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This study investigates the enhancement of disposal efficiency for deep geological repositories (DGRs) based on three design factors: decay heat optimization, increased thermal limit of the buffer, and double-layer concept using coupled thermo-hydro-mechanical (THM) numerical simulations. Decay heat optimization is achieved by iteratively emplacing spent nuclear fuels having the maximum and minimum decay heat in a canister. Disposal areas can be reduced by 20 % to 40 % compared to the current reference disposal system in Korea (KRS+) in accordance with the combinations of the three design factors, alleviating challenges in site selection for the DGR. This study additionally identifies an optimal layer spacing of 500 m for the double-layer concept in the viewpoint of the buffer temperature, where thermal interaction between the upper and lower layers nearly disappears. However, determining the ultimate disposal and layer spacing requires engineering judgement, considering not only the thermal performance of the DGR but also various factors such as cost and difficulties of the construction and rock mass stability. DGRs designed with an increased thermal limit of the buffer poses a greater probability of rock mass failure around disposal tunnels and deposition holes due to elevated thermal stresses. Densely arranged heat sources for the DGRs with enhanced disposal efficiency lead to larger temperature increase even at the far-field scale, raising a possibility of thermally driven fracture shear activation with associated hydraulic, mechanical, and seismic changes.

Bentonite is utilized as a barrier material in high-level nuclear waste repositories due to its superior low permeability and swelling properties. However, its engineering properties are influenced by the chemical composition of the infiltrating pore water during operation. Understanding the effect of salt solution on the mechanical properties of bentonite is crucial for evaluating the performance of buffer and backfill barriers in deep geological repositories for nuclear waste. In this study, various concentrations and types of salt solutions were used to treat Na-bentonite samples, which were then subjected to free swell test, no loading swelling ratio test, Atterberg limits test, compaction test, and analysis of the content of exchangeable cations. The results showed that the content of counterbalance cations changed significantly after the addition of salt solution, and the decrease in free swelling rate increased gradually with the increase of solution concentration. The effect of different types of salt solutions on swelling was primarily determined by the type of cation that governs charge level and hydration capacity. The inhibition of the free swelling rate was stronger for high-concentration low-valence salt solution than that for low-concentration high-valence one. Bentonites undergoing cation exchange exhibited a decreased plasticity index, a decreased maximum dry density, and an increased optimum water content. This was mainly due to the cation exchange that occurred between bentonite layers under the action of the salt solution, which affected the crystal layer structure, double electric layer structure, and intergranular stress. Finally, the van’t Hoff equation was used to quantitatively characterize the differences in swelling in the test results. [ABSTRACT FROM AUTHOR]

In the search for a repository site for high-level radioactive waste in Germany, the perception of safety and trust in the actors are central to public acceptance. In communicating safety, methods of safety assessment and the role of uncertainties need to be addressed. Given the complexity of the issue, there is a need for indicators that are suitable both for assessing the long-term safety of repositories and for communicating with the general public. Similarly, there is a requirement to communicate uncertainties in an accessible manner. The TRANSENS project provides basic research in nuclear waste management (NWM) and utilizes a transdisciplinary approach: Non-experts who are not directly affected by the site selection process and who have no stated interest in NWM are involved in the research process, as are practice actors. A series of four transdisciplinary workshops was specifically designed to explore the perspectives of individuals with a high level of disciplinary knowledge but no system knowledge of NWM. Participants were selected from doctoral students in science and technology who had no prior knowledge in this area. Two of these workshops address the questions stated above and are presented here. The article describes the considerations underlying the workshop planning and implementation phases, and the content developed in the workshops on indicator selection and visualisation of uncertainties. The participants compiled a list of desirable indicator properties, which showed a high degree of congruence with the relevant literature. A proposal for a database to collect, administer and assess uncertainties shows similarities with the approach followed by the German implementer and complements it with an interactive visualisation. Transdisciplinary work is resource-intensive and its use in a research context must be carefully considered for each individual application. A transdisciplinary approach was successfully used for the purposes of method validation, method optimisation and the development of disciplinary impulses. An application of transdisciplinary approaches for optimising the Safety Case of nuclear repositories is feasible. [ABSTRACT FROM AUTHOR]

As a solution to the problem of spent nuclear fuels (SNFs), the disposal of SNF has gained attention from nations using nuclear energy because of hazards posed to the ecosystem. Among many proposed solutions, the most promising method is to dispose of SNF in a deep geological repository (DGR) which utilizes the multi-barrier concept developed by Finland and Sweden. Here, a new fully-coupled Thermal-Hydraulic (TH) code HADES (High-level rAdionuclide Disposal Evaluation Simulator) is developed using the MOOSE framework. This new code suggests basic numerical tools, such as a non-linear solver and finite element discretization, to assess the safety performance of disposal systems. The new TH code considered various TH behavior using Richards' flow approach, assuming gas pressure is constant. The HADES showed promising results when it was compared to various TH codes validated from DECOVAELX-THMC projects. When the single-canister model was utilized to estimate the TH behavior of the Korean Reference disposal System, although it showed significant saturation reduction due to the evaporation of water, the temperature was maintained under the thermal criteria limit, which is 100 °C. In addition, the new code estimated temperature and degree of saturation of the multi-canisters model, considering two or three canisters, it showed a slightly lower temperature, 5 °C, than the single-canister model. From these results, the following are concluded: (1) the new TH code contribute to an additional integrity by estimating TH behavior of KRS; (2) however, due to limitations in single-canister simulation, it is recommended to use multi-canisters simulation to estimate TH behavior accurately. Therefore, this model is anticipated not only to help licensing applications and estimation of various multi-physics phenomena and multi-canister at the disposal site.

Copper canisters are commonly employed to contain HLW for the long-term, making it crucial to understand how corrosion affects the canister. This study conducted a comparative analysis of two widely used calculation methods for modeling canister corrosion within a 3-D DGR domain. The first method, termed transport-limited corrosion, assumes an immediate sulfide-copper reaction and has been traditionally used due to its conservative nature. The second method, known as the potential-limited corrosion, considers coupled redox reactions at the canister surface and computes corrosion rates through anodic current density. From the results, we found that the edge of the canister geometry and the omission of electrochemical kinetics impose critical limitations with the transport-limited corrosion method. These limitations include the singularity problem, excessive sensitivity to the curvature of the canister's edge, and an inability to evaluate the distribution of corrosion rate over the canister surface as a function of the sulfide concentration. On the other hand, the potential-limited corrosion method avoided the limitations found in the other method. Since the factors relating to these limitations are critical to the design and optimization of the copper disposal canister, careful consideration when selecting appropriate calculation methods for corrosion will be required.

Because of its swelling capacity, compacted bentonite clay is a suitable buffer material in deep geological repositories for high-level nuclear waste. However, this only applies if the swelling capacity is maintained. Accordingly, bentonites have to be stable to changing temperature, humidity, infiltrating fluids or microbial activity. In batch experiments, we investigated combined microbial and thermo-hydro-geochemical effects on the swelling capacity of uncompacted bentonite MX-80. Bentonite was exposed to fluids of different ionic strength and the bacterium Stenotrophomonas bentonitica. Bacterial growth was monitored by counting colony-forming units while the swelling capacity of bentonite was evaluated using in situ XRD at varied temperatures and humidity. The presence of bentonite prolonged the survival of S. bentonitica. However, electron microscopy, XRD and ICP-OES analyses showed neither an interaction of S. bentonitica with bentonite, nor significant changes in the swelling capacity or element composition. The swelling capacity and diffraction peak intensity were, however, strongly reduced by the ionic strength of the fluid and the exposure time. The study highlights that bentonite is affected by thermo-hydro-geochemical and microbial processes to different degrees and that the complexity of different co-occurring factors in potential nuclear waste repositories is important to consider in safety assessments. [ABSTRACT FROM AUTHOR]

Natural analogues are systems that have evolved over geological timescales with features similar to one or several components of a deep geological repository (DGR). Natural analogues complement short-duration laboratory studies since they are existing reflections of many long-term processes that might affect the performance of a repository. Mathematical models are often used for the post-closure safety assessment of a DGR. Confidence in the models' predictions is enhanced when the models successfully simulate the past evolution of a natural analogue. This paper summarizes the Canadian Nuclear Safety Commission's (CNSC's) recent research on natural analogues to inform on (1) glacial erosion, (2) engineered barrier system, and (3) uranium reactive transport in the context of DGRs for radioactive wastes. Glaciation and its erosion are prominent factors impacting the performance of future DGRs at high latitudes in the northern hemisphere. The authors have reviewed the field data from the Greenland Analogue Project, developed a conceptual and mathematical model for the simulation of the thermal conditions within the Greenland ice sheet, as well as the thermal-hydraulic conditions at its base and the ice sheet velocity, and eventually estimated the erosion rate at the site. The Cigar Lake Analogue demonstrates the long-term radionuclide containment capability of the illite clay zone enveloping the ore body, serving as an analogy to the engineered clay barriers. The CNSC and University of Ottawa analyzed 129I in the Cigar Lake core samples, and modeled and correlated the diffusion-dominated transport of radionuclides over the geological evolution of the Cigar Lake deposit. The results provide information on the mobility of fission products and significant radionuclides in conditions analogous to the source, engineered barriers, and near-field host rock of a DGR. The reactive transport and geochemistry of the Kiggavik-Andrew Lake uranium deposit mineralization and remobilization was another natural uranium deposit analogue studied by the CNSC. A reactive transport model was established according to the conceptualized geochemical processes and run under specified boundary and initial conditions to validate the geochemical processes. The geometry, timing, geochemistry, and fluid composition were used as model constraints. [ABSTRACT FROM AUTHOR]

Bentonite is an integral part of the engineered barrier system (EBS) in deep geological repositories (DGR) for nuclear waste, but its indigenous microorganisms may jeopardize long-term EBS integrity. To predict microbial activity in DGRs, it is essential to understand microbial reactions to the early hot phase of DGR evolution. Two bentonites (BCV and MX-80) with varied bentonite/water ratios and saturation levels (compacted to 1600 kg.m− 3 dry density/powder/suspension), were subjected to heat (90–150 °C) and irradiation (0.4 Gy.h− 1) in the long-term experiments (up to 18 months). Molecular-genetic, microscopic, and cultivation-based techniques assessed microbial survivability. Exposure to 90 °C and 150 °C notably diminished microbial viability, irrespective of bentonite form, with negligible impacts from irradiation or sample type compared to temperature. Bentonite powder samples exhibited microbial recovery after 90 °C heating for up to 6 months but not 12 months in most cases; exposure to 150 °C had an even stronger effect. Further long-term experiments at additional temperatures combined with the mathematical prediction of temperature evolution in DGR are recommended to validate the possible evolution and spatial distribution of microbially depleted zones in bentonite buffer around the waste canisters and refine predictions of microbial effects over time in the DGR. [ABSTRACT FROM AUTHOR]

AbstractTemperature distribution plays a crucial role in the safety performance assessment and thermal dimensioning design in a deep geological repository for disposing high-level waste. In this study, a two-dimensional axisymmetric model of a single container for heat transfer was created. The fully analytical solution to temperature distribution in the repository was derived by utilizing the methods of separation of variables, impulse theorem, and Fourier transform.The fully analytical solution was validated by comparing with the existing semi-analytical solution and line heat source solution. The temperature change in the near field around the container was analyzed using the present solution, and the influences of different parameters on the container surface temperature were investigated. Furthermore, the proposed fully analytical solution was used to predict the results of the in situ test.The findings indicate that the temperature in the buffer layer rapidly increases and reaches its peak value within the first 2 years, then gradually decreases thereafter with time. The thickness of the bentonite pellet layer had a greater effect on the container surface temperature than that of the bentonite block layer. A comparison between the fully analytical solution and the results of the in situ heating test demonstrated that the proposed fully analytical solution can accurately predict the temperature variations in the in situ heating test. [ABSTRACT FROM AUTHOR]

The Canadian deep geological repository (DGR) design consists of copper coated used fuel containers (UFCs) placed within a highly compacted bentonite (HCB) buffer surrounded by a suitable host rock. Although the copper is thermodynamically stable in oxygen-free environments, it is potentially susceptible to microbiologically influenced corrosion from bisulfide (HS-). Therefore, understanding HS- corrosion is important to ensure long-term performance of UFCs. Various reactions in the bentonite barrier of the DGR can affect HS- transport through the HCB and therefore the extent of copper corrosion caused by HS-. Since HS- transport and reactive processes are interconnected, numerical models are required to assess the complex HS- reactive transport dynamics and quantify the influence of reactive processes on HS- transport and corrosion. In this paper, various HS- transport models were coupled with (i) a key geochemical reaction between HS- and iron (i.e., simulating HS- retardation due to iron sulfide formation) or (ii) HS- adsorption. Since HS- is an anion, anion exclusion was also explored. Valuable insight was obtained through validation, comparison, and sensitivity analyses of these models. A comparison between experimental and modelled HS- transport dynamics showed that HS- is being retained by the bentonite due to reactive processes and anion exclusion is occurring. Lastly, HS- transport was simulated for the entire DGR lifespan and was found to be delayed (≈50–800 years) due to FeS formation or HS- adsorption. However, these predicted HS- diffusion delays are relatively short in a DGR lifespan (i.e., 1 million years) and do not impact long-term HS- corrosion.

In the search for a repository site for high-level radioactive waste in Germany, the perception of safety and trust in the actors are central to public acceptance. In communicating safety, methods of safety assessment and the role of uncertainties need to be addressed. Given the complexity of the issue, there is a need for indicators that are suitable both for assessing the long-term safety of repositories and for communicating with the general public. Similarly, there is a requirement to communicate uncertainties in an accessible manner. The TRANSENS project provides basic research in nuclear waste management (NWM) and utilizes a transdisciplinary approach: Non-experts who are not directly affected by the site selection process and who have no stated interest in NWM are involved in the research process, as are practice actors. A series of four transdisciplinary workshops was specifically designed to explore the perspectives of individuals with a high level of disciplinary knowledge but no system knowledge of NWM. Participants were selected from doctoral students in science and technology who had no prior knowledge in this area. Two of these workshops address the questions stated above and are presented here. The article describes the considerations underlying the workshop planning and implementation phases, and the content developed in the workshops on indicator selection and visualisation of uncertainties. The participants compiled a list of desirable indicator properties, which showed a high degree of congruence with the relevant literature. A proposal for a database to collect, administer and assess uncertainties shows similarities with the approach followed by the German implementer and complements it with an interactive visualisation. Transdisciplinary work is resource-intensive and its use in a research context must be carefully considered for each individual application. A transdisciplinary approach was successfully used for the purposes of method validation, method optimisation and the development of disciplinary impulses. An application of transdisciplinary approaches for optimising the Safety Case of nuclear repositories is feasible.

Currently, the production of radioactive waste from nuclear industries is increasing, leading to the development of reliable containment strategies. The deep geological repository (DGR) concept has emerged as a suitable storage solution, involving the underground emplacement of nuclear waste within stable geological formations. Bentonite clay, known for its exceptional properties, serves as a critical artificial barrier in the DGR system. Recent studies have suggested the stability of bentonite within DGR relevant conditions, indicating its potential to enhance the long-term safety performance of the repository. On the other hand, due to its high resistance to corrosion, copper is one of the most studied reference materials for canisters. This review provides a comprehensive perspective on the influence of nuclear waste conditions on the characteristics and properties of DGR engineered barriers. This paper outlines how evolving physico-chemical parameters (e.g., temperature, radiation) in a nuclear repository may impact these barriers over the lifespan of a repository and emphasizes the significance of understanding the impact of microbial processes, especially in the event of radionuclide leakage (e.g., U, Se) or canister corrosion. Therefore, this review aims to address the long-term safety of future DGRs, which is critical given the complexity of such future systems. [ABSTRACT FROM AUTHOR]

AbstractThe Brazilian nuclear program includes nuclear power plants fueled by enriched uranium that require a final disposal solution. A viable option for the underground disposal of spent nuclear fuel (SNF) is a deep geological repository (DGR). This study simulates the thermo-hydro-mechanical (T-H-M) performance of rock media subjected to the radiogenic heat loads from spent nuclear fuel in a deep geological repository using a multibarrier concept. At maximum temperatures as high as 131.79 °C and vertical stresses greater than 55 MPa between fuel containers the isolation buffer effectively reduced the temperature and stress between the containers and rock media. The behavior of gas and liquid phases showed to be more complex than observed in strictly thermal analysis, and the prescribed separation between canisters, originally aimed to mitigate radionuclides migration, showed to be as effective for long-term stability of the DGR. [ABSTRACT FROM AUTHOR]

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The safety and robustness of Deep Geological Repositories (DGRs) are of paramount importance for the long-term management of spent nuclear fuel from electricity generation. The introduction of a multi-barrier system, which includes the host rock formation and an engineered barrier system (including the bentonite buffer), has been a widely used approach to ensure the safety of DGRs. The assessment of the long-term safety of DGRs involves the mathematical modeling of the coupled thermal–hydraulic–mechanical–chemical (THMC) processes that occur in the near-field of the DGRs and their impact on the behaviour and engineering properties of the bentonite buffer. This paper presents a review of the THMC-coupled processes that arise in the bentonite buffer as well as a mathematical model governing such coupled processes. The model is verified against existing analytical solutions and validated against measured data of a thermal diffusion experiment in a sand bentonite column. Also, scoping analyses were performed to assess the influence of coupled THM processes on solute transport in clayrocks. The results of the numerical model closely matched those of the analytical solutions and experimental data demonstrating the capability of the provided mathematical model as well as the numerical approach in enhancing our comprehension of DGR behaviour. This enhanced comprehension will be valuable for safety prediction and assessment in the context of DGRs. The work presented in this paper is part of the Canadian Nuclear Safety Commission's (CNSC) regulatory research to gain independent knowledge on the safety of the geological disposal of radioactive waste. [ABSTRACT FROM AUTHOR]

A typical deep geological repository (DGR) design consists of a multi-barrier system, including the natural host rock and the engineered barrier system. Understanding the swelling behavior of bentonite-based sealing materials (BBSM), as a candidate material for the engineered barrier system, is crucial for DGR's long-term safety. In this study, a hydromechanical (HM) column-type test was designed to model the hydration of BBSM from the underground water and determine the resulting swelling pressure in vertical and radial directions. Five hydration tests were carried out on identical compacted samples of 70% bentonite and 30% sand (70-30 bentonite-sand) mixtures with a dry density of 1.65 g/cm3 for varied durations of hydration, between 1 day and 120 days. The experiments were performed parallel to the compaction direction. Following each HM column-type test, the advancement of the wetting front was determined for each test. After 120 days, 56,339 mm3 of water infiltrated the sample and the wetting front reached over 50% of the sample height. The evolution of axial swelling pressure revealed an initial increase in swelling pressure with time in all tests, followed by a reduction in the rate at later times. After early stages of swelling, radial sensors showed an increase in swelling pressure. After 120 days, the radial pressure sensor closest to the hydration front showed 52% more radial pressure than the axial swelling pressure. [ABSTRACT FROM AUTHOR]

Over the past decade, the Nuclear Waste Management Organization has conducted a thorough proof test plan (PTP) to evaluate their novel copper‐coated used fuel container and bentonite buffer box underground emplacement concept for use in a deep geological repository (DGR). This PTP has included the development of new technologies as well as feasibility studies related to engineered barrier production, underground emplacement, and safety assessment of the technologies within a DGR. Although the PTP was winding down in 2022, many work packages continue, particularly those associated with the evaluation of the corrosion performance of the used fuel container, the Nuclear Waste Management Organization (NWMO)'s copper corrosion allowance. This work evaluates the extent of corrosion that may result from oxic‐, radiolytic‐, anoxic‐, and sulfide‐induced corrosion which may occur in the Canadian DGR. Particular attention is paid to assessing the potential for localized corrosion phenomena across each project. This article provides an overview of these work packages which support a lifetime corrosion expectation of 270 µm and an extreme upper bound of corrosion of 1204 µm over one million years. [ABSTRACT FROM AUTHOR]

A 1‐D reactive‐transport model has been developed to describe the production and transport of sulfide in a deep geological repository in crystalline rock and the subsequent corrosion of the copper canister. The model accounts for various processes, including: (i) the microbial reduction of sulfate by organotrophic and lithotrophic sulfate‐reducing bacteria, (ii) the supply of sulfate from both the ground water and from the dissolution of gypsum present as an accessory mineral in the bentonite buffer, (iii) diffusive transport of reactants and products, and (iv) sequestration of a fraction of the microbially produced sulfide by precipitation as mackinawite. The results of a base case simulation and of sensitivity analyses indicate that the extent of uniform corrosion is approximately 0.2 mm after 1 million years and that the maximum flux of sulfide to the canister surface is below the threshold for localized corrosion or stress corrosion cracking. [ABSTRACT FROM AUTHOR]

To better understand the Thermal-Hydraulic-Mechanical (THM) response of the rock caused by the high-level radioactive waste (HLW) released heat in the Callovo-Oxfordian formation (COx) in the near-field and far-field areas, a series of coupled THM modelling is performed. This study presents a case study based on the French geological repository for HLW (Cigéo project) to better assess modelling of deep geological repositories (DGR) within the DECOVALEX-2019 framework. In this study, the proposed coupled THM model, implemented in the platform of COMSOL, is validated by comparing the modelled THM results for a point heat source in an infinite rock mass with an analytical solution. It is also validated by comparing its thermal component for the Base Case with accurate results calculated using another numerical method. To shorten the calculation time, the DGR is modelled using six cuboid blocks representing 162 HLW cells and only six central HLW cells of interest modelled in detail. This simplification is validated. This model can easily be used to model not only the near-field THM response of the rock but also the far-field THM response. The influence of the boundary conditions applied on the gallery wall, HLW cell walls, and external surfaces of the model and the influence of different vertical dimensions of the geometry are studied. The sensitivity analyses of the THM parameters of COx on the THM response at different locations are performed. The influence of using a 2-dimensional (2D) model to represent a 3-dimensional (3D) repository is also investigated.

Evaluation of the stability of placement rooms and excavation damage in the surrounding geological formations of a deep geological repository (DGR) for high-level nuclear waste are essential components when assessing the overall repository performance. Assessment of the geomechanical performance of placement rooms for the duration of the regulatory period (typically 1 million years) usually involves simulations using numerical models capable of simulating coupled thermo-hydro-mechanical (THM) processes induced by different natural and repository-induced perturbations over different time and length scales. The numerical constitutive model used to represent the host medium should be capable of capturing highly nonlinear responses to stress changes, including development of damage and fracturing. In this paper, a novel approach for simulating the evolution of damage and fracturing around the repository rooms considering coupled THM processes is presented, including validation examples for the approach. The important and novel feature of this modeling approach is the use of the bonded-block model based on the distinct element method (DEM) to represent deformation, damage, and fracturing of the surrounding rock that could potentially result in rock fragmentation and collapse of the rock surrounding the placement rooms. As part of the study, the predictions of damage around placement rooms obtained using the new approach are compared with the conventional continuum mechanics model. Subsequently, the capability of the bonded-block model to simulate damage caused by in-situ and thermally induced stresses is validated by comparison with monitoring data and observations from field-scale experiments. Highlights: A novel numerical approach for modeling thermo-hydro-mechanical processes taking place around deep geological repositories is presented. The approach uses a coupling scheme that allows simulations of times relevant for performance assessment of the repositories. Formation of stress-induced microcracks and their effect on permeability changes and long-term stability of the rooms are explicitly simulated. The methodology is validated by comparison with field data and with a different numerical method. [ABSTRACT FROM AUTHOR]

The Revell batholith is located within the Western Wabigoon terrane of the Superior Province, Northwestern Ontario, Canada, and is a potential site for a deep geological repository (DGR). This batholith is considered to have favourable geoscientific characteristics for hosting a DGR, including a sufficient volume of relatively homogenous rock. The subsurface geometry of the batholith plays an important role in determining its volume, as well as assessing regional-scale hydraulics, rock mechanics, and glacial stress disturbances on the bedrock, which are other important features and processes that can impact the batholith over the timeframes of concern for long-term storage of used nuclear fuel. Subsurface geometry is complicated to unravel, and surface mapping alone is inadequate to obtain the information at depth. However, gravity, magnetic, or seismic data can be used to enhance understanding by approximating the geometry.This study aims to refine the subsurface geometry and distribution of the Revell batholith from a constrained forward and inverse geophysical model, incorporating high-resolution geophysical data together with a compilation of historic and recent geological field data. The Revell batholith was previously cited as a flat-based pluton with a depth of 1.6 km, where our findings suggest the batholith is deeper than previously thought, with an uneven contact geometry at its base that extends slightly deeper than 3.5 km. Model uncertainties were assessed by varying probabilistic constraints on volume overlap/commonality and shape within GeoModeller™. Results indicate that overall batholith-greenstone contact is generally unchanged when the geological constraints are varied, providing a high degree of confidence that the Revell batholith has a sufficient volume of relatively homogeneous bedrock.

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

The increase of temperature of a low-permeability, fluid-saturated media may trigger significant thermal pressurization, where the expansion of pore fluid cannot be accommodated by the thermal expansion of the pore space. With the aid of a coupled thermohydromechanical numerical simulator, we investigate the possible impact of thermal pressurization during the life of a deep geological repository (DGR) for high-level radioactive waste, characterized by the emplacement of radioactive material-filled canisters in a series of parallel tunnels, excavated in a low-permeability clay formation. We represent the fault as a planar structure embedded in a thermoporoelastic material and shear activation evaluated by a strain-softening Mohr-Coulomb failure criterion. The results show that stress changes caused by temperature and thermal pressurization of a rock mass around the emplacement tunnels may trigger a slip event on a fault plane in proximity of the geological disposal site: Rupture nucleates at depth, hundreds of meters below the DGR. Stress transfer plays a key role, while a direct hydraulic connection between the repository and the nucleation zone is not necessary in order to trigger rupture. A low stress ratio may favor the occurrence of slip up to a distance of 600 m of the fault from the outermost tunnel. These results highlight the need of investigating hydromechanical properties and local stress conditions at depth to characterize the geomechanical response of weak planes located in the surroundings of a high-level radioactive waste repository and to provide sufficient knowledge for the safe development of the DGR site.

The disposal area of a deep geological repository (DGR) for the disposal of spent nuclear fuels (SNFs) is estimated considering the spacing between deposition holes and between disposal tunnels, as determined by a thermal analysis using the decay heat of a reference SNF. Given the relatively large amount of decay heat of the reference SNF, the disposal area of the DGR is found to be overestimated. Therefore, we develop a computer program using MATLAB, termed ACom (Assembly Combination), to combine SNFs when stored in canisters such that the decay heat per canister is evenly distributed. The stability of ACom was checked and the overall distribution of the decay heat per canister was analyzed. Finally, ACom was applied to disposal scenarios suggested in the conceptual design of a DGR for SNFs, and it was confirmed that the decay heat per canister could be evenly distributed and that the maximum decay heat of the canister could be much lower than that of a canister estimated using a reference SNF. ACom can be used to improve the disposal efficiency by reducing the disposal area of a DGR for SNFs by ensuringg a relatively even distribution of decay heat per canister.

Multinational geological repositories (Multinational repositories: MNRs) will inevitably be features of the international radioactive waste management landscape in future decades. They will involve more complex requirements for long-term information management than national deep geological repositories (DGRs) but the considerations involved in managing these requirements also point to wider needs for transnational information management for any country with a national deep geological repository. This article looks at what information needs to be propagated into the future for both DGRs and MNRs, and for how long. It is argued that the critical requirements are quite limited and are readily achievable, with the most important period being the coming few hundred years. The transience of organisations and national boundaries, issues being addressed for MNRs, also affect any national programme, but are generally overlooked. It is concluded that there is a need to move towards international oversight of all geological disposal facilities, including a common system of regulations and information archiving, and that providing these is a potential role for the IAEA.

To date, the increasing production of radioactive waste due to the extensive use of nuclear power is becoming a global environmental concern for society. For this reason, many countries have been considering the use of deep geological repositories (DGRs) for the safe disposal of this waste in the near future. Several DGR designs have been chemically, physically, and geologically well characterized. However, less is known about the influence of microbial processes for the safety of these disposal systems. The existence of microorganisms in many materials selected for their use as barriers for DGRs, including clay, cementitious materials, or crystalline rocks (e.g., granites), has previously been reported. The role that microbial processes could play in the metal corrosion of canisters containing radioactive waste, the transformation of clay minerals, gas production, and the mobility of the radionuclides characteristic of such residues is well known. Among the radionuclides present in radioactive waste, selenium (Se), uranium (U), and curium (Cm) are of great interest. Se and Cm are common components of the spent nuclear fuel residues, mainly as 79Se isotope (half-life 3.27 × 105 years), 247Cm (half-life: 1.6 × 107 years) and 248Cm (half-life: 3.5 × 106 years) isotopes, respectively. This review presents an up-to-date overview about how microbes occurring in the surroundings of a DGR may influence their safety, with a particular focus on the radionuclide-microbial interactions. Consequently, this paper will provide an exhaustive understanding about the influence of microorganisms in the safety of planned radioactive waste repositories, which in turn might improve their implementation and efficiency. [ABSTRACT FROM AUTHOR]

Deep geological repositories are the best way to high-level radioactive waste and spent nuclear fuel disposal, which involves the safety and security aspects and the requirements of providing passive protection for future periods after their closure. The aim of this study is to present knowledge about host rock engineering geological and geotechnical properties obtained from preliminary engineering geological investigation on two the most perspective sites for the construction of the deep geological repository in Slovakia. In the crystalline environment, it is the central part of Tribeč Mts.; in the sedimentary environment, it is the southern part of Slovakia (area between Rimavská kotlina basin and Cerová vrchovina Upland). The use of several different testing methods proves to be necessary for the reliable characterization of the host rock environment. Results of laboratory tests on determination of key characteristics are presented and compared with results of field tests realised in comparable rock environments. Since laboratory tests on small specimens are often less reliable to predict the deformability of rock masses, in situ tests are necessary. Dilatometer and pressuremeter testing in comparable crystalline rock environments are analysed, focused on the deformation modulus determination. Statistical analysis to develop relations between the in-situ detected value of Rock Quality Designation or macroscopically estimated degree of faulting in the borehole and deformation parameters is presented. However, dilatometer field tests are universal for various geological conditions and these test methods can be consistently applied to later site investigation stages process. [ABSTRACT FROM AUTHOR]

This article deals with the development of concretes intended to be used as construction materials in the Czech deep repository for radioactive waste. The basic requirements for this concrete are a reduced pH value, which must maintain a constant reduced pH with a value of around 11 in the long term, and mechanical properties comparable to conventional concrete. The raw materials for the production of the proposed low pH concrete come exclusively from the Czech Republic. Material characteristics were measured on fresh mixtures and concrete after 28, 56, and 90 days of curing. In addition to the basic raw materials (aggregate, cement, water), plasticizers, microsilica or slag, and defoaming agents were added to the mixture. The aggregate:binder ratio was approximately 5:1 and the w/c water coefficient was approximately 0.6. The mechanical properties of the final concrete were similar to the reference recipe of conventional concrete (the decrease was less than 10%) and the pH value was even below 11 after 90 days. The issue of a sprayed variant of the LPC mixture was solved within the project. [ABSTRACT FROM AUTHOR]

Compacted bentonite–sand (B/S) mixtures have been used as a barrier material in engineered barrier systems (EBSs) of deep geological repositories (DGR) to store nuclear wastes. This study investigates the individual and combined effects of different chemical compositions of deep groundwaters (chemical factor) at potential repository sites in Canada (the Trenton and Guelph regions in Ontario), heat generated in DGRs (thermal factor), dry densities and mass ratios of bentonite and sand mixtures (physical factors) on the swelling behavior and ability of bentonite-based materials. In this study, swelling tests are conducted on B/S mixtures with different B/S mix ratios (20/80 to 70/30), compacted at different dry densities (ρd = 1.6–2 g/cm3), saturated with different types of water (distilled water and simulated deep groundwater of Trenton and Guelph) and exposed to different temperatures (20 °C–80 °C). Moreover, scanning electron microscopy (SEM) analyses, mercury intrusion porosimetry (MIP) tests and X-ray diffractometry (XRD) analyses are carried out to evaluate the morphological, microstructural and mineralogical characteristics of the B/S mixtures. The test results indicate that the swelling potential of the B/S mixtures is significantly affected by these physical and chemical factors as well as the combined effects of the chemical and thermal factors. A significant decrease in the swelling capacity is observed when the B/S materials are exposed to the aforementioned groundwaters. A large decrease in the swelling capacity is observed for higher bentonite content in the mixtures. Moreover, higher temperatures intensify the chemically-induced reduction of the swelling capacity of the B/S barrier materials. This decrease in the swelling capacity is caused by the chemical and/or microstructural changes of the materials. The results from this research will help engineers to design and build EBSs for DGRs with similar groundwater and thermal conditions.

In order to assess sites for a deep geological repository for storing high-level nuclear waste safely in Germany, various numerical models and tools will be in use. For their interaction within one workflow, their reproducibility, and reliability version-controlled open-source solutions and careful documentation of model setups, results and verifications are of special value. However, spatially fully resolved models including all relevant physical and chemical processes are neither computationally feasible for large domains nor is the data typically available to parameterize such models. Thus, simplified models are crucial for the pre-assessment of possible sites to narrow down the list of suitable candidates for which detailed site investigations and fully resolved models will be done at a later stage. Still, the accuracy of these simplified models is of importance as the pre-assessment of suitable sites will be based on them. In this study, we compare the modelling capabilities of TransPyREnd, a one-dimensional transport code based on finite differences, specifically developed for the fast estimation of radionuclide transport by the German federal company for radioactive waste disposal (BGE), with OpenGeoSys, which is a modelling platform based on finite elements in up to three spatial dimensions. Both codes are used in the site selection procedure for the German nuclear waste repository. The comparison of the model results of TransPyREnd and OpenGeoSys is augmented by comparisons with an analytical solution for a homogeneous material. For the purpose of numerical benchmarking, we consider a geological profile located in southern Germany as an example where the hypothetical repository is located in a clay-stone formation. TransPyREnd and OpenGeoSys yield overall similar results. However, both codes use different discretizations which impact is the highest for strongly sorbing compounds, while the difference gets negligible for less sorbing and more diffusive compounds as higher diffusion tends to blur the initial conditions. Overall, the OpenGeoSys model is more exact whereas the TransPyREnd model has considerable faster run times. We found in our example, that significant substance amounts are only leaving the host rock formation, if apparent diffusion is high, for which case both codes give similar results, while relative differences are considerable for strongly sorbing compounds. However, in the latter case no significant substance amount of radionuclides leaves the host-rock formation, thus deeming the differences in the model results minor for the overall safety assessments of sites. • A deep geological repository can provide a safe containment of radionuclides. • Diffusion, sorption, radioactive decay, and advection are modelled accurately over a simulated time of 1 million years. • The results of two numerical codes are compared with each other, consistency with analytical solutions is shown. • Spatial and temporal discretizations may influence radionuclide transport results significantly. [ABSTRACT FROM AUTHOR]

To date, the increasing production of radioactive waste due to the extensive use of nuclear power is becoming a global environmental concern for society. For this reason, many countries have been considering the use of deep geological repositories (DGRs) for the safe disposal of this waste in the near future. Several DGR designs have been chemically, physically, and geologically well characterized. However, less is known about the influence of microbial processes for the safety of these disposal systems. The existence of microorganisms in many materials selected for their use as barriers for DGRs, including clay, cementitious materials, or crystalline rocks (e.g., granites), has previously been reported. The role that microbial processes could play in the metal corrosion of canisters containing radioactive waste, the transformation of clay minerals, gas production, and the mobility of the radionuclides characteristic of such residues is well known. Among the radionuclides present in radioactive waste, selenium (Se), uranium (U), and curium (Cm) are of great interest. Se and Cm are common components of the spent nuclear fuel residues, mainly as 79Se isotope (half-life 3.27 × 105 years), 247Cm (half-life: 1.6 × 107 years) and 248Cm (half-life: 3.5 × 106 years) isotopes, respectively. This review presents an up-to-date overview about how microbes occurring in the surroundings of a DGR may influence their safety, with a particular focus on the radionuclide-microbial interactions. Consequently, this paper will provide an exhaustive understanding about the influence of microorganisms in the safety of planned radioactive waste repositories, which in turn might improve their implementation and efficiency.

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., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: All authors report financial support was provided by Japan Ministry of Economy Trade and Industry, as part of “The project for validating near-field system assessment methodology in geological disposal (FY2019-2022, JPJ007597)”., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Deep geological disposal is currently the preferred method of treating high-level nuclear waste (HLW). A compacted bentonite buffer layer is established between the HLW canister and the surrounding rock to prevent nuclear leakage. However, the liquid in the cracks of surrounding rocks usually contains a variety of chemical substances, so it is necessary to evaluate the mechanical properties of bentonite in a chemical environment and the changes in its microstructure. Ensure the long-term stability of the repository in complex underground systems. To figure out the effect of groundwater pore solution on the mechanical properties and microstructure of bentonite, the compacted GMZ07 bentonite specimens were saturated and consolidated at different concentrations of Na2SO4 solutions, followed by a direct shear test. The laser particle analysis (LPA) and scanning electron microscope (SEM) were conducted to evaluate the microstructural changes. The results showed that the surface of bentonite particles was fine-grained in Na2SO4 solutions. As the concentration of the solution increases, the bentonite particle size decreases, which makes the connection of the bentonite particles tighter and easier to flocculate. Macroscopically, it shows the decrease in volume and the increase in strength of bentonite specimens. [ABSTRACT FROM AUTHOR]

High-level radioactive waste needs to be safely stored for a long time in a deep geological repository by using a multi-barrier system. In this system, suitable barrier materials are selected that ideally show long-term stability to prevent early radionuclide release into the biosphere. In this study, different container matals (copper and cast iron) and pore water compositions (Opalinus Clay pore water and saline cap rock solution) were combined with Bavarian bentonite in static batch experiments to investigate microbial-influenced corrosion. The increasing concentration of iron and copper in the solution as well as detected corrosion products on the metal surface are indicative of anaerobic corrosion of the respective metals during an incubation of 400 days at 37 °C. However, although the intrinsic microbial bentonite community was stimulated with either lactate or H 2 , an acceleration of cast iron- and copper corrosion did not occur. Furthermore, neither corrosive bacteria nor conventional bacterial corrosion products, such as metal sulfides, were detected in any of the analyzed samples. The analyses of geochemical parameters (e.g. ferrous iron-, iron-, copper- and potassium concentrations as well as redox potentials) showed significant changes in some cast iron- and copper-containing setups, but these changes did not correlate with the microbial community structure in the respective microcosms, as confirmed by statistical analyses. Hence, the analyzed Bavarian bentonite (type B25) showed no significant contribution to cast iron and copper corrosion under the applied conditions after 400 days of incubation. From this perspective, bentonite B25 could be a suitable candidate as a geotechnical barrier in future repositories. [Display omitted] • Selecting geotechnical barrier material (e.g. bentonite) relevant for the storage of nuclear waste remains important. • Microcosms were used to analyze the microbial effects of B25 bentonite on metal corrosion. • Indigenous microorganisms in B25 bentonite did not affect cast iron- and copper corrosion. • Corrosion was mainly attributed to abiotic factors of Opalinus Clay pore water and cap rock solution. • Statistical analysis (e.g. dbRDA) is a powerful tool to evaluate the microbial influence. [ABSTRACT FROM AUTHOR]

Wyoming-type bentonite clay (MX-80; Wyoming, USA) will be used in a deep geological repository (DGR) for the long-term storage of used nuclear fuel in Canada. The natural organic matter (NOM) found in bentonite may serve as a microbial nutrient source and potentially compromise the performance of the used fuel containers. Previous investigations indicate that NOM is present in low concentrations in MX-80 and has undergone extensive diagenetic alteration, though limited knowledge is available regarding NOM chemistry under simulated DGR conditions. Of particular concern is the possibility for gamma-radiation to alter NOM dissolution and reactivity due to the presence of reactive species generated by water radiolysis in Wyoming-type bentonites. In this study, NOM chemistry was investigated using complementary molecular-level techniques following exposure to a total gamma-radiation dose of 100 kGy (1.08 kGy/h for 93 h) at varied moisture levels (20%, 40%, 60% and 80%) and room temperature. Treated samples exhibited relatively low total organic carbon concentrations (0.074–0.232%), with no evidence of any major changes in total, organic, and inorganic carbon concentrations. Solid-state 13C NMR spectroscopy detected no changes in solid-phase NOM chemistry after irradiation. Solubilization of NOM increased significantly with radiation exposure at 80% moisture, suggesting higher levels of water saturation may enhance dissolved organic matter (DOM) production. Solution-state 1H nuclear magnetic resonance (NMR), and UV–Vis analyses did not identify any significant differences in DOM composition. More sensitive, targeted compound analysis revealed significant decreases in total n -alkanol concentration at lower moisture content levels (20% and 40%). Several individual compound concentrations also differed significantly at the nanogram-level, including n -octacosanol and several n -alkanoic acids at elevated moisture levels (60% and 80%). These findings suggest the majority of NOM in MX-80 remains chemically stable under the anticipated initial conditions of the proposed DGR. • MX-80 carbon concentrations were insensitive to irradiation with varied moisture. • Irradiation of saturated MX-80 slightly increased NOM solubilization. • No changes in the chemical composition of MX-80 DOM were observed. • MX-80 solid-phase NOM chemistry was not altered by radiation and moisture. [ABSTRACT FROM AUTHOR]

• Buffer plays an important role in the engineered barrier system for deep geological repository. • It is crucial to predict the buffer peak temperature for the safe disposal of HLW. • A T-H coupling model is developed to simulate the temperature and saturation of buffer. • Present model is validated with the work in the DECOVALEX 2015 Task B2. • Simulation results show the advantage of T-H coupling model in design of repository arrangement. The buffer plays an important role in a engineered barrier system for the deep geological repository of high-level radioactive waste (HLW). High temperature may result in degradation of buffer performance. It is crucial to investigate the buffer temperature behavior so that its peak temperature is not higher than the criterion of 100 °C. In the present work, the thermal-hydraulic (T-H) coupling model is developed to simulate the histories of temperature and saturation in the disposal repository. This model is validated with the previous simulation works in the DECOVALEX 2015 Task B2 in order to simulate the geological repository of HLW with confidence. The whole repository of HLW in the present works can be reasonably simplified as a single hole with the symmetric boundary conditions on its peripheral sides. Effects of tunnel spacing and hole spacing of repository on the histories of buffer peak temperature are considered since the layout with various spacing is strongly related to the peak temperature of buffer. Based on the comparison of simulation results from the T-H coupling model and T-only model for the different layout of whole HLW repository, the maximum value of peak temperature in the buffer predicted by the T-H coupling model is lower than that from the T-only model. In other words, less area is needed for the selection of repository, rendering that the present T-H coupling model can assist in the design of HLW repository. [ABSTRACT FROM AUTHOR]

Compacted bentonite is used as a buffer or backfill material in deep geological repositories. However, the seepage of pore water containing various salt and minerals can affect the hydro-mechanical performance of the bentonite. To investigate these effects, samples of compacted bentonite with initial dry densities of 1.50 Mg·m−3 and 1.70 Mg·m−3 were subject to saturation by distilled water, and solutions of NaCl and NaOH (0.1 M and 1.0 M), and the changes in swelling pressure and microstructure were monitored. The swelling pressure was seen to reduce with greater concentrations of the salt solution or when the initial dry density was decreased. At lower salt concentrations, the variation of the swelling pressure exhibited a double-peak, while further increases in the salt concentration caused the swelling pressure curve to exhibit only a single-peak. Compared to samples with a 1.7 Mg·m−3 dry density, the large swelling pressure decreased following the initial peak, and a smaller second peak was observed for samples with a 1.5 Mg·m−3dry density. The swelling pressure of samples saturated with NaOH solutions was notably lower, particularly after the second peak in the double-peaked curves. Applicable empirical kinetic equations were proposed to fit the relationship between the swelling pressure (p, MPa) and time (t, h) of the compacted sample saturated with DW and aqueous NaCl and NaOH. Microscopic analyses provide evidence that this reduction of swelling pressure could be attributed to the osmotic consolidation, ionic exchange, and mineral dissolution of bentonite. [ABSTRACT FROM AUTHOR]

High-level heat-emitting long-lived vitrified radioactive waste produced during recycling of the spent nuclear fuel is under consideration for permanent disposal in deep geological formations with appropriate thermomechanical, hydrogeological and geochemical properties. The capability of these rock formations ensuring long-term confinement and isolation of such waste from the environment is significantly controlled by their efficiency in smoothly dissipating the heat emanating from the waste. A number of rock types such as basalt, granite, clay stones, volcanic tuff, argillites, etc. are being evaluated worldwide as well as in India. In this study, a granite from Jalore and bentonite from Barmer, both from Rajasthan, India, have been evaluated for their heat dissipation capacity. The study revealed that the temperature within granite at the centre of the canister reached 55.21°C, resulting in a thermal stress of 25.50 MPa. Bentonite experienced a temperature of 67.42°C in the central part with maximum thermal stress and displacement of 1.78 MPa and 0.446 mm respectively. A displacement of 0.997 mm was recorded at the granite-bentonite interface. Thus, no significant microcrack formation or undesirable displacement was observed within the granite as well as in bentonite, suggesting their capability to isolate and confine the heat-emitting source for extended periods. [ABSTRACT FROM AUTHOR]

Deep geological repositories are the sole option for safeguarding the atmosphere from hazardous radiation from high-level radioactive waste. Hyperalkaline pore fluids are produced when fluids from host rocks penetrate through the concrete/cement layers of such repositories, the layers created as bulkheads in vaults or to facilitate gallery access between buffers and the host rock. Such fluids also disperse through the compacted bentonite layers placed around the steel canisters that contain the waste in the repositories. The corrosion of steel canisters inside such repositories can be observed over a certain time. Chemical corrosion can lead to harmful leachates that can seep through the compacted bentonite layers that surround the steel canisters, altering the bentonite properties. In addition, these compacted bentonite layers are subjected to thermal histories due to the canisters emitting continuous heat (150°C to 200°C). These compacted bentonite layers have to hold up under these three conditions in repositories, all of which can affect the swelling properties of the bentonite. Two bentonite samples (B1 and B2) from Rajasthan's Barmer area were mixed with corrosion products and compacted to dry densities of 1.5, 1.75, and 2.0 Mg/m3 for this investigation. Thermal histories were induced at 110°C and 200°C for 3 h to prepare samples that were then saturated with hyperalkaline cement water. The presence of corrosion products resulted in a decrease in swelling pressure values when compared with samples without adding corrosion products. This decrease was more significant with an increase in the temperature of the induced thermal history. [ABSTRACT FROM AUTHOR]

Deep geological repositories (DGRs) are considered the most promising technology for the long-term management of nuclear wastes. One of the major functions of the buffer or barrier material used in DGRs for nuclear waste is to prevent the release of high-level radioactive chemicals into the environment in the event of failure. To accomplish this, the buffer is typically designed to have very low hydraulic conductivity. The effects of mix-composition, swelling condition, temperature and groundwater chemistry on the hydraulic conductivity of compacted bentonite–sand barrier material were investigated in this study. Permeability tests were carried out using flexible wall permeameter after flooding the compacted samples in simulated groundwater. The obtained results showed that the saline groundwater prevailing in the Guelph region of Canada as well as high temperature have a negative impact on the swelling potentials of the buffer material, consequently increasing the hydraulic conductivity. The saline water reduces the thickness of the diffuse double-layer which is accompanied by a weak repulsion between the clay minerals. Similarly, elevated temperature decreases the basal spacing between the bentonite clay minerals which also results in less swelling. The combination of high temperature and salt concentration have an increased negative influence on the hydraulic conductivity. Furthermore, it was observed that restricting the swelling of the bentonite–sand mixture produces a well-packed material with a very low hydraulic conductivity compared to the free swelling. High confining pressure generates higher swelling stress from the hydration of clay minerals, which expand to fill the voids between the sand particles. It was also observed that increasing the percentage of bentonite and the initial dry density of the mix reduced the hydraulic conductivity by eliminating interparticle voids in the material. The research findings presented in this manuscript provide valuable information that will contribute to gain a deeper insight into the impact of field conditions (temperature, groundwater chemistry, confinement) and material characteristics (composition, dry density) on the permeability of bentonite–sand barrier, which is essential for a safe long-term management of radioactive wastes. [ABSTRACT FROM AUTHOR]