Geological Disposal Of Nuclear Waste Research Articles

Deep geological disposal of plastic waste: Pros and cons

Plastic production is growing, plastic waste is accumulating, and current waste management methods (recycling, incineration, etc.) are not yet able to solve this waste problem. Proposals and projects aimed at solving or facilitating the problem of plastic waste are therefore relevant. This article proposes a deep geological disposal method for plastic waste projects. The prototype for this project was the deep geological disposal of nuclear waste. Although plastic waste and nuclear waste are fundamentally different, the adequacy of this approach is justified by the fact that plastic waste, such as nuclear waste, has a a long degradation period and poses great danger to the environment, animals, and humans. The article is conceptual and examines the aspects necessary for the implementation of the project, such as the establishment of a special international UN agency for plastic waste management, specific sources of funding, giving plastic waste a monetary value, applying free market principles, and using existing potential opportunities for deep geological disposal of plastic waste. This article also discusses the proposed concept for finding more optimal options.

Retention of 226Ra in the sandy Opalinus Clay facies from the Mont Terri rock laboratory, Switzerland

The safety assessment of deep geological disposal of nuclear waste requires a sound understanding of radionuclide retention in the repository host rocks. Here, we investigate the retention behaviour of the safety relevant radionuclide 226Ra in the heterogeneous sandy facies of the Opalinus Clay (OPA-SF) at the Mont Terri underground rock laboratory. Various rock samples were selected from a drill core (BAD-1) located in the lower sandy facies, which were representative for its textural and mineralogical heterogeneity. The samples were carefully characterized with respect to mineralogy and chemistry using X-Ray diffraction and electron microscopy. The amounts of quartz, carbonates and clay minerals in the samples ranged between 58 wt.% – 65 wt.%, 13 wt.% – 35 wt.% and 5 wt.% – 25 wt.%, respectively, underpinning the mineralogical heterogeneity. Rare (Ba,Sr)-sulphate precipitates were identified in the clay matrix by electron microscopy. The uptake behaviour of Ra was evaluated using batch sorption experiments. Corresponding to the mineralogical variability of the rock samples, the measured distribution ratios Rd for the 226Ra retention varied between 20 L·kg-1 and 260 L·kg-1 after 120 days. The experimental results and thermodynamic modelling approaches suggest that various processes contribute to the uptake of 226Ra on different time scales: fast cation exchange and surface complexation reactions by clay minerals and the (slower) formation of solid solutions during recrystallization of (Ba,Sr)-sulphates as well as retention by carbonate minerals; the latter mechanism still requiring further investigation.

Atomistic Computer Simulations of Uranyl Adsorption on Hydrated Illite and Smectite Surfaces

A quantitative understanding of the molecular-scale mechanisms of radionuclide sorption on different clay minerals is crucial for the development and safe implementation of geological nuclear waste disposal technologies. We apply classical molecular dynamics (MD) computer simulations to study the adsorption of uranyl on the external basal surfaces of two typical clay models. In the illite model, negative charge is primarily localized in the tetrahedral sheets, while in the lower-charge smectite model, the isomorphic substitutions are introduced in the octahedral sheet. The comparison of atomic density distributions at the clay surfaces and adsorption-free energies profiles as a function of distance from these surfaces demonstrates that overall U behavior at the basal clay surface is quite similar for illite and smectite. Uranyl is sorbed as a mixture of outer-sphere aqua complexes [UO2(H2O)5]2+ and hydrolyzed aqua complexes [UO2(H2O)4–5OH]+ on both surfaces. The structural and compositional differences between the models do not greatly affect the uranyl’s nearest coordination environment and are mainly reflected in the specific localization and orientation of the uranyl ions at both surfaces and in the magnitude of the adsorption-free energies. The observed quantitative characteristics of uranyl interactions with illite and smectite surfaces will help to better understand U behavior during the sorption process on clay minerals for the entire range of mixed-layer illite–smectite structures. A comparison of two versions of the ClayFF force field in the simulations made it possible to more accurately and quantitatively evaluate some subtle features of the uranyl–clay interactions and to obtain a more precise composition of uranyl complex with the modified ClayFF force field (ClayFF-MOH).

Insight to the cesium entrapment by natural shale: implications to geological disposal of nuclear waste

ABSTRACT The knowledge of radionuclides sorption behavior of various site-specific heterogeneous geosystems is essential for developing different clean-up strategies and predicting the radiological risks for long term. The present study is aimed at understanding the mineralogical and textural characteristics of natural shale rock (sampled from Kachchh Basin of Gujarat, India) and its correlation with cesium (Cs) sorption behavior. The modal mineralogical composition, textural attributes, petrophysical properties, surface functionalities and morphology of shale were evaluated. Clay minerals (smectite, kaolinite and illite) constitute more than 40% of the shale along with the major component- quartz. A comprehensive Cs batch sorption study performed at varying experimental parameters demonstrated that the maximum Cs retention of shale was 34.80 ± 2.85 mg g−1 in the studied concentration range. Sorption equilibrium data agreed well with Freundlich isotherm, highlighting the heterogeneity of shale. Variation in solution pH (in the range of 6–10) during sorption demonstrated the buffering of solution in near neutral range indicating the role of cations (available in the interlayers of clay minerals) in Cs uptake. Alternatively, the presence of illite mineral in the shale with known frayed edge sites contributed to the irreversibility of sorption event. The present studies illustrate the potential of shale as host rock for its application in nuclear waste management.

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.

What the Flux? – Water-Rock-Microbe Interactions and Crustal Gases in the Deep Subsurface

The deep, dark fracture zones of the continental crust host a fascinating interplay between water, rocks, and microbes, resulting in the production and consumption of gases, including methane, volatile organic compounds (VOCs), and hydrogen. Various geological factors influence the formation and release of these crustal gases, including the local rock type with its concentration of radioactive elements and carbon, temperature, and the connectivity and dynamics of fracture systems with each other and to the surface. To understand the formation, accumulation, and release of crustal gases, methodologies of hydrogeochemistry, biogeochemistry, and isotope geochemistry can be employed. Sample collection from drill holes and mines, coupled with on-line monitoring of gas flux rate and composition, provides important data. Furthermore, the integration of molecular biological methods enhances our understanding of the water-rock-microbe interactions that shape the deep subsurface gas realm. Crustal gases have crucial implications for life in extreme environments, including those outside of our planet Earth, but potentially also pose significant challenges to drilling, mining, and their environmental impact. Moreover, crustal gases hold relevance for the energy sector, contributing to both the long-term safety of geological disposal of nuclear waste, carbon footprint of geothermal wells, and the exploration of hydrogen as a sustainable energy resource.

Laboratory Study of Effective Stress Coefficient for Saturated Claystone

Claystone is potentially the main rock formation for the deep geological disposal of high-level radioactive nuclear waste. A major factor that affects the deformation of the host medium is effective stress. Therefore, studying the effective stress principle of claystone is essential for a stability analysis of waste disposal facilities. Consolidated drained (CD) tests were carried out on claystone samples to study their effective stress principle in this paper. Firstly, two samples were saturated under a specified confining pressure and pore pressure for about one month. Secondly, the confining pressure and pore pressure were increased to a specified value simultaneously and then reverted to the previous stress state (the deformations of the samples were recorded during the whole process). Different incremental combinations of the confining pressure and pore pressure were tried at this step. Finally, the effective stress coefficients of the samples were obtained through a back analysis. Furthermore, some potential influencing factors (the neutral stress and loading rate) of the effective stress coefficient were also studied through additional tests. Some interesting results are worth mentioning: (1) the effective stress coefficient of claystone is close to one; (2) the neutral stress and loading rate may have little effect on the effective stress coefficient of claystone.

Application of machine learning for modeling brønsted-guggenheim-scatchard specific ion interaction theory (SIT) coefficients

Machine learning methodologies can provide insight into Brønsted-Guggenheim-Scatchard specific ion interaction theory (SIT) parameter values where experimental data availability may be limited. This study develops and executes machine learning frameworks to model the SIT interaction coefficient, ε. Key findings include successful estimations of ε via artificial neural networks using clustering and value prediction approaches. Applicability to other chemical parameters is also assessed briefly. Models developed here provide support for a use-case of machine learning in geologic nuclear waste disposal research applications, namely in predictions of chemical behaviors of high ionic strength solutions (i.e., subsurface brines).

Real-time permeability evolution of limestone under high temperature and triaxial stresses

In deep underground geological disposal of nuclear waste, geothermal energy development, underground coal gasification, geothermal resources development and other underground high temperature projects, they are all closely related to the permeability and thermal expansion characteristics of rocks in high temperature high-pressure real-time state. In this experiment, the real-time evolution of permeability of limestone under high-temperature and triaxial stresses condition was studied using the HTHP rock mechanics triaxial test system and compared with the permeability of limestone after high temperature cooling treatment. The test results show that: the effect of temperature on limestone permeability facilitation under high temperature cooling condition is significant, and the permeability increases by 1–2 orders of magnitude relative to the initial permeability for every 100 °C increase in the preheat temperature after 300 °C. Under high-temperature and high-pressure in real time condition, the difference values of permeability at different temperature points relative to room temperature always do not more than one order of magnitude. During the heating process at 500 °C, the permeability curves of the triaxial pressures range from 15 to 35 MPa are very different. Under the pressure condition of 15 MPa, the permeability curve increases monotonically and rapidly with the increasing temperature; When the pressure reaches 25 MPa pressure, there is a peak value area in the permeability curve; the permeability decreases monotonically with temperature when the pressure up to 35 MPa. Compared with the only thermal effect, the thermal- mechanical coupling effect has a significant inhibitory effect on limestone permeability development, and the permeability disparity between the two types of heat treatment rocks become more and more significant with increasing temperature and hydrostatic pressure.

An experimental methodology to assess the impact of desaturation on gas diffusion in clay based materials

The transport of gas in clay-based materials is dominated by diffusion. SCK CEN has studied and recorded an extensive data set of diffusion coefficients of gases in various saturated clays (Boom Clay, Opalinus Clay, Callovo-Oxfordian claystone and bentonites) used in Europe as host formations or in engineered barrier systems in the context of deep geological disposal of high and intermediate level nuclear wastes. However, partially saturated conditions may exist during the life cycle of a radioactive waste repository. The current study aims at establishing an experimental method to measure the diffusion coefficients of various gases in partially saturated clay-based materials by means of double through diffusion tests (cross diffusion of gases across the porous sample, driven by concentration gradients) and to assess the impact of desaturation on gas diffusivity. Historically, gas diffusion measurements have been performed under dissolved conditions with continuous water and dissolved gas flow around the clay sample. However, since the saturation degree of the sample has to be constant, a novel setup has been designed and optimized to operate under a pure gas phase under controlled relative humidity (RH). The experimental concept consists of cross diffusion of gas mixtures from two reservoirs at a fixed relative humidity on either side of the sample cell, namely, the upstream and downstream reservoirs. The RH is controlled using the same saturated salt solutions in both reservoirs. During the experiment, gases will diffuse from the upstream to the downstream reservoir. By measuring the change in gas composition in the downstream reservoir with a gas analyzer equipped with a mass spectrometer (which expresses the gas composition in terms of partial pressure), the gas flux and hence the diffusion coefficient is estimated by applying steady state Fick’s first law. The experimental methodology has been validated by performing an experiment on a sandy-clay sample of known petrophysical properties, at full saturation.

Temperature effect on non-Darcian flow in low-permeability porous media

The Darcy’s law specifies a linear relation between the Darcy velocity of a fluid flow and the pressure gradient that drives the flow. However, in low-permeability porous media, the velocity of a fluid flow exhibits a nonlinear dependence on the imposed pressure gradient, which is referred to as the non-Darcian flow behavior. Temperature has been postulated to affect the threshold pressure gradient of a non-Darcian flow; however, the supporting data is very limited due to the challenges in conducting flow experiments in low-permeability porous media. In this study we for the first time report a systematic measurement of the threshold pressure gradient under various permeabilities and temperatures. The results show that a higher temperature leads to a lower threshold pressure gradient under the same permeability and a faster reduction of the threshold pressure gradient with increasing permeability. The experimental data are fitted to a two-parameter model we previously developed to determine the parameters, h0 and a, which characterize the interfacial fluid-solid interactions and the transition between the Darcy and non-Darcian regimes. Both h0 and a decrease with an increasing temperature. The lower value of h0 under a higher temperature leads to the faster decline of threshold pressure gradient with increasing permeability. This is the first comprehensive experimental study that measured the threshold pressure gradient under multiple different permeabilities and temperatures, which generated the first laboratory dataset of this kind. The experimental data and the model constrained here allow for systematic predictions of both Darcy and non-Darcian flows for a general set of low-permeability porous media under various temperatures and pressure gradients, which have important applications to geological disposal of nuclear waste, hydrocarbon extraction from shale, and contaminant remediation in clay-rich formations.

A Review of Techniques for Measuring the Biot Coefficient and Other Effective Stress Parameters for Fluid-Saturated Rocks

Abstract Predicting the behavior of a saturated rock with variations in pore fluid pressure during geo-energy production and storage, deep geological disposal of nuclear wastes, etc. with skeletal mechanical behavior in the linear elastic range is carried out using the isothermal theory of poroelasticity that incorporates Biot's effective stress principle. For conditions that are not within linear elasticity, other effective stress coefficients are used. Several experimental methods for determining Biot's and other effective stress coefficients have been documented in the literature. The objective of this study is to review the fundamentals of these techniques, their advantages and disadvantages, and to include several case studies. Current techniques for Biot's coefficient are based on different premises: jacketed and unjacketed bulk moduli or compressibility values; volume changes of the bulk and pore fluid from a drained triaxial test on a saturated sample; isotropic-isochoric compression tests on a saturated sample; matching volumetric strains for dry and saturated samples; estimation of the Biot coefficient from other poroelastic parameters; and approximation of the jacketed bulk modulus from ultrasonic wave velocities and/or unjacketed bulk modulus from the mineralogical compositions. Other effective stress coefficients are based on matching failure envelopes for dry and saturated samples and variations of rock properties (such as volumetric strain, permeability, and ultrasonic wave velocities) with respect to confining stress and pore pressure. This article discusses variations in Biot's and other effective stress coefficients produced using the different techniques and how factors such as pore geometry, test conditions, stress path, and test temperature affect the coefficients.

Bentonite swelling into a void under suction or water flow

In the context of the deep geological disposal of nuclear waste and to improve the understanding of the homogenisation process of bentonite barriers, in particular with regard to the filling of technological voids, tests were performed with compacted FEBEX bentonite samples hydrated under limited axial swelling conditions. The samples were saturated from the top surface using the vapour transfer technique (imposing suctions of 6 and 0.5 MPa), or with deionised water injected at a low flow rate either from the bottom surface or from the gap on top. The final water content of the samples saturated via vapour transfer was related to the suction imposed during the tests according to the water retention curve. Thus, the final water content of the samples tested under suction 6 MPa was lower and not enough to allow sufficient bentonite swelling to close the gap. In contrast, the samples saturated under suction 0.5 MPa reached higher water contents and were able to fill the gap before the equilibrium water content had been reached. In the tests performed with liquid water supplied through the gap, the samples were able to swell easily into the open void and the gap closed earlier than when hydration took place from the opposite end. In all the tests, the final water content of the bentonite was higher and the dry density lower towards the hydration surface. These gradients were more remarkable and persistent as the initial strain was larger. No completely homogeneous density or water content distribution was observed in any of the tests. These changes were also reflected in the pore size distribution. The pores of size larger than the upper limit of mercury intrusion porosimetry (~ 550 µm) were also quantified, which allowed a better representation of the pore size distribution of the more swollen samples. Although the void ratio corresponding to pores smaller than 200 nm (em) was initially higher, over time the volume of macropores (eM) increased more, giving place to an overall decrease in the em/eM ratio, which tended to be constant along the samples in the longest tests. In the first stages of hydration, the macropore void ratio and size considerably increased close to the gap in the tests with saturation from it.

Sensitivity analysis of generic deep geologic repository with focus on spatial heterogeneity induced by stochastic fracture network generation

Geologic Disposal Safety Assessment Framework is a state-of-the-art simulation software toolkit for probabilistic post-closure performance assessment of systems for deep geologic disposal of nuclear waste developed by the United States Department of Energy. This paper presents a generic reference case and shows how it is being used to develop and demonstrate performance assessment methods within the Geologic Disposal Safety Assessment Framework that mitigate some of the challenges posed by high uncertainty and limited computational resources. Variance-based global sensitivity analysis is applied to assess the effects of spatial heterogeneity using graph-based summary measures for scalar and time-varying quantities of interest. Behavior of the system with respect to spatial heterogeneity is further investigated using ratios of water fluxes. This analysis shows that spatial heterogeneity is a dominant uncertainty in predictions of repository performance which can be identified in global sensitivity analysis using proxy variables derived from graph descriptions of discrete fracture networks. New quantities of interest defined using water fluxes proved useful for better understanding overall system behavior.

Co-transport of less soluble accessory minerals during expansion of compacted bentonite and its impacts on erosion

Compacted bentonite is considered as an engineered barrier in geological disposal of nuclear waste in fractured rocks. One concern with this concept is that the expansion of bentonite into fractures as the system saturates may lead to its erosion by flowing water. The loss of bentonite through erosion over long times may compromise the barrier. Commercial bentonite contains less soluble accessory minerals and experimental observations have shown that these are co-transported with clay minerals during expansion and form a mineral film at the eroding boundary. Quantifying the effects of the mineral film on the erosion rate is important for the assessment of the long-term sealing capacity of bentonite barriers. The paper presents a model for the extrusion of bentonite into fractures that considers key processes governing the co-transport of less soluble accessory minerals and explains why and where the mineral film forms and how it affects the erosion rate.The model is tested against two sets of published experimental data. It is demonstrated that the model predicts the phenomena observed in experiments: the build-up of a mineral film near the extrusion boundary; and the progressive growth of the mineral film thickness. Importantly, the model allows for investigating the effects of key environmental and material parameters – water chemistry, filtration coefficient, and mineral fraction – on the expansion and erosion. The results of such investigations are presented. It is shown that higher water ion concentration, larger filtration coefficient, and larger fraction of accessory minerals retard the swelling and extrusion of bentonite, thus reducing the overall erodibility. It is further shown that the presence of less soluble accessory minerals may reduce the loss of bentonite by a factor of 2 compared to a pure bentonite.

Experimental and Numerical Investigation of the Flow Behaviour of Fractured Granite under Extreme Temperature and Pressure Conditions

As a result of negligible connected porosity—and thus, minimal matrix permeability—the fluid-transport characteristics of crystalline rocks are strongly influenced by the fractures at all scales. Understanding the flow behaviour of fractured rock under extreme stress and temperature conditions is essential for safe and effective deep geo-engineering applications, such as deep geothermal recovery, geological nuclear waste disposal, oil and gas extraction, geological storage and deep mining operations. Therefore, this study aims to investigate the flow characteristics of mechanically fractured Australian Strathbogie granite under a wide range of stress (confining pressures 1–80 MPa) and temperature conditions (20 °C to 350 °C). The study utilised a sophisticated high-temperature, high-pressure tri-axial setup capable of simulating extreme geological conditions, followed by a numerical simulation. According to the experimental results, a linear increment in the steady-state flow rate was observed, with increased injection pressure for the experimental conditions considered. Therefore, linear laminar Darcy flow was considered, and the fracture permeability was calculated using the cubic law. It was found that stress and temperature strongly depend on the flow of fluid through fractures. The steady-state flow rate decreased exponentially with the increase in normal stress, showcasing fracture shrinkage with an increment in effective stress. With regard to permeability through the fractures, increasing temperature was found to cause an initial reduction in fracture permeability due to an increased interlock effect (induced by thermal overclosure), followed by increments because of the thermally induced damage. Furthermore, the increasing temperature caused significant non-linear increments in the fluid flow rates due to the associated viscosity and density reduction in water. Considering the laboratory-scale flow-through exercises, a fully coupled numerical model that can predict hydro–thermo–mechanical variations in the reservoir rocks was developed using the COMSOL Multiphysics simulator. The developed model was calibrated, utilising the temperature- and pressure-dependent properties of granite rocks and fluid (water); was validated against the experimental results; and was used to predict the permeability, pressure development and strain of rock samples under extreme conditions, which were difficult to achieve in the laboratory.

Effect of spatial variability of creep rock on the stability of a deep double-lined drift

This work aims at investigating the effect of aleatoric uncertainty of creep rock properties on the stability of an underground structure. This uncertainty relates to the spatial variability of the mechanical parameters representing the time-dependent behavior of geological rock formation due to the change in its mineralogy. The chosen methodology consists of representing the aleatoric uncertainty of rock properties by random fields, written as correlation functions with respect to the spatial correlation length. The adaptation of the well-known Expansion Optimal Linear Estimationmethod (EOLE) is performed to account for the cross-correlation of the random fields of the viscoplastic parameters of the host rock. Then, the Kriging-based reliability analysis is undertaken with respect to the discretized random fields, which allows elucidating the effect of spatial variability. As an application, the proposed approach is chosen to study the stability in the long-term of a deep double-lined drift within the geological disposal facilities (Cigeo project) conducted by the French National Radioactive Waste Management Agency (Andra). The drift will be excavated in Callovo-Oxfordian (COx) claystone (if the Cigeo project is licensed), considered as a potential host rock for the deep geological nuclear waste disposal in France. The results show that the chosen Kriging metamodel for the reliability analysis can be appropriate for the case of high correlation length represented by a moderate number of random variables (up to about 50) after the discretization of random fields. Further, the consideration of aleatoric uncertainty exhibits a lower probability of exceedance in comparison with the case where spatial variability is ignored. Still, more investigations need to be conducted in the future to conclude this observation.

Investigations of effect of underground excavations using hydrologic and tracer transport modeling

Understanding the effect of underground excavations and barrier capability of the near field are relevant to the geologic disposal of nuclear waste. For a repository in crystalline host rock characterization of the naturally fractured rock and the excavated disturbed zone are of importance to the study of migration of radionuclides. Excavations could weaken the repository components that are part of the engineered barrier system. Permeable excavated disturbed zones could also develop around the repository walls that could create conduits for radionuclides away from the repository. Experimental hydrology and tracer transport data from the Mizunami Underground Research Laboratory in Central Japan have been used to develop a site-scale model for the study area. Upscaled permeability and porosity data of a previously developed discrete fracture network model were used. In this study development and utilization of the model is presented. The model was used to reproduce hydrology and non-reactive transport in the modeling area. The modeling work includes prediction of inflow of water into the research tunnel during excavation of tunnel parts and related draw-down. The modeling work also includes prediction of pressure recovery and changes in tracer distribution as a result of water filling of a tunnel chamber. Modeling results were successfully compared with the project hydrology and tracer concentration histories. Modeling results of inflow and recovery were complementary and together with fracture characterization combined to give a better constraint on the system. The study provides experimental and modeling methods to study the effect of excavations in an underground research laboratory.

Effects of Corrosion Products Deposited on 304 Stainless Steel on Reduction of Se (IV/VI) in Simulated Groundwater.

Selenium (Se) is a key mobile fission product in the geological disposal of nuclear waste. It is necessary to analyze the reductive deposition behavior of iron-based materials to Se(IV) and Se(VI) in groundwater. In the present work, the corrosion behavior of 304 stainless steel in simulated groundwater (SG) and the effects of corrosion products on the dissolution of Se were investigated by electrochemical and immersion tests. Experimental results revealed that passivation films formed on 304 stainless-steel samples were destroyed by polarization measurements, forming corrosion products consisting of Fe(II) compounds, such as Fe3O4 and FeO. Corrosion products deposited on the surface of steel samples previously treated by polarization measurements in SG + CaCl2/Na2CO3/Na2SiO3 solutions effectively reduced soluble Se(IV) and Se(VI) during immersion tests, depositing FeSe2 on sample surfaces.

Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering. Under the action of groundwater solution infiltration and external stress, the hydro-mechanical (HM) behaviour of bentonite-sand mixtures, i.e. the swelling characteristics and permeability, will change. Once the anti-seepage and filtration effect is weakened or lost, the pollutants will spread to the biosphere. Therefore, it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechano-chemical (MC) effect and to establish corresponding prediction model. For this reason, swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%, 70% and 90% sand contents, the compression tests are carried out on saturated samples, and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi's one-dimensional consolidation theory. The concepts of true effective stress p e , montmorillonite void ratio e m and critical sand content α s are introduced to determine the e m - p e relationship and finally the k - e m relationship of bentonite-sand mixtures. It is found that when the sand content α ≤ α s , the e m - p e relationship of the mixture is linear and independent of the salt solution concentration, and when α > α s , the e m - p e relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress p esα as the intersection. In addition, the e m - k relationship also shows the linear trend when α ≤ α s , and the slope of the line increases with the increase of the salt solution concentration. When α > α s , the k - e m relationship will deviate from the linear relationship. Moreover, the larger the sand content is, the farther the deviation is. On the basis of summing the regularity, a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed. By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures, the predictive model is verified. The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.