Ben Laurich

In the context of the disposal of high‑level nuclear waste (HLW) in clay rock formations, crushed clay‑rock‑based materials can be used to backfill repository drifts and shafts and to construct sealing elements, as their expansive clay content provides favourable properties, such as the development of swelling pressure and the reduction of hydraulic conductivity. The installation of backfill and sealing elements serves to limit the propagation of the excavation damaged zone (EDZ) by stabilizing the surrounding rock formation and by inhibiting fluid transport between the emplacement units and the accessible biosphere. Once installed, these elements complement the host rock and contribute to ensuring the long‑term integrity of the repository.The behavior of backfill and sealing elements - particularly their volume‑change and fluid transfer behavior - is significantly controlled by the initial (or as‑compacted) dry density. This parameter, in turn, depends on the material’s mineralogy, grain‑size distribution, initial (or as‑compacted) water content, and the applied compaction energy.This study presents a simplified approach using the compressibility index to derive the initial dry density of crushed clay‑rock‑based materials from the applied axial stress, initial water content, and expansive clay content.The approach is validated by static compaction experiments in a drained oedometer setup. Prior to compaction, crushed clay rock is mixed with sodium bentonite at three wet‑weight ratios, and each mixture is prepared to six target water contents. The results show that, for low expansive clay contents, the compressibility index exhibits a quadratic dependence on the initial water content, which transitions to a linear dependence as the expansive‑clay content increases. Overall, the findings underline the relevance of this simplified approach for the installation of backfill and sealing elements, particularly with respect to the selection of appropriate installation techniques.

Permeability of tight rocks is an important subject where fluid-rock interaction is concerned. It is of particular importance in the site selection process for a deep geological repository for high-level radioactive waste. According to the German site selection act the containment-providing rock zone shall isolate the waste and prevent or limit transport of radionuclides from the repository into the biosphere for 1 Ma. Consequently, the permeability of the host rock needs to be very low.However, permeability measurements of tight rocks are a great challenge for rock physics laboratories. There is a wide variety of measurement methods and analytical equipment. Therefore, the data from different laboratories are generally hard to compare and it is difficult to build a trustworthy, comprehensive and reproducible permeability data base for possible host rocks of a disposal site for nuclear waste. This is of particular importance because trust in the applied data base is one key factor for a successful site selection process. Against this background it is desirable to:develop a calibration standard with uniform and consistent rock physical properties test this standard at various laboratories which apply different permeability testing methods and experimental setups. Such a round robin test with a well-defined, homogeneous and stable standard would allow a reliable comparison of measurement results of the applied methods and support comparability of permeability measurements of tight materials.The research project Low Perm, which we present here, is geared towards both these objectives and aims to improve the objective comparability of measured permeability values of tight rocks.In a first step a calibration standard on corundum (Al2O3) basis is currently developed. Corundum is chemically resistant, has high mechanical strength, high temperature stability and low thermal expansion. The aim is to produce a ceramic with a porosity of roughly 30 % with pore diameters averaging between 10 and 100 nm and – most importantly – a permeability of 10-18 to 10-20 m². For this purpose, the Al2O3 granulate was first uniaxially formed into a cylindrical shape and then isostatically compacted at pressures between 100 and 300 MPa. Finally, the samples were sintered at temperatures between 1300° and 1600°C.Quality control of the test specimens, i.e. determination of porosity and permeability, is done by a combination of He pycnometry, sorption measurements as well as BIB-SEM imaging, radial diffusion and gas permeability measurements. So far, specimens sintered at 1350°C and 300 MPa are closest to the desired permeability with 9.438 x 10-18 m² (at a porosity of 38 %). A specimen at 1400°C / 300 MPa is currently investigated. It is expected that the permeability of this specimen is close to 10-19 m², which in turn should make it a suitable calibration standard.The developed specimens will be sent to five laboratories (yet to determine) for a round robin test. Each of them will carry out permeability measurements of five samples. Each laboratory will apply its own permeability testing methodology and equipment. This will be done to evaluate the permeability testing concepts of the participants. The results of the benchmark will eventually be discussed jointly, evaluated and published under open access.

Processed claystone-bentonite mixtures are considered as potential backfill material for drifts and shafts in deep geological high-level radioactive waste repositories, foremost due to their strong self-sealing and radionuclide retardation properties. Self-sealing is mainly facilitated by the materials’ swelling capacity when getting in contact with water from the surrounding host rock. A timely sealing of fractures and voids is of high significance in the isolation of radioactive nuclides. However, a specific duration for a fracture to close upon wetting, the exact processes involved (next to interlayer water uptake in smectite) and the necessary width of a swelling zone are unclear. In order to investigate the self-sealing, past research focused almost exclusively on the (free) swelling capacity of bentonites or claystone and hence neglected other processes for permeability reduction upon saturation. More recently, µCT techniques have been used to overcome the lack of direct evidence of fracture closure in clay rocks and to provide a mechanistic understanding of self-sealing processes. To obtain a deeper understanding of the self-sealing behavior of crushed claystone-bentonite mixtures, we map the swelling induced strain in a simple 2D approach using digital image correlation (DIC). For this purpose, an artificial fracture in specimen with different claystone/bentonite ratios but defined initial bulk density and initial water content is saturated and a camera taking pictures of the sample surface every two minutes documents the wetting front as well as the developing strain. For DIC the free software GOM is used that allows tracking strains beneath the order of pixel resolution, which in our setup is about 10 µm. In previous experiments with Opalinus clay localized tiny fissures appeared when the wetting front spread into the specimen and contributed to the closure of an a-priori induced fracture. Further, a straining zone, advancing ahead of the wetting front, lead to a more evenly volume expansion that finally closed not only the artificial fracture but also the newly formed tiny fissures. In general, we assume a somehow similar self-sealing behavior in the claystone/bentonite mixtures, but we expect different responses depending on the claystone/bentonite ratio. Additional tests (e.g. wettability, BIB-SEM, zeta potential) could be used to understand the observed behavior.

For the underground disposal of high-level nuclear waste in rock salt formations, the safety concept includes the backfilling of open cavities with crushed salt. For the prognosis of the sealing function of the backfill for the safe containment of the radioactive waste, it is crucial to have a comprehensive process understanding of the crushed-salt compaction behavior. The crushed-salt compaction process is influenced by internal properties (e.g., grain size, mineralogy, and moisture content) and boundary conditions (e.g., temperature, stress state, and compaction rate) and, therefore, involves several coupled thermal–hydro–mechanical (THM) processes (Hansen et al., 2014; Kröhn et al., 2017). With the paradigm shift from the limited release of radionuclides to safe containment due to the German Repository Site Selection Act passed in 2017, the importance of crushed salt as geotechnical barrier has increased, with a focus on the evolution of its hydraulic properties. Based on the knowledge gaps in the current process understanding, the “Compaction of crushed salt for safe containment” (KOMPASS) projects were initiated to improve the scientific basis behind using crushed salt for the long-term isolation of high-level nuclear waste within rock salt repositories. The efforts to improve the prediction of crushed-salt compaction begun during the first phase of the KOMPASS projects (Czaikowski et al., 2020) and were followed up in a second phase ending in June 2023. The primary achievements of the projects are as follows (Czaikowski et al., 2020; Friedenberg et al., 2022): specification of the KOMPASS reference material, an easily available and reproducible synthetic crushed-salt material, for generic investigations; development of pre-compaction methods and successful production of samples in the short term and under in situ loading conditions; formulation of an extended laboratory program addressing the isolated investigation of known relevant factors influencing the compaction behavior of crushed salt (Düsterloh et al., 2022); execution of long-term compaction tests addressing isotropic and deviatoric load changes, temperature, and compaction state; construction of a backfill body using the KOMPASS reference material in the Sondershausen mine through collaboration with the SAVER (Entwicklung eines salzgrusbasierten Versatzkonzepts unter der Option Rückholbarkeit) project (Schaarschmidt and Friedenberg, 2022); advancement of the tools for microstructure investigation methods (Svensson and Laurich, 2022); generation (first stages) of a microphysical process list combining literature research with our own findings; benchmarking of long-term compaction test for model development and optimization of various existing models as well as the development of new models; application of a virtual demonstrator (2D model representing a backfilled drift in rock salt) for the visualization of developments and the quantification of the models (Rabbel, 2022). In summary, the KOMPASS projects contributed to the reduction of uncertainties and the strengthening of the safety case for using crushed salt within rock salt repositories.

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].

The testing of low-permeable materials is challenging. Yet, for the disposal of radioactive waste, it is essential, too. This workshop is aimed at gathering ambitious scientists to discuss and to collaborate on their experiences in the laboratory testing of low-permeable materials. The focus here is on the methods: What method is best for what kind of low-permeable host rock (salt/clay) and for what kind of technical barrier material (bentonite/crushed salt)? How can measurement errors be correctly determined? What are the crucial “bottlenecks” in the device setups? How can high porous but low permeable samples best be pre-saturated? How can coupled flow and cumbersome gas traps in the tests be dealt with? What is the best-practice analysis of permeability from pressure decay recordings? Is there a hope of defining a standardized procedure for low-permeability testing? These points will be reflected in the light of radioactive waste disposal and in the need to find a best-practice solution when it comes to eventual evaluation and comparison of potential underground disposal sites.

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.

In Germany, rock salt formations are possible host rock candidates for a repository for heat-emitting radioactive waste. The safety concept of a repository in salt bases on a multibarrier system consisting mainly of the geological barrier salt and geotechnical seals ensuring safe containment. Crushed salt will be used for backfilling of cavities and sealing measures in drifts and shafts due to its favourable properties and its easy availability (mined-off material). The creep of the rock salt leads to crushed salt compaction with time. Thereby, the crushed salts' porosity is reduced from the initial porosity of 30 %–40 % to a value comparable to the porosity of undisturbed rock salt (≤1 %). In such low porosity ranges, technical impermeability is assumed. The compaction behaviour of crushed salt is rather complex and involves several coupled THM processes (Kröhn et al., 2017; Hansen et al., 2014). It is influenced by internal properties like humidity and grain size distribution, as well as boundary conditions such as temperature, compaction rate or stress state. However, the current process understanding has some important gaps referring to the material behaviour, experimental database and numerical modelling. It needs to be extended and validated, especially in the low porosity range. The objective of the KOMPASS project was development of methods and strategies for the reduction of deficits in the prediction of crushed salt compaction leading to an improvement of the prognosis quality. Key results are as follows (KOMPASS Phase 1, 2020): selection of an easily available and permanently producible synthetic crushed salt mixture, acting as a reference material for generic investigations; development and proof of different techniques for producing pre-compacted samples for further investigations; establishment of a tool of microstructure investigation methods to demonstrate the comparability of grain structures of pre-compacted samples with in-situ compacted material for future investigations; execution of various laboratory experiments using pre-compacted samples, e.g. long-term creep tests which deliver reliable information about time- and stress-dependent compaction behaviour; development of a complex experimental investigation strategy to derive necessary model parameters considering individual functional dependencies. Its technical feasibility was successfully verified; benchmarking with various existing numerical models using datasets from three different triaxial long-term tests. The result was not entirely satisfactory; however, the number of influencing factors is small and further validation work has to be done. Overall, the KOMPASS project has made significant progress in the approaches to solving the outstanding question, building the basis for further investigations.