The Saint-Ursanne earthquakes of 2000 revisited: evidence for active shallow thrust-faulting in the Jura fold-and-thrust belt

The interpretation of seismotectonic processes within the uppermost few kilometers of the Earth’s crust has proven challenging due to the often significant uncertainties in hypocenter locations and focal mechanisms of shallow seismicity. Here, we revisit the shallow seismic sequence of Saint-Ursanne of March and April 2000 and apply advanced seismological analyses to reduce these uncertainties. The sequence, consisting of five earthquakes of which the largest one reached a local magnitude (ML) of 3.2, occurred in the vicinity of two critical sites, the Mont Terri rock laboratory and Haute-Sorne, which is currently evaluated as a possible site for the development of a deep geothermal project. Template matching analysis for the period 2000–2021, including data from mini arrays installed in the region since 2014, suggests that the source of the 2000 sequence has not been persistently active ever since. Forward modelling of synthetic waveforms points to a very shallow source, between 0 and 1 km depth, and the focal mechanism analysis indicates a low-angle, NNW-dipping, thrust mechanism. These results combined with geological data suggest that the sequence is likely related to a backthrust fault located within the sedimentary cover and shed new light on the hosting lithology and source kinematics of the Saint-Ursanne sequence. Together with two other more recent shallow thrust faulting earthquakes near Grenchen and Neuchâtel in the north-central portion of the Jura fold-and-thrust belt (FTB), these new findings provide new insights into the present-day seismotectonic processes of the Jura FTB of northern Switzerland and suggest that the Jura FTB is still undergoing seismically active contraction at rates likely < 0.5 mm/yr. The shallow focal depths provide indications that this low-rate contraction in the NE portion of the Jura FTB is at least partly accommodated within the sedimentary cover and possibly decoupled from the basement.

Impact of the heterogeneity of the sandy Opalinus clay facies at the Mont Terri underground research laboratory on radionuclide migration

The assessment of the safety of a deep geological repository (DGR) for high-level radioactive wastes over assessment time scales of up to 1 million years requires an in-depth understanding of the multi-scale coupled processes that affect the repository system evolution over time, to reduce uncertainties and conservatism in safety analyses. This is in particular required with respect to the challenges of a comparative assessment of different repository concepts in different host rocks within the process of a site selection for a DGR for heat-generating radioactive wastes in Germany. The collaborative project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCross)” conducted jointly by five research centres of the Helmholtz Association and co-funded by the Initiative and Networking Fund of the Helmholtz Association and the Federal Ministry of Education and Research (BMBF) has been initiated with the overall objective to improve the understanding of coupled thermal-hydraulic-mechanical-chemical-(micro)biological (THMCB) processes and to develop simulation tools that enable a holistic close to reality description of the long-term evolution of the repository system. Geological formations, such as those foreseen as potential host rocks for DGRs, and their surroundings are heterogeneous on various length scales ranging from nanometers to kilometers. Therefore, the aim of this work in the context of iCross is to evaluate the effects of mineralogical, geochemical and microstructural heterogeneities of repository host rocks on radionuclide transport in the repository far field, using the sandy facies of the Opalinus clay (SF-OPA) from the Mont Terri underground research laboratory (St. Ursanne, Switzerland) as an example. Here, we address in particular the migration behaviour of Ra-226 as an important radionuclide to be considered in safety cases for deep geological disposal of spent nuclear fuel. To assess the impact of the heterogeneities in SF-OPA on radionuclide transport, a complementary approach combining microstructural characterisation methods, experimental techniques for the determination of transport parameters of the rock matrix and the mobility of Ra-226 with innovative developments in reactive transport modelling on the pore and continuum scales was pursued. One of the results was that although the limited clay content in SF-OPA decreases the total amount of Ra bound to the illite phase, the solid solutions of sulphate and carbonate compensate for this and provide a major fixation mechanism.

From process to system understanding with multi-disciplinary investigation methods: set-up and first results of the CD-A experiment (Mont Terri rock laboratory)

A potential repository site for high-level radioactive waste should ensure the highest possible safety level over a period of one million years. In addition to design issues, demonstrating the integrity of the barrier is essential as it ensures the long-term containment of radioactive waste. Therefore, a multi-disciplinary approach is necessary for the characterization of the surrounding rock and for the understanding of the occurring physical processes. For site selection, however, the understanding of the respective system is essential as well: Do fault zones exist in the relevant area? Are these active and relevant for interpreting system behavior? What is the role of the existing heterogeneities of the claystone and how do these site-dependent conditions influence the physical effects? To answer these questions, the site-selection procedure requires underground exploration, which includes geophysical and geological investigations on milli- to decameter scales. Their results serve as the basis for numerical modelling. This combined, multi-disciplinary interpretation requires extensive knowledge of the various methods, their capabilities, limitations, and areas of application. In the cyclic deformation (CD-A) experiment in the Mont Terri rock laboratory, the hydraulic–mechanical effects due to excavation and the climatic conditions within the rock laboratory are investigated in two niches in the Opalinus Clay. The twin niches differ mainly with regard to the relative humidity inside them, but are also characterized by different boundary conditions such as existing fault zones, the technical construction of the neighboring gallery, etc. In order to gain insights into the relevance of the individual influences, comparative studies are being carried out on both niches. The presented results provide a first insight into the initial experimental years of the CD-A long-term experiment and illustrate the benefits of multi-disciplinary investigations in terms of system understanding and the scale dependency of physical effects. Amongst other effects, the assessment of the impact of heterogeneities on the deformation behavior and the evolution of pore water pressure is very complex and benefits from geological interpretation and measurements of for example deformation, water content, and pore pressure. The numerical modeling allows statements about the interaction of different processes and thus enables an interpretation of the overall system, taking into account the knowledge gained by the multi-disciplinary investigation.

Anaerobic corrosion of carbon steel in compacted bentonite exposed to natural Opalinus clay porewater: Bentonite alteration study

Carbon steel is a potential canister material for the disposal of high-level radioactive waste in deep geological repositories in clays and clay rocks. Bentonite is considered as a potential backfill material for those multi-barrier systems. To predict the long-term performance and for safety assessment the knowledge of canister corrosion behavior is important. The corrosion products formed and mineralogically altered bentonite at the canister/bentonite interface can potentially provide an additional barrier against radionuclide migration. In-situ corrosion experiments were performed at the Mont Terri underground research laboratory. Coupons of carbon steel were embedded in Volclay MX-80 bentonite with controlled densities, installed in a borehole under simulated repository and anaerobic conditions and exposed to natural Opalinus clay porewater for a period up to 5.5 years (Smart et al., 2017). In the present study, the bentonite layer at the canister/bentonite interface was characterized by complementary microscopic and spectroscopic techniques (XPS, SEM-EDX, µXANES) under anoxic conditions. The interface revealed reddish-brown staining up to 2 mm depth into the bentonite in the zone adjacent to the steel in all three obtained samples. The XPS analysis revealed formation of sulfides at the interface consisting of iron and other trace metals present in the steel. The SEM-EDX analyses of the interface (embedded cross-cut with steel removed) showed different degrees of calcium enrichment in the bentonite adjacent to the metal for various samples. The µXRF analysis performed on the bentonite at the interface showed a scarce or distinct calcium-enriched rim up to 100 µm into the bentonite and iron-enriched rim depending on the sample (one sample in Fig. 1), while µXANES analysis revealed formation of iron silicate compounds in the reacted reddish-brown zone. The iron appears to displace calcium from the interlayer sites in montmorillonite. The calcium then precipitates at the interface as calcite. The extent of this process seems to be strongly related to the bentonite density. The steel coupon was removed prior to embedding, with the location marked as resin in Fig. 1. A line scan from the edge towards the bulk bentonite did not indicate any systematic gradient in the Fe2+/3+ ratio. The formation of mixed Fe2+/3+ silicate compounds appears to be heterogeneous. This work contributes to an increasing understanding of steel corrosion mechanisms in clay, which can improve the robustness of canister lifetime predictions.

An interdisciplinary view of the long-term evolution of repository systems across scales: the iCROSS project

The interdisciplinary project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCROSS)” combines research competencies of Helmholtz scientists related to the topics of nuclear, geosciences, biosciences and environmental simulations in collaborations overarching the research fields energy and earth and environment. The focus is to understand and analyze close-to-real long-term evolutionary pathways of radioactive waste repositories across nanoscales to repository scales. The project is subdivided into work packages dealing with laboratory studies, field experiments in underground research laboratories (URLs), advanced modelling studies and the integration and alignment of data and information using virtual reality methods. In this sense, the project structure aims at a holistic view on relevant processes across scales in order to comprehensively simulate potential repository evolutions. Within the multi-barrier system of a repository for heat-generating radioactive waste, a number of complex reactions proceed, including dissolution, redox processes, biochemical reactions, gas evolution and solid/liquid interface and (co)precipitation reactions. At the same time, thermal and external mechanical stress has an impact on the conditions in a deep geological repository. All those processes are highly coupled, with multiple interdependencies on various scales and have a strong impact on radionuclide mobility and retention. In recent years, substantial progress was achieved in describing coupled thermal-hydro-mechanical-chemical-biological (THM/CB) processes in numerical simulations. A realistic and concise description of these coupled processes on different time and spatial scales is, at present, a largely unresolved scientific and computational challenge. The close interaction of experimental and simulation teams aims at a more accurate quantification and assessment of processes and thus, the reduction of uncertainties and of conservative assumptions and eventually to a close-to-real perception of the repository evolution. One focus of iCROSS is directed to relevant processes in a clay rock repository. In this context, the iCROSS team became a full member of the international Mont Terri consortium and worked in close collaboration with international and German institutions in URL projects. Respective experiments specifically deal with coupled processes at the reactive interfaces in a repository near field (e.g. the steel/bentonite and bentonite/concrete interfaces). Within iCROSS, the impact of secondary phase formation on radionuclide transport is investigated. At Mont Terri, experiments are in preparation to study radionuclide transport phenomena in clay rock formations within temperature gradients and in facies exhibiting significant heterogeneities on different scales (nm to cm). Beside those studies, high resolution exploration methods for rock characterization are developed and tested and the effect of temperature and other boundary conditions on the strength, creep properties and healing of faults within Opalinus clay are quantified. Multiphysics models coupled to reactive transport simulation have been further developed and applied to laboratory and field experiments. Results are digitally analyzed and illustrated in a visualization center, in order to enhance the comprehension of coupled processes in repository systems across scales. The present contribution provides an overview on the project and reports selected results. The impact of considering complex coupled processes in repository subsystems for the assessment of the integrity of a given (generic) repository arrangement is discussed.

Large-scale testing of a sandwich shaft-sealing system at the Mont Terri rock laboratory

Shaft-sealing systems for nuclear waste repositories are constructed to limit fluid inflow from the adjacent rock during the early stage after closure of the repository and to delay the release of possibly contaminated fluids from the repository at later stages. Current German concepts of shaft seals contain the hydraulic sandwich sealing system as a component of the lower seal in host rock (Kudla and Herold, 2021). The KIT-developed sandwich sealing system consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES) that are characterized by a high hydraulic conductivity. Within the ES, fluid is evenly distributed over the cross section of the seal. Water bypassing the seal via the excavation-damaged zone or penetrating the seal inhomogeneously is contained, and a more homogeneous hydration and swelling of the DS is obtained. The functionality of such a system was proven in laboratory and semi-technical-scale experiments (Schuhmann et al., 2009). After a joint international pre-project (Emmerich et al., 2019) dedicated to the planning of a large-scale in situ test that demonstrates the feasibility and effectiveness of the sandwich shaft-sealing system in interaction with the host rock, the large-scale experiment was launched at the Mont Terri rock laboratory in July 2019 with partners from Germany, Switzerland, Spain, UK, and Canada. It consists of two experimental shafts of 1.18 m diameter and 10–12.6 m depth, constructed using a core drilling technique with a custom-made drill rig in a new niche in the sandy facies of the Opalinus Clay. The seal in shaft 1 consists of four DS (calcigel) of 1 m thickness and five ES (fine-grained quartz sand), each 30 cm thick (Fig. 1). Shaft sinking began in August 2020 and was completed in November 2020. In the following months, the sealing system and instrumentation of shaft 1 were installed. The sealing system is saturated from a pressure chamber located at the shaft bottom via an inclined lateral feeding borehole. Hydration of the system started in May 2021. Shaft 2 will host a slightly modified system emplaced 1–1.5 years later, in order to integrate experience obtained during the early operation phase of shaft 1. In contrast to shaft 1, the excavation-damaged zone around shaft 2 will have had time to develop. The seals and the surrounding rock are intensely monitored. Measurements in the rock (geophysics, pore pressure, and total stress) were started between August 2019 and March 2020. Characterization of the excavation-damaged zone along the wall of shaft 1 was performed by geophysical and surface packer measurements prior to seal emplacement. Measurements inside the shaft comprise water content, relative humidity, and temperature, pore pressure, stress, and displacements. The in situ work is backed by laboratory testing and model simulation. Data and experience obtained to date will be presented. The sandwich experiment is funded by the German Federal Ministry for Economic Affairs and Energy under contract 02E11799.

In situ corrosion of canister materials in bentonite: the IC-A experiment at Mont Terri

Since 2012, a long-term in situ corrosion experiment (IC-A) has been conducted in the Mont Terri Underground Research Laboratory in Switzerland. The aims of the project with international partners are to confirm the long-term anaerobic corrosion rate of carbon steel and copper in compacted bentonite under repository-relevant environmental conditions, to gather in situ corrosion data supporting canister lifetime predictions, to provide confirmation of the effect of the bentonite buffer on microbial activity and microbially influenced corrosion, and to study the effects of welding (steel) and deposition technique (copper) on the corrosion properties of these candidate materials for disposal canisters. To date, carbon steel and cold sprayed and electrodeposited copper coatings have been retrieved after different exposure periods up to 3 years and characterised to establish the composition of the corrosion product, the morphology of the corroded surface, and to measure the rate of corrosion. For carbon steel specimens, a complex corrosion product was identified, consisting predominantly of magnetite. Low average anaerobic corrosion rates were measured for carbon steel and a very modest amount of alteration was identified on copper. The density and the initial form of the bentonite had a small influence on the rate of corrosion, across all materials.

Building the bridge between safety requirements and numerical modeling: an example considering crack development of Opalinus clay in laboratory and field scales

Salt, crystalline and clay formations are under discussion as potential host rocks for storage of heat-generating radioactive waste. Each of these rocks has a different structure and composition, and consequently a different material behavior. The latter needs to be studied and evaluated with respect to the main aim: to find a place to store the waste in a safe and sustainable manner. Several requirements in the context of the safety of a repository need to be fulfilled, concerning the long-term as well as the operational phase. One key point in this matter is the integrity, which refers to retention of the isolating rock zone's containment capabilities. With the focus on some experimental and numerical investigations on the excavation influenced near-field behavior of Opalinus clay (OPA), this contribution aims to illustrate an example for the role of numerical modeling in safety assessment. Once, e.g. anthropogenic action such as excavation starts, the natural state of equilibrium in the formation is disturbed. Trying to restore it, the rock deforms (convergence) and/or releases energy in other ways such as cracking. This could lead to loss of integrity since crack nucleation and propagation can affect the mechanical stability and create paths to transport contaminants. During operation in the excavated rock, environmental changes, e.g. temperature and humidity, further affect its behavior. The understanding of these dynamic phenomena ideally needs to occur at the in situ scale; however, performing an experiment in the spatial and time scales of interest is not always possible. For this reason, the in situ problem needs to be formulated, abstracted and mathematically modeled. The interpretation of the results must take place with simplifying assumptions and complementary laboratory scale experiments can be used to improve understanding of the system. The real problem is approached stepwise, each step associated to the size of the model and its complexity. The gradually obtained knowledge is necessary to achieve a better understanding of the process and to evaluate the capacities and limitations of the models. This contribution aims at showing the basic practical steps for numerical modeling with particular focus on the preparation and interpretation of the models and results, e.g. model calibration, verification and validation. As an example, the OPA at the Mont Terri site is chosen. The material parameters are obtained either experimentally or from the literature. We choose and perform laboratory scale simulations that are related to nearly the same mechanism as in the in situ scale. To have a first impression on the latter, a simplified, large-scale numerical model is prepared. The mechanism in study is drying and wetting, which is associated with shrinkage and swelling. We analyze the pore pressure and stress development in both scales. Thus, hydraulic mechanically coupled approaches are essential. The concept of effective stress is used, which combines the contributions of the solid and fluid phases (gas and liquid). In the current modeling approach, the gas pressure remains constant (atmospheric pressure) and during drying, the liquid pressure induces capillary pressure development and decrease of saturation. The laboratory scale simulation is important to evaluate the model of choice and to assess potential numerical problems. Furthermore, it can be used to perform a sensitivity study of material and numerical parameters. This step is necessary during the development or extension of numerical models as well as to evaluate their applicability on new research questions. The simplified in situ scale numerical model is then extended. In this phase the numerical model is evaluated once again, especially with respect to its complexity. Furthermore, specific questions related to this scale are posed: overall behavior of the rock, influence of the excavation, seasonal and long-term effects. In this contribution we deal with the long-term cyclic deformation (CD-A) experiment. The CD-A experiment has been taking place in the Mont Terri Rock Laboratory since October 2019. It consists of twin niches, a closed and an open niche, subjected to either high air humidity or seasonal humidity changes leading to saturation/desaturation during summer/winter in the OPA, respectively. Several parameters are periodically or continuously measured, including relative air humidity, convergence and crack development. We attempt to transfer the knowledge and numerical models developed in the small scale to the large scale and to evaluate the possibilities and limitations of the chosen approaches by comparing the numerical and experimental results.

Potential Uranium Migration within the Geochemical Gradient of the Opalinus Clay System at the Mont Terri

Transport properties of potential host rocks for nuclear waste disposal are typically determined in laboratory or in-situ experiments under geochemically controlled and constant conditions. Such a homogeneous assumption is no longer applicable on the host rock scale as can be seen from the pore water profiles of the potential host rock Opalinus Clay at Mont Terri (Switzerland). The embedding aquifers are the hydro-geological boundaries, that established gradients in the 210 m thick low permeable section through diffusive exchange over millions of years. Present-day pore water profiles were confirmed by a data-driven as well as by a conceptual scenario. Based on the modelled profiles, the influence of the geochemical gradient on uranium migration was quantified by comparing the distances after one million years with results of common homogeneous models. Considering the heterogeneous system, uranium migrated up to 24 m farther through the formation depending on the source term position within the gradient and on the partial pressure of carbon dioxide pCO2 of the system. Migration lengths were almost equal for single- and multicomponent diffusion. Differences can predominantly be attributed to changes in the sorption capacity, whereby pCO2 governs how strong uranium migration is affected by the geochemical gradient. Thus, the governing parameters for uranium migration in the Opalinus Clay can be ordered in descending priority: pCO2, geochemical gradients, mineralogical heterogeneity.