This study presents a coupled thermo-hydro-mechanical (THM) model for simulating the heating and hydration behavior of bentonite, a buffer material in deep geological repositories (DGRs). The model incorporates a new temperature-dependent soil water retention curve which captures the thermal-induced shift in water retention behavior. It also distinguishes between liquid and gas permeability, modeling intrinsic gas permeability as a function of accessible porosity to improve vapor transport and desaturation predictions. The model was validated against two large-scale tank tests, demonstrating good agreement with measured temperature, relative humidity, and water inflow data. It revealed a complex porosity evolution driven by thermal expansion, vapor movement, vapor condensation, and hydration-induced swelling during heating and hydration processes. The simulation results also suggest that the permeability of the hydration layer plays a critical role in controlling water intake. Clogging of this layer can significantly reduce the volume of water inflow during the hydration phase. While the model effectively captures key THM behavior, further development of the mechanical constitutive law is required to account for possible thermo-elasto-plastic volume changes and microstructural effects. Overall, the model provides a robust tool for evaluating the evolution of bentonite-based barrier material in DGRs.

Hydration hysteresis is a ubiquitous feature of water exchange between porous solids and humid vapors. Whether one or both of the hydration hysteresis loop branches is metastable remains debated due to the many possible hydration mechanisms that can cause hysteresis. Here, we show that both endothermic and exothermic phase transitions among hydration states in smectites, model compliant porous media, can be optically activated during both hydration and dehydration, indicating that both branches are metastable. A net free energy difference of +15 kJ/mol of water = 6R at 298 K between adsorption and desorption branches is observed over a full hydration-dehydration loop in isothermal, isobaric hydration calorimetry. Hysteresis vanishes, and the reversible sorption energy is 5R per water molecule under continuous-wave laser illumination. In situ Raman spectra confirm that hydrogen bonding between water and structural hydroxyl in the metal oxide framework screens the interlayer charge, weakening interactions and releasing heat. The latent heat of hydration is stored in this immobilized water, which provides the mechanical dilation necessary to nucleate distinct hydration states when water vibrational modes are optically activated. Our findings show that water sorption hysteresis is caused by delicate interfacial phase transitions, which visible light at ambient levels is sufficient to promote. These findings may have important consequences for the fate of sorbents, such as carbon, mineral nutrients, and environmental contaminants associated with clays in soils since the retention and mobility of these species depends strongly on the hydration state of the mineral surface.

The long-term containment of high-level radioactive waste in geological disposal repositories relies on Engineered Barrier Systems (EBS), with bentonite clay emerging as a candidate material due to its unique properties. Understanding moisture dynamics within bentonite buffers is crucial for EBS performance, as it directly influences the material's swelling capacity, thermal and hydraulic conductivity, mechanical properties, and long-term evolution under complex thermal-hydrological-mechanical (THM) processes. This study develops an advanced Electrical Resistivity Tomography (ERT)-based framework to quantitatively monitor moisture dynamics under THM conditions. Our framework extends the Waxman-Smits model to incorporate the coupled effects of temperature, water content, fluid chemistry, and mechanical changes on bentonite's electrical properties. Utilizing HotBENT-Lab data from our companion paper, which includes electrical conductivity, CT density, and thermocouple measurements, this study offers a novel methodological framework bridging different scales of the model. Our results show that the extended model can estimate water content from ERT data, capturing spatial and temporal variations in moisture distribution within bentonite columns. However, the model tends to overestimate water content compared to CT density-derived measurements. We address this discrepancy by incorporating a simplified swelling effect model, which improves agreement between ERT and CT density-based water content estimates. We also discuss model limitations, including simplified treatment of swelling and micropore effects, and propose a conceptual framework for transitioning from laboratory to field applications, addressing challenges such as parameter scalability, field validation methods, and integration of diverse data sources. This ERT-based framework can potentially advance real-world moisture monitoring of bentonite-based EBS in nuclear waste repositories.

More than 50 countries worldwide are currently exploring options for radioactive waste (RW) disposal, considering the link between geoscience fundamentals and the safety of RW disposal sites. The presentation will focus on the lessons learned from representative RW disposal projects worldwide (such as those conducted in Sweden, Finland, France, Spain, Switzerland, Japan, and the USA), summarizing the existing deep geological repository (DGR) concepts, including potential DGR site selection and characterization, as well as long-time modeling predictions. A comparative assessment of models of coupled thermo-hydro-mechanical-chemical (THMC) processes has been performed within the scope of the international project DECOVALEX, which has helped advance the understanding of THMC processes in geological systems. The experimental and modeling studies of the DGR concepts are ultimately linked to assessing the safety of RW disposal. The presentation will provide critical references and case studies related to the representative national disposal programs, which would interest geoscientists, engineers, and decision-makers working on national RW disposal programs. We will summarize and compare challenging geological problems and experiences in siting nuclear waste repositories in different host rocks, such as hard rock (crystalline and sediments), clayey, and salt formations. The availability of rock in a country limits the choice of rock type for the DGR. The interplay of geological conditions with technical feasibility, an engineering design for different rock types and operational and post-closure safety is critical in technical evaluating potential sites. We will also present summaries of the progress in international cooperation studies and testing of the design of buffer and backfill materials, the development of the concept of RW disposal in deep boreholes, the R&D research in Underground Research Laboratories (URL), and multi-national RW repository initiatives. Acknowledgments: LBNL work was supported under Contract Number DE-AC02-05CH11231 with the US DOE. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Bentonite clay is widely used in engineered barrier systems for the permanent disposal of high‐level radioactive waste due to its low permeability, high swelling capacity, and thermal stability. However, the complex thermal‐hydrological‐mechanical (THM) processes induced by heating from decaying radioactive waste and hydration from surrounding rock can lead to heterogeneous changes that are difficult to measure and predict. This study develops an Electrical Resistivity Tomography (ERT)‐based framework for monitoring THM processes, progressing from sample‐scale to bench‐scale tests, to inform field‐scale applications. Sample‐scale tests analyzed small bentonite samples under controlled variations in water content, temperature, and porosity to establish fundamental resistivity relationships. Bench‐scale tests involved larger bentonite columns subjected to heating (up to 200°C) and hydration under controlled pressure, simulating repository conditions. ERT measurements, complemented by X‐ray CT imaging, temperature monitoring, and tracing sensors, revealed coupled THM processes, such as hydration‐induced compression, swelling, and thermal gradients, leading to complex resistivity patterns. The results demonstrate the potential of ERT for capturing THM‐induced resistivity changes, though challenges remain in upscaling and quantitative analysis. This study evaluates laboratory test capabilities and proposes future improvements for understanding THM‐induced resistivity responses. A conceptual framework for ERT implementation in field‐scale monitoring is presented, synthesizing findings from both scales and exploring how ERT data can inform long‐term modeling and reduce prediction uncertainties. Overall, this ERT‐based framework offers a robust method for monitoring bentonite buffers, aiding in early issue detection and supporting the safe long‐term disposal of radioactive waste in geological repositories, while highlighting the need for future development.

The distinctive properties of salt, such as low permeability, high thermal conductivity, and self-sealing features, make it a suitable medium for storing heat-generating radioactive waste. Understanding the thermal, hydrological, and mechanical (THM) processes, including permeability evolution and brine migration around the heat source, is crucial for safety. The Brine Availability Tests in Salt (BATS) at the Waste Isolation Pilot Plant were conducted to investigate these processes, employing an array of sensors and techniques like electrical resistivity tomography (ERT), fiber optic distributed temperature sensing (DTS), and strain sensing (DSS). These techniques monitored temperature changes, brine movement, and stress conditions in the salt. This presentation highlights the results from ERT, DTS, and DSS in controlled heating experiments, focusing on the response differences across various heating events. The analysis, augmented by Discrete Element Models (DEM) simulations, showed that resistivity changes were sensitive to temperature and brine movement. A significant decrease in resistivity, especially beyond temperature effects, indicated brine migration or permeability changes. The DTS and DSS data captured the evolving thermal and mechanical responses of the salt to heating and cooling cycles, including salt deformation, and creeping towards the drift wall. Joint analysis of ERT, DTS, and DSS data provided an integrated understanding of THM processes in salt during heating. Consistent heating and brine migration patterns were observed across different events. Ongoing work aims to combine all monitoring data for a deeper insight into the coupled behaviors in salt formations, offering valuable calibration and benchmarking for numerical models.

The coupled thermo-hydrological-mechanical-chemical (THMC) behavior of rock within an Excavation Damaged Zone (EDZ) is critical for the safety and long-term performance of a geological repository for high-level radioactive wastes. While many laboratory experiments have been conducted to investigate EDZ rocks, the flow and deformation characteristics resulting from anisotropic rock textures and microcrack distribution under triaxial loading and elevated temperatures remain poorly understood. Particularly, cracks at various scales serve as fast paths for fluid flow and solute transport and present as focal points of mechanical weakness, which complicate the coupled THMC processes in anisotropic EDZ rocks and challenge modeling predictions. In this study, a series of core-scale experiments was conducted on three granite samples under repository-relevant conditions. These rock samples were obtained from the Grimsel Underground Research Laboratory (URL), featured by anisotropic minerals (represented by bedding layers) and microcrack distributions and coarse cm-scale grain sizes. During the experiments, samples were subjected to an elevated temperature at 90 °C and different triaxial loading conditions either by radial (normal to bedding layers) or axial (parallel to bedding layers) compaction. Water was injected into the samples, and the rock permeability evolutions and effluent water chemistry were monitored closely. For intact samples, thermal expansion of minerals at 90 °C resulted in a large, 75

In deep geological repositories for high-level radioactive waste, the bentonite buffer undergoes thermal, hydrological, mechanical, and chemical (THMC) processes. This study aims to clarify the significance of the mechanical process in the THC evolution of bentonite by comparing a THC model with a coupled THMC model for a generic repository. The comparison shows that the impact of the mechanical process on hydrological and geochemical changes is significant during the early unsaturated stage but diminishes in the later stages once the bentonite is fully saturated. This suggests that considering mechanical processes may not be essential for long-term radionuclide migration models.

Bentonite buffer in the geological repository for high-level radioactive waste undergoes the heating from the waste package and hydration from the geological formation and goes through coupled thermo-hydro-mechanical-chemical (THMC) changes over the life span of a repository. For a better understanding of such process under higher temperatures, we report bench-scale laboratory experiments with heating up to 200◦C and the corresponding THM model. The bench-scale laboratory experiments included two test columns, with the non-heated control column undergoing only hydration, and a heated column experiencing both heating in the center up to 200◦C and hydration from a sand-clay boundary surrounding the column. During the experiment, we took frequent X-ray CT images to provide insight into the spatio-temporal evolution of THMC due to heating, hydration, bentonite swelling/compression. Based on the experiment setup, 2-D axisymmetric simulations were performed for the heated column and the mechanical changes were investigated in 3-D. The model first matched the temperature evolution with step-wise temperature boundary conditions at the heater and calibrated the thermal conductivity and specific heat of the materials. Then model interpreted the spatio-temporal distribution of bulk density by considering the combined effect of hydration, fluid pressure, and porosity change due to swelling/compression.

The disposal of high-level nuclear waste (HLW) has been one of the most challenging issues for nuclear energy utilization. In this study, we have explored the potential of extracting decay heat from HLW, taking advantage of recent advances in the technologies to utilize low-temperature geothermal resources for the co-generation of electricity and heat. Given that geothermal energy entails extracting heat from natural radioactivity within the Earth, we may consider that our approach is to augment it with an anthropogenic geothermal source. Our study—for the first time—introduces a conceptual model of a binary-cycle geothermal system powered by the heat produced by HLW. TOUGHREACT V3.32 software was used to model the heat transfer resulting from radioactive decay to the surrounding geological media. Our results demonstrate the feasibility of employing the organic Rankine cycle (ORC) to generate approximately 108 kWe per HLW canister 30 years after emplacement and a heat pump system to produce 81 kWth of high-potential heat per canister for HVAC purposes within the same timeframe. The proposed facility has the potential to produce carbon-free power while ensuring the safe disposal of radioactive waste and removing the bottleneck in the sustainable use of nuclear energy.

Abstract. More than a decade ago, the United States disposal program discontinued all research activities focused on the unsaturated fractured tuff formation at Yucca Mountain as the geologic disposal site for spent fuel and high-level radioactive waste. A new research and development (R&D) program was initiated to provide a sound technical basis for alternative disposal options across clay, crystalline, and salt rocks. The goals of this broad program were (and still are) to (1) increase confidence in the robustness of generic disposal concepts, (2) develop the science and engineering tools needed to support disposal concept implementation, and (3) conduct R&D on the direct disposal of existing dual-purpose (storage and transportation) canisters. Recognizing the benefits of international collaboration toward the common goal of safely and efficiently managing the back end of the nuclear fuel cycle, the program emphasized international cooperation as an effective strategy for sharing information and knowledge. In a multi-laboratory effort coordinated by Lawrence Berkeley National Laboratory, the United States Department of Energy (DOE) program established formal and informal cooperation partnerships with several international initiatives and institutions and developed a number of collaborative R&D activities in important research areas, such as engineered barrier integrity, near-field perturbations, radionuclide transport, performance assessment, and methods for characterization and monitoring of engineered and natural barriers. This presentation gives an overview of these R&D activities, with a specific focus on activities that improve our current understanding of the coupled thermal–hydrological–mechanical–chemical (THMC) processes occurring in engineered and natural barriers. We start with a brief review of selected international collaboration initiatives and then describe a few specific collaboration projects. We focus specifically on such studies that use experimental data sets provided by international research cooperation for joint modeling work to increase confidence in performance-relevant predictions of coupled processes. Overall, the focus on international collaboration has allowed deep engagement of US researchers with the international waste management R&D community in terms of best practices, new scientific advances, state-of-the-art simulation tools, new monitoring and performance confirmation approaches, and lessons learned. The joint R&D with international researchers, worldwide sharing of knowledge and experience, and access to relevant data and experiments from a variety of host rocks have helped our researchers to significantly improve their understanding of the current technical basis for disposal in a range of potential host rock environments. International collaboration also provides ample opportunity for training and educating junior staff that are well suited to move the United States disposal research program forward into the next decades, a promising avenue for developing a next-generation workforce of disposal scientists.

Under the thermodynamics framework, a thermo-hydro-mechanical coupling model was established by a multi-scale homogenization technique for fractured rock mass containing both small-scale arbitrarily-distributed microcracks and large-scale grouped-directionally-distributed fractures. The proposed model simultaneously considers the deformation evolution and permeability and thermal conductivity variations induced by the damage growth, compression closure and frictional sliding of microcracks and the friction sliding and shear dilatancy of fractures under coupled thermo-hydro-mechanical loading. Model numerical implementation was carried out by the linkage between the self-developed FEM-based micromechanical code and TOUGH2, and verified with analytical solutions on a solid column under thermal consolidation. Laboratory compression test data with thermal conductivity measurements on Beishan granite was also used to validate the proposed model, with good agreement between the predicted and measured results. Finally, the proposed model was utilized to numerically study the heat extraction responses of a large scale three-dimensional enhanced geothermal reservoir, demonstrating the importance of considering the thermo-hydro-mechanical coupling behavior and the variations of both small-scale microcracks and large-scale fractures for better understanding the heat transfer and water pressure evolution responses and deformation-induced reservoir permeability change during heat mining.

Abstract. Bentonite buffer in a geological repository will be simultaneously heated from decaying radioactive waste and hydrated from the surrounding host rock, triggering complex and coupled THMC (thermal–hydrological–mechanical–chemical) processes. Understanding the THMC behavior of bentonite-based engineered barrier system (EBS) is key to the evaluation and prediction of its long-term performance. Studies on the THMC process have been focused on conditions under 100 ∘C, as most design concepts impose a thermal limit of 100 ∘C in bentonite. Recently, studies under high-temperature conditions have been conducted to evaluate the possibility of raising the thermal limit and expanding the data/knowledge base to increase the confidence level. In this abstract, we present a series of bench-scale laboratory experiments at high temperatures (up to 200 ∘C) and the corresponding modeling work. Two sets of column tests were conducted, and each set consisted of two test columns: a control column undergoing only hydration (non-heated) and an experiment column experiencing both heating and hydration (heated). During the experiment, frequent X-ray computed tomography (CT) images were collected to provide a 3D visualization of the density distribution and present the spatiotemporal evolution of (1) hydration/dehydration, (2) clay swelling/shrinkage, (3) displacement, and (4) mineral precipitation. The two sets of tests differ with respect to several experimental conditions, such as bentonite type, compacted density and water content, water chemistry, and hydration pressure, but the important difference is that the first set used bentonite powder with a dry density of 1.28 g cm−3, whereas the second set used granulated bentonite (mixture of pellets and powder) with a dry density of 1.45–1.5 g cm−3. In both sets of experiments, a comprehensive post-dismantling characterization of bentonite samples was carried out after the column tests had been running for 1.5 years. Comparing non-heated and heated columns, the temperature gradient led to lower degree of homogenization of bentonite after bentonite became fully saturated; comparing the first and second sets, granulated and powdered bentonite exhibited drastically different hydration behavior. A THM model with a 2D axisymmetric grid system was used to interpret the data from the first set of tests. The model considers the combined impact of saturation, fluid pressure, and porosity change due to swelling/compression on the spatiotemporal distribution of bulk density and movement of the thermocouple modules. Observations from the tests help us understand the early perturbation of bentonite buffer under high temperature, and data from these tests improve the calibration of key constitutive hydrological and mechanical models and, therefore, enhance the modeling capability with respect to calculating the long-term evolution of bentonite buffer.

The behavior of heated bentonite buffer is critical for the security and long-term performance of a geological repository for high-level radioactive waste (HLW). While laboratory column experiments have been conducted to investigate compacted bentonite and coupled THMC (thermal-hydro-mechanical and chemical) processes for a moderate temperature range of up to 100 degrees C, data for a higher temperature range are limited. Understanding bentonite behavior and coupled THMC processes under higher temperatures (e.g., up to 200 degrees C) could allow for a more economic repository design and would expand the data and knowledge base for more reliable modeling. In this study, a bench-scale experiment was conducted in a compacted bentonite column experiencing both heating up to 200 degrees C in the center and hydration from a sand-clay boundary surrounding the column. During the experiment run for 1.5 years, frequent X-ray computed tomography (CT) scanning of bentonite provided insights into the spatiotemporal evolution of (1) hydration/dehydration, (2) clay swelling/shrinkage, (3) displacement, and (4) mineral precipitation. After the experiment, a comprehensive post-dismantling characterization of bentonite samples was conducted. Results showed that the bentonite hydration was axi-symmetrical despite the initial heterogeneity due to packing, confirming the ability of bentonite to seal fast flow/transport paths. Compared to a non-heated control experiment, the heated column showed greater CT density variations along the radial distance, indicating that homogenization of bentonite might be more difficult if a temperature gradient is maintained in the repository. Precipitation of an anhydrite layer occurred in the inner hot zone, pointing to potential concerns about salt precipitation causing canister corrosion. Ultimately, the experiments provided a high-resolution window into the strongly dynamic and coupled behavior of bentonite exposed to heating, hydration and swelling, which will be valuable for improving modeling of coupled processes, especially for the early state of a HLW repository.

This Special Issue “Clay Mineral Transformations after Bentonite/Clayrocks and Heater/Water Interactions from Lab and Large-Scale Tests” covers a broad range of relevant and interesting topics related to deep geological disposal of nuclear fuels and radioactive waste [...]