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.

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.

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.

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.

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 (T_1/2 229 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.