Waste Repository Research Articles

Mechanical properties and macro–micro failure mechanisms of granite under thermal treatment and inclination

The coupling effects of inclination angle (θ) and high temperature (T) on rock degradation present significant construction challenges for underground engineering projects, such as underground coal gasification, inclined mining pillars, and nuclear waste repositories. This study is built on a novel combined compression and shear test system, investigating granite’s physical and mechanical properties and macroscopic failure characteristics under varying temperatures and inclination angles. Additionally, acoustic emission technology was used to analyze the impact of inclination and temperature on the micro-failure behavior of the specimens. The experimental results indicate that as the inclination angle increases, the specimens’ peak compressive strength and elastic modulus decrease linearly. In contrast, the shear stress increases non-linearly, suggesting that additional shear stress accelerates the initiation and propagation of cracks. Furthermore, these mechanical parameters initially increase with rising temperatures before decreasing, with a temperature threshold identified at 400 °C. The effect of inclination angle on the failure mode of the specimens is more pronounced than that of temperature; as the inclination angle increases, the failure mode transitions from splitting failure to shear failure at any given temperature. Acoustic emission results reveal that the specimens’ microcrack initiation (CI) and damage (CD) thresholds reduce with increasing inclination, while the corresponding shear stress increases. As temperature rises, the CI and CD thresholds exhibit an initial increase attended by a decrease. Finally, founded on the experimental findings, a multivariate equation was established to accurately predict the peak compressive strength and elastic modulus about θ and T. The results of this study provide valuable insights and references for the construction of underground thermal engineering under complex conditions.

Two-phase reactive transport modeling of heterogeneous gas production in a low- and intermediate-level radioactive waste repository

Deep geological disposal is a widely proposed approach for safe storage of low- and intermediate-level radioactive waste (L/ILW) for tens to hundreds of thousands of years. Although the use of cement materials will maintain a high pH environment that is favorable for radionuclide retention, slows down metal corrosion, and suppresses microbial activity, different gases may still be produced by chemical reactions, such as pH-dependent anoxic corrosion of metals, and the degradation of organic matter as well. Both reactions consume water and may lead to the formation of a free gas phase within and around the repository.In order to investigate the controlling factors of this gas production processes, a coupled reactive transport model of component-based two-phase flow in the OpenGeoSys framework is adopted here. Building on the previous work by Huang et al. (2021), this work extends the geometric configuration of the model from a single drum to the scale of a waste container (12 drums) in order to more realistically model the water and gas fluxes. The geochemical evolution of a container filled with cemented steel drums and surrounded by mortar and host rock is simulated in a two-dimensional setup over 500 years. The relative humidity in the pore space as measure of water availability determines the chemical reactivity.We have conducted a sensitivity study for the influence of mortar capillarity and permeability on gas production over time in the waste drums. As expected, the amount of gas produced within the first several years depends primarily on the initial water content within the waste drum and the amount of fast degradable waste stored in a drum. Later, this feedback system is forced into a new equilibrium, where the water transport has to match the water consumption rate at the waste package. Both, the mortar capillarity and permeability control the point in time when the equilibrium is reached and the subsequent rate of gas production.This study shows how reactive transport models are capable to help with the understanding of couplings between chemical and transport processes that control gas generation in L/ILW waste repositories, especially in barrier systems with different material properties. Future studies could benefit from more accurate parameterization of the chemical reactions, depending on the availability of new experimental data and/or more realistic process-based model approaches.

Radionuclide transport and retardation in uplifting granitic rocks: Part 2 – Modelling coupled processes in uplift scenarios

Uplifting fractured granitic rocks occur in substantial areas of countries such as Japan. Some of these areas might be considered when siting a deep geological repository for radioactive wastes. A repository site would be selected in such an area only if it is possible to make a safety case, accounting for the changing conditions during uplift. The safety case must include robust arguments that chemical processes in the rocks around the repository will contribute sufficiently to minimise radiological doses to biosphere receptors. Numerical modelling is an important aspect of making these arguments. To provide confidence in the safety arguments, numerical models need to be sufficiently realistic, but also parameterised conservatively (pessimistically). However, model development is challenging because uplift involves many complex couplings between groundwater flow, chemical reactions between water and rock, and changing rock properties. The couplings would affect radionuclide mobilisation and retardation, by influencing diffusive radionuclide fluxes between groundwater flowing in fractures and effectively immobile porewater in the rock matrix (rock matrix diffusion, RMD) and radionuclide partitioning between water and solid phases, via: (i) mineral precipitation/dissolution; (ii) mineral alteration; and (iii) sorption/desorption. It is difficult to represent all this complexity in numerical models while showing that they are parameterised conservatively. Here we present a modelling approach, illustrated by simulation cases for some exemplar radioelements, to identify realistically conservative process conceptualisations and model parameterisations.

Effects of alkaline solution and aging time on thermal conductivity of MX80 powder-granule mixtures

In the design of high-level nuclear waste (HLW) repositories, granular bentonite is esteemed as an effective sealant for interfacing the spaces that exist between the bentonite blocks and adjacent geological bodies. When degraded cement dissolves in groundwater, it generates an alkaline solution with a high pH. Therefore, determining whether the thermal conductivity of granular bentonite changes under the long-term effect of alkaline solution is essential for the repository safety. In this study an experimental investigation was conducted on the changes in the thermal conductivity of granular bentonite with an alkaline solution (NaOH solution) over time, with the effects of aging time, particle size distribution, alkaline solution content and concentration being considered. X-ray diffraction (XRD) technique was applied for examining the variations in mineral composition, while the pores and cracks analysis system (PCAS) was utilized to process previous SEM images, revealing the change in porosity. The test results are as follows. Increasing the alkaline concentration, average particle size or aging time leads to a decline in thermal conductivity, whereas a higher alkaline solution content enhances it. In descending order of effect, the factors influencing thermal conductivity are ranked as the alkaline solution content, particle size distribution, alkaline concentration, and aging time. The interaction effects exist between these different factors. The decrease of thermal conductivity is not only related to the increase in porosity caused by the dissolution of montmorillonite, but also to the decrease in quartz content. The evolution of thermal performance can be a reference for the design and construction of HLW repositories.

Microbial Corrosion of Copper Under Conditions Simulating Deep Radioactive Waste Disposal.

This paper presents the results of microbial corrosion tests on M0-grade copper under conditions simulating a geological repository for radioactive waste at the Yeniseisky site (Krasnoyarsk Krai, Russia). The work used a microbial community sampled from a depth of 450 m and stimulated with glucose, hydrogen and sulfate under anaerobic conditions. It was shown that the maximum corrosion rate, reaching 9.8 µm/y, was achieved with the addition of sulfate (1 g/L) with the participation of microorganisms from the families Desulfomicrobiaceae, Desulfovibrionaceae and Desulfuromonadaceae. It was noted that the most important factor leading to copper corrosion was the accumulation of hydrogen sulfide during the activation of sulfate-reducing microorganisms of the genera Desulfomicrobium, Desulfovibrio and Desulfuromonas. During the development of the microbial community under these conditions, the content of copper can have a significant toxic effect at a concentration of more than 250 mg/L.

Retention of Nickel and Cobalt in Boda Claystone Formation

The Boda Claystone Formation (BCF) is considered to serve as a natural barrier to the potential high-level radioactive waste repository in Hungary. In order to evaluate the radionuclide retention capacity of the albitic claystone of the BCF, the adsorption and diffusion properties of the rock for Ni2+ and Co2+ cations (activation products) were investigated separately and in competitive conditions when the two ions were simultaneously added. Batch sorption experiments were performed with powdered and conditioned albitic claystone samples in synthetic pore water to obtain adsorption isotherms. In addition, adsorption tests were performed on petrographic thin sections to check the transferability between dispersed and compact systems. Correlation analysis of microscopic X-ray fluorescence elemental maps recorded on thin sections suggested that nickel is primarily bound to clay minerals (mainly illite and chlorite), which was confirmed by (scanning) transmission electron microscopy measurements. Around illite particles, a newly formed nickel-rich few atomic layer thick phyllosilicate phase was identified. The discrepancy between the experimental and modeled adsorption isotherm at high concentrations could be explained with this nickel-rich new phase. Apart from Cin = 10−3 M and only Ni2+ or Co2+ in the source, the apparent diffusion coefficients of Ni2+ and Co2+ (Cin = 10−3–10−2 M) were found to be similar. Overall, the BCF shows promising capabilities to retain the studied radionuclides.

Assessing future ice-sheet variability for long-term safety of deep geological repositories

Abstract. In many regions considered for deep geological repositories (DGR) for nuclear waste, repeated glaciations could occur within the time frame relevant to their long-term post-closure safety (up to 1 million years; Myr). Ice sheets can affect the long-term safety of a DGR by elevating the hydrostatic pressure at DGR depth, altering groundwater flow and chemistry, influencing the frequency and severity of earthquakes, and causing surface bedrock erosion. Therefore, DGR safety assessments must account for uncertainties in future ice-sheet variability, including the timing, frequency and duration of ice sheets at the DGR site. Using coupled ice sheet-climate models to constrain uncertainties in ice-sheet variability over the next 1 Myr is not feasible due to the long timescales involved and substantial computational requirements. Instead, we propose a simplified methodology to assess future ice-sheet variability at potential DGR sites using reconstructions of past ice sheets and global simulations of future climate change. The simulations are conducted using a conceptual climate model driven by changes in insolation and atmospheric CO2 concentrations resulting from anthropogenic emissions. The model is calibrated with 500 000 years (500 kyr) of climate proxy data inferred from deep-ocean sediments. Applying this methodology to the planned Swedish DGR site intended for disposal of spent nuclear fuel in Forsmark suggests that the onset of the next glaciation at the site will not occur until 100 kyr after present, even in the absence of anthropogenic CO2 emissions. However, anthropogenic emissions have the potential to delay the next glaciation in Forsmark by several hundred thousand additional years. Following the initial glaciation, our results suggest that the frequency and duration of subsequent glaciations in Forsmark resemble those observed over the last 800 kyr. Considering uncertainties in anthropogenic emissions and future climate evolution, a wide range of possible future glacial developments is identified. At the extremes of this uncertainty range – developments with a low likelihood of occurrence but relevant for evaluating the robustness of the DGR – we find that Forsmark could experience ice-sheet coverage for nearly half of the next 1 Myr or remain almost entirely ice-free throughout this period. The proposed methodology is easy to implement and applicable to any potential DGR site with a recorded history of glaciations.

In situ static elastic properties assessment and validation with pressuremeter testing using a formation tester tool

Pressuremeter testing (PMT) is a formation test that consists of inflating a cylindrical packer inside a borehole while measuring the radial deformation or injected fluid volume as a function of packer pressure. Provided that the stiffness of the packer measuring system is known and large enough compared to that of the formation, changes in packer pressure associated with changes in injected fluid volume provide a direct measurement of formation stiffness. In turn, the in situ static shear modulus is obtained from the formation stiffness at a length scale similar to that of the packer. Here, we report on the first field-scale campaign of PMTs in deep boreholes performed using a wireline formation tester (WFT) tool. We carried out PMT measurements as part of the characterization and appraisal of potential sites for a deep geological repository for radioactive waste in Switzerland. We performed multiple PMT inflation cycles to infer in situ static shear moduli at six stations spread across four boreholes. PMT-derived static shear moduli results were consistent with static shear moduli derived from sonic logs using independent dynamic-to-static elastic moduli transformations. PMT-derived static shear moduli and laboratory-derived static elastic moduli using samples from coring performed at the depths of the PMT stations were consistent, with slightly lower laboratory values. Furthermore, we report dynamic-to-static shear moduli transformations by using laboratory-scale data obtained on cores and field-scale derived from sonic logs and PMT. We observed differences between static and dynamic shear moduli derived from laboratory scale using cores and field scale using sonic logs and PMT. We report linear trend slopes of about 0.5 for the laboratory data and 0.7 for the field data. These first results show the viability of in situ PMT in deep boreholes with a WFT tool, as it can be performed at multiple depths in a single run, in a time-efficient manner, and in combination with micro-hydraulic and sleeve fracturing stress tests for an integral approach to in situ geomechanical assessment.

Soft interface instability and gas flow channeling in low-permeability deformable media

Understanding gas percolation through a clay layer or a shale formation is of great importance for the development of a geologic repository for nuclear waste disposal, a subsurface system for gas storage, and an engineering approach for hydrocarbon extraction from unconventional reservoirs. Gas injection experiments have revealed complex dynamic behaviours of gas percolation through water saturated compacted bentonite, characterized by a high breakthrough pressure, rapid breakthrough, a pressure/stress decay after the breakthrough, a relatively high migration rate, high-frequency periodic/nonperiodic variations in flow rate, stepwise rate reductions during relaxation, and low gas saturation over the whole process, all indicating channelling nature of the processes. Using linear stability analyses, we show that this channelling can autonomously emerge from the instability of the deformable interface between the injected gas and the compacted bentonite matrix driven by local stress concentration, pore dilation, and hydrologic gradient. Channel patterns formed would possess a fractal geometry. We further show that, once a percolating channel is established, the gas injected would percolate through the channel in a chain of gas bubbles, also due to the interface instability, resulting in periodic/chaotic variations in gas flow rate. Our work provides a unified explanation for key features observed for gas percolation in low-permeability deformable media. The work also suggests a possibility of designing an engineered barrier system for a nuclear waste repository that can have controllable gas release while limit water transport.

Comparing modelling approaches for a generic nuclear waste repository in salt

This paper contains a comparison of five modelling approaches for a simplified nuclear waste repository in a domal salt formation. It is the result of a four-year collaboration between five international teams on Task F of the DECOVALEX-2023 project on performance assessment modelling. The primary objectives of Task F are to build confidence in the models, methods, and software used for performance assessment (PA) of deep geologic nuclear waste repositories, and/or to bring to the fore additional research and development needed to improve PA methodologies. This work demonstrates how these objectives are accomplished through staged development and comparison of the models and methods used by participating teams in their PA frameworks. Participating teams made a wide range of model assumptions, ranging from compartmentalized networks to full 3D models of the salt formation and repository. Despite differences in the modelling strategies, all models indicate that salt compaction and diffusion of radionuclides in brine are key processes in the repository. For the isothermal spent nuclear fuel and vitrified waste scenario with multiple early failures considered, all models indicate little of the disposed radionuclides will migrate beyond the repository seal over the 100,000-year simulations. In general, the model output quantities have the largest differences over the short term and near the waste. Disparities between the models are believed to be due to differing simplifications from the conceptual model.

A Parametric Study of Mathematical Model for Long-Term Localized Corrosion of Copper in Canadian Deep Geological Repository

The concept of Adaptive Phased Management, approved by Canadian federal authorities and implemented by the Nuclear Waste Management Organization, forms a comprehensive strategy for the long-term safe management of used nuclear fuel in Canada.1 The used nuclear fuel will ultimately be placed within a container in a deep geological repository (DGR). Current container designs employ steel vessels with a protective copper outer layer, strategically selected to reduce corrosion damage. The copper-coated container may be subjected to various corrosion mechanisms over time as the DGR environment evolves. Specific to this work and in the early aerated environment of the DGR, efflorescing salt impurities present on the container surface may lead to the formation of an Evan’s droplet. To understand the behavior of the copper-coated container with respect to localized corrosion and ensure the effectiveness of this protection strategy, a preliminary two-dimensional axisymmetric time-dependent model for the corrosion of copper under an Evans droplet was developed.2, 3 The mathematical model was developed using the finite-element method (COMSOL Multiphysics). Some unique features of the model are inclusion of six heterogeneous and fifteen homogeneous reactions, implicit calculation of nm-scale films, and treatment of the influence of films on surface concentrations and potentials. The model shows the time-dependent localized corrosion rates and depths, calculated for an elapsed time of ten years. It also shows time-dependent radial distributions for current density and surface coverage of films. The influence of oxygen conditions and temperature was included, and the model accounted for the influence of films on reaction rate constants. Preliminary results show that the corrosion of copper in the Evan’s droplet is almost uniform on the electrode surface. Temperature and oxygen concentration was shown to have a strong contribution to copper corrosion.The current version of the mechanical model has many parameters. Some parameters are obtained from PHREEQC thermodynamic software,4 some are selected to yield good results, and some are defined using expert judgement from the literature.While the number of parameters is very large, the impact of each parameter is constrained by the model. A parametric study is necessary to identify reasonable ranges of values and explore different conditions that may lead to localized corrosion of copper. For example, to switch the mechanism from kinetic control to mass-transfer control, the overall rate constants were increased by different orders of magnitude. The influence of both oxide film and salt film on rate constants reduction were also studied. Combining both the blocking effect of films on the copper surface and the control mechanism shift, simulations were completed to study the parameter space. The present work shows that the corrosion rate increased as rate constants were increased, and currents became limited by mass transport of oxygen.References NWMO, “Choosing a Way Forward. The Future Management of Canada’s Used Nuclear Fuel. Final Study,” Nuclear Waste Management Organization, Toronto, Ontario, 2005.C. You, S. Briggs, and M. E. Orazem, “Model Development Methodology for Localized Corrosion of Copper,” Corrosion Science, 222 (2023), 111388.C. You, Y. Chuai, S. Briggs, and M. E. Orazem, “Model for Corrosion of Copper in a Nuclear Waste Repository,” Corrosion Science, 226 (2023), 111658.PHREEQC Version 3, United States Geological Survey, 202, https://www.usgs.gov/software/phreeqc-version-3. AcknowledgementThis work was supported by the Nuclear Waste Management Organization, Canada, under project 2000904.

A methodology for deciding on well seal options for abandonment

The energy transition to achieve net zero anthropogenic CO 2 emissions will require permanently sealing and abandoning wells or boreholes drilled for many different purposes, including: underground CO 2 storage; hydrogen storage; geothermal energy extraction; site characterization for radioactive waste repositories; and hydrocarbon extraction. It is necessary to objectively decide what sealing materials, combination of materials and methods of emplacement are optimal for a given well sealing project, taking into account the properties of lithologies penetrated, the physical and geochemical conditions around the well, the geometry of the well, and characteristics of well engineering. A workflow that would allow for a structured approach to designing well seals during well decommissioning and a set of complementary tools that will enable an informed, structured, transparent and traceable process for designing a well decommissioning sealing solution for a particular well in a given geological environment is presented in this paper. The tools include: (1) a database of Features Events and Processes; (2) a decision tree for evaluating different well sealing options and for documenting the rationale for decisions; (3) an approach for comparing options using multiple decision trees; (4) and a suite of simplified numerical models to inform judgements.

Effect of cellulose degradation products on 63Ni sorption on Portland cement

Concepts of nuclear waste repositories make use of large quantities of cementitious materials, especially near surface disposal facilities in which cement, mortar, and concrete are used both as structural materials and waste immobilization matrices. The retardation of radionuclides by the cementitious barriers of these repositories is crucial for the safe long-term management of the waste. Yet both the degradation of the cementitious materials and the presence of complexing ligands originating from the waste can affect the sorption of radionuclides. Cellulosic materials present in nuclear waste degrade under irradiation and under the alkaline conditions generated by the cementitious materials and they release organic compounds. Within the cellulose degradation products, α-isosacharinic acid (α-ISA) is generally assumed to be the strongest radionuclide complexant and to have the most impact on radionuclide sorption, though the presence of other ligands in the mixture could also have an effect. In this work, the sorption of 63Ni was assessed on fresh and state III hardened cement paste (HCP). The effect of α-ISA on the 63Ni sorption at these two degradation states was investigated and compared to the effect of a complete mixture of cellulose degradation products generated by irradiation and alkaline degradation of cellulosic tissues. A higher sorption of 63Ni was observed on state III HCP (Rd = (8.6 ± 3.3) × 103 L/kg) than on fresh HCP (Rd = (1.0 ± 0.3) × 103 L/kg), and, in both cases, the sorption was higher than previously observed on pure C–S–H (calcium silicate hydrate) suggesting the involvement of other mechanisms than the sole surface complexation on C–S–H. The effect of α-ISA on the sorption of 63Ni depended on the cement degradation state, ranging from no effect on fresh cement to a maximal sorption reduction factor of 8 ± 3 on degraded cement for 10−2 M α-ISA. The effect of the mixture of degradation products from cellulosic tissues on the sorption of 63Ni was found to be considerably higher on both fresh and degraded HCP, with a sorption reduction factor of 582 ± 972 and 718 ± 327 for the fresh and degraded cement, respectively, for a corresponding α-ISA concentration of 10−2 M.

Investigating temperature effects on the shear behavior of clays: Molecular dynamics simulations

The shear behavior of clays is critically important for the stability and safety of nuclear waste repositories and clay gouges. These contexts expose clayey geomaterials to high pressures and temperatures. Under such varying conditions, the shear behavior of these materials becomes complex, necessitating thorough investigations. This study aims to elucidate the effect of temperature on the shear behavior of three clayey materials: kaolinite, illite, and montmorillonite—through Molecular Dynamics. The research replicates a geotechnical shear setup at the molecular scale, varying the temperature (including sub-freezing temperatures below 300 K and elevated temperatures 400-500 K and hydrostatic pressures. The results reveal stick-slip behavior, enabling the calculation of nanoscale cohesion, friction angle, and shear modulus across different temperatures. Thermal effects are notably significant for illite and kaolinite, both exhibiting a marked decrease in shear properties with increasing temperature. Kaolinite demonstrates a high shear modulus of up to 40 GPa, indicating substantial shear strength. Illite displays the highest friction angle among the studied materials. For montmorillonite, thermal effects on shear behaviour is comparatively less pronounced. This study provides critical insights into the mechanical behaviour of clays under varying thermal conditions, contributing to the broader understanding of geomaterial stability in high-stake environments.

Novel dissolved organic 14C analyses method applied in a case study at a LILW waste repository

Abstract A routine chemical procedure was developed at the Ede Hertelendi Laboratory of Environmental Studies (HEKAL), in Debrecen which can measure the dissolved organic radiocarbon content of groundwater as well as the inorganic and total fraction. The typical background of this non-purgeable dissolved organic radiocarbon preparation is 0.73 ± 0.14 percent modern carbon (pMC), using a carbon contamination correction on fossil dissolved material (potassium hydrogen phthalate) samples.

DecTree: a physics-based geochemical surrogate for surface complexation of uranium on clay

Abstract. Geochemistry is usually the computational bottleneck in coupled reactive transport simulations, which hampers the complexity of the systems and of the processes they can investigate. In recent years, promising speedups have been obtained by substituting the numerical solution of geochemical models with approximated surrogates borrowed from artificial intelligence and machine learning (AI/ML). In the framework of the DONUT/EURAD project a set of benchmarks were defined to assess the performance and the accuracy of different surrogate approaches in settings relevant to the safety assessment of nuclear waste repositories, such as the surface complexation and exchange of U(VI) on clay. In this context, this work introduces am original surrogate modelling approach based on recursive partitioning of parameter space, which exploits prior domain knowledge for the training. The surrogate, which can be represented as a decision tree, hence the DecTree name, performs dimensionality reduction by identifying functional relationships between outputs and input variables using a straightforward non-monotonic extension of the Spearman's rank correlation coefficient. New predictions are then interpolated from the partitioned training data. Applied to a low-dimensional geochemical model, DecTree shows virtually no training time and excellent accuracy, ensuring a throughput of around 500 000 predictions per second on a single CPU core.

Geophysical visualization of water content distribution in bentonite by joint seismic and radar tomography

SUMMARY Bentonite is often considered as buffer material for deep geological radioactive waste repositories. To support decision making and safety assessment of radioactive waste repositories, international agencies and research institutions proposed the implementation of monitoring programmes. While the overall concepts of such monitoring programmes have been largely developed, the selection of key observations parameters, such as temperature, pressure and water content, and the technical implementation are still under development. The direct measurement of such parameters requires the placement of sensors inside a repository, which can significantly affect its safety functions and only provides information at the typically sparse sensor locations. Geophysical tomography can help gaining valuable insights into the state of the repository non-invasively by providing images of the distribution of geophysical parameters from measurements that are purely taken from the outside. However, the extracted geophysical parameters are often difficult to interpret and the geophysical tomography problem is non-unique, meaning that there exist multiple models that explain the data equally well. Here, we demonstrate that this non-uniqueness can be significantly reduced by simultaneously employing multiple geophysical methods in a joint tomography scheme. We simultaneously invert seismic and ground penetrating radar (GPR) traveltimes and amplitudes by imposing structural similarity constraints on the tomographic velocity and attenuation images. The resulting, estimated geophysical parameter maps show a strongly improved correlation when compared to results obtained from individual inversions, which in turn facilitates the establishment of constitutive relationships between the geophysical parameters (seismic and GPR velocity and attenuation) with the water content, as key parameter for the evaluation of the state of a radioactive waste repository. Using data from the full-scale emplacement (FE) experiment, we employ a supervised machine-learning model that enables the translation of the tomographic velocity and attenuation images obtained in bentonite to an image of the distribution of the water content inside the repository, where the machine learning model is trained using direct point measurements of the water content at sparse locations inside the tomographic plane. Due to the lack of direct water content sensors in the FE experiment, we use neutron log data (which are directly linked to water content) to train the machine learning model. Ultimately, this enables us to extrapolate the sparse neutron log data to a spatially cohesive distribution inside the repository corresponding to a visualization of the spatial distribution of water content.

Coupled thermo-hydro-mechanical analysis of porous rocks: Candidate of surrounding rocks for deep geological repositories

Deep geological sequestration is widely recognized as a reliable method for nuclear waste management, with expanded applications in thermal energy storage and adiabatic compressed air energy storage systems. This study evaluated the suitability of granite, basalt, and marble as reservoir rocks capable of withstanding extreme high-temperature and high-pressure conditions. Using a custom-designed triaxial testing apparatus for thermal-hydro-mechanical (THM) coupling, we subjected rock samples to temperatures ranging from 20 °C to 800 °C, triaxial stresses up to 25 MPa, and seepage pressures of 0.6 MPa. After THM treatment, the specimens were analyzed using a Real-Time Load-Synchronized Micro-Computed Tomography (MCT) Scanner under a triaxial stress of 25 MPa, allowing for high-resolution insights into pore and fissure responses. Our findings revealed distinct thermal stability profiles and microscopic parameter changes across three phases—slow growth, slow decline, and rapid growth—with critical temperature thresholds observed at 500°C for granite, 600 °C for basalt, and 300 °C for marble. Basalt showed minimal porosity changes, increasing gradually from 3.83% at 20 °C to 12.45% at 800 °C, indicating high structural integrity and resilience under extreme THM conditions. Granite shows significant increases in porosity due to thermally induced microcracking, while marble rapidly deteriorated beyond 300 °C due to carbonate decomposition. Consequently, basalt, with its minimal porosity variability, high thermal stability, and robust mechanical properties, emerges as an optimal candidate for nuclear waste repositories and other high-temperature geological engineering applications, offering enhanced reliability, structural stability, and long-term safety in such settings.

Multiphase flow and reactive transport benchmark for radioactive waste disposal

Compacted bentonite is part of the multi-barrier system of radioactive waste repositories. The assessment of the long-term performance of the barrier requires using reactive transport models. Here we present a multiphase flow and reactive transport benchmark for radioactive waste disposal. The numerical model deals with a 1D column of unsaturated bentonite through which water, dry air and CO2(g)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\hbox {CO}_{2{(g)}}}$$\\end{document} may flow and with the following reactions; aqueous complexation, calcite and gypsum dissolution/precipitation, cation exchange and gas dissolution. INVERSE-FADES-CORE V2, DuMuX\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {DuMu}^X$$\\end{document}, TOUGHREACT and iCP were benchmarked with 6 test cases of increasing complexity, starting with conservative tracer transport under variably unsaturated conditions and ending with water flow, gas diffusion, minerals and cation exchange. The solutions of all codes exhibit similar trends. Small discrepancies are found in conservative tracer transport due to differences in hydrodynamic dispersion. Computed CO2(g)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\hbox {CO}_{2{(g)}}}$$\\end{document} pressures agree when a sufficiently refined grid is used. Small discrepancies in CO2(g)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\hbox {CO}_{2{(g)}}}$$\\end{document} and pH are found near the no-flow boundary at early times which vanish later. Discrepancies are due differences in the formulations used for gas flow at nearly water-saturated conditions. Computed CO2(g)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\hbox {CO}_{2{(g)}}}$$\\end{document} pressures show a fluctuation between 10-4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{-4}$$\\end{document} and 10-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{-3}$$\\end{document} years which slows down the in-diffusion of CO2(g)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\hbox {CO}_{2{(g)}}}$$\\end{document}. This fluctuation is associated with chemical reactions involving CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\hbox {CO}_{2}}$$\\end{document}. There are discrepancies in solute concentrations due to differences in the Debye–Hückel (DH) formulation. They are overcome when all codes use the same DH formulation. The results of this benchmark will contribute to increase the confidence on multiphase reactive transport models for radioactive waste disposal.

Trace element partitioning between natural barite and deep anoxic groundwaters: Implications for radionuclide retention in host rocks of nuclear waste repositories

Safety assessments for deep geological repositories involve risk calculations for the release of radionuclides like Ra, U, Pu and trivalent actinides from the storage containers to the groundwater. The retention of radionuclides through water-mineral interaction along the groundwater flow path could be a crucial factor in case of a repository failure. Barite (BaSO4) assumes significance in this context, as it has the potential to (re)crystallize and incorporate significant quantities of radioactive elements under relevant physico-chemical conditions.The assessment of mineral-fluid partition coefficients provides a means to evaluate the uptake potential of elements into the mineral. Usually, partition coefficients are determined under well-defined and controlled experimental conditions in laboratories. However, these results have shown discrepancies to partitioning coefficients determined from natural systems.Furthermore, effects like diagenesis or changes in the chemical fluid parameters might lead to a secondary alteration of the phases and affecting the retention ability.Here we investigate the incorporation of trace elements in natural barite from a borehole at 415 m depth in the Äspö Hard Rock Laboratory (Sweden). High-resolution LA-ICP-MS enables the quantitative determination of field-based partition coefficients through the integrated pixel average of selected zones in element distribution maps, combined with existing fluid concentration data. Similarities between the solid solution systems (Ra,Ba)SO4 and (Sr,Ba)SO4 allowed the combination of Sr partitioning data with density-functional theory simulations for an estimation of the partition coefficient value for Ra in natural barite. Values in the 10−2 range were determined, showing a deviation from those reported in previous experimental studies in the 10° range.Moreover, lanthanum serves as an analogue element for the radioactive trivalent actinides. The partition coefficient values for La in natural barite were determined in the range of 10−2 and 10−1, aligning well with experimental partition coefficients. Although density functional theory simulations cannot directly convert a La partition coefficient into a partition coefficient for trivalent actinides, it is assumed that these elements exhibit comparable behavior. Besides primary growth zonation, La also exhibits strong secondary enrichment, probably resulting from groundwater mixing and late fluid-mediated element transport through connected intracrystalline pore space oriented at cleavage plane systems. Additional SIMS analysis provides insights into the temporal variation of the sulfate source at the sampling site reflecting the influence of different waters during mineral growth.This study demonstrates a discrepancy between natural and synthetic PRa for barite and emphasizes that secondary processes significantly impact radionuclide retention in barite. Including these effects in reactive transport models will improve the reliability and applicability of integrative risk models.