Underground Research Laboratory Research Articles

Using multiple isotopes to determine groundwater source, age, and renewal rate in the Beishan preselected area for geological disposal of high-level radioactive waste in China

Understanding hydrogeological conditions is crucial for selecting and assessing the long-term safety performance of a high-level radioactive waste (HLW) disposal repository. Utilizing environmental isotopes as effective markers for analysing groundwater movement, this study investigates groundwater recharge sources, age, and renewal rates using multiple isotopes in China's potential HLW repository site, the Beishan area. The results indicated deep bedrock groundwater primarily derives from ancient precipitation infiltration under cold climatic conditions. A noteworthy distinction is that loose sedimentary groundwater exhibits higher tritium content (>10 TU) compared to bedrock groundwater (<3.2 TU). Groundwater within the recharge area, especially within gullies and piedmont slope deposits, is relatively youthful, with an age of less than 30 years and an annual renewal rate exceeding 5 %. In contrast, the shallow groundwater age in the intermountain basins and depressions of the discharge area generally exceeds 50 years, with an annual renewal rate often falling below 0.5 %. At the Beishan underground research laboratory site, deep groundwater at the disposal repository depth displays a corrected 14C age exceeding 8,000 years, indicating an extremely slow movement and alteration rate. As a result, the hydrogeological conditions in the Beishan area are expected to be relatively beneficial for ensuring the safety of HLW repository.

Countermeasure for reduction of groundwater inflow and construction of the Mizunami Underground Research Laboratory

Countermeasure for reduction of groundwater inflow and construction of the Mizunami Underground Research Laboratory

A thermomechanical model for argillaceous hard soils–weak rocks: application to THM simulation of deep excavations in claystone

The paper presents the enhancement of an existing constitutive model for argillaceous hard soils–weak rocks to incorporate non-isothermal conditions, to be used in coupled thermo-hydro-mechanical (THM) simulations of underground excavations subjected to temperature variations within the context of deep geological nuclear waste disposal. The proposed thermo-elastoplastic extension accounts for the effect of temperature on the yield and plastic potential functions and on the elastic stiffness. The resulting model is validated through the simulation of relevant non-isothermal laboratory tests reported in the literature. The model is then applied to the coupled THM simulation of an in situ heating test conducted at the Meuse/Haute-Marne underground research laboratory in Bure, France, excavated in the Callovo-Oxfordian claystone. Results show that the incorporation of thermal effects into the constitutive description of the host rock plays a significant role in the behaviour of the excavation when subjected to thermal loading, particularly in the evolution of the excavation fractured zone.

Evaluation of EDZ characterization technology for URL based on field investigation at BET

Detailed characterization of the Excavation Damage Zone (EDZ) is an important issue for the high-level radioactive waste (HLW) geological disposal engineering, which is closely related to the long-term safety of the HLW repository. With drill-blast experiment in Beishan Exploration Tunnel (BET, a platform for the field experiment and validation of potential technologies used for China’s HLW disposal engineering in crystalline rock), systematic field tests were carried out to evaluate the suitability of techniques in characterizing EDZ, and to study EDZ characteristic in crystalline tunnel. A series of investigation technologies were used to delineate EDZ and to examine the process of its formation, which includes: p-wave velocity measurement, ground penetrating radar (GPR), acoustic emission (AE) monitoring and borehole televiewer. P-wave velocity measurement along borehole reveal that wave velocity decreased significantly (more than 10%) in 30 cm to 50 cm from the excavation surface, and this zone is defined as EDZ. Clear images of borehole televiewer were obtained, which is helpful for EDZ delineation. GPR and borehole radar were also used for EDZ delineation. Strong reflection signals were detected in the range of 10 cm to 25 cm by GPR with high frequency antenna, indicating significant changes of physical properties of rock-mass in the zone, which is defined as Highly Damaged Zone (HDZ). Moreover, the formation process of EDZ induced by blast in crystalline rock tunnel was analyzed using AE monitoring. Damage process of surrounding rock was divided into blasting damage stage and progressive damage stage. AE events location show the induced cracking activities were clustering within 30 cm to 100 cm from excavation surface. With the experience from field investigation and result analysis, the feasibility and suitability of the techniques for EDZ characterization is evaluated. Finally a specialized EDZ characterization technique system is proposed for underground research laboratory construction in crystalline host rock in Beishan area.

Geophysical Survey Applications in Underground Research Laboratories and Deep Disposal Sites of Spent Nuclear Fuel

Geophysical Survey Applications in Underground Research Laboratories and Deep Disposal Sites of Spent Nuclear Fuel

High‐Resolution Characterization of Excavation‐Induced Fracture Network Using Continuous and Discrete Inversion Schemes

AbstractThe Meuse/Haute‐Marne Underground Research Laboratory provides the location for the experiment designed to investigate the induced fracture network around open or sealed galleries and drifts. The objective of this study is to investigate and reconstruct the hydraulic properties and the geometry of the induced fracture network to improve the insights and validate the conceptual model of the induced fracture network due to the stress redistribution during tunnel excavations. Within the presented study, the cross‐hole responses of the pneumatic tests were analyzed in the first step with an equivalent porous media—3D travel time‐based tomographic approach. In the next step selected 2D profiles of the 3D model domain were inverted using a discrete fracture network inversion approach. The database of the tomographic analysis is based on 18 gas injection tests and 151 pressure interferences, which were recorded between nine closely spaced boreholes. The travel time‐based inversion approach allowed for the reconstruction of the 3D gas diffusivity distribution between nine boreholes with high‐resolution. The applied discrete fracture network inversion approach is based on a transdimensional Markov Chain Monte Carlo (MCMC) methodology and it operates with reversible model updates (jumps) that change the problem dimensions, that is, the number, length and position of fractures within the model domain after each iteration step. The synthesis of the results between the reconstructed 3D diffusivity tomogram and the 2D fracture tomograms improved the insights into the spatial geometry of the induced fracture network around galleries.

Stable microbial community in compacted bentonite after 5 years of exposure to natural granitic groundwater.

The Materials Corrosion Test (MaCoTe) at the Underground Research Laboratory in Grimsel, Switzerland, assesses the microbiology and corrosion behavior of engineered barrier components of a deep geological repository (DGR) for long-term disposal of high-level nuclear waste. Diversity and temporal changes of bentonite-associated microbial community profiles were assessed under DGR-like conditions for compacted Wyoming MX-80 bentonite (1.25 g/cm3 and 1.50 g/cm3 targeted dry densities) exposed to natural groundwater. Using culture-dependent and molecular techniques, samples taken from the outside layer of 5-year borehole modules revealed up to 66% and 23% of 16S rRNA gene sequences affiliated with Desulfosporosinus and Desulfovibrio, respectively. Putatively involved in sulfate reduction, these taxa were almost undetectable within the bentonite core. Instead, microbial profiles of the inner bentonite core were similar to uncompacted bentonite used to pack modules years earlier, and were consistent with a previously published 1-year time point, revealing no detectable microbial growth. Abundances of culturable aerobic and anaerobic heterotrophic bacteria in the uncompacted bentonite were relatively low, with less than 1,000 and 100 colony-forming units (CFUs) per gram dry weight, respectively. Nearly 5 years after emplacement, culturable heterotrophic bacterial CFUs and sulfate-reducing bacteria did not change significantly inside the bentonite core. Phospholipid fatty acid data indicated similar lipid abundance, and corresponding cell abundance estimates, for inner 5-year MaCoTe bentonite samples compared to those previously obtained for 1-year incubations. Collectively, our results provide complementary evidence for microbial stability inside highly compacted bentonite exposed to conditions that mimic engineered barrier components of a deep geological repository. IMPORTANCE The long-term safety of a deep geological repository for used nuclear fuel is dependent on the performance of the engineered and natural barriers. Microbial activity can produce chemical species that can influence the corrosion of the disposal containers for used nuclear fuel. Although previous studies have evaluated the microbiology of compacted bentonite clay within subsurface environments, these have been limited to relatively short incubations (i.e., 1 year). The current study provides a unique 5-year perspective that reinforces previous findings of growth inhibition for bentonite clay exposed to in situ subsurface conditions.

Numerical modeling of site-scale groundwater flow with stochastic parameterized hydraulic conductivity fields for geological disposal of high-level radioactive waste in China

Deep geological disposal is internationally accepted as a feasible and safe approach for high-level radioactive waste (HLW) disposal. To evaluate hydrogeological conditions in support of safety assessments for HLW disposal at the Xinchang preselected site in China, stochastic inverse models of site-scale groundwater flow were developed in this study to generate highly parameterized hydraulic conductivity fields for the fractured medium by adopting pilot point and null-space Monte Carlo methods. The hydraulic influence zones of the faults were carefully considered to have homogenous parameter values across the zones. A total of 120 multiple random realizations were generated to characterize the 3D spatial variations in hydraulic conductivity by conditioning to 1218 fixed pilot points. The simulated hydraulic heads of the realizations were in good agreement with observational data. The hydraulic conductivity fields close to the boreholes could be characterized in detail and had relatively low uncertainties. Furthermore, the hydraulic conductivity (K) values around underground research laboratory (URL) at the HLW disposal zone depth were less than 10−9 m/s. The K values obtained for most of the influence zones of these faults at this depth were consistent with hydraulic test data and ranged from 10−10 to 10−9 m/s, thereby indicating that the surrounding rock has low permeability and is suitable for HLW disposal. These findings offer new insights into hydrogeological conditions by providing key information on the permeability characteristics that need further focus on HLW disposal.

Performing science, performative science: a science and technology studies (STS) reflection on the pioneering pathways of the HADES underground research laboratory (URL)

Abstract. Developing and implementing a project for managing high-level radioactive waste over extensive periods of time is a multifaceted challenge. Among others, it entails demonstrating the feasibility, safety and overall performance of the proposed waste management strategy. However, how to tackle such a demonstration when the periods over which the waste needs to be isolated and contained extend over thousands of years? In the development of geological disposal facilities, a key role in this matter is played by underground research laboratories (URLs), which aim to provide a setting to study how different techniques and materials perform in a realistic environment. Insights gained into URLs are expected to be translatable to the performance and timescales of an actual repository, hence offering a tool to make a seemingly distant future comprehensible today. In this contribution, we use the HADES (High-Activity Disposal Experimental Site) URL in Mol, Belgium, as an example to illustrate how URLs are producing knowledge and insights, and how in this way they are also actively performing the possible future(s) of radioactive waste management. Building on insights gained in the field of science and technology studies (STS), this paper emphasizes how knowledge production is a complex process characterized by the interplay of heterogeneous – and sometimes rather trivial – elements and actors interlinked through a range of explicit and implicit decisions. We argue that tracing these elements and decisions, making them explicit and reflecting upon them are of key importance, as knowledge production not only describes a possible future but also actively contributes to “making the future” by emphasizing what is considered possible and imaginable – and what is not. This paper will therefore also focus on what the implications of making the future could entail from an ethical perspective, with a particular focus on deliberation and intergenerational justice.

Determination of the in situ hydraulic parameters of the Callovo–Oxfordian clay rock in a long-term hydraulic interference testing at Andra's Underground Research Laboratory (CMHM)

Abstract. Between 2009 and 2015, Andra has conducted a unique long-term hydraulic interference test in two parallel boreholes 1 m apart from each other and air core drilled in the undisturbed Oxfordian Clay rock at Andra's Underground Research Laboratory (CMHM) in Bure, France. The experimental set-up consisted of six test intervals positioned along the parallel boreholes drilled from a drift located within the Callovo–Oxfordian clay at around 500 m b.g.l. (below ground level). The 20 m boreholes were oriented around 30∘ downwards. The pressure sensors linked to six intervals of 1 m length each allowed for continuous pressure measurement over 6 years and a slanted borehole for the observation of rock displacements (see Fig. 1). Additionally, flow rates were measured during water injection tests that lasted weeks or months. This extensive set of hydraulic data allowed, for the first time, a precise determination of the in situ Callovo–Oxfordian clay rock permeability and anisotropy and its specific storage at the metre to decametre scale. It was achieved based on an automated calibration, using a 3D flow model and explicitly considering all the relevant hydraulic elements acting on the flow system (such as rock mass, borehole disturbed zone and sealing elements) and the drift acting as a steady sink due to the underground ventilation (see Fig. 2). The history matching was performed on the transient pressure measurements. The obtained numerical calibration results were in close agreement with the hydraulic parameter values of the Callovo–Oxfordian clay determined at a smaller scale (single borehole tests and rock core samples in the laboratory), therefore demonstrating the reliability of the very low permeability of the Callovo–Oxfordian clay at the metre to decametre scale. Furthermore, the modelling illustrated the impact of the measurement set-up and its details on the observed hydraulic behaviour. The analysis methodology and the main findings will be presented.

Methods for the quantification of uncertainties in thermo–hydro–mechanical simulations for safety analyses and influence of modelling decisions

Abstract. The simulation of thermal, hydraulic, and mechanical coupled processes can be a decisive factor in the integrity assessment of geotechnical and geological barriers. Modelling decisions, such as the representation of heterogeneity and the constitutive models used, significantly impact the simulation outcome (Wagener and Pianosi, 2019). Furthermore, numerical inputs to the simulation, i.e. material parameters and boundary conditions, are subject to uncertainty. This results in a lower confidence level of the outcome, even if the overall simulation framework is well validated. To derive robust conclusions from such analyses, it is important to quantify the relative impact of modelling decisions and inputs on certain quantities of interest. Parameter uncertainties can be quantified by their forward propagation through the discretized problem (Helton, 1994), providing a natural frame of reference for quantifying structural uncertainty (Bond et al., 2007), such as the representation of heterogeneity, and for model validation. This contribution will focus on the latter aspects. We present research on workflows for the unification of evaluating uncertainty in experimental data and certain modelling decisions. We first focus on parameter uncertainty quantification and the resulting conclusions concerning the chosen modelling approach. Hereafter, scale questions are addressed in the context of heterogeneity and anisotropy, based on selected case studies. We close by discussing two example applications, namely one at the Underground Research Laboratory (URL) scale (Mount Terri Full-Scale Emplacement (FE) experiment) and one at the repository scale (ANSICHT Ton Nord model). This work is done as part of the Bundesministerium für Umwelt, Naturschutz, nukleare Sicherheit und Verbraucherschutz (BGE URS) research cluster.

Features, events and processes (FEP) analysis of the interactions between repository monitoring systems and multi-barrier systems

Abstract. In line with the European waste directive 2011/70/EURATOM, the European Joint Programme on Radioactive Waste Management (EURAD) was launched in June 2019. EURAD aims to make a step change in European collaboration between advanced and early-stage waste management programmes. Monitoring Equipment and Data Treatment for Safe Repository Operation and Staged Closure (MODATS) is Work Package (WP) 17 of the EURAD programme. This WP aims to address a range of research needs relating to repository monitoring data and technologies. This abstract summarises research relating to the interactions between repository monitoring systems and the multi-barrier systems in which they are emplaced. A detailed features, events and processes (FEP) analysis is being undertaken to identify and describe the manner in which monitoring systems may interact with multi-barrier systems. Monitoring system components are generically considered in this research; they include data acquisition technologies, power technologies, data transmission technologies and data loggers. No specific disposal concepts have been selected; however, a range of commonly engineered barrier system materials and typical host rocks are considered. This FEP analysis research will provide an understanding of the impacts of monitoring systems, which could be used to aid monitoring system design optimisation, for example by supporting the screening of monitoring technologies (White and Scourfield, 2019). It may also provide evidence to demonstrate that monitoring systems do not unacceptably impact the safety functions of the multi-barrier system. Potential interactions may include the creation of gas migration pathways along monitoring power or data transmission cables positioned in the multi-barrier system. This process has been observed in underground research laboratory experiments, such as the Large Scale Gas Injection Test (LASGIT) experiment in the Äspö Hard Rock Laboratory, which involved a series of gas injection tests in a full-scale KBS-3V deposition hole. Other possible interactions could include (non-exhaustive) corrosion of metallic components, degradation of non-metallic components, microbial activity, gas generation, void introduction and formation and volume changes. The final output of this research will be a catalogue of FEPs describing the interactions between generic monitoring systems and multi-barrier systems. Similar to the Nuclear Energy Agency FEP lists, this FEP catalogue is intended to be a starting reference for waste management organisations to use to understand the potential interactions between their specific monitoring systems and multi-barrier systems.

Geomechanical in situ testing of fault reactivation in argillite repositories

Abstract. Pressurization of natural faults as a result of repository-induced effects can lead to their reactivation and permeability generation in case such features are present near disposal tunnels. Potential driving forces for such pressurization are the temperature increase caused by heat-producing high-level radioactive waste and the generation of hydrogen and other gases due to corrosion of engineered materials. We are primarily concerned about pressurization and fault activation in host rocks and faults that have very low natural permeability for pore pressure increases to dissipate, such as the argillite rocks currently investigated in Switzerland, France and other countries. This presentation discusses a series of in situ experiments of fault activation by fluid injection conducted in the argillite rock (Opalinus clay) at the Mont Terri underground research laboratory in Switzerland. A multi-model monitoring test bed was installed at Mont Terri that includes distributed fiber optics for strain, temperature, and acoustics; local fault pore pressure and three-dimensional displacement sensors; active seismic imaging; and passive seismic monitoring. The fault experiments (and their subsequent analysis via hydromechanical modeling) provide a new fundamental understanding of the coupling between pore pressure, fault deformation and permeability generation as a function of time and allow exploring how seismic and aseismic events may impact the integrity of faulted argillite host rock. Our experimental observations demonstrate that significant flow (and transport) can occur along the initially impermeable argillite fault when rupture is activated. However, the fault permeability decreases to almost its pre-activation value when fluid injection ceases and fluid pressure drops. Dilatant slip on the fault plane alone does not explain the observed increase in fault permeability; pressure-induced fault opening also plays a role, favored by the softness of the shale along with the fact that the structure of the fault zone prevents fluids from diffusing into the adjacent damage zone. Rupture initiation and permeability generation is initially aseismic, associated only with an increase in noise level and emerging tremors. Micro-earthquakes are initiated later in the experiments and typically occur away from the fluid-pressurized area. Hydromechanical models show that stress transferred from the initial aseismic deformation can build up to stress criticality and later induce seismic rupture. After presenting the experimental results, we will close the presentation with an outlook to future experimental campaigns using the fault test bed at Mont Terri. We are currently planning a controlled thermal stimulation of the fault, where instead of fluid injection as a trigger mechanism we will heat up the nearby rock volume and measure potential effects on fault stability. Lessons learned from our past and future experiments will help inform the safety assessment of geologic disposal in argillite host rock.

Modeling the convergence of deep drifts in fractured rock based on a continuous equivalent model.: Application to drifts excavated in the Callovo-Oxfordian claystone.

The French National Radioactive Waste Management Agency (ANDRA) is conducting a research program including in-situ experiments at the Meuse/Haute-Marne Underground Research Laboratory (MHM URL), which aims to demonstrate the feasibility of constructing and operating a deep geological radioactive waste disposal in the Callovo-Oxfordian (COx) claystone formation. A drift network of about 2km has been excavated essentially following minor and major horizontal stress directions in the MHM URL since 2000. Continuous monitoring shows that the convergence of the drifts has not stabilized yet, but the convergence rate decreases with time. Moreover, the convergence response strongly depends on the drift orientation. Understanding the time-dependent behavior of the host rock is the key issue for optimizing the design of the disposal. This paper aims to present a constitutive model including threefold anisotropies (elasticity, plasticity and viscosity) to describe the behavior of COx claystone, with a focus on the time-dependent behavior. Transverse isotropic elasticity is considered. Anisotropic Mohr–Coulomb criterion is used to represent the plastic behavior of the host rock. Anisotropic Lemaitre creep law is proposed to model the time-dependent behavior of the COx claystone. Anisotropy is elaborated by introducing an equivalent stress tensor. Model parameters are calibrated based on laboratory tests on sample scale and then adjusted against the field observation in MHM URL. The proposed model is finally applied for modeling the anisotropic convergence observed in the drifts GED and GCS which are respectively excavated in the direction of the minor principal horizontal stress and in the direction of the major principal horizontal stress. The model proved to be able to reproduce the observed convergences for the two directions of the drifts.

Modelling the viscoplastic behaviour of Callovo-Oxfordian claystone with consideration of damage effect

In order to evaluate the performance of deep geological disposal of radioactive waste, an underground research laboratory (URL) was constructed by Andra in the Callovo-Oxfordian (COx) claystone formation at the Meuse/Haute-Marne (MHM). The construction of URL induced the excavation damage of host formations, and the ventilation in the galleries desaturated the host formation close to the gallery wall. Moreover, it is expected that the mechanical behaviour of COx claystone is time-dependent. This study presents a constitutive model developed to describe the viscoplastic behaviour of unsaturated and damaged COx claystone. In this model, the unsaturation effect is considered by adopting the Bishop effective stress and the van Genuchten (VG) water retention model. In terms of the viscoplastic behaviour, the nonstationary flow surface (NSFS) theory for unsaturated soils is used with consideration of the coupled effects of strain rate and suction on the yield stress. A progressive hardening law is adopted. Meanwhile, a non-associated flow rule is used, which is similar to that in Barcelona basic model (BBM). In addition, to describe the damage effect induced by suction change and viscoplastic loading, a damage function is defined based on the crack volume proportion. This damage function contains two variables: unsaturated effective stress and viscoplastic volumetric strain, with the related parameters determined based on the mercury intrusion porosimetry (MIP) tests. For the model validation, different tests on COx claystone under different loading paths are simulated. Comparisons between experimental and simulated results indicated that the present model is able to well describe the viscoplastic behaviour of damaged COx claystone, including swelling/shrinkage, triaxial extension and compression, and triaxial creep.

About this title - Geological Disposal of Radioactive Waste in Deep Clay Formations: 40 Years of RD&amp;D in the Belgian URL HADES

Over the past four decades, research in the HADES underground research laboratory has demonstrated that the geological disposal of radioactive waste in clay can provide a safe and feasible disposal solution. This Special Publication presents the main contributions of the HADES laboratory to Belgian and international research into geological disposal.

Potential influence of microorganisms on the corrosion of carbon steel in the French high‐level long‐lived radioactive waste disposal context at 80°C

AbstractIn this study, experiments were carried out to assess the microbially influenced corrosion (MIC) risk in the context of the French high‐level radioactive waste disposal. The exposures were carried out at 80°C in different repository relevant conditions, including the presence of different cement‐grout mixtures as filling material. Biotic conditions with nutrient and nonsterile conditions with indigenous microbes added from Callovo Oxfordian clayey rock and without nutrients were considered. For biotic conditions, specific preparations of microbial inoculum were carried out from samples collected at Andra's Underground Research Laboratory and microorganisms from microbial culture collection centers. Corrosion kinetics were determined using traditional coupons and completed with real‐time corrosion sensors. Microbiological characterizations consisted of cultural approach, quantitative polymerase chain reaction, and next‐generation sequencing. The obtained results show no significant MIC and a reduced risk with the use of more alkaline filling material.

Understanding parameters impacting the mechanism leading to spalling around underground excavations in massive rocks under high stress

Underground excavation is required for mining engineering and nuclear waste repository. As the excavation progresses, the stress increases, and brittle failures frequently exist. Brittle failures are arranged into two types: spalling and rock bursting. Spalling in the shape of a V-notch may severely damage the machine and workers involved in its development. This potential hazard can be mitigated by fully representing all parameters that cause spalling in the model. Thus, optimized tunnel design can be achieved. As a result, the company can strike a balance between profit and safety. Several possible results can be proposed by conducting a series of sensitivity studies on a credible model based on a real-world case study of spalling in the Underground Research Laboratory (URL) in Canada. It includes predicting the Depth of Failure (DOF) of spalling by changing two main parameters, particularly strength (tensile and crack initiation) and stress (σ1). The result reveals that increasing tensile strength over UCS decreases DOF, increasing percentages of crack initiation (CI) over UCS decreases DOF, and increasing σmax over UCS increases DOF. Moreover, systematic errors in the generated model can lead to inaccurate DOF estimation. This problem must be approached with engineering judgment in order for the solution to correspond to the actual spalling phenomenon.

A predicted CRISPR-mediated symbiosis between uncultivated archaea.

CRISPR-Cas systems defend prokaryotic cells from invasive DNA of viruses, plasmids and other mobile genetic elements. Here, we show using metagenomics, metatranscriptomics and single-cell genomics that CRISPR systems of widespread, uncultivated archaea can also target chromosomal DNA of archaeal episymbionts of the DPANN superphylum. Using meta-omics datasets from Crystal Geyser and Horonobe Underground Research Laboratory, we find that CRISPR spacers of the hosts Candidatus Altiarchaeum crystalense and Ca. A. horonobense, respectively, match putative essential genes in their episymbionts' genomes of the genus Ca. Huberiarchaeum and that some of these spacers are expressed in situ. Metabolic interaction modelling also reveals complementation between host-episymbiont systems, on the basis of which we propose that episymbionts are either parasitic or mutualistic depending on the genotype of the host. By expanding our analysis to 7,012 archaeal genomes, we suggest that CRISPR-Cas targeting of genomes associated with symbiotic archaea evolved independently in various archaeal lineages.

Clay-associated microbial communities and their relevance for a nuclear waste repository in the Opalinus Clay rock formation.

Microorganisms are known to be natural agents of biocorrosion and mineral transformation, thereby potentially affecting the safety of deep geological repositories used for high-level nuclear waste storage. To better understand how resident microbial communities of the deep terrestrial biosphere may act on mineralogical and geochemical characteristics of insulating clays, we analyzed their structure and potential metabolic functions, as well as site-specific mineralogy and element composition from the dedicated Mont Terri underground research laboratory, Switzerland. We found that the Opalinus Clay formation is mainly colonized by Alphaproteobacteria, Firmicutes, and Bacteroidota, which are known for corrosive biofilm formation. Potential iron-reducing bacteria were predominant in comparison to methanogenic archaea and sulfate-reducing bacteria. Despite microbial communities in Opalinus Clay being in majority homogenous, site-specific mineralogy and geochemistry conditions have selected for subcommunities that display metabolic potential for mineral dissolution and transformation. Our findings indicate that the presence of a potentially low-active mineral-associated microbial community must be further studied to prevent effects on the repository's integrity over the long term.