ERE4.3

The future deep geological repository for radioactive waste in the Czech Republic will be constructed in a suitable crystalline rock mass around 500 metres below the earth’s surface. The commencement of operation is planned for 2065. The current DGR development phase is devoted principally to the determination of the optimum disposal concept and the selection of the most suitable site. A total of nine potential sites have been assessed with the aim of reducing their number to four.

The data set subjected to assessment included site descriptions from the geological point of view (3D geological and hydrogeological model), and long-term site stability (seismotectonic, climate and erosion) and geomechanical data. A further assessed dataset included information on construction issues and on the evaluation of both environmental characteristics and the presence of groundwater resources. All the assessed characteristics were derived from surface-based exploration without the need for borehole drilling.

The key criteria reflected the three main areas of concern i.e. long-term and operational safety (including geological and hydrogeological indicators), technical feasibility and environmental impacts. The assessment of the sites was performed in two stages. The first stage involved the assessment of the probability of fulfilling the exclusion criteria (total 26), while the second stage involved the mutual comparison of the sites in terms of the defined key criteria (total of 13, divided into 38 indicators). The second stage involved the determination of weightings for the various criteria and indicators via the application of the SAATY method for the expert comparison of the significance of criteria. This method distinguished between relatively strongly weighted and less weighted criteria. The sites were graded with respect to the value estimation of the criteria; moreover, the grading of the sites considered various types of data.

A total of eight calculations were performed for five scenarios using various procedures for the estimation of the weightings and for data normalisation purposes. The first assessment stage indicated that all the sites fulfilled the DGR site assessment methodology requirements. The second stage, which comprised the assessment of the comparison of the site calculations (assessment grades) for each of the sites, was based on the levels of significance of the indicators and criteria and the resulting representative values for each site. The results of the subsequent comparison calculations indicated that the same four sites always occupied the first four positions with only minor variations in the order. The differences in the gradings of the four most suitable sites and the four relatively less suitable five sites ranged between 11% and 17.8% (between the fourth and fifth sites), which convincingly differentiated between the two groups of sites. One site was always in last position according to the calculations. In compliance with the assessment results, the four  sites were subsequently recommended to the Government of the Czech Republic for further follow-up research and analysis. Those sites that were not recommended for the next stage of research will continue to be considered as reserve (i.e. backup) sites.

How to cite: Vondrovic, L., Augusta, J., Vokal, A., Konopacova, K., Popelova, E., and Urik, J.: Multi-criteria site assessment process for candidate deep geological repository sites: Case study from the Czech Republic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16491, https://doi.org/10.5194/egusphere-egu21-16491, 2021.

The Federal Company for Radioactive Waste Disposal mbH (BGE mbH) is as Germans waste management organization responsible to implement the search for a site with the best possible safety for the disposal of high-level radioactive waste for at least one million years, following the amendments of the Repository Site Selection Act in 2017. The selection procedure is meant to be a participatory, transparent, learning and self-questioning process based on scientific expertise.

This contribution will provide an overview of the methodology of the forthcoming preliminary safety assessments as a major part of the next steps in the site selection procedure. This procedure overall consists of three phases with increasing level of detail for identification of the best site. The first phase consists of two steps. The objective of the first step was to determine sub-areas in the three considered host rocks, salt (halite), clay and crystalline rock, by applying legally defined exclusion criteria, minimum requirements and geoscientific weighing criteria. 90 sub-areas that cover approximately 54 % of the area of Germany were identified due to their general suitable geological conditions. The result was published in September 2020.

The second step of phase one is currently in progress and consists of representative preliminary safety assessments that aim to assess the safety of the repository system as well as its robustness. The requirements for the preliminary safety assessments in the site selection procedure are defined by a governmental directive released in October 2020. Representative preliminary safety assessments have to be performed for each sub-area and consist of the compilation of all geoscientific information relevant to the safety of a repository, the development of preliminary safety and repository concepts and the analysis of the repository system. In addition, a systematically identification and characterization of uncertainties has to be undertaken and the need for exploration, research and development must be determined. The application of the representative preliminary safety assessments as well as the following renewed application of geoscientific weighing criteria will lead to the identification of siting regions within the larger sub-areas of step one. These regions will be considered, first for surface-based geoscientific and geophysical exploration, including i.e. seismic exploration and drilling of boreholes. Subsequently the last phase of the site selection will proceed with subsurface exploration. Finally, all suitable sites will be proposed and the German government will decide the actual site. This process is expected to be finalized in 2031.

How to cite: Hoyer, E.-M., Behrens, C., Bjorge, M., Dannemann, J., Gawletta, D., Kreye, P., Lohser, T., Luijendijk, E., Müller, P., Panitz, F., Schlüter, F., Wengorsch, T., and Rühaak, W.: Preliminary safety assessments in the high-level radioactive waste site selection procedure in Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9429, https://doi.org/10.5194/egusphere-egu21-9429, 2021.

According to the 'Act on the Organizational Restructuring in the Field of Radioactive Waste Disposal' the BGE was established in 2016. The amended 'Repository Site Selection Act' (StandAG) came into force in July 2017 and forms the base for the site selection by clearly defining the procedure. According to the StandAG the BGE implements the participative, science-based, transparent, self-questioning and learning procedure with the overarching aim to identify the site for a high-level radioactive waste (HLW) repository in a deep geological formation with best possible safety conditions for a period of one million years.

The German site selection procedure consists of three phases, of which Phase 1 is divided into two steps. Starting with a blanc map of Germany, the BGE completed Step 1 in September 2020 and identified 90 individual sub-areas that provide favorable geological conditions for the safe disposal of HLW in the legally considered host rocks; rock salt, clay and crystalline rock. Based on the results of Step 1, the on-going Step 2 will narrow down these sub-areas to siting regions for surface exploration within Phase 2 (§ 14 StandAG). Central to the siting process are representative (Phase 1), evolved (Phase 2) and comprehensive (Phase 3) preliminary safety assessments according to § 27 StandAG.

The ordinances on 'Safety Requirements' and 'Preliminary Safety Assessments' for the disposal of high-level radioactive waste from October 2020 regulate the implementation of the preliminary safety assessments within the different phases of the siting process. Section 2 of the 'Safety Requirements' ordinance provides requirements to evaluate the long-term safety of the repository system; amongst others, it states that all potential effects that may affect the long-term safety of the repository system need to be systematically identified, described and evaluated as “expected” or “divergent” evolutions. Additionally, the ordinance on 'Preliminary Safety Assessments' states in § 7, amongst others, that the geoscientific long-term prediction is a tool to identify and to evaluate geogenic processes and to infer “expected” and “divergent” evolutions from those. Hence, considering the time period of one million years for the safe disposal of the HLW and the legal requirements, it is essential to include long-term climate evolution in the German site selection process to evaluate the impact of various climate-related scenarios on the safety of the whole repository system.

To better understand and evaluate the influence of climate-related processes on the long-term safety of a HLW repository, climate-related research will be a part of the BGE research agenda. Potential research needs may address i) processes occurring on glacial – interglacial timescales (e.g. the inception of the next glaciation, formation and depth of permafrost, glacial troughs, sub-glacial channels, sea-level rise, orbital forcing) and their future evolutions, ii) effects on the host rocks and the barrier system(s) as well as iii) the uncertainties related to these effects but also to general climate models and predictions.

How to cite: Wengler, M., Göbel, A., Hoyer, E.-M., Liebscher, A., Reiche, S., Völkner, E., and Rühaak, W.: Significance of long-term climate evolution and associated impacts on the long-term safety of a high-level radioactive waste repository within the German siting process, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15771, https://doi.org/10.5194/egusphere-egu21-15771, 2021.

The political and social debate on nuclear energy in Germany has been characterized for many decades by a high potential for conflict and dissatisfaction. Especially the controversies surrounding the Gorleben salt dome gained international attention and changed the relationship between citizens and political decision-makers from the local to the national level. With the Repository Site Selection Act of 2013 (StandAG, first amendment in 2017) a new approach was chosen to implement a participative, inclusive and transparent search process for the best possible repository for high-level radioactive waste in Germany. In this context, a self-learning process was proclaimed, based on a white (unbiased) map, which should give citizens an active role. However, the first interim report of the Federal Company for Radioactive Waste Disposal and the publication of the colorful map, in which geologically suitable areas were identified on a large scale, already revealed a massive potential for conflict. Many citizens and activists who were already protesting against the Gorleben salt dome criticized in this early phase of the process, the lack of transparency and opportunities to have a say on the possible sitting regions.

To address this criticism, we want to provide an interactive map as an online platform that presents existing geographic data, that enables people to contribute spatially-located information (geological, on-surface), and thus a possibility for people to interact and participate regarding the possible siting regions. Therefore, we collect existing spatial data that is relevant to the ongoing process, such as possible siting regions (declared by the Federal Company for Radioactive Waste Disposal), nuclear power plants (active/inactive, research facilities, etc.), storage facilities (on-site, central, interim, etc.), historically relevant locations (places of protest, uranium enrichment & processing facilities, etc.) as well as basic data for orientation. We implement two possibilities for participatory interaction: (1) adding spatially-located notes that contain own experiences or local knowledge (e.g. reports, concerns, suggestions) and (2) initiating a platform for a spatially-located discussion. Against the background of transdisciplinary research, in an iterative process, we want to evaluate the participatory value of this application by consulting civic as well as scientific actors. We, therefore, employ focus groups with a transdisciplinary support group of citizens beforehand and surveys after using the application. For this panel we want to present our primary results from a first test with the aforementioned focus groups.

Aside from testing the suitability of such a mode of participation, we aim to analyze where problems emerge, and which information is necessary and/or might lead to conflict. Finally, we want to gain insight into how such modes of participation influence the quality of dialogue and how it contributes to the overall perception of a procedurally just process.

How to cite: Schwarz, L. and Bräuer, P.: Improving participation for the German search for a nuclear waste repository site: an interactive map as a transdisciplinary approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10969, https://doi.org/10.5194/egusphere-egu21-10969, 2021.

The safety assessment (SA) of the spent nuclear fuel repository is often based on lumped-parameter models (LPM) of radionuclide transport between the source term and the biosphere (under various terminology like a compartment model, or a channel model). It profits e.g. from computational efficiency when used with stochastic data. Useful property of LPM is intuitively clear influence of most of its parameters, in terms of “the larger/smaller the better”. On the other hand, some parameters are not clearly defined and their values depend on expert choice.

Gradual improvements of computing hardware and simulation software also allow using physically-based models on real geometry for SA application. Migration of radionuclides is simulated by means of groundwater flow and advective-dispersive transport with linear sorption on the input (hydro)geological configuration of site. Defining a LPM based on the 3D transport input data and results, it actually represents a model upscaling method and can keep the LPM advantages with avoided compromises of their input data definition.

As the LPM, we consider a generic 1D channel with analytical advection-diffusion-sorption solution, in particular implemented in GoldSim software as “Pipe” object. The 3D flow and transport are solved with Flow123d simulation code (open-source developed at author’s institute), but the presented principles are theoretically applicable to any finite-element or finite-difference code.

We derived a procedure of integral processing of 3D model velocity field and trajectories and the fictitious pulse or step input breakthrough curve between the repository and the output to the biosphere. Four tracers have been used at the same time: non-sorbing, less/more sorbing, and decaying. The relevance of estimation was verified by optimization of the LPM parameters to the best fit between the 3D model and the LPM for all kinds of tracers. The optimization decrease the fit criterion by a small factor, but graphically, all three curves (3D transport, postprocessed LPM, and optimized LPM) are similar.

The resulting data (path length, cross-section, flow rate, travel time, dilution factor, etc.) are obtained with little computing cost compared to the optimization. With a reasonable precision, they can serve for quick comparison of candidate sites, without explicitly running the 3D model or the lumped-parameter model with full source term temporal evolution. On the other hand, some of the parameters are questionable whether physical realistic, which is a consequence of possible model oversimplification. Therefore, other LPM configurations with more blocks representing the real conceptual path segments are evaluated – two serial, three serial and two serial couples in parallel. Due to more constraints, the breakthrough curve fit between LPM and 3D is little worse but with important advantage of physically realistic parameters.

The method was demonstrated on hydrogeological configuration of 9 anonymized (and with partly synthetic features) candidate sites in Czechia.

The project leading to this result has received funding from the EU’s Horizon 2020 programme under grant agreement No 847593. Computational resources were supplied by the project “e-Infrastruktura CZ” (e-INFRA LM2018140).

How to cite: Hokr, M. and Landa, J.: Deriving lumped parameters for DGR safety assessment by 3D transport model postprocessing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13424, https://doi.org/10.5194/egusphere-egu21-13424, 2021.

The kinetics of mineral dissolution plays a key role in many environmental and technical fields, e.g., weathering, building materials, as well as host rock characterization for potential nuclear waste repositories. Mineral dissolution rates are controlled by two parameters: (1) transport of dissolved species over and from the interface determined by advective fluid flow and diffusion (transport control) and (2) availability and distribution of reactive sites on the crystal surface (surface reactivity control). Reactive transport models (RTM) simulating species transport commonly calculate mineral dissolution by using rate laws [1]. However, the applied rate laws solely depend on species concentration in the fluid. While the effect of transport-controlled processes is addressed in current RTM approaches, the intrinsic variability of surface reactivity is neglected. Experimental studies under surface-controlled dissolution conditions have shown that surface reactivity is heterogeneously distributed over the surface [e.g., 2]. This heterogeneity in reactivity is largely caused by nanotopographical structures on the crystal surface, such as steps and etch pits. These structures are generated through defects in the crystal lattice. At these structures, the high density of reactive kink sites is leading to a local increase in surface reactivity observable through high dissolution rates.

In this study, we test whether the current rate calculation approach applied in RTMs is sufficient to reproduce experimentally observed rate heterogeneities. We apply a standard RTM approach combined with the measured surface topography of a calcite single crystal [2]. Calcite is an important mineral component in the sandy facies of the Opalinus clay formation, that is under investigation for nuclear waste storage. The modeled surface dissolution rate maps are compared to experimentally derived rate maps. Results show that the current RTM is not able to reproduce the measured rate heterogeneities on the calcite surface. To improve the predictive capabilities of RTMs over the large time scales required for the safety assessment of nuclear waste repositories, the surface reactivity that is intrinsic to the mineral needs to be implemented into future rate calculations. Investigating calcite surface reactivity in the context of dissolution can also yield information about other kinetic surface processes such as the adsorption of radionuclides during transport. We show the integration of surface reactivity into rate calculation by using a proxy parameter. The slope of the crystal surface at the nm scale is applied. We show that by adding a factor based on the slope to the rate law the RTM is able to approximate experimental rate maps. Other proxy parameters such as surface roughness could yield similar results as well. The implementation of surface reactivity proxy parameters will allow for a more precise prediction of host rock-fluid interaction over large time scales in RTMs, relevant for safety assessment of nuclear waste repositories.

[1] Agrawal, P., Raoof, A., Iliev, O. and Wolthers, M. (2020), Advances in Water Resources, 136, 103480. [2] Bibi, I., Arvidson, R.S., Fischer, C. and Lüttge, A. (2018), Minerals, 8, 256.

How to cite: Schabernack, J. and Fischer, C.: Impact of Surface Reactivity on the Simulation of Mineral Dissolution Rates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11079, https://doi.org/10.5194/egusphere-egu21-11079, 2021.

As the final barrier of the multi-barrier deep geological repository (DGR) for radioactive waste (RAW)  the rock environment fulfils the primary safety function by limiting the transport of radionuclides to the biosphere via the low hydraulic conductivity of the rock mass compared to other rock massifs (1). Moreover, the various properties and characteristics of the rock environment comprise important considerations with respect to the DGR safety assessment.

Samples of the various types of igneous and metamorphic rocks present in the Bohemian Massif were collected as part of the Research Support for the Safety Assessment of the DGR project (SURAO). The study of the rock materials also included that of the fracture fillings, the characteristics of which supplemented the input data set for the future DGR safety assessment. All the rock samples were subjected to both mineralogical (X-ray analysis) and petrological characterisation (2).

Fracture fillings (e.g. clay minerals, biotite, Fe oxyhydroxides, calcite) generally evince higher specific surface areas and cation exchange capacities than do the rocks themselves, i.e. properties that are able to significantly influence the sorption of radionuclides

The sorption experiments performed with radionuclides revealed differing degrees of sorption on the rock and fracture filling samples (e.g. ¹³⁴Cs, ⁸⁵Sr, U, Se). The initial experiments on the fracture filling materials determined that their presence can to significantly enhance the capture of radionuclides (e.g. ¹³⁴Cs) during their migration towards the biosphere, and thus to enhance the safety function of the rock environment (2).

The diffusion characteristic values were determined experimentally using the through diffusion method (2). With respect to the diffusion characteristics (the effective diffusion coefficient D_e), although the samples were taken from different parts of the Czech Republic and from differing rock types, the effective diffusion coefficients were found to lie within a relatively narrow range: for ³H (4–10) · 10⁻¹³ m² s⁻¹, for ³⁶Cl (1–10) · 10⁻¹³ m² s⁻¹ and for ¹²⁵I (1–4) · 10⁻¹³ m² s⁻¹. Anionic exclusion was demonstrated for the metamorphic rock samples, which led to the determination of lower D_e values for ³⁶Cl and ¹²⁵I in comparison to  ³H (2)

The experimental results were determined as part of the Research Support for the Safety Assessment of the Deep Geological Repository project, financed by SURAO (SO2014-061-01), and the EURAD WP FUTURE project.

How to cite: Havlová, V., Zuna, M., Rosendorf, T., Galeková, E., Jankovský, F., and Hofmanová, E.: Influence of rock environment on radionuclide migration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14456, https://doi.org/10.5194/egusphere-egu21-14456, 2021.

The use of underground repositories excavated in low-permeability formations, such as rock salt, to store high-level, radioactive waste requires the analysis of its isolation properties. Underground excavation disturbs the original stress state of the rocksalt, resulting in a deviatoric stress distribution around the walls of excavated galleries and boreholes. At high deviatoric stresses and low confinement, dense rocksalt produces an increase in porosity and permeability. The extent of dilatancy in this disturbed zone, as well as the impact of dilatancy on the transport properties, are important for assessing the safety of radio-active waste disposal as well as the integrity of salt caverns and boreholes in the context of energy storage, brine cavern abandonment and gas well abandonment.  

The stress conditions at which dilatancy occurs have been measured experimentally, and are typically determined on the basis of macroscopic (sample-scale) measurements of volumetric strain and permeability, and/or acoustic velocity changes or emissions. However, the detailed mechanisms causing dilatancy at the grain-scale are poorly understood. We have developed a microphysical model for dilatancy in rocksalt, both under dry and wet conditions, including mechanisms such as slip and opening of grain boundaries. This model enables us to describe and predict the dilatancy behaviour of rocksalt based on physical, mechanical and chemical processes. The model is presently being independently verified through comparison with existing literature data, and new experiments.

How to cite: van Oosterhout, B., Spiers, C., and Hangx, S.: Understanding dilatancy in rocksalt: a microphysical model of rocksalt at the grain-scale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15894, https://doi.org/10.5194/egusphere-egu21-15894, 2021.

Argillaceous rocks have great potential as possible geological host medium to store radioactive waste.  Andra is leading the design of a deep geological nuclear waste repository to be located in the Callovo-Oxfordian formation. In the framework of this project, excavations of large diameter galleries are contemplated to access and to store intermediate-level long-lived nuclear waste at repository main level. The closure of the repository will be realized by building sealing structures of expansive material.

The response of such structures is affected by several thermo-hydro-mechanical coupled processes taking place in the near and far field of the argillaceous formations. They include the formation of an excavation induced damaged zone around the galleries, the impact of the thermal load on host rock pressures and deformations, the long-term interaction with support concrete structural elements and the hydration and swelling of sealing materials. As a result, the study of their performance requires to perform simulation works of increasing complexity in terms of coupling equations, problem geometry and material behaviour. As well, challenging computational aspects, as the ones related to fractures creation and propagation, have to be considered for a representative analysis of the problem.

This work presents advanced large scale THM numerical models to provide keys about the response of the host rock around large diameter galleries during excavation and further thermal load as well as to analyse the performance of large diameter sealing structures. Particular features of the models include on one hand advanced constitutive laws to capture the development of the fractured zone around excavations, the behaviour of host rock/gallery support interfaces and the multi-scale response of bentonitic backfill. On the other hand, simulations consider geometries including constructive details of interest at decimetre scale within large discretization domain covering the whole formation stratigraphic column.

These challenging simulations provided qualitative and quantitative results on key aspects for natural and engineered barrier integrity, like extension of the damaged zone, impact of the thermal load and water pressure variations in the surrounding geological layers, duration of natural hydration phase, swelling pressure development and seals global stability.

How to cite: Alonso, M., Vaunat, J., Vu, M.-N., and Gens, A.: THM analysIs of natural and engineered barriers for large excavations in deep nuclear waste repository, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7588, https://doi.org/10.5194/egusphere-egu21-7588, 2021.

Numerical studies on integrity of the geological barriers in heat generating radioactive waste disposal remain a challenging topic involving modelling of thermal, hydraulic and mechanical (THM) processes within complex geometries, as well as particularly long simulation time intervals . Due to this, unfeasible computational complexity emerges for many three-dimensional problems, resulting in the need of further model assumptions and simplification for many types of simulation. To make use of results of such simulations reliably as a tool in the decision-making process, uncertainties introduced by the modelling have to be addressed in the framework of safety assessment.

Consequently, the system describing partial differential equations are dependent on a set of parameters, each parameter possibly subject to uncertainty resulting from reduced knowledge or imprecise measurement. The treatment of uncertainties introduces additional dimensions into the physical system, resulting in a dramatic increase of computational complexity for each parameter considered uncertain.

For general applicability, the method chosen for uncertainty quantification should be problem-independent, i.e. an arbitrary set of stochastic input data is propagated through the physical system, while the output is again a freely selectable quantity of interest. To this end, sampling-based methods like Monte-Carlo methods and stochastic collocation seem to be favourable.

Since a full stochastic model is never computable, it is amenable to include only the most sensitive parameters into stochastic analyses, retaining all other parameters as deterministic, in order to spend available computational power efficiently. With aim of finding such a suitable set of stochastic parameters, preliminary studies of simplified two-dimensional models with less complex geometries and a less complex TH-process seem to be appropriate.

In this contribution, a simplified two-dimensional model of a radioactive waste disposal in clayey rock is proposed, as a starting point, and its results of the thermal induced increase in pore water pressure is compared with more sophisticated and established models for a set of deterministic input parameters. It will be demonstrated that the simplified two-dimensional model is suitable for first stochastic investigation of pore water induced tensile or shear failure.

Subsequently, the results of different stochastic simulations for this model are presented, giving rise to a better understanding of stochastic modelling as well as stochastic post-processing in discretized problems for computational safety assessment of radioactive waste disposal. In detail, sensitivity of the quantity of interest to changes in the input parameters can be studied and in addition, worst-case scenarios within the parameter interval can be found. Given known probability density functions for each input parameter, probability of occurrence of each scenario as well as expected values and variances can be calculated.

How to cite: Bittens, M., Maßmann, J., and Thiedau, J.: Preliminary stochastic investigations of pore water induced loss of host rock integrity in radioactive waste disposal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7401, https://doi.org/10.5194/egusphere-egu21-7401, 2021.

From previous studies it is evident that decoupled simulations lack the ability to capture certain coupled effects, such as the Noordbergum effect or the Mandel-Cryer effect in a hydraulic-mechanical context. Thus, for detailed simulations of geotechnical or geological system, coupled simulations are usually chosen. For example, thermal-hydraulic-mechanical (THM) coupled systems, and even chemical and biological couplings (THMCB), are considered in simulations used to assess barrier integrity over long time spans in the context of geological waste disposal.

This paper is restricted to coupled hydraulic-mechanical (HM) systems. A monolithic approach is both stable and accurate for strongly coupled systems. However, as site-scale models of geological disposal facilities are also large in spatial dimensions, it is worth to investigate how staggered methods may cut down the computational costs. The fixed-stress split appears to be a promising approach for staggered schemes in terms of stability, consistency, accuracy, and efficiency.

While adding another iteration level in comparison to monolithic schemes, staggered schemes allow for lower-order approximation spaces, whereas monolithic schemes require Taylor-Hood elements resulting in a larger number of degrees of freedom per element. Both coupling schemes are implemented in the the open-source finite-element (FE) software OpenGeoSys and used to simulate a large-scale model, which is oriented towards a real site in planning in Russia. Simulation results are compared in terms of accuracy, coupling effects and performance.

How to cite: Kern, D., Magri, F., Malkovsky, V., and Nagel, T.: Does a Staggered Scheme Pay Off on Large-scale Hydraulic-mechanical Simulations?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7823, https://doi.org/10.5194/egusphere-egu21-7823, 2021.

Co-authors: Francesco Parisio, Thomas Nagel

Glaciation cycles affect the long-term evolution of geosystems by crustal deformation, ground freezing and thawing, as well as large-scale hydrogeological changes. In order to properly understand the present and future conditions of potential nuclear waste repository sites, we need to simulate the past history. 
For this, a sedimentary basin is considered here as a large-scale hydrogeological benchmark study. The long-term evolution during one glacial cycle is simulated using the open-source multi-field finite element code OpenGeoSys. The impact of the glacial loading (weight and induced shear) is taken into account using appropriate time-dependent stress boundary conditions. As a preliminary study, the hydro-mechanically coupled problem and the thermal problem are considered separately. For comparison with a previously published study by Bense et al. (2008), the entire displacement field is prescribed and the groundwater evolution (hydraulic problem) is regarded. Then, the displacement is only prescribed by means of boundary conditions. The impact of different constitutive assumptions on the deformation and hydraulic behavior is analyzed. The thermal problem is used to simulate the evolution of frost bodies in the subsurface beneath and ahead of the glacier.

V. F. Bense and M. A. Person. Transient hydrodynamics within intercratonic sedimentary basins during glacial cycles. Journal of Geophysical Research,
113(F4):F04005, 10 2008.

How to cite: Silbermann, C.: Hydrogeological simulation of sedimentary basin evolution during a glacial cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13299, https://doi.org/10.5194/egusphere-egu21-13299, 2021.

Bentonite is a swelling clay, consisting mainly of montmorillonire,  being planned to be used as a backfill material in the nuclear waste repository. It contains indigenous microbial populations that can negatively influence the long-term safety of the geological repository due to their metabolic activity (canister corrosion, illitization of bentonite, gas production, degradation of cementitious materials). However, reliable detection of microorganisms in clayish material is generally very difficult. Although the compactness of bentonite will undoubtedly limit the microbial activity, in the extremely long-time frame of repository lifetime this condition can fail. It is thus crucial to understand the potential of the naturally present microbial community in bentonite to compromise the safety of repository, if not limited by the compactness. Higher metabolic activity can be mainly expected at the interfaces or in the places with a lower density of bentonite.

Here we present an optimized cell extraction method enabling direct estimation of bacterial density and viability in bentonite. Indigenous bacterial cells were extracted from bentonite suspensions by an improved step-wise protocol and their viability was detected using live/dead staining and epifluorescence microscopy. We used dispersant (2.5 mM natrium pyrophosphate-based solution or 1% methanol) to partially disintegrate the bentonite and detach the vital and dead microbial cells from its surface. The dispersed material was subsequently stepwise centrifuged over two high-density media (sucrose and Histodenz) to remove most of the heavy bentonite particles while keeping the light bentonite particles and cells in the final extract. We were able to detect and enumerate the cells concentrated at the surface of the light bentonite particles, which served as a sieve to retain all free cells during centrifugation.    

Different extraction procedures were tested and their efficiency was estimated by comparing live/dead ratios of resulting extracts and was also proved by implementing both NGS and quantitative PCR. The results show that most of the microbial genera present in the original suspension are also present in extracts but as proved by Deseq2 analysis some genera tend to settle down with heavier bentonite particles during the first centrifugation step.

To conclude, we present a protocol for extraction and detection of metabolically active cells in clayish material – bentonite. The quality of the extraction procedure was estimated both by a combination of fluorescent microscopy and genetic methods. The protocol was successfully tested on different bentonite types showing general applicability of this approach for clay materials.

How to cite: Hlavackova, V., Cerna, K., Kejzlarova, L., Bartak, D., Shestra, R., and Sevcu, A.: Detection of living bacterial cells in clay - bentonite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16492, https://doi.org/10.5194/egusphere-egu21-16492, 2021.