
|
On-site presentation
Katja Schmeide, Thimo Philipp, Nina Huittinen, Claudia Sieber, and Jérôme Kretzschmar

The safe disposal of radioactive waste from operation and decommissioning of nuclear power plants in geological repositories requires the application of multiple barriers to isolate the waste from the biosphere. Bentonite and cementitious materials are foreseen as buffer and borehole sealing material or for stabilization purposes. Pore waters of the North German clay deposits are characterized by high ionic strengths up to 4 M [1,2]. The contact of such saline formation waters with concrete will result in an enhanced corrosion of concrete and to the evolution of highly alkaline cement pore waters (10 < pH < 13), which in turn, can react with the bentonite buffer as well as with the clay host rock, changing their retention potential towards radionuclides. Moreover, the role of organics (as admixtures in cement-based materials or waste constituents [3]) on actinide retention has to be studied.

The U(VI) retention on Ca-bentonite in mixed electrolyte solutions (‘diluted Gipshut solution’, I = 2.6 M) was found to be very effective at pH>10, even in the presence of carbonate and despite the prevalence of anionic aqueous uranyl species [4]. By means of luminescence and X-ray absorption spectroscopy, the U(VI) speciation could be clarified. A substantial contribution of calcium (aluminum) silicate hydrates (C-(A-)S-H), formed as secondary phases in the presence of Ca due to partial dissolution of alumosilicates at hyperalkaline conditions, to the retention of anionic actinide species in clayey systems was shown [5]. Citrate and 2-phosphonobutane-1,2,4,-tricarboxylate (PBTC) were found to reduce U(VI) retention only when present at high concentrations.

The U(VI) and Eu(III)/Cm(III) retention by C-A-S-H, formed due to Al-rich additives in cement formulations, was studied applying samples with Ca/Si molar ratios of 0.8, 1.2 and 1.6, representing different alteration stages of concrete, and with increasing Al/Si molar ratios of 0, 0.06 and 0.18 in each series. Furthermore, the impact of temperature (25°C, 100°C, 200°C) on both the C-A-S-H structure and the actinide retention mechanism was studied. Solid-state 27Al and 29Si NMR spectroscopy along with XRD revealed that enhanced temperatures increase the crystallinity of the material with the appearance of neoformed crystalline phases. Surface-sorbed, interlayer-sorbed or incorporated actinide species were detected by luminescence spectroscopy. Actinide mobilization due to high ionic strengths or presence of organics (gluconate, PBTC or nitrilotriacetate (NTA)) was very low [6,7].

The results show that both bentonite and cementitious material constitute an important retention barrier for actinides under hyperalkaline conditions at increased ionic strengths and in presence of organics.

Acknowledgement. This work was supported by the German Federal Ministry for Economic Affairs and Energy (BMWi) within the GRaZ II project (no. 02E11860B) and by the European Union’s Horizon 2020 Research and Innovation Programme (CORI project, no. 847593).

References:

[1] Lommerzheim, A. et al. TEC-08-2014-Z. DBE Technology (2014).

[2] Brewitz, W. GSF-T 136 (1982).

[3] Altmaier, M. et al. EPJ Nuclear Sci. Technol. 8, 27 (2022).

[4] Philipp, T. et al. Sci. Tot. Environ. 676, 469-481 (2019).

[5] Philipp, T. et al. Sci. Total Environ. 842, 156837 (2022).

[6] Wolter, J.-M. et al. Sci. Rep. 9, 14255 (2019).

[7] Dettmann, S. et al. Front. Nucl. Eng. (under review).

How to cite: Schmeide, K., Philipp, T., Huittinen, N., Sieber, C., and Kretzschmar, J.: Bentonite and concrete: Efficient barrier materials for actinide retention under hyperalkaline conditions at increased ionic strengths and in presence of organics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17145, https://doi.org/10.5194/egusphere-egu23-17145, 2023.

15:20–15:30
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EGU23-1086
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ERE3.3
|
On-site presentation
Josep M. Soler, Deby Jurado, Maarten W. Saaltink, Lurdes Martínez, Juan J. Hidalgo, G. William Lanyon, and Andrew J. Martin

Within the GTS-LTD project (Grimsel Test Site – Long-Term Diffusion), a field radionuclide tracer transport experiment in unfractured granitic rock was performed. Grimsel groundwater containing several tracers (3H as HTO, 36Cl-, 22Na+, 134Cs+, 133Ba2+) was continuously circulated through a packed-off borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of 1266 days (05/03/2014 – 22/08/2017). Additionally, tracer breakthrough was monitored in an observation borehole a few cm away.

Initial modeling of the experiment (1D radial) showed that the evolution of tracer concentrations seemed to depart from the expected trend after some time, with concentrations in the injection borehole decreasing faster than expected from pure diffusive transport. Additional 2D calculations (section normal to the boreholes) were performed to check the possible effect of advection through the rock matrix. Advection could explain the evolution of concentrations in the injection borehole, but could not fully explain the measurements in the observation borehole.

Rock samples around the experimental section were collected right after the end of the experiment, allowing the measurement of tracer distributions in the rock. The observed patterns for the non-sorbing tracers (HTO, 36Cl-) showed clear preferential transport directions, consistent with advective flow towards the gallery from which the boreholes were drilled. Final 3D modeling of the experiment can explain the measured concentrations in the boreholes and in the rock. Tracer transport is affected by both diffusion and advection through the granitic rock matrix. Also, in situ accessible porosities (about 0.0014) are smaller than those measured in rock samples (about 0.009), pointing to the destressing of the rock samples after drilling.

How to cite: Soler, J. M., Jurado, D., Saaltink, M. W., Martínez, L., Hidalgo, J. J., Lanyon, G. W., and Martin, A. J.: Advection in granitic rock matrix: Modeling a radionuclide tracer transport experiment at the Grimsel Test Site., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1086, https://doi.org/10.5194/egusphere-egu23-1086, 2023.

15:30–15:40
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EGU23-9478
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ERE3.3
|
On-site presentation
Cornelius Fischer, Johannes Kulenkampff, Maria Cardenas-Rivera, Wenyu Zhou, and Jann Schöngart

The predictive power of numerical approaches for the analysis of flow fields, e.g. for  radionuclide migration, depends on the quality of the underlying pore network geometry. Validation of the obtained simulation results can only be performed with a limited number of methods. Positron emission tomography (PET) is a suitable technique that has been established in geomaterial sciences in recent years. The use of suitable radiotracers allows the analysis of advective transport and diffusive flux in a variety of complex porous materials. In addition to the visualization of time-resolved transport patterns, the statistical analysis of transport controlling parameters is currently in the focus of investigations using PET techniques.

Using potential host rock types with low permeability for underground radioactive waste repositories as examples, we have analyzed the heterogeneity of the flow field at laboratory scale.1 Diagenetic and sedimentary components and their pore size distributions and pore network geometries are responsible for the flow field properties. The resulting generalized pore network geometries are used in digital rock models to calculate effective diffusivities, using a combined upscaling workflow for transport simulations from the nanometer to the micrometer scale.2 For advective transport in fractured crystalline rocks, PET provides evidence for the influence of fracture wall geometries over a wide range of the length scale. Surface building blocks from nm to mm size are responsible for the observed changes in breakthrough curve behavior. Finally, another hot topic is the testing of reactive PET tracers for materials analysis. In addition to the use of conservative tracers described above, reactive tracers provide insight into the density of reactive surface sites in complex porous materials.

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.

2Yuan, T.; Fischer, C., The influence of sedimentary and diagenetic heterogeneity on the radionuclide diffusion in the sandy facies of the Opalinus Clay at the core scale. Applied Geochemistry 2022, 146, 105478.

How to cite: Fischer, C., Kulenkampff, J., Cardenas-Rivera, M., Zhou, W., and Schöngart, J.: Tomographic analysis of advective flow and diffusive flux toward improved migration predictability in host rocks for radioactive waste, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9478, https://doi.org/10.5194/egusphere-egu23-9478, 2023.

15:40–15:45

Posters on site: Tue, 25 Apr, 16:15–18:00 | Hall X4

Design & Site Evaluation
X4.154
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EGU23-8846
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ERE3.3
Dominik Gottron, Thomas Lohser, Niklas Bertrams, Dennis Gawletta, Frederik Fahrendorf, Stephen Klimke, Florian Schlüter, Michael Werres, and Wolfram Rühaak

The Federal Company for Radioactive Waste Disposal (BGE mbH) is responsible for the execution of the German site selection procedure for high-level radioactive waste. The aim is to identify a repository site that ensures the best possible safety for the disposal of high-level radioactive waste for at least one million years. Three potential host rocks are considered for disposal in Germany: rock salt, claystone and crystalline rock. The German site selection procedure consists of three consecutive phases with a continuously increasing level of detail. The first step of phase I served to determine sub-areas based on different geoscientific criteria. In this process 90 suitable areas were identified. The second step of phase I comprises representative preliminary safety assessments to further narrow down the sub-areas (see Figure 1).

Figure 1: German Site Selection Procedure

For these assessments, a preliminary concept and dimensioning of the deep geological repository is required, among other aspects. The primary input data for the preliminary design of the repository are the respective host rock properties, the inventory data of the high-level radioactive waste as well as a preliminary safety concept for the disposal system. Since there are only limited area-specific data available as no exploration is part of this early stage of the selection procedure, a two-stage method has been developed for the determination of the required area of the potential repository, consisting of a host rock specific and a site specific part. The objective of the first, host rock specific stage is to perform an analysis irrespective of the prevailing geological conditions at the considered locations. The aim is to obtain the areal extend of a potential repository as a function of the depth, the initial temperature in the host rock and mechanical properties of the host rock. Another important topic is the assessment of favorable depth ranges of the repository. Within the second stage, the function is used with available site-specific data to obtain the possible size of the repository at this specific site.

This contribution will provide an overview of the first stage within the aforementioned methodology for the development of the preliminary design of the repository as part of the representative preliminary safety assessments. 

How to cite: Gottron, D., Lohser, T., Bertrams, N., Gawletta, D., Fahrendorf, F., Klimke, S., Schlüter, F., Werres, M., and Rühaak, W.: Preliminary design of the repository for high-level radioactive waste in Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8846, https://doi.org/10.5194/egusphere-egu23-8846, 2023.

X4.155
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EGU23-17181
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ERE3.3
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Highlight
Jiří Šťástka

The Centre of Experimental Geotechnics (CEG) is an experimental department of the Faculty of Civil Engineering of the Czech Technical University (CTU) in Prague. The CEG operates the Josef Underground Laboratory (the Josef UL) situated in a former exploration mine (the Josef Gallery) that was excavated in connection with potential gold mining. This facility provides practical courses for students and also provides a unique location for the conducting of experimental and research projects. The Josef Gallery was substantially reconstructed to form the Josef Underground Laboratory in 2007. The most important role of this underground research facility is to provide practical in-situ training in the fields of geotechnical engineering, geology, geochemistry, radiochemistry, radioecology etc. The training of future experts in this authentic underground setting involves the participation of several other Czech universities and numerous experienced specialists from outside the academic sphere. The IAEA (International Atomic Energy Agency) has added the Josef UL to its prestigious list of international training centres and the Josef Underground Laboratory is a member of the Underground Research Facilities network.

The Josef underground complex provides excellent opportunities for both the conducting of in-situ research and teaching. The advantages of the facility include its varied geological conditions (tuffs, tuffites and granitic rocks) and excellent infrastructure which, together, form a modern and unique research and teaching facility. The total length of the reconstructed and ready-for-use galleries is currently over 5 km and the rock cover varies from 0m to 170m. Support for the experimental research conducted in the underground complex is provided by an experimental hall, laboratories and administration facilities that are housed in a specially-reconstructed surface building located near to the entrance to the Josef Gallery.

The Josef complex has been designed as a multi-disciplinary rather than a single topic-oriented facility. The size of the Josef underground gallery and its unique geological diversity make it ideal for the conducting of experimental research across a wide spectrum of scientific disciplines.

The research projects in which CEG staff participate and which are conducted or partially conducted at the Josef Underground Laboratory fall under the areas of underground and surface structures, modern tunnelling technologies, crash testing, ecology, waste storage and disposal, gas and heat storage, underground architecture and robotics.

How to cite: Šťástka, J.: Josef Underground Research Laboratory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17181, https://doi.org/10.5194/egusphere-egu23-17181, 2023.

X4.156
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EGU23-10713
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ERE3.3
Myungsun Kim, Yeonguk jo, and In Hwa Park

Among many parameters needed to characterize subsurface rock mass related to underground storage and disposal projects, an important one is the heat production rate of the bedrock itself.

The rock produces heat from the decay of radioactive isotopes consisting of minerals and its gamma-ray emissions, of which the magnitude is dominated by contents of major radioactive isotopes, e.g., uranium, thorium, and potassium. The heat production rate is generally calculated from the rock density and the radiogenic isotope contents, which can be measured from the spectrometry of drilled cores or rock fragments. However, such approaches are rarely available in deep boreholes because recovering rock samples from several hundred meters to a few kilometers is quite difficult.

A recent geophysical logging technique for deep boreholes is available where the uranium, thorium, and potassium contents are measured from the gamma-ray spectrum. However, this technique requires the density to be measured separately, and the measurement depth of the equipment is still limited. As an alternative method, a natural gamma-ray logging tool was adopted to estimate briefly the heat production from the total gamma activity, which is relatively easy to measure. This study introduces the development of the proposed method for evaluating the heat production of a granitic rock mass with domestic commercial borehole logging tools, as well as its application and verifications in deep boreholes.

How to cite: Kim, M., jo, Y., and Park, I. H.: Estimation of the radiogenic heat production rate of fractured granitic rock mass based on in-situ natural gamma logging, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10713, https://doi.org/10.5194/egusphere-egu23-10713, 2023.

X4.157
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EGU23-14400
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ERE3.3
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ECS
Jonas Suilmann, Andrea Perin, Matteo Broggi, Thomas Graf, and John W. Molson

Long-term safety has to be assured for the disposal of high-level nuclear waste to prevent contamination of the biosphere. A numerical framework for the probabilistic assessment of hazardous events regarding nuclear waste disposal in salt domes is developed here. The goal is to numerically simulate transport times and mass fluxes of radionuclides into the biosphere, and to couple this with a probabilistic framework for reliability assessment. This model can deal with uncertainties such as the impact of external events (major climate changes, human activities, earthquakes) on subsurface structure, material properties and boundary conditions. To quantify the impact of external events, a numerical model of the far field of a salt dome disposal site is generated. It includes the simulation of density-driven (thermohaline) flow, heat transport, transport of dissolved salt and a radionuclide in discretely-fractured porous media using the FE-code HEATFLOW-SMOKER Version V3/TC2.

As a first step in quantifying the effects of uncertain parameters in the context of nuclear waste disposal in salt domes, the salt dome problem (HYDROCOIN level 1 case 5) is further investigated in terms of groundwater age. Groundwater age is one of the exclusion criteria in the site selection process for nuclear waste deposits and therefore of great importance. Groundwater age is here calculated as a transport problem using steady-state flow velocities. This problem is density-driven due to transport of salt into the model domain and the steady-state solution is highly dependent on dispersion.  A sensitivity analysis is carried out to quantify the effect of uncertain dispersion on flow and salt distribution and resulting groundwater age. Preliminary simulation results demonstrate that both salt distribution and groundwater age in steady-state are significantly affected by longitudinal and transversal dispersivity. Resulting flow velocities are higher with increased dispersion and therefore lead to a decreased maximum groundwater age in the model domain.

How to cite: Suilmann, J., Perin, A., Broggi, M., Graf, T., and Molson, J. W.: Risk-based Assessment of Salt Domes as Disposal Sites for Nuclear Waste: Uncertainty of Groundwater Age in the Salt Dome Problem, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14400, https://doi.org/10.5194/egusphere-egu23-14400, 2023.

X4.158
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EGU23-2875
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ERE3.3
Torben Weyand, Michael Jendras, Nele Bleyen, Katrien Hendrix, and Christoph Borkel

In several countries, claystone is a possible host rock for a radioactive waste repository. The reducing capacity of claystone has a strong impact on the speciation, solubility, retention and transport properties of redox-sensitive radionuclides. Since redox-sensitive radionuclides are more mobile in a more oxidised state, the reducing capacity of claystone is an important feature regarding the long-term safety of nuclear waste disposal.

To study the influence of oxidising components such as nitrate (which is for example a component of bituminised waste) on the reducing capacity of claystone and radionuclide mobility, ongoing in situ tests are carried out in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland) as part of the Bitumen-Nitrate-Clay interaction (BN) experiment. These in situ tests are performed in a vertical borehole containing three packed-off intervals (water chambers), which allow to monitor and change the hydrochemical conditions in the different intervals. Moreover, in recent years, the fate of selenate (SeO42-) in the Opalinus Clay has been studied within the BN experiment. This includes the possible microbial reduction of Se(VI) to Se(IV) (in presence of nitrate), as this could contribute to the retention of selenium within the geosphere. Selenate was selected since 79Se is one of the main dose-contributing radionuclides in long-term safety analyses for radioactive waste disposal in claystone.

In each of the past in situ tests, artificial pore water with a pH of approximately 7.8 and containing nitrate and/or selenate and sometimes electron donors (acetate and H2), is injected in the borehole. Thereby all ongoing processes are studied at a near-neutral pH. In future experiments, the influence of an increased pH on the fate of nitrate (and later on selenate) and on the reducing capacity of the claystone will be investigated, since cement, for example used as an engineered barrier in a repository, may cause alkaline conditions. Therefore, it is envisaged to increase the pH of the water within the borehole slowly in a stepwise manner - first up to pH 9-10 - by injecting solutions containing NaOH, KOH or cementitious pore water.

In this study, geochemical calculations are performed using the program PHREEQC to evaluate the increased pH in the intervals on the surrounding claystone and the borehole water. The result of this modelling will show the aqueous speciation and mineral dissolutions and precipitations. An evaluation of mineral precipitations under the given conditions is necessary since clogging of the water lines in the experiment should be avoided. Therefore, an understanding of the geochemical interactions of the injected aqueous alkaline solutions with the surrounding claystone and pore water is necessary. Within the context of nuclear waste disposal in claystone, the database ThermoChimie provides suitable thermodynamic data and will be used for the simulations. To assess the precipitation of secondary phases for cementitious materials, the thermodynamic database CEMDATA might be applied for comparison. Finally, the range of pH that can be reached in the interval solutions will be assessed and the amount of precipitated minerals will be calculated.

How to cite: Weyand, T., Jendras, M., Bleyen, N., Hendrix, K., and Borkel, C.: Modelling the geochemical behaviour of aqueous solutions in the Opalinus Clay as part of the Mont Terri BN Experiment using PHREEQC, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2875, https://doi.org/10.5194/egusphere-egu23-2875, 2023.

X4.159
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EGU23-5144
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ERE3.3
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Highlight
Hui Ding, Maximilian Scholze, Eva Schörner, Sonia Sortan, Thomas Hertweck, Lars Houpt, Thomas Bohlen, Niklas Kühne, Tomi Jusri, Felix Hlousek, and Stefan Buske

3D seismic measurements are a tool for high-resolution structural exploration of the subsurface using artificially generated seismic waves. Depending on the acquisition geometry and the seismic source, structures can be examined at depths ranging from a few meters to more than 10 km. The accuracy of the geological layer boundaries in the depth image resulting from the field data depends heavily on the quality and the details of the physical earth model and the imaging algorithms used.

The "DOSIS" project was initiated in order to design an optimized, combined and high-resolution method to facilitate finding answers for important geophysical and geological questions in the exploration of repository sites in Germany in the future. In the framework of the "DOSIS" project, the simulation of elastic waves using finite differences and the associated full-waveform inversion as well as the Fresnel-volume migration are further developed, so that seismic anisotropy and inelastic attenuation can be taken into account for a detailed characterization of the subsurface. These methods are tested and validated using both, synthetic data and real data in the form of a massive high-quality 3D seismic data set acquired in 2020 in the area of the "Asse" salt structure in Lower Saxony and provided by BGE.

How to cite: Ding, H., Scholze, M., Schörner, E., Sortan, S., Hertweck, T., Houpt, L., Bohlen, T., Kühne, N., Jusri, T., Hlousek, F., and Buske, S.: Development of an optimized, combined and high-resolution imaging method for the site investigation of radioactive repositories (DOSIS), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5144, https://doi.org/10.5194/egusphere-egu23-5144, 2023.

Engineered Barrier System & Excavation Damage Zones
X4.160
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EGU23-10687
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ERE3.3
|
ECS
Deuk-Hwan Lee, Seok Yoon, and Gi-Jun Lee

For the safe disposal of high-level radioactive waste, several performances of the compacted buffer block are required, such as low hydraulic conductivity, high thermal conductivity, adequate swelling pressure, and good mechanical properties. Furthermore, it is well recognized as the performance of these buffers is highly correlated with the density of buffer blocks. Therefore, it is an essential task to fabricate the compacted buffer block with a homogenous density distribution. In this study, the manufacturing characteristics of bentonite-sand buffer were evaluated to identify the required density relationship in accordance with the pressure of floating die press and the homogeneity of the dry density distribution in the buffer blocks.

How to cite: Lee, D.-H., Yoon, S., and Lee, G.-J.: Evaluation of the density distribution in bentonite-sand buffer blocks manufactured by floating die press method, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10687, https://doi.org/10.5194/egusphere-egu23-10687, 2023.

X4.161
|
EGU23-12223
|
ERE3.3
Seok Yoon, Gi-Jun Lee, and Deuk-Hwan Lee

The compacted bentonite buffer is one of the most important components in an engineered barrier system(EBS) to dispose of high-level radioactive waste (HLW) produced by nuclear power generation. The design temperature of the bentonite buffer is below 100 °C. However, if the design temperature can be increased above 100 °C, the disposal area can be reduced. For this reason, it is necessary to investigate its properties at 
temperatures above 100 °C to increase the target temperature of the buffer. Although some studies have investigated the thermal-hydraulic properties above 100 °C, few have evaluated the water suction of compacted bentonite. Therefore, this study developed experimental system to measure water suction above 100 °C, and measured relative humidity for compacted Korean bentonite between 25 and 150 °C with initial saturations of 0, 0.22, and 0.47 under constant saturation conditions. Water suction decreased as the temperature increased. In particular, there was an approximately 5–20% decrease in the water suction between 100 and 150 °C.

How to cite: Yoon, S., Lee, G.-J., and Lee, D.-H.: Development of Experimental System to Measure Waster Suction of Compacted Bentonites beyond 100 °C, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12223, https://doi.org/10.5194/egusphere-egu23-12223, 2023.

X4.162
|
EGU23-8722
|
ERE3.3
Wei-Hsing Huang and Guo-Liang Ren

With the extra long term consideration for safety in a geological disposal repository, the creep behavior of the buffer material in a deposition hole can have an effect on the long-term stability of the engineered barrier. In this study, the hydro-mechanical properties and creep modeling parameters were determined experimentally on MX-80 bentonite. These include swelling pressure, water conductivity, and shear stress-strain behavior obtained from constant-stress direct shear test. The experimental works involved testing the bentonite in 3 different modes, namely, unsaturated, saturation under constant volume, and saturation after pre-swell. The pre-swelling process was designed to simulate an expansion of the buffer material due to mass losses of bentonite upon resaturation in a deposition hole.

          The experimental results show that in the constant volume mode and pre-swell mode, the swelling curve each has its own characteristics and, as the density of buffer decreases, the swelling pressure continues to decrease but in different trends. The shear strength obtained in the 3 modes is different while the values for the latter 2 modes are close. In the constant stress shear test, the higher the shear stress level, the faster the increase in shear strain rate. The shear strain rate is found to be higher in the pre-swell mode. Thus, for the long term safety of the deposition hole, the mass loss of buffer material should be limited to prevent degraded performance against creep deformation.

          Finally, numerical simulation was implemented to predict the creep deformation in a deposition hole for extended periods of time, in order to provide a reference for the design of the deposition hole and disposal tunnel. It is estimated that, at the junction of the buffer material and the backfill material, the vertical displacement increases continuously and reaches its maximum at 100,000 years.

How to cite: Huang, W.-H. and Ren, G.-L.: Experimental study on the creep behavior of buffer materials in the deposition hole of a repository, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8722, https://doi.org/10.5194/egusphere-egu23-8722, 2023.

X4.163
|
EGU23-4037
|
ERE3.3
Kyung-Woo Park, Byeong-Hak Park, and Sung-Hoon Ji

This presentation describes the results of a study on the change in hydrogeological properties that can occur in natural barriers after excavating a disposal hole. We performed a series of hydraulic tests at a borehole in the KURT area to understand the change in the hydrogeological characteristics around the drilling-induced damage zone after excavating the disposal hole. As a result, it was confirmed that the transmissivity of fractured rock was increased from several times to tens of times. It was caused by the drilling-induced damage generated around the borehole surface by expanding the diameter of the borehole. In particular, the change in transmissivity was increased in the test section with low transmissivity. In the end, the permeability in the borehole damage zone had a high transmissivity overall, and the hydraulic connection within the borehole damage zone was expanded. Additionally, it was confirmed that the range of hydrogeological influence of the borehole damage zone decreases as the distance from the borehole hole. The results, which analyzed the hydrogeological effect of the borehole damage zone by the borehole expansion, differ in size from the actual disposal hole in the repository. However, compared to large-scale research on an excavation damage zone conducted in tunnels, researchers can easily access it. Therefore, it will be used in various studies to analyze qualitative phenomena.

How to cite: Park, K.-W., Park, B.-H., and Ji, S.-H.: Hydrogeological Change in Borehole Damage Zone (BDZ) by Expanding Diameter of Borehole, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4037, https://doi.org/10.5194/egusphere-egu23-4037, 2023.

X4.164
|
EGU23-17192
|
ERE3.3
Pooya Hamdi, Peter Achtziger, Seyyedmohammad Moulaeifard, Antonio Rinaldi, Virginie Durand, Linus Villiger, Florian Amann, and Stefan Wiemer

Development of brittle damage around nuclear waste repository tunnels is a common phenomenon in massive rocks in highly-stressed conditions. The time-dependent brittle fracturing may lead to an interconnected fracture network (i.e. excavation damage zone; EDZ) and induced seismicity. Within the excavation damage zone (EDZ), the permeability is often enhanced and – in the framework of nuclear waste disposal – may provide preferential pathways for radionuclide migration. Therefore, a comprehensive understanding of the brittle fracturing requires multi- multidisciplinary monitoring systems to allow for spatial and temporal characterization of the EDZ. Recently, the Swiss Federal Institute of Technology (ETH) Zurich established a new Underground Research Laboratory (URL) in Southern Switzerland in the old Bedretto Gallery. Within the PRECODE experiment, we will establish a new, experimental tunnel as a branch from the existing tunnel, which will be densely instrumented with strain, pore pressure and acoustic emission sensors prior to the excavation. The main objectives of the PRECODE experiment are to understand: (1) short-term rock mass behavior and EDZ formation during tunneling; (2) long-term fracture propagation within the EDZ associated with environmental conditions (fluctuations in humidity and temperature); (3) permeability changes with time around an open excavation and (4) the impact of tunneling on the nearby fault zones. This study outlines an overview of the project objectives, details of the planned monitoring systems, and some preliminary results obtained from a baseline study of characterization of the +40-year EDZ from the existing Bedretto Tunnel.

Bedretto Team: The team involves more than 30 people from ETHZ and 10 research institutes and companies involved in the Bedretto Laboratory (see http://www.bedrettolab.ethz.ch/en/home/ for more details)

How to cite: Hamdi, P., Achtziger, P., Moulaeifard, S., Rinaldi, A., Durand, V., Villiger, L., Amann, F., and Wiemer, S.: Progressive Excavation Disturbance Zone Evolution during and Post Mine-by Tunneling (PRECODE) – Insight into New Underground Research Laboratory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17192, https://doi.org/10.5194/egusphere-egu23-17192, 2023.

X4.165
|
EGU23-1794
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ERE3.3
|
Highlight
Jai-Yong Park and Dae-Sung Cheon

In order to provide basic data for selecting candidate sites for high-level radioactive waste geological disposal, the drilling project is being carried out to investigate the deep environment for each type of bedrock existing on the Korean Peninsula. Securing 10 boreholes with a depth of 750 m is the final goal of the drilling project, which is planned for 5 years from 2020 to 2024. Two boreholes are being secured for sedimentary, plutonic, volcanic, metamorphic rock, respectively, and two additional boreholes will be secured for the rock types that require further investigation. As of December 2022, three boreholes were secured for sedimentary rocks, two for plutonic rocks, and one for metamorphic rocks. The boreholes were targeted for shale, granite and gneiss, respectively. To provide precise and reliable basic data, multidisciplinary (geological, geophysical, geochemical, hydrogeological, rock mechanics, etc.) studies have been conducted using drilling holes and cores. These research results are expected to be used as important basic data for selecting candidate sites for high-level radioactive waste geological disposal in Korea.

How to cite: Park, J.-Y. and Cheon, D.-S.: Drilling project for deep environmental geoscientific investigation for each type of bedrock for HLW geological disposal in Korea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1794, https://doi.org/10.5194/egusphere-egu23-1794, 2023.

Radionuclide migration
X4.166
|
EGU23-2754
|
ERE3.3
|
ECS
Theresa Hennig and Michael Kühn

In the context of safety assessments for nuclear waste repositories, it is essential to quantify potential radionuclide migration. This can only be done by the application of numerical simulations due to the required spatial (>100 m) and temporal scales (1 Ma), whereby resulting migration lengths highly depend on the underlying model concept and data [1-4].

Migration of uranium, the main component of spent fuel, is used here in regard to the potential host rock Opalinus Clay as an example for models close to a real case application. For this, one-dimensional diffusion simulations were conducted with PHREEQC applying mechanistic surface complexation models to account for sorption processes as a function of the geochemical conditions [1].

Extensive numerical studies for the hydrogeological system at the underground rock laboratory Mont Terri (Switzerland) have shown that migration lengths can vary from 5 m using an experimentally determined distribution coefficient Kd (m³/kg) up to 80 m applying more advanced approaches [1-4]. However, these results represent maximum scenarios. At Mont Terri, geochemical gradients established towards the embedding aquifers due to the Jura folding and associated erosion history [2, 3]. For a potential disposal site, more constant conditions without a gradient are favoured. Furthermore, the impact of the engineered barriers and with that a reduction of the source term was not taken into account in previously. Therefore, simulations are conducted for a site with less steep geochemical gradients compared to Mont Terri as well as for decreased source term concentrations.

First, measured pore water profiles from Schlattingen (Switzerland) were modelled, where geochemical gradients are less pronounced. Second, they serve as initial conditions for subsequent uranium migration driven by decreasing source term concentrations. The comparison of resulting migration lengths with the mentioned maximum scenarios shows that uranium migration is decreased by several metres. Consequently, the selection of initial and boundary conditions is essential for a reliable quantification of radionuclide migration.

References:

[1] Hennig, T. et al. (2020): Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay. Applied Geochemistry 123, 104777. DOI: 10.1016/j.apgeochem.2020.104777

[2] Hennig, T. and Kühn, M. (2021): Potential uranium migration within the geochemical gradient of the Opalinus Clay system at the Mont Terri. Minerals 11 (10), 1087. DOI: 10.3390/min11101087

[3] Hennig, T. (2022): Uranium migration in the Opalinus Clay quantified on the host rock scale with reactive transport simulations, PhD Thesis, Potsdam: Universität Potsdam, 161 p. DOI: 10.25932/publishup-55270

[4] Hennig, T. and Kühn, M. (2022): Reactive transport simulations of uranium migration in the Opalinus Clay depend on ion speciation governed by underlying thermodynamic data. Advances in Geosciences 58, 11–18, DOI: 10.5194/adgeo-58-11-2022

How to cite: Hennig, T. and Kühn, M.: Uranium migration lengths in Opalinus Clay depend on geochemical gradients, radionuclide source term concentration and pore water composition, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2754, https://doi.org/10.5194/egusphere-egu23-2754, 2023.

X4.167
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EGU23-14909
|
ERE3.3
Alwina Hoving and Jasper Griffioen

Clay minerals play an important role as barrier in radioactive waste storage. In addition to their function as hydraulic barrier, clay minerals can also immobilize radionuclides by adsorption and redox reactions with radionuclides. Reduction of redox-active radionuclides such as  technetium and selenium by smectites has already been shown. Many clay minerals contain iron (Fe) in their structure, however, not all of this structural Fe is accessible for redox reactions. This can depend on multiple factors such as the quantity and coordination of Fe in the clay mineral structure. While the redox-activity of Fe in smectites has received quite some attention, other common types of clay minerals present in clay host rocks have received less attention. In this study we investigated redox capacities of a variety of common clay minerals using mediated electrochemistry. Results show that most clay minerals have electrochemically‑active Fe to some extent, but there is a large variation in the fractions of electrochemically-active Fe between the different clay minerals. Smectites had the highest redox-active Fe fraction (~100%), followed by mixed illite-smectite (~40%), glauconite (~15%), illite (~10%) and chlorite (~1%). Within the same clay mineral type there was also variation in the redox-active fraction. Batch experiments showed that even for clay minerals in which only a small fraction of Fe was redox-active, reduction of selenite to elemental selenium took place.

How to cite: Hoving, A. and Griffioen, J.: Redox capacities of different types of clay minerals and their potential to immobilize redox-active radionuclides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14909, https://doi.org/10.5194/egusphere-egu23-14909, 2023.

X4.168
|
EGU23-7753
|
ERE3.3
|
Highlight
Wenyu Zhou and Cornelius Fischer

Low-permeability granites are considered as host rocks for nuclear waste repositories. Understanding fluid flow and solute transport in granite fractures are essential in assessing the feasibility and safety of a nuclear waste repository. The internal variability of fractures, such as aperture distribution and asperities, dictates the hydrodynamics of reactive fluid, thus affecting the dispersion and retention of radionuclides. Numerical studies using 2-D models have focused on the heterogeneity of aperture distribution, but the effects of fracture asperities and additional surface features on the evolution of flow paths have not been systematically examined. In this study, the nonreactive solute transport behavior in a single fracture was numerically investigated considering the effects of fracture aperture and surface asperity by comparing 2.5-D and 3-D modeling results on a realistic fracture. The additional motivation here was to identify the limitations of model simplification. The 3-D fracture geometry was extracted from a micro-computed tomography of a natural fracture several centimeters long. Then, 2.5-D models were generated by mapping the aperture distribution of the 2-D fracture geometry on the x-y plane. Flow simulations were performed in both numerical models to detect the respective effects of fracture shape and surface asperities. For validation, we performed a sensitivity analysis by decreasing the 3-D fracture geometry mesh according to the quadric edge-collapse strategy, simulating the solute transport behavior under different fracture surface properties. The size variability of the isometric grid blocks ranges from 6.5 µm to 2.2 mm. Thus, we provide a function that can be used to quantitatively estimate the concentration error due to the simplification of the geometry mesh. The results show which fracture asperities and surface properties can significantly affect the solute transport behavior. Above a certain geometry complexity, the 3-D model results show less retention in the rather stagnant zones and thus better agreement with breakthrough curves (BTCs) of experiments compared to the 2.5-D model approaches. The results of the 3-D models also agree well with previous studies that less pronounced tailing is observed in the case of lower surface roughness. Simplifying the model geometry leads to more distorted results, with the 3-D model being more sensitive than the 2.5-D model. Moreover, based on a function summarized from the BTCs, the error in the simulated concentration due to mesh simplification can be estimated within a certain range that varies with the fracture geometry. The results presented show the capabilities and limitations of using 2.5D models in comparison with more elaborate 3-D models in predicting fluid dispersion in fractured crystalline rocks. Our study can serve as a guideline for the construction of fracture geometry and model design in future reactive transport modeling.  

How to cite: Zhou, W. and Fischer, C.: 3D modeling of solute transport through natural fractures with microrough walls in crystalline host rocks: The effects of asperities and surface characteristics on hydrodynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7753, https://doi.org/10.5194/egusphere-egu23-7753, 2023.

Thermal-Hydraulic-Mechanical (THM) Modeling
X4.169
|
EGU23-3988
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ERE3.3
|
ECS
|
Highlight
|
Aqeel Afzal Chaudhry, Chao Zhang, Oliver Ernst, and Thomas Nagel

When modeling the material properties of host rocks for THM simulations for the purpose of barrier integrity investigations in deep geological disposal of radioactive waste, there are numerous modeling aspects to consider. We consider the simple case of stationary Darcy flow in a two-dimensional medium to convey the basic phenomena when constructing a suitable model for host rock permeability with a special focus on the treatment of anisotropy occurring at different spatial scales and the associated uncertainty. If complete information were available, permeability would be a known function of space, and hydrological features such as inhomogeneity (spatial variability) and anisotropy (direction-dependence) can be expressed by spatially varying and tensor-valued permeability coefficients. In reality, uncertainty is present and needs to be considered. One approach is to model the rock medium as piecewise homogeneous in such a way that the permeability values in each homogeneous subregion (e.g. geological layer) are modeled as random variables. The permeability values at any two locations of the same subregion are then completely correlated. Randomness with a more general structure can be modeled by random fields, the realizations of which are functions of space which are not necessarily constant. A common modeling choice is that of a Gaussian random field for log permeability, which is completely determined by its mean and two-point correlation function. Anisotropy can now occur both in the statistical covariance structure, resulting in different correlation lengths along principal axes, and in the hydraulic properties, leading to tensor-valued random fields the realizations of which are anisotropic tensors for which the underlying covariance structure may, in addition, display statistical anisotropy.

In this work we present a number of simulations designed to illustrate the effects of inhomogeneity, randomness as well as statistical and hydraulic anisotropy.

How to cite: Chaudhry, A. A., Zhang, C., Ernst, O., and Nagel, T.: Effects of Inhomogeneity and Anisotropy on THM Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3988, https://doi.org/10.5194/egusphere-egu23-3988, 2023.

X4.170
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EGU23-7365
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ERE3.3
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Highlight
|
|
Dominik Kern, Christian B. Silbermann, Fabien Magri, Rebekka Steffen, Holger Steffen, and Thomas Nagel

Important aspects for subsurface installations, such as Deep Geological Repositories (DGRs), in crystalline rock are the presence and evolution of fractures and faults, since they control the subsurface flow regime. According to climate extrapolation, it is expected that cold and warm period will alternate, accompanied by ice sheet progression and regression. The large moving mass of an ice sheet causes a dynamic response of the earth's crust, referred to as glacial isostatic adjustment (GIA) [1]. GIA changes the displacement and stress field not only under and near the ice sheet but also in its far-field. In view of the long-term assessments, we apply boundary conditions derived from an established GIA model [2] in order to analyze induced far-field stress and pore pressure changes and their impacts on existing faults in a hydromechanical simulation. As indicator for permeability changes we apply the Coulomb failure stress criteria. To quantify the consequences on the subsurface flow we run a component transport simulation before and another after the fault reactivation, revealing how the faults canalize the radionuclid propagation. For both kinds of simulations, hydromechanical and component transport, we apply Finite-Element methods (FEM) [3].
The INFRA project is funded by the DFG under grants NA1528/2-1 and MA4450/5-1.
 
[1] Holger Steffen, Patrick Wu. "Glacial isostatic adjustment in Fennoscandia - a review of data and modeling". Journal of geodynamics 52.3-4, p. 169-204, 2011. https://doi.org/10.1016/j.jog.2011.03.002
[2] Georg Kaufmann. "Program package ICEAGE". Manuscript, Institut für Geophysik der Universität Göttingen, vol. 40. p. 840, 2004.
[3] OpenGeoSys 6.4.3. Lars Bilke, Thomas Fischer, Dmitri Naumov, Christoph Lehmann, Wenqing Wang, Renchao Lu, Boyan Meng, Karsten Rink, Norbert Grunwald, Jörg Buchwald, Christian Silbermann, Robert Habel, Linda Günther, Mostafa Mollaali, Tobias Meisel, Jakob Randow, Sophia Einspänner, Haibing Shao, Kata Kurgyis, Olaf Kolditz, Jaime Garibay. 2022. https://doi.org/10.5281/zenodo.7092676

How to cite: Kern, D., Silbermann, C. B., Magri, F., Steffen, R., Steffen, H., and Nagel, T.: Consequences of Fault Reactivation on Subsurface Flow in Crystalline Rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7365, https://doi.org/10.5194/egusphere-egu23-7365, 2023.

X4.171
|
EGU23-12486
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ERE3.3
Jörg Buchwald, Sonja Kaiser, Wenqing Wang, Olaf Kolditz, and Thomas Nagel

In deep underground storage facilities, coupled thermo-hydro-mechanical models are used to simulate the changes in temperature, pore pressure, and stress surrounding canisters containing high-level radioactive waste. Their numerical modeling is often computationally highly demanding, especially if an investigation is conducted that requires many model evaluations, like a sensitivity analysis or a parameter identification. In these analyses, the thermally driven pore pressure evolution and the subsequently altered flow processes are usually the primary targets of interest. 
In our contribution, we show the derivation of a hydro-thermal (HT) model with consistent incorporation of thermo-mechanical effects, allowing it to profit from its computational efficiency while maintaining most of its accuracy. We show its applicability to a 3D model of the FE experiment, a full-scale multiple heater experiment at the URL site in Mt. Terri.

How to cite: Buchwald, J., Kaiser, S., Wang, W., Kolditz, O., and Nagel, T.: Improved predictions of thermal fluid pressurization in hydro-thermal models based on consistent incorporation of thermo-mechanical effects - Application study of the FE-Experiment at Mt. Terri, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12486, https://doi.org/10.5194/egusphere-egu23-12486, 2023.

X4.172
|
EGU23-16485
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ERE3.3
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ECS
Florian Zill, Christian Silbermann, Tobias Meisel, Friederike Tiedtke, Dominik Kern, Anton Carl, Andreas Jockel, Thomas Nagel, Olaf Kolditz, Heinz Konietzky, and René Kahnt

To properly assess the present and future conditions of potential nuclear waste repository sites, understanding their evolution in the past is mandatory. Here, glaciation cycles strongly affect the long-term thermo-hydro-mechanical (THM) evolution of the geosystem.

The AREHS project studies the effects of time-dependent boundary conditions on the evolution of large-scale hydrogeological systems. The focus is on numerical long-term modeling taking into account thermal-hydraulic-mechanical couplings. On the basis of generic geological models for different host rock formations, complex 3D-THM-simulations are performed. The long-term evolution during glacial cycles is simulated using the open-source multi-field finite element code OpenGeoSys, as well as multiple pre-and postprocessing tools integrated into an automatized workflow. This workflow facilitates testing/benchmarking and improves reproducibility as well as overall software quality in a sense of modularization. The impact of the glacial THM loading and atmospheric temperature evolution is taken into account using appropriate time-dependent THM boundary conditions. The simulation results are analyzed with respect to potential safety-critical parameters, such as maximum temperature, hydraulic pressure, subsidence, equivalent effective stress and strain. Some general conclusions will be drawn for the host rock salt in Germany.

How to cite: Zill, F., Silbermann, C., Meisel, T., Tiedtke, F., Kern, D., Carl, A., Jockel, A., Nagel, T., Kolditz, O., Konietzky, H., and Kahnt, R.: Automatized large-scale 3D THM simulations capturing glacial cycle effects on German nuclear waste repositories in salt rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16485, https://doi.org/10.5194/egusphere-egu23-16485, 2023.

X4.173
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EGU23-7016
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ERE3.3
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ECS
|
Feliks Kiszkurno, Jörg Buchwald, Olaf Kolditz, and Thomas Nagel

Existing literature suggests the importance of the thermo-osmosis (TO) for an accurate simulation of pore pressure evolution in heater tests for nuclear waste disposal in clay rock. However, there is limited consensus regarding the appropriate choice of parameters controlling TO and the extent of its physical impact. Uncertainty of parameters describing the host rock and their spatial variability further add to  both the complexity of this consideration. This study will use the ATLAS in-situ full-scale heating experiment from the HADES underground research laboratory in Mol, Belgium, to investigate the impact of TO on the thermal pressurisation in Boom Clay and its significance for the evaluation of barrier integrity.

The ATLAS experiment was simulated with an inelastic thermo-hydro-mechanical model implemented in OpenGeoSys. After comparison to published data, assisted-history-matching was performed to obtain a good representation of the in-situ measurements with and without taking into account TO. The comparison of both groups allows a clearer discussion of the influence of TO on temperature and pressure evolution in the studied system. The final step -- uncertainty quantification of the TO parameterisation -- puts the results in the context of the large parameter uncertainty documented in the literature. Subsequently, the impact of TO and said uncertainties on barrier integrity was estimated in terms of area and duration of potential integrity violation using statistical and geometrical methods.  

How to cite: Kiszkurno, F., Buchwald, J., Kolditz, O., and Nagel, T.: Numerical investigation on the impact of thermo-osmosis on fluid pressurisation and barrier integrity in Boom clay – a case study of the ATLAS in-situ full-scale heating experiment., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7016, https://doi.org/10.5194/egusphere-egu23-7016, 2023.

Posters virtual: Tue, 25 Apr, 16:15–18:00 | vHall ERE

vERE.9
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EGU23-9029
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ERE3.3
|
ECS
Sonja Kaiser, Michael Pitz, Wenqing Wang, Jörg Buchwald, Aqeel Afzal Chaudhry, Olaf Kolditz, and Thomas Nagel

The theory of non-isothermal Richards flow with mechanics (TRM) assumes that the mobility of gases in unsaturated porous media is always large enough to allow sufficiently fast drainage so that the gas pressure does not increase significantly but remains constant. The benefit of an implementation using this simplified approach is a faster numerical model compared to an implementation using the more general theory of non-isothermal two-phase two-component flow with mechanics (TH2M), which represents the gas phase explicitly. The Full-scale Emplacement (FE) experiment conducted at the Mont Terri Underground Rock Laboratory was designed to simulate an emplacement tunnel for high-level radioactive waste at full scale using the reference repository design of the National Cooperative for the Disposal of Radioactive Waste (Nagra) of Switzerland. In this experiment, which is numerically studied in Task C of the Decovalex-2023 project, water vaporisation in the bentonite backfill near the heaters might potentially increase the gas pressure. Thus, justification for the applicability of Richards' assumption for this environment should be demonstrated before attempting to use a TRM process to model the coupled thermo-hydro-mechanical problem of this experiment. We demonstrate the applicability for the given conditions by comparing model results from OpenGeoSys (OGS-6) obtained by a TRM process with those obtained by a TH2M process implementation. The comparison of the TRM with the TH2M process provides the basis for subsequent extensive analyses using the TRM process of OGS-6 for modelling the FE experiment with large 3D meshes with more than one million degrees of freedom in parameter variation studies. This model has 75 parameters and many initial / boundary conditions, of which single ones or several at a time are varied. By comparing model results of these variations at 200 observation points with each other and to five years of hourly measurement data using statistical methods, sensitivities to single parameters are studied. But also relevant features not included in the current setup are revealed by using parameter settings that act as proxies for these anticipated missing features. These proxies give results that fit well to the observations but are without physical justification themselves. Due to the great possibilities for validation of existing coupled THM models arising thereof, this also leads to significant further development of model capabilities for the use in integrity evaluations of geotechnical and geological barriers.

How to cite: Kaiser, S., Pitz, M., Wang, W., Buchwald, J., Chaudhry, A. A., Kolditz, O., and Nagel, T.: Coupled THM modelling of the FE Experiment at Mt. Terri in Decovalex Task C, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9029, https://doi.org/10.5194/egusphere-egu23-9029, 2023.

vERE.10
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EGU23-7159
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ERE3.3
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ECS
|
Majedeh Sayahi, Theresa Hennig, Vinzenz Brendler, and Michael Kühn

Migration of neptunium, a minor component of high-level nuclear waste but with a long half-life (237Np: 2.14·106 years) and high radiotoxicity, is assessed for safety of potential nuclear waste disposal sites. Argillaceous rocks, like the Opalinus Clay (OPA), are preferred host rocks because of their low hydraulic conductivity and high sorption capacity. Numerical simulations are required to quantify radionuclide migration in the context of safety assessments.

One-dimensional diffusion simulations are conducted with PHREEQC to quantify the general neptunium migration processes. Sorption of Np(V) on the clay minerals is taken into account using surface complexation models. Carbonate minerals, mainly calcite, govern the pH, while the redox potential (pe) is controlled by pyrite. Therefore, both are considered in the mineral assemblage for atmospheric pCO2 conditions. Numerical simulations as a function of mineralogy and the associated changes in redox state and pore water composition are compared for two diffusion laboratory experiments conducted by Fröhlich et al. (2013) and Wu et al. (2009). Although both experiments were performed with the same setup, the determined transport parameters differ by one order of magnitude. Np(V) was applied via a synthetic pore water with pH 7.6 and pe 6 under atmospheric conditions, but to different OPA core samples. The Fröhlich experiment has already been modelled, and therefore this numerical setup is now applied to the Wu experiment to assess quantitatively whether the differences are due to variations in clay mineralogy or geochemistry.

Impact of clay mineralogy is quantified for maximum and minimum weight percentages of kaolinite and illite. Our results revealed that mineralogical variations only have a minor impact on the migration, and therefore cannot explain the difference between the two experiments. Np speciation is highly sensitive to pe and is partitioned between Np(IV) and Np(V). In the experiments, a pe of 6 is applied resulting in a redox disequilibrium within the sample compared to in-situ conditions with a pe of -3.8. Simulated migration is underestimated at high pe due to increasing sorption and overestimated at low pe, where Np(V) does not sorb. Results are consistent with the Fröhlich experiment when the pe is in an intermediate range between in-situ and experiment. Applying different redox conditions compared to Fröhlich, results also coincide with the Wu experiment. Therefore, variations in neptunium sorption are mainly attributed to different redox conditions.

We conclude that neptunium migration is governed less by clay mineralogy and more by redox. Therefore, redox conditions need to be accurately controlled in laboratory experiments as they determine neptunium speciation, and hence sorption. Distribution coefficients (Kd) can vary significantly, for instance, by one order of magnitude between two experiments with the same set up. This might hinder the applicability of experimentally determined Kd to assess neptunium migration for in-situ conditions.

 

References

Fröhlich, D. R., et al. (2013). Radiochimca Acta, 101(9), 553-560. DOI:10.1524/ract.2013.2059

Wu, T., et al. (2009). Environmental Science & Technology, 43(17), 6567-6571. DOI: 10.1021/es9008568

 

How to cite: Sayahi, M., Hennig, T., Brendler, V., and Kühn, M.: Simulation of neptunium migration as a function of clay mineralogy and redox conditions , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7159, https://doi.org/10.5194/egusphere-egu23-7159, 2023.