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ERE3.3

EDI
Towards a safe nuclear waste repository – assessment of barrier integrity, geoscientific, technological, societal and regulatory challenges and approaches

The successful implementation of safe deep geological disposal of spent fuel, high-level waste and other long-lived radioactive waste is one of the currently most pressing environmental challenges in several countries worldwide. Site investigation and selection are primarily geoscientific tasks that require interdisciplinary collaboration of different disciplines, like geophysics, hydrogeology, (hydro-)geochemistry, mineralogy, geomechanics, material science, and geological as well as THMC modelling. Moreover, successful and socially accepted site selection and disposal implementation depend not only on geoscientific state-of-the-art results, sound engineering and R&D programs but to a large extent on well-designed public outreach and public involvement/participation activities as well as on suitable regulatory frameworks.


As for other subsurface technologies such as the storage of thermal energy and other energy carriers, or the deposition of chemotoxic waste or carbon dioxide, barrier integrity is a crucial aspect for the assessment of nuclear waste disposal. Numerical simulations, in conjunction with experimental studies are an integral part of safety and environmental-impact assessment concepts involving barrier integrity as a key component. Reliable comparative analyses of potential technological options require coupled THMC models capturing the particularities of each rock type and associated repository concept to a comparable level of sophistication. Structural as well as process complexity are often met by data scarcity and variability, necessitating the treatment of uncertainties and variability.


Aside from geoscientific and technological aspects this interdisciplinary session also addresses social and regulatory challenges by welcoming contributions from research and technical support organizations, waste management organizations, regulatory bodies, and NGOs. The session provides a platform for the exchange of i) geoscientific, geochemical, geotechnical and material science knowledge for assessing the integrity of multi-barrier systems considering equally conceptual, theoretical, computational and experimental aspects as well as ii) safety assessment strategies and tools, disposal concepts, national and transnational public outreach and involvement programs, siting approaches and relevant regulatory frameworks. Presentations related to other subsurface technologies that face comparable challenges are also welcome.

Convener: Thomas Nagel | Co-conveners: Václava Havlová, Axel Liebscher, Jobst Maßmann, Klaus-Jürgen Röhlig
Presentations
| Tue, 24 May, 13:20–15:55 (CEST)
 
Room 0.96/97

Tue, 24 May, 13:20–14:50

Chairpersons: Thomas Nagel, Václava Havlová, Axel Liebscher

13:20–13:26
Session Introduction

13:26–13:33
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EGU22-1925
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ECS
Jayjayanti Basumallik et al.

Across the world, about thirty countries are generating power from nuclear energy, but adopting different strategies to deal with high-level radioactive waste (HLW) produced during the process. A brief comparative analysis of four countries, namely, Finland, Germany, China, and India, is conducted in order to understand the key geopolitical, technical, and social factors that drive these different strategies. There are significant differences in their preparedness related to planning and implementing a final disposal facility for the HLW in deep geological repositories. The research explores the extent of public awareness and acceptance of the nuclear waste management strategies in these four countries. Of special interest is the scope of public participation in the decision-making process related to repository site-selection, and the safety assurances of the proposed HLW disposal techniques. The analysis finally delves into the subject of protracted timelines of the ongoing HLW management projects and the associated challenges.

How to cite: Basumallik, J., Rühaak, W., and Schreurs, M.: Different strategies in attempting nuclear waste disposal – A comparative study based on four countries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1925, https://doi.org/10.5194/egusphere-egu22-1925, 2022.

13:33–13:40
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EGU22-7801
Katharina Bairlein et al.

According to the 'Repository Site Selection Act' (a governmental law, called in German the Standortauswahlgesetz – StandAG), the German National Waste Management Organisation (BGE) has been assigned to be the implementer for the German site selection procedure. As such, the BGE is responsible to identify a site for a high-level radioactive waste repository in a deep geological formation with best possible safety conditions for a period of at least one million years.

The German site selection procedure is an iterative process and consists of three phases with an increasing level of detail while the survey area becomes smaller during the process. An immanent part is the repetitive application of exclusion criteria, minimum requirements and the geoscientific weighting criteria (Sections 22 – 24 StandAG) during each phase. Starting with an empty, so-called white map of Germany, the BGE completed Step 1 of Phase I in September 2020 with the submission of the sub-areas interim report. Therein, 90 individual sub-areas were identified, where a favourable geological condition for the safe disposal of radioactive waste is possible. According to the site selection act the host rocks claystone, rock salt and crystalline rock are considered. In the current Step 2 of Phase I, both the representative preliminary safety assessments (Section 27 StandAG) and the repeated application of the above-mentioned criteria and requirements as well as the planning-scientific weighing criteria according to Section 25 StandAG are applied to localise siting regions within the 90 sub-areas.

Surface exploration, including geophysical surveys, geological mapping, hydrogeological investigation and drilling of boreholes, will take place within these siting regions in the scope of Phase II. Currently, exploration targets are defined, which arise from the safety assessments and the mentioned requirements and criteria. Existing data from former exploration activities such as seismic and borehole data, but also other geoscientific data are considered. These data have to be selected, procured and possibly reprocessed with a new focus on a high-level radioactive waste repository. After comparison of the principal demand of information as mentioned above and the already existing data, the exploration demand for every siting region will be derived by a thorough gap analysis. Based on that, the surface exploration programs will be developed.

Phase III is characterized by higher-grade, more detailed subsurface exploration activities, which again provide the base for preliminary safety assessments. Phase III ends with a proposal for a site for a high-level radioactive waste repository.

How to cite: Bairlein, K., Perner, M., Meier, F., and Schamp, J.: The site selection procedure for a high-level radioactive waste repository in Germany: an overview of the process and upcoming exploration activities, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7801, https://doi.org/10.5194/egusphere-egu22-7801, 2022.

13:40–13:47
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EGU22-9127
Heini Reijonen et al.

A proposed strategy for the use of natural analogue (NA) studies in Nuclear Waste Services1 (NWS) UK geological disposal facility programme and safety case has been developed based on an extensive review of past and current international practices. In the review, it was found that information on the strategic implementation of NA information is not readily available. This may be partly due to lack of strategic planning for the utilisation of NAs, or simply because waste management organisations do not generally publish internal strategic discussions. Nevertheless, it is seen of importance that the topic is discussed in public to increase the visibility of NA information and its implementation in geological disposal programmes. Another main conclusion concerns the cultural aspects of treatment of NAs in international fora, which seems to set NA information in the category of complementary/alternative evidence rather than equal part of the knowledge base. Often, the emphasis is set on the uncertainties and qualitative nature of NA. However, this ignores the fact that uncertainties are also present in laboratory, URL (underground rock laboratory) and modelling studies: for example, NA uncertainties are related generally to ill-defined boundary conditions, while short term experiments have uncertainties in both spatial and temporal scales. When NAs are used only as “alternative” lines of evidence, there is a risk that the information is devolved from the other evidence and this creates a risk of over or underestimating processes if extrapolated.

The strategy presented highlights the importance of:

  • knowledge management and systematic approaches to the use of data from the broadest range of sources (i.e. laboratory, URL, modelling and NA) possible
  • the continuous review of existing NA information
  • the potential benefits of new NA projects in the future national programme
  • the significance and experience of communication with NA information (at various levels across a broad range of stakeholder groups)

The strategic study also includes an update of RWM’s NA catalogue (Milodowski et al. 2015), a starting point of the NWS’s NA knowledge base. Development of a strategic approach to utilise NA information naturally leads to activities to be undertaken in the immediate future and one of the activities emphasised is the potential use of regional (or self) analogues in support of the site characterisation programme. The emphasis and requirements for NA research will evolve and change as the focus of the geological disposal programme develops naturally as the programme matures so, for example, preliminary discussion on the operational phase is also included. 

References

Milodowski, A.E., Alexander, W.R., West, J.M., Shaw, R.P., McEvoy, F.M., Scheidegger, J.M. & Field, L.P., 2015. A Catalogue of Analogues for Radioactive Waste Management. BRITISH GEOLOGICAL SURVEY COMMISSIONED REPORT CR/15/106. Keyworth, Nottingham British Geological Survey 2015. 1849p.

1 Following a merger with Low Level Waste Repository Limited, Radioactive Waste Management (RWM) is now part of Nuclear Waste Services (NWS)

How to cite: Reijonen, H., Alexander, R., and Norris, S.: Natural Analogues – A proposed strategy for implementation within the Nuclear Waste Services (NWS), UK, programme of geological disposal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9127, https://doi.org/10.5194/egusphere-egu22-9127, 2022.

13:47–13:54
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EGU22-11278
Axel Liebscher et al.

One of the key requirements for the deep geological disposal of high-level nuclear waste is the assessment of its long-term performance and safety. As any other barrier of the disposal system, waste containers must fulfil their respective safety functions for the required duration, which can vary from a few hundreds of years to several hundreds of thousands of years, depending on disposal system requirements. Sufficient corrosion resistance under repository conditions is one key requirement for container material to provide complete waste containment. Copper is an important part of many waste packaging and disposal concepts, e.g. KBS-3 developed in Sweden and Finland and Mark II developed in Canada. Much of the data available regarding its behaviour under repository conditions comes from short-term investigations, such as laboratory experiments at different scales and under controlled conditions. Observations made from copper analogue studies provide additional information on copper behaviour during the assessment time scale and under real geological environments. By this, they can support the argumentation in the safety case.

Keweenaw native copper occurrences (Lake Superior, US) reflects more than one billion years of deposit evolution covering various geological (from bedrock to sediments and even anthropogenic mine site remnants) and geochemical environments (e.g., brines to meteoric water, anoxic vs. oxic, sulphur-free vs. sulphur-bearing). These deposits have been mined for a long time and there is a great deal of knowledge related to them as well as samples collected. However, data to be used in process based safety assessments for geological disposal is lacking and no formal review has been made from the geological disposal point of view. The current MICA Project Phase I systematically collect and review the existing literature and data on the Michigan copper analogue sites and available sampling potential. Based on the outcome, MICA Project Phase II will then study and analyse prospective sites and samples to address relevant questions regarding long-term behaviour of copper under disposal conditions. The MICA Project thus will provide a unique complementary data source to estimate processes governing behaviour of metallic copper and to support safety cases.

How to cite: Liebscher, A., Reijonen, H., Aaltonen, I., Liu, X., Lilja, C., Norris, S., Waffle, L., Keech, P., and Diomidis, N.: Michigan International Copper Analogue (MICA) project – assessment of long-term behaviour of copper in repository relevant environments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11278, https://doi.org/10.5194/egusphere-egu22-11278, 2022.

13:54–14:01
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EGU22-1936
Wolfram Rühaak et al.

Based on representative data for the geologic situation in Germany the temperature evolution around a nuclear waste canister was modeled in detail. The surrounding rock data for this specific case study represent crystalline rock properties. Heat production was estimated based on the German radioactive waste inventory. Using FEFLOW a high-resolution 3D numerical thermal model of one single canister was set up. The respective canister design was selected according to the Scandinavian KBS-3V concept. Based on this model representative temperature data in space and time were computed. By using these detailed reference results a strongly simplified model with just single node heat sources representing a repository section with 15 emplaced canisters was calibrated.

Tests showed that this approach allows to compute large arrays of canisters with a good quality.

Results of the study will be shown and further potential improvements are discussed.

How to cite: Rühaak, W., Werres, M., Lohser, T., and Röhlig, K.-J.: Generalizing the simulation of temperature distributions in a deep geological nuclear waste repository, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1936, https://doi.org/10.5194/egusphere-egu22-1936, 2022.

14:01–14:08
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EGU22-2019
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ECS
Merle Bjorge et al.

The presented work investigates aspects of uncertainty quantification in thermal design calculations of deep geological repositories for nuclear waste. The expected evolution of thermal conditions is a key element in the design process of the repository layout. Due to the radioactive decay and the associated emission of heat, temperatures increase in the repository system, potentially affecting processes relevant to the repository negatively. In order to quantify the temperature evolution and assess its effects on the various barriers, such as the host rock, models are set up and thermal calculations are conducted. Often specific distributions are assigned to model parameters, which are not known precisely.

To achieve a robust understanding and design despite this limited knowledge, it is necessary to assess the uncertainties associated with both parameters and models as part of these calculations. However, an uncertainty quantification, which includes calculations based on full distributions is expensive. To compare different uncertainty quantification methods applied to thermal design calculations, a benchmark is created. This benchmark is based on an analytical solution for a 1-D, thermal heat conduction problem using Python. Results of uncertainty quantification calculations based on full distributions, utilizing statistical moments or employing series expansion such as the first-order-second-moment method are compared.

This benchmark can help assess methods of uncertainty quantification in context of thermal design calculations.

How to cite: Bjorge, M., Chaudhry, A. A., Rühaak, W., and Nagel, T.: Comparing uncertainty quantification methods based on distributions or statistical moments in thermal design calculations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2019, https://doi.org/10.5194/egusphere-egu22-2019, 2022.

14:08–14:15
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EGU22-3295
Kaveh Sookhak Lari and Dirk Mallants

A potentially novel disposal pathway for small volumes of radioactive waste originating from reprocessing of spent nuclear fuels is in deep boreholes (DBs). The waste packages are stacked in a disposal zone at depths that would be considerably deeper than the typical depth of conventional mined repositories. DB disposal still requires considerable research and development to bring the science and technology to a similar level as the conventional disposal geological disposal facilities [1]. The half-life of several of the radionuclides in reprocessed spent nuclear fuel is on the order of 105 - 109 years. Such wastes generate heat for hundreds of years. Containers should have a lifetime long enough to survive at least the heat-production period [2]. In the geosphere surrounding the borehole, nonlinear interactions between transport phenomena and long time scales necessitate modelling as the most realistic tool to assess the risks. Coupling the flow, natural hydrostatic and temperature profiles with heat and solute mass transport is not computationally trivial.      

We study a complex domain including geological faults, stratified lithology, and an engineered DB with its surrounding damaged zone and backfilling materials. The system has an assumed groundwater flow above the low-permeable host rock, a diagonally-oriented fault and a cylindrical representation of the borehole. Such objects require a domain which is neither axisymmetric nor 2D Cartesian. A structured mesh to represent such a domain in 3D, with the required fine-scale near-field resolutions essential for simulation of the slow advective and diffusive transport, significantly increases the computational cost. For the first time a fully-3D unstructured Voronoi mesh was developed to represent such a layout with various petrophysical features and engineering objects to conduct a preliminary safety assessment.  

We present a coupled heat-solute mass transport modelling framework, subjected to depth-dependent temperature, pressure, and viscosity profiles - assuming an instantaneous release of radionuclides, as the most conservative “what if” scenario. Several scenarios of heat-generation were investigated to test if the additional heat produced by the waste affects radionuclide migration, e.g., by generating convection-driven transport.  The state-of-the-art TOUGHREACT-OMP was run on the CSIRO supercomputer to model the 3D Cartesian domain 3 km in length, 400 m in width and 3 km in depth. We present our meshing approach through linking voro2mesh, T2Viewer and in-house codes to develop a fully-3D unstructured mesh, compatible with the core model [3].

We tested the effect of lithology on the diffusion process (effective diffusion) for quantifying the radionuclide concentrations and annual dose rates for a potential human receptor (e.g., through a well).  We show the dose rate is highly sensitive to the diffusion parameter and fault configuration, while the effect of heat generation on convection-driven transport is of lesser importance.

 

1-Mallants, D., et al., The State of the Science and Technology in Deep Borehole Disposal of Nuclear Waste. Energies, 2020. 13(4).

2-IAEA, Geological disposal facilities for radioactive waste. 2011, International Atomic Energy Agency: Viena.

3-Bonduà, S. and V. Bortolotti, TOUGH2Viewer 2.0: A multiplatform tool for fully 3D Voronoi TOUGH grids. SoftwareX, 2020. 12: p. 100596.

How to cite: Sookhak Lari, K. and Mallants, D.: Coupled heat-mass transport modelling for safety assessment of deep borehole disposal of long-lived radioactive waste in a complex 3D petrophysical condition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3295, https://doi.org/10.5194/egusphere-egu22-3295, 2022.

14:15–14:22
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EGU22-9735
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ECS
Michael Kröhn and Lars Fromme

Bentonite is a versatile material that is, among other things, envisaged for designs for radioactive waste repositories to protect the waste canisters against groundwater. The thermo-hydraulic-mechanical (THM) coupled process of bentonite saturation is commonly based on two-phase flow. As an alternative to the formulations in THM-models, a thermo-hydraulic coupled saturation model for confined conditions (as expected in a repository) has been developed and realised for 1D-problems in the FORTRAN-code VIPER. In order to enhance the inherent limited range of possible applications of this code, the underlying partial differential equations have been transferred to COMSOL Multiphysics®. This has been done for the most simple, isothermal form at first, and subsequently for the non-isothermal formulation by coupling the hydraulics to the heat transfer interface of COMSOL.

The model concept for the hydraulics, qualified for a number of different problems using the VIPER code, is comparatively simple. Vapour diffusion in the pore space is coupled to diffusive water migration in the interlamellar space by instantaneous hydration. This results in a double-continuum model that calculates re-saturation rather efficiently for cases where the bentonite is subject to confined conditions and only little internal swelling occurs.

At first, a COMSOL-model has been developed that matches an isothermal water uptake test performed at GRS as well as the earlier performed simulations with code VIPER. An important motivation for the transfer of the VIPER-concept to COMSOL Multiphysics is broadening the range of possible applications from 1D to 3D modelling tasks.

After successfully completing this first step, further terms in the balance equation as well as equations of state were added which are required for calculating non-isothermal water transport. Furthermore, the simplified treatment of heat flow in VIPER could now be replaced by implementing a full coupling of the hydraulic formulations to the heat flow interface included in COMSOL Multiphysics.

The new implementation in COMSOL has been checked on the basis of temperature and humidity measurements from the FEBEX in-situ experiment performed at the Grimsel hard rock laboratory in Switzerland. The FEBEX-experiment was intended to represent the deposition of heat-producing nuclear waste canisters that were enclosed in a layer of compacted bentonite blocks in a tunnel in granitic rock. Wetting of the bentonite buffer was provided by the rather highly conductive host rock. Measured and simulated temperature evolution as well as wetting dynamics in the buffer matched each other reasonably well.

The presented work opens up the possibility to apply the alternative re-saturation concept to a wide range of three-dimensional problems not solvable by code VIPER. However, a number of further enhancements of the new model for instance concerning boundary conditions for water vapour or swelling into free space are conceivable as a follow-up.

How to cite: Kröhn, M. and Fromme, L.: An alternative thermo-hydraulic model for bentonite re-saturation exclusively based on diffusive water transport and its implementation in 3D, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9735, https://doi.org/10.5194/egusphere-egu22-9735, 2022.

14:22–14:29
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EGU22-11557
Václava Havlová et al.

The “Assessment of the Chemical Evolution of ILW and HLW Disposal Cells” (abbreviation ACED) Work Package of the EURAD EJP project concerns the assessment of chemical evolution at the disposal cell scale involving component/material interactions and thermal, hydraulic and/or chemical gradients via the consideration of ILW and HLW disposal concepts that are representative of the various approaches being followed in Europe. The general objective of the modelling approach presented here is to create a geochemical and coupled reactive transport model for the assessment of the geochemical evolution along a carbon-steel canister/Ca-Mg bentonite boundary in terms of corrosion rates and geochemical alterations.

The modelling, conducted in the PHREEQC geochemical code, focused on the representation of 3 completed corrosion experiments (temperatures of up to 90°C) that provided data on the corrosion trends of C-steel under compacted Ca-Mg bentonite conditions under both laboratory and in-situ conditions.  

To date, the development of the geochemical modelling in the PHREEQC code includes: 

  • The modelling of the bentonite pore water chemistry
  • The equilibrium modelling of the canister - bentonite boundary
  • The kinetic modelling of corrosion processes at the canister - bentonite boundary

The final step will comprise the development of a complex reactive transport model, including the consideration of Fe migration into the bentonite.

It is planned that the model will also be used for the evaluation of experiments conducted at temperatures of up to 150°C which are currently underway as part of the ConCord WP of the EURAD EJP project.

How to cite: Havlová, V., Klajmon, M., Mendoza, A., Dobrev, D., and Vokal, A.: Modelling of carbon steel canister and Ca-Mg bentonite boundary development , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11557, https://doi.org/10.5194/egusphere-egu22-11557, 2022.

14:36–14:43
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EGU22-1728
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ECS
Carlos Guevara Morel et al.

Safe deep geological disposal of heat generating nuclear waste requires a detailed assessment of the geotechnical integrity of potential host rocks. In Germany, several repository systems are under discussion, in which clay stone, salt or crystalline rock could serve as host rock. This contribution proposes a modelling concept for the numerical analysis of the thermal, hydraulic and mechanical (THM) coupled processes used for the barrier integrity assessment of a nuclear waste repository in crystalline rock. In particular, focusing on repository systems where the Containment Providing Rock Zone (CRZ) represents the essential barrier. This implies that understanding the potential changes in the geological barrier caused by the disposal of the waste is fundamental in order to avoid the creation of preferential flow paths for the disposed waste into the biosphere. Furthermore, in Germany, the host rocks must comply with the safety requirements stipulated by the German Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection in 2020.

Crystalline rock, in contrast to clay stone and salt rock, is usually characterised by fractures and other types of discontinuities. Therefore, it cannot be assumed that a single sufficiently large unfractured area can be found in which all of the nuclear waste could be emplaced. In consequence, multiple smaller CRZs, each providing undisturbed rock, have to be defined. Moreover, the fracture network is expected to influence the hydraulic behaviour 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.

The proposed modelling concept is applied to a generic geological model reflecting a realistic geological situation. A Discrete Fracture Network (DFN) model is used to determine the hydraulic properties that are then upscaled and mapped into a continuum Finite Element (FE) model. The preliminary numerical results (e.g. stresses and pore fluid pressures) from the parameterized continuum model are used to exemplary assess the barrier integrity using criteria which take into account the dilatancy strength and fluid pressure.

How to cite: Guevara Morel, C., Maßmann, J., and Thiedau, J.: Numerical assessment of the barrier integrity from a generic nuclear waste repository in crystalline rock, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1728, https://doi.org/10.5194/egusphere-egu22-1728, 2022.

14:43–14:50
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EGU22-12179
Filip Jankovský et al.

The EURAD European Joint Project (www.ejp-eurad.eu) is a European project supported by the European Commission via the Horizon 2020 framework programme. WP 5: Fundamental understanding of radionuclide retention (FUTURE) concerns the quantification of the long-term retention of key radionuclides in solid phases aimed at developing models of reactive transport in the host rock in close cooperation with the Development and improvement of numerical methods (DONUT) WP. The Mobility of radionuclides in crystalline rock Task of the WP is aimed at the observation of the retention of DGR-relevant radionuclides in a crystalline host rock fracture filling and the evaluation of its contribution to the safety function of the crystalline host rock. A calcite fracture filling, nickel as the radionuclide of interest and caesium as the reference radionuclide were selected so as to allow for the study of sorption processes in the crystalline fracture environment. Samples of a natural calcite fissure infill and migmatite host rock extracted from the Bukov URF, Czech Republic, were distributed to the various project partners for experimental research purposes. Batch sorption experiments were subsequently performed on both materials using Ni, Cs and other elements in synthetic ground water and CaCl2 for the calcite. The results of Ni sorption on natural calcite revealed a lower level of retention than that of the surrounding host rock (migmatite).

How to cite: Jankovský, F., Kočan, K., Hofmanová, E., Havlová, V., Zuna, M., and Smutek, J.: Retention of radionuclides in a fracture infill, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12179, https://doi.org/10.5194/egusphere-egu22-12179, 2022.

Tue, 24 May, 15:10–16:40

Chairpersons: Klaus-Jürgen Röhlig, Jobst Maßmann, Thomas Nagel

15:10–15:17
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EGU22-8733
Dominik Kern et al.

One of the most important aspects for Deep Geological Repositories (DGRs) in crystalline rock is the presence and evolution of fractures and faults, since they dominate the subsurface flow regime and thus the possible transport of contaminants. In the considered period of one million years, it is expected that cold and warm period alternate, accompanied by ice sheet progression and regression. The large moving mass of an ice sheet causes a dynamic response of the earth's crust, referred to as glacial isostatic adjustment (GIA). GIA changes the displacement and stress field not only under and near the ice sheet but also in its far-field. In view of the long-term safety assessments for DGRs, we analyze GIA-induced far-field stress and pore pressure changes and their impacts on existing faults.

For that purpose, we use Finite-Element methods (FEM) to simulate the hydromechanical processes around an exemplaric DGR of the Yeniseiskiy Site, Russia, applying boundary conditions derived from established GIA models [1,2]. As result, we obtain the Coulomb failure stress for varying instances of assumed faults.

The INFRA project is funded by the DFG-RFBR program:
DFG funds: NA1528/2-1 and MA4450/5-1
RFBR funds: 20-55-12009, АААА-А20-120012190168-5

References

[1] Patrick Wu. “Using commercial finite element packages for the study of earth deformations, sea levels and the state of stress”. In: Geophysical Journal International 158.2 (2004), pp. 401–408.

[2] G. Spada et al. “A benchmark study for glacial isostatic adjustment codes”. In: Geophysical Journal International 185.1 (2011), pp. 106–132.

How to cite: Kern, D., Silbermann, C. B., Magri, F., Steffen, R., Steffen, H., Malkovsky, V., and Nagel, T.: Fault reactivation in crystalline rock as consequence of glaciation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8733, https://doi.org/10.5194/egusphere-egu22-8733, 2022.

15:17–15:24
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EGU22-8693
Christian Silbermann et al.

While guidelines for the location and licensing of a deep geological repository (DGR) for high-level radioactive waste depend both on national government policies and international regulations, it is mandatory to select a site where the hydrogeological setting provides sufficiently safe natural conditions for long-term waste isolation from groundwater flow. Therefore, safety assessments of a suitable location of a DGR require the evaluation of future external events and processes that may affect its long-term evolution.
Here, glaciation cycles are of special importance: Ice sheets evoke crustal deflections  (including deformation), and impose pronounced hydraulic heads, both of which change the large-scale hydrogeological conditions. To properly assess the present and future conditions of a DGR site, its evolution in the past should be understood. For this, a sedimentary basin [3] is considered here as a large-scale hydrogeological benchmark. The evolution during one glacial cycle is simulated using the open-source multi-field finite element code OpenGeoSys. The hydraulic-mechanical impact of the glacial loading is taken into account using appropriate time-dependent boundary conditions. For comparison with a previously published study [3], the same (heuristic) displacement field is prescribed and the groundwater evolution is regarded. Then, a more realistic displacement field obtained from large-scale GIA simulations [1,2] is prescribed. Using a one-sided mechanical-hydraulic coupling with a staggered solution scheme it is possible to consider not only the hydraulic head from the glacier and the crustal deflection but also the crustal compression. Especially in regions at the margin of the glacier this is could have an impact on the hydraulic behavior at the depth of a DGR.

References & Funding

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

[1] Patrick Wu. “Using commercial finite element packages for the study ofearth deformations, sea levels and the state of stress”. In: Geophysical Journal International 158.2 (2004), pp. 401–408.

[2] G. Spada et al. “A benchmark study for glacial isostatic adjustment codes”. In: Geophysical Journal International 185.1 (2011), pp. 106–132.

[3] V.F. Bense and M.A. Person. “Transient hydrodynamics within intercratonic sedimentary basins during glacial cycles”. In: Journal of Geophysical Research: Earth Surface 113.F4 (2008).

How to cite: Silbermann, C., Kern, D., Steffen, R., Steffen, H., Bense, V., and Nagel, T.: On the role of crustal deflection in the hydraulic-mechanical simulation of sedimentary basins during glacial cycles, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8693, https://doi.org/10.5194/egusphere-egu22-8693, 2022.

15:24–15:31
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EGU22-1012
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ECS
Marco Brysch et al.

A material's porosity controls all of its physical properties. Hence, many studies have dealt with its exact determination. In researching potential host rocks for radioactive waste disposal, there is a particular interest in pore distribution and connectivity as this defines the rock's permeability and rheological behavior, e.g. in [Houben et al., 2013], [Hemes et al., 2011], [Klaver et al., 2012], and [Keller, 2021]. All these studies made use of specialized scanning electron microscopy (SEM). This analysis allows the evaluation of pores by size, location, orientation, and frequency by using a binary segmentation mask. The preparation of these masks is associated with some difficulties caused by the interpretation margins, non-uniform procedures, and, especially, by the resolution limit of the SEM. In addition, the overlap of gray values of grain and pores renders conventional methods such as pixel-thresholding unfeasible.

We present a method that deals with this problem in two stages. The first stage consists of an up-sampling technique of the SEM images. Here, the resolution of the images is artificially enhanced. In the second stage, this up-sampling is combined with an algorithm that computes a probability field using multiple learning classifiers (MLC). First, we trained and implemented an enhanced super-resolution generative adversarial network (ESRGAN) [Wang et al., 2018] for upsampling SEM images. The enhanced images show a much finer detail of pore edges, making even the smallest pores more apparent and detectable. In the second stage, nine different MLC's have been trained and examined for their segmentation results. Here, the characteristic segmentation properties of the trained MLC's are particularly noticeable. Their differences show that no single MLC alone can provide sufficient segmentation quality compared to manual interpretation. Hence, a voting classifier combines the individual MLC-masks into a probability field. This combination allows the derivation of confidence levels that reduce spurious pore segmentation and capture pore edges more organically and uniformly. 

Finally, we have combined the voting classifier and the super-resolution to the so-called Super-Segmentation (SSM). The segmentation of the pores is now performed on the artificially enhanced SEM image. Eventually, the final binary segmentation mask is down-sampled to the resolution of the input image. Compared to other segmentation methods, SSM shows a clearer detection of the pore edges with enhanced quality even for the smallest pores. In a test case on Opalinus clay, we were able to detect pores that were undetected or insufficiently segmented. We discuss the result and ongoing work to improve the reliability of MLCs with ESRGAN images with the goal to lower the truncation limit [Bonnet et al., 2001].

How to cite: Brysch, M., Laurich, B., Schettler, C., and Sester, M.: Pore Super Segmentation in Opalinus Clay on Artificially Enhanced SEM Images with Voting Classification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1012, https://doi.org/10.5194/egusphere-egu22-1012, 2022.

15:31–15:38
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EGU22-2343
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ECS
Chaofan Chen et al.

Radionuclide migration in clay-rich formations is dominated by molecular diffusion due to the low permeability of the claystone. Accurate estimation of radionuclide migration in host rock using numerical tools plays a key role in the safety assessment of disposal concepts for nuclear waste. In the sandy facies of the Opalinus Clay (SF-OPA), the spatial variabilities of the pore network and compositional heterogeneities at the pore scale (nm to µm) cause heterogeneous diffusion at the core scale (cm to dm). Such heterogeneous diffusion patterns affect the migration of radionuclides in the various sedimentary layers even above the core scale (~m). Small-scale heterogeneities of diffusive transport could play an important role in upscaling to larger length scales of SF-OPA, particularly because of differences in sedimentary and diagenetic facies. Therefore, a meaningful estimation of radionuclide migration in the host rock above the core scale requires a comprehensive study of the influence of sedimentary layers on the heterogeneous diffusion.

In this work, we study the heterogeneous diffusion of radionuclides based on a two-dimensional (2D) structural model from the geological data of SF-OPA in the Mont Terri rock laboratory at the m-scale. As key parameters for the diffusive transport calculation, the effective diffusion coefficients in different sedimentary layers are quantified based on the developed upscaling workflow from pore- to core-scale simulation combined with the multi-scale digital rock models [1]. The heterogeneous effective diffusivities are then implemented into the large-scale structural model for diffusive transport simulation using the FEM-based OpenGeoSys-6 simulator. Results show that the various heterogeneous effective diffusivities under different mesh resolution (length scales) in the large-scale simulations strongly affect the evolution of radionuclides concentration in SF-OPA, especially in the vicinity of the canister. The sensitivity analysis focuses on the effects of length, bedding angle and thickness of the sedimentary layer on the spatio-temporal evolution of radionuclide concentrations. The numerical results provide insight into the heterogeneous diffusion of radionuclides, contributing to enhanced long-term predictability of radionuclide migration in the host rock of the deep geological repository.

[1] Yuan, Tao, and Cornelius Fischer. "Effective Diffusivity Prediction of Radionuclides in Clay Formations Using an Integrated Upscaling Workflow." Transport in Porous Media 138.2 (2021): 245-264.

How to cite: Chen, C., Yuan, T., Lu, R., Fischer, C., Montoya, V., Kolditz, O., and Shao, H.: The influence of sedimentary heterogeneity on the diffusion of radionuclides in the sandy facies of Opalinus Clay at the geological scale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2343, https://doi.org/10.5194/egusphere-egu22-2343, 2022.

15:38–15:45
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EGU22-4115
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ECS
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Theresa Hennig and Michael Kühn

Any safety assessment of radioactive waste disposal sites is done based on the simulation of migration lengths of radionuclides through the host formation. This is done by the application of transport parameters obtained from experiments with homogeneous, e.g. geochemically and mineralogically constant, conditions. However, such an assumption of homogeneity is no longer applicable on the host rock scale (>200 m). Consequently, experimentally determined transport parameters might no longer represent the host rock on larger scales.

Uranium, as main component of spent fuel, is used here as an example to evaluate the impact of heterogeneous systems on the total migration lengths compared to homogeneous ones. For this, the hydrogeological system of the Swiss Opalinus Clay, a potential host rock, is modelled in one-dimensional diffusion simulations with PHREEQC. Since sorption and hence migration of uranium is primarily governed by pCO2 > Ca2+ > pH > pe > clay mineral quantity [1], the focus is on the simulation of the geochemically heterogeneity, and thus on the hydrogeological system. Sorption is quantified with mechanistic surface complexation models and cation exchange. At Mont Terri, geochemical gradients established towards the embedding aquifers due to diffusive exchange over millions of years as a consequence of the Jura folding and associated erosion history [2].

First, measured pore water profiles were confirmed by the simulations. They served as starting profiles for the subsequent uranium migration that was quantified in a second step. By comparing migration lengths after a simulation time of one million years with results of homogeneous simulations, it has been shown that uranium migration is enhanced by up to several tens of meters depending on the pCO2. Consequently, the entire hydrogeological system needs to be taken into account and the governing parameters can be prioritized as follows: pCO2 > hydrogeology > mineralogy.

 

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

[2] Mazurek M., Alt-Epping P., Bath A., Gimmi T., Waber H. N., Buschaert S., De Cannière P., De Craen M., Gautschi A., Savoye S., Vinsot A., Wemaere I. and Wouters L. (2011): Natural tracer profiles across argillaceous formations. Applied Geochemistry 26 (7), 1035–1064.DOI:10.1016/j.apgeochem.2011.03.124

 

How to cite: Hennig, T. and Kühn, M.: Migration lengths of uranium in the Opalinus Clay are determined by the pore water geochemistry and hydrogeological setting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4115, https://doi.org/10.5194/egusphere-egu22-4115, 2022.

15:45–15:52
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EGU22-5660
Christoph Behrens et al.

The current phase of the site selection procedure for a high-level nuclear waste repository in Germany includes representative preliminary safety assessments of potential suitable areas that cover approximately half of the territory of Germany. These safety assessments rely in part on numerical models of radionuclide transport. The modelling of radionuclide transport faces several challenges: 1) Estimation of radionuclide transport for a large set of potentially suitable areas with different hydrogeological settings and 2) scarcity of site-specific hydrogeological data for the parameters that govern radionuclide transport. To overcome these challenges we discuss potential workflows that combine the analysis of hydrogeological data and numerical models of radionuclide transport. We introduce a suite of numerical model codes that combines computationally efficient one-dimensional codes with more computationally demanding 2D and 3D codes. The 2D and 3D codes will be employed to simulate regional groundwater flow fields. These are subsequently combined with large sets of 1D model runs to quantify the effects of parameter variation and future geological developments on radionuclide transport. We discuss methods to combine site-specific data with global data to constrain the bandwidths of hydrogeological parameters and the range of radionuclide transport. Our contribution aims for an open discussion of model strategy and exchange with the scientific community.

How to cite: Behrens, C., Luijendijk, E., Kreye, P., Panitz, F., Bjorge, M., Gelleszun, M., Renz, A., Miro, S., and Rühaak, W.: Challenges in modelling radionuclide transport in the German nuclear waste repository search, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5660, https://doi.org/10.5194/egusphere-egu22-5660, 2022.

15:52–15:55
Closing Remarks