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Copyright of World Nuclear Geoscience is the property of World Nuclear Geoscience Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In high-level nuclear waste (HLW) repositories, concrete and compacted bentonite are designed to be employed as buffer materials, which may raise a problem of interactions between concrete and bentonite. These interactions would lead to mineralogy transformation and buffer performance decay of bentonite under the near field environment conditions in a repository. A small-scale experimental setup was established to simulate the concrete-bentonite-site water interaction system from a potential nuclear waste repository in China. Three types of mortars were prepared to correspond to the concrete at different degradation states. The results permit the determination of the following: (1) The macro-properties of Gaomiaozi (GMZ) bentonite (e.g. swelling pressure, permeability, the final dry density, and water content of reacted samples); (2) The composition evolution of fluids from the synthetic site water-concrete-bentonite interaction systems; (3) The sample characterization including Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD). Under the infiltration of the synthesis Beishan site water (BSW), the swelling pressure of bentonite decreases slowly with time after reaching its second swelling peak. The flux decreases with time during the infiltrations, and it tends to be stable after more than 120 d. Due to the cation exchange reactions in the BSW-concrete-bentonite systems, the divalent cations (Ca and Mg) were consumed, and the monovalent cations (Na and K) were released. The dissolution of minerals in the bentonite such as albite causes Si increasing in the pore water. It was concluded that the hydro-mechanical property degradation of bentonite takes place when it comes into contact with concrete mortar, even under low-pH groundwater conditions. The soil dispersion, the uneven water content, and the uneven dry density in bentonite samples may partly contribute to the swelling decay of bentonite. Therefore, the direct contact with concrete has an obvious effect on the performance of bentonite.

Gaomiaozi (GMZ) bentonite as a back-filling (buffer) material for repositories of high-level nuclear waste (HLW) has an obvious multiple-pore structure. The particle characteristics of its constituent minerals have significant impacts on its macroscopic physical and mechanical properties. After purification, the nanoscale montmorillonite mineral particle morphology of GMZ bentonite in suspension was observed by atomic force microscope (AFM). The particle size, particle shape and stacking characteristics of particles were statistically analysed. The effects of the hydration process, ambient temperature, and interlayer cation types on its particle characteristics are explored. The particle gradation of GMZ bentonite particles is discontinuous. The large particles have a long stripe shape with an equivalent particle size of 1200 ~ 2500 nm; the small particles have an elliptical shape with an equivalent particle size of 5 ~ 600 nm. Larger the particles particle size, more irregular the particle shape. The complete hydrated bentonite particles contain generally less than 20 layers. The layer thickness is about 0.98 nm, and the layer basal distance is about 1.15 ~ 1.55 nm. The particles with more layers would have a smaller layer basal distance. Low ambient temperature could increase the layer basal distance. Calcium ion exchange increases the layer basal distance, and the opposite is true for sodium ions. The research would be facilitating the understanding of the evolution of multiple pore structures during the bentonite hydration process. [ABSTRACT FROM AUTHOR]

The joints of buffer material composite blocks as potential weak parts in the engineering barrier system of a high-level radioactive waste (HLW) repository must be studied in depth. Therefore, a laboratory experiment device suitable for unsaturated composite bentonite samples was developed. The evolution of temperature and volumetric water content at different locations of Gaomiaozi (GMZ) composite bentonite samples with time before and after simulated water inflow was measured by the experiment device. According to the experimental results, the thermal conductivity and hydraulic conductivity of the joint location after healing of the composite bentonite samples were obtained. The experimental results show that the change in the internal temperature of the composite bentonite samples is mainly affected by the temperature boundary and that the change in the internal water has little effect on it. In a short period of time, the loading of hydraulic boundary conditions only makes the volumetric water content of the soil near the hydraulic boundary increase significantly but has little effect on other locations. And, affected by the temperature boundary, the volumetric water content of the soil near the temperature boundary gradually decreases with time. The process of hydration swelling of the composite bentonite sample is accompanied by the adjustment of stress. The composite bentonite samples are continuously squeezed to the joint area after hydration swelling, the whole composite samples are generally homogenized, and the joints between the composite bentonite samples tend to heal. The thermal conductivity and permeability of the joint location after healing can meet the requirements of the engineering barrier of the HLW repository. [ABSTRACT FROM AUTHOR]

During the long-term operation of a deep geological repository, hydrogen, methane and carbon dioxide, etc. could be generated and accumulated in bentonite around canisters, threatening integrity and safety of the engineering barrier systems. In this work, self-designed test apparatuses were developed. Gas diffusion tests were conducted on GMZ bentonite specimens under both rigid and flexible boundary conditions with consideration of initial saturations and gas pressures. After experienced the gas diffusion tests, the specimens were cut and submitted for the mercury intrusion porosimetry (MIP) tests. Results revealed that under same initial saturation and gas pressure conditions, diffusion coefficient of the specimen tested under rigid boundary conditions was higher than that tested under flexible boundary conditions. This is because that compared to that of rigid boundary conditions, the effective pore volume for gas diffusion was more easily to be compressed under flexible boundary conditions, resulting in a relatively poor pore connectivity and low gas diffusion coefficient. Regardless of constraint conditions, diffusion coefficient of the specimens with same initial saturations decreased with increases of the initial gas pressure, and the decreasing rate slowed down with increasing initial gas pressure. Explanations could be that increases of gas pressure reduced the mean free path of gas molecules, affecting the diffusion efficiency. Meanwhile, the diffusion coefficient decreased with increasing initial saturations and same initial gas pressures for the flexible boundary specimens, while an opposite trend was recorded for the rigid boundary ones. Explanations to this observation could be that under the rigid boundary conditions, specimens with higher initial saturations have larger diameter macro-pores, facilitating the gas diffusion. [ABSTRACT FROM AUTHOR]

Bentonite, as a buffering and backfilled material in high level radioactive waste disposal repository, will produce colloids when interacting with groundwater in near field. Colloids play an important role on the diffusion and migration behavior of radioactive nuclides. Therefore, study on bentonite releasing colloids is one of important issues in disposal safety assessment. GMZ bentonite, short for Gaomiaozi bentonite, is identified as the most promising backfill material in China. GMZ bentonite was taken as research object in this paper, and factors and mechanisms affecting bentonite releasing colloids were studied using turbidimetric method because of good linear relationship between turbidity and colloid concentration. The effects of ion strength, ion types, acid-base property and temperature on releasing colloids were researched. Sample bottle of turbidimeter was special customized in order to avoid disturbance caused by frequent sampling which would affect measurement results. Bentonite was pressed by tableting firstly, then placed in a restricted container made of granite for simulating the real situation of disposal repository. Relationship curves of ion type, ionic strength, pH and temperature with turbidity were obtained, and the mechanisms of all factors affecting colloids releasing were analyzed. The results show that order of cationic species inhibiting the release of colloids from bentonite is Li+＜Na+＜K+＜NH4+＜Zn2+＜Mg2+＜H+＜Ca2+. The cations with higher charge have stronger suppression ability except H+ which could protonate the edge of montmorillonite and reduce the swelling ability of bentonite significantly. The effect of cations on bentonite releasing colloid depends on the ion hydration radius, agglomeration capacity, ability to neutralize negative charges, and adsorption capacity. OH- among four kinds of anions could promote the release of colloids significantly, while others have little effect on the release of colloids basically. The ability of solution to inhibit bentonite releasing colloids would be stronger, when the ionic strength of the solution is higher. Bentonite generates colloids much easier in neutral condition, weak acid and weak base conditions take second place, while strong acid and alkali conditions have the strongest inhibitory effect on colloid release. Acidic environment is more effective in inhibiting colloid release from bentonite than alkaline environment. Increasing the temperature is conducive to the release of bentonite colloid under 20-50 ℃. The reason lied in raising temperature could increase the internal energy of montmorillonite, improve probability of collision between colloidal particles,and reduce the ability of neutralizing negative charges which is beneficial for the swelling of montmorillonite. The results have important guiding significance for analyzing the retardation of nuclide migration by bentonite colloid and understanding the geochemical behavior of bentonite in the near-field correctly.

Gas migration behaviour in saturated, compacted bentonite, especially under rigid-boundary conditions, is controversial. Gas breakthrough phenomena, observed under higher pressure gradient conditions in laboratory experiments, are described in literatures by adopting visco-capillary or dilatancy-controlled flow concept. Since, under rigid-boundary conditions, volumetric expansion is restricted and/or water dissipation is not detected, these concepts cannot be implemented satisfactorily. Instead, a diffusion and solubility-controlled (DSC) flow concept was previously found to be adequate for describing the behaviours at lower temperatures (20 °C). The DSC concept describes gas breakthrough as a function of gas solubility. Breakthrough occurs when concentration of dissolved gas reaches or surpasses the solubility limit in the entire specimen. In this work, the DSC flow concept is applied to validate gas migration and breakthrough experiments conducted at higher temperatures, e.g. 40 and 60 °C. Good agreements are observed between the experimental and predicted results, suggesting that the DSC flow concept can be applied to describe gas migration behaviour satisfactorily in rigidly confined saturated bentonites (under constant volume conditions) for various temperature regimes. Results also show that helium dissolution and diffusion processes in saturated bentonite are sensitive to test temperature and pressure conditions. The processes become more stable with increasing gas injection pressure and ambient temperature. [ABSTRACT FROM AUTHOR]

Incorporating the nuclear magnetic resonance (NMR) technique, a newly developed infiltration instrument constructed from polytetrafluoroethylene is designed for the rapid determination of unsaturated hydraulic conductivity of bentonite materials. The proposed approach is founded on measuring the variation of the NMR signal in numerous cross sections of the specimen during the infiltration test, enabling the determination of water content distribution. Through integration with the instantaneous profile method, unsaturated hydraulic conductivity can be obtained using Darcy's law. The mechanism underlying the observed U-shaped trend of unsaturated hydraulic coefficient with decreasing suction can be attributed to the relatively high value controlled by water vapor diffusion at higher suction stages, as well as the hydraulic conductivity increasing near saturation, similar to the behavior of most unsaturated soils. The consistency of test results with those acquired from conventional measurements, regarding amplitude level and evolutionary pattern, provides evidence of the reliability of the proposed method. The primary improvement lies in the real-time acquisition of the water content profile using the spatial encoding technique in the NMR in the infiltration test. Simultaneously, the approach substantially reduces the test duration from thousands of hours to dozens of hours while maintaining measurement accuracy. [ABSTRACT FROM AUTHOR]

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

As the buffer/backfill material for geological disposal of high-level radioactive waste, the sealing performance of compacted bentonite is affected by its volume deformation characteristics. During the operation of the geological disposal repository, the suction and temperature of bentonite changes due to the groundwater level and heat released by radioactive waste. On this premise, to study the thermal effect on the volume change of bentonite induced by suction variations, a series of wetting/drying tests were conducted on cubic bentonite samples under controlled temperatures of 20℃, 40℃ and 60℃. The corresponding microstructure changes during the test were investigated by the mercury injection porosity (MIP) technique. The results show that the increasing temperature accelerated the change in water content and weakened the water retention capacity of bentonite. Additionally, the volume deformation induced by suction change, regardless of swelling or shrinkage, was inhibited by heating. During the process of suction equilibrium, the compacted bentonite showed significant anisotropy, which was positively correlated with temperature and negatively correlated with suction. The sample deformation was due to the changes in the microstructure of the inter-aggregate pores and intra-aggregate pores. The total volume of macro pores decreased obviously with increasing suction, while, the volume of micropores remained almost unchanged. Both the peak value and total volume of the macro pores were reduced by the rising temperature. Furthermore, a threshold suction of approximately 90 MPa was observed, where the temperature effect changed from inhibition of dilation to inhibition of contraction, indicating the suction-dependent temperature effect. [ABSTRACT FROM AUTHOR]

To construct an engineering barrier in a deep geological repository, bentonite generally should be laid out in blocks, which are commonly pre-prepared through uniaxial static compactions. The way of compaction and installation of the bentonite blocks will inevitably influence swelling performance of the bentonite compacted and eventually buffer functions of the engineering barrier system constructed. In this article, a new experimental apparatus was developed and cubic GMZ bentonite specimens with different initial suctions and dry densities were statically compacted. Swelling pressures were measured parallel to and perpendicular to the compaction direction/axis of the cubic specimens prepared in different ways. Results confirmed existence of the residual lateral stress and sidewall friction. The swelling pressure parallel to is higher than that perpendicular to the compaction axis, and its anisotropy coefficient tends to decrease to a certain degree upon hydration. Specimen with higher initial suction swells much quicker and exhibits a more significant intermediate period. The initial suction effects on anisotropy coefficient depend on hydration. The measured swelling pressure significantly increases with increasing dry density with about a 9.6- times increase as the density increased from 1.5 to 1.9 Mg/m3. The anisotropy coefficient of swelling pressure increases at first and then decreases a little bit with increasing dry density (e.g., the final anisotropy coefficient exhibited a 1% decrease while dry density changed from 1.8 to 1.9 Mg/m3). [ABSTRACT FROM AUTHOR]

Copyright of World Nuclear Geoscience is the property of World Nuclear Geoscience Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of World Nuclear Geoscience is the property of World Nuclear Geoscience Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The pore structure evolution characteristics of bentonite under thermal effects are the basis for determining its various hydraulic properties and thus are of vital importance for evaluating the long-term safety of repository operations. Therefore, the current research has been carried out to characterize the pore structure evolution of GMZ bentonite during hydration at different temperatures. ESEM fixed-point scanning was adopted at various relative humidities (RHs) and temperatures on unconfined compacted GMZ samples with an initial dry density of 1.70 g/cm3. It was found that the pore structure characteristics evolved in a significant moisture-dependent and temperature-dependent pattern. During hydration at 20 °C, large deflections of the structure are evident in the low RH, while crack formation and extension are more likely to occur in high RH. With the increase in temperature, the inter-aggregate porosity shows an increasing phenomenon, while for high moisture samples, the inter-aggregate porosity occurs first with an increase followed by a decrease and again increased phenomenon. As the temperature rises, the bentonite undergoes two processes of water loss: inter-aggregate pore water loss and intra-aggregate pore water loss, while this process also faces the effects of thermal expansion, leading to complex changes in porosity and pore size distribution under the competing effects. [ABSTRACT FROM AUTHOR]

During the construction and operation of a geological repository, with the infiltration of groundwater, highly compacted bentonite blocks hydrate and swell freely to fill up technological voids and gradually turn to hydrate under a constant-volume condition. Additionally, these processes are inevitably influenced by the pore chemistry of the groundwater. Therefore, it is necessary to investigate the hydration process of compacted bentonite under the combined influences of chemistry and technological voids. In this study, hydration tests were conducted on compacted GMZ bentonite with artificial annular gaps with infiltration of deionized water, 1 M NaCl, and 1 M CaCl2 solutions, respectively. Variation of swelling pressure and hydraulic conductivity with time was measured during hydration. The distribution of dry density, water content, and microstructure feature in the specimens was determined. Results show that sealing of the technological voids resulted in a heterogeneous distribution of dry density and water content of the barrier formed. Compared to that of deionized water, infiltration of salt solutions weakened swelling capacity and increased the permeability, leading to more significant heterogeneity in dry density distribution. During hydration, the specimen could be divided into swelling and compression zones according to variations of dry density with time. The boundary between the swelling and compression zones moved with hydration time. Evolutions of microstructure features were also affected by solutions. For specimens infiltrated with NaCl, more macro-pores and meso-pores were observed, while for CaCl2 infiltration, more undetectable pores and less meso-pore pores could be detected. After the tests, all the specimens were still in non-homogeneous states. [ABSTRACT FROM AUTHOR]

Bentonite, as buffer material, is a pivotal part of the engineered barrier, which retards and adsorbs leaky nuclides. The adsorption performance of natural bentonite, however, is still questionable to maintain the operational target of a high-level radioactive waste repository for over 10,000 years. Modification of bentonite to improve the adsorption capacity of nuclides is of great significance for preventing nuclide leakage. Accordingly, a laboratory testing program was designed to prepare the polyacrylate-bentonite nanocomposites (PAANa-Bent) by means of grafting a sodium polyacrylate chain. The synthesized PAANa-Bent samples were characterized using X-ray diffraction (XRD), BET pore analysis, and scanning electron microscopy (SEM). Through a series of adsorption experiments, the effects of contact time, pH, temperature, Eu3+ concentration, and ion strength on the adsorption capacity were investigated. Results indicated that the preparation method of sodium polyacrylate grafted bentonite is feasible and can effectively realize the nanocomposite of polymer and bentonite minerals. The grafted polymers encapsulate and protect the bentonite from acid and salt ions, improving the overall specific surface area and mesoporous content. Most importantly, the adsorption capacity of the PAANa-Bent for Eu3+ ions is greatly improved from 25.34 to 69.22 cmol/kg, due to the increase of the carboxylic group (adsorption site). Different from the Langmuir isothermal adsorption model of raw bentonite, the isothermal adsorption curve of PAANa-Bent is S-shaped and has adsorption buffering properties, less susceptible to ionic strength on Eu3+ adsorption. [ABSTRACT FROM AUTHOR]

Gaomiaozi (GMZ) bentonite is an excellent backfill material for high-level nuclear waste repositories. The bentonite minerals possess a laminated structure and carry a negative charge. These surface charge characteristics play a crucial role in the development of the electric double layer at the liquid-solid interface and the adsorption of nuclear elements during the hydration process. Following the purification of GMZ bentonite, the surface electric potential of bentonite mineral colloids in suspension and compacted bentonite blocks was measured using atomic force microscopy (AFM). Additionally, the influence of various factors on its surface charge characteristics was examined, including the types of interlayer cations, ambient temperature, number of layers, and dry density. The findings reveal that at 20 °C, the surface electric potential of Na-bentonite quasicrystals averages around −225.2 mV, while that of Ca-bentonite quasicrystals averages approximately −155.3 mV. Additionally, the surface electric potential increases logarithmically with the number of layers. In the compacted state, its surface potential would decrease. An increase in the dry density of compacted bentonite results in a decrease in the surface electric potential. High ambient temperatures can increase the surface electric potential. The research findings offer valuable data and a theoretical foundation for predicting colloid formation, nuclear element adsorption, and swelling pressure development in bentonites used in high-level nuclear waste repositories. [Display omitted] • Nanoscale surface electric potential test for bentonite minerals. • Surface charging characteristics of compacted bentonite. • The effect of hydration degree and temperature on surface electric potential of bentonite. • Cross-scale connections in the surface charging properties of compacted bentonite. [ABSTRACT FROM AUTHOR]

• Increasing dry density or decreasing porewater salinity can inhibit Eu(Ⅲ) diffusion. • Dry density and porewater salinity affect Eu(Ⅲ) diffusion by altering triple pore structure. • A mathematical expression based on triple pore structure is proposed to predict Eu(Ⅲ) diffusion. Compacted bentonite is microscopically characterized by a triple pore structure composed of inter-aggregate pores, inter-particle pores and interlayer pores. These three types of pores are available for diffusive transport of cations through different mechanisms: free water diffusion, surface diffusion, and interlayer diffusion. This study investigated the effects of dry density and porewater salinity on Eu(Ⅲ) diffusion in compacted GMZ bentonite from the perspective of triple pore structure. A series of through-diffusion (TD) experiments were conducted as a function of the density and porewater salinity. Then the triple pore structure was characterized using SEM and MIP tests. On this basis, a mathematical expression about the dependence of Eu(Ⅲ) diffusion on the triple pore structure was developed. Results indicated that increasing dry density inhibited the Eu(Ⅲ) diffusion, as evidenced by decreases in the effective diffusion coefficient D e , capacity factor α , and apparent diffusion coefficient D a. This inhibition is attributed to the reduction in inter-aggregate pores, inter-particle pores, and interlayer pores with increased density, thereby suppressing free water diffusion, surface diffusion, and interlayer diffusion. Conversely, increasing porewater salinity facilitated the Eu(Ⅲ) diffusion. This is likely because the relatively high concentration of Eu(Ⅲ) in the TD tests makes free water diffusion as the dominant pathway. Although surface diffusion and interlayer diffusion were inhibited owning to the decrease of inter-particle pores and interlayer pores, their negative effects on total diffusion flux were less than the positive effects of enhanced free water diffusion resulting from the increase of inter-aggregate pores. Finally, by comparing the calculated total diffusion fluxes and experimental values, the applicability of the proposed expression for predicting Eu(Ⅲ) diffusion in compacted bentonite with varying dry densities and salinities was discussed. [ABSTRACT FROM AUTHOR]

Anisotropic microstructure would be generated in the bentonite block during unidirectional compactions. Working as buffer materials, the compacted bentonite will inevitably experience shearing processes during the long-term operation of geological repositories. In this paper, high-pressure triaxial tests were conducted on unsaturated compacted GMZ bentonite specimens with different water contents, dry densities and confining stresses, with the compaction surface of the specimens oriented in both horizontally (H-type) and vertically (V-type) configurations. Results demonstrate that an increase in water content leads to a reduction in both peak strength and residual strength, while higher confining stresses enhance these strength parameters. Normally consolidated and lightly over-consolidated bentonite specimens display substantial shear deformation, whereas heavily over-consolidated specimens tend to experience brittle failure. Water content plays a significant role in shaping both the critical state line (CSL) and the Hvorslev surface (HS), with increasing water content resulting in decreased slope parameters for both and an increased intercept parameter for the HS. Generally, V-type specimens demonstrate a steeper CSL and an outwardly extended HS in contrast to that of H-type specimens. The critical state ratio for V-type specimens is about 10 % higher, and the friction angle is 2.8° greater, than that of the H-type ones. Moreover, this difference appears to increase with increasing water content. The difference of the HS slope parameter between the two specimens is minor, while the intercept parameter is higher for the V-type specimens. • Triaxial tests were conducted on unsaturated specimens with compaction surface horizontally (H) and vertically (V) • Normally and lightly over-consolidated specimens display substantial shear deformation, heavily ones experience brittle failure • Increasing water content leads to reductions in peak and residual strength, while high confining stress enhances strength • V specimens present steeper critical state lines and outwardly extended Hvorslev surface than that of H ones [ABSTRACT FROM AUTHOR]

Compacted GMZ bentonite/sand mixture is considered as a potential buffer/backfill material for high-level radioactive waste disposal in China. Because buffer/backfill material can be exposed both to groundwater and in situ stresses, further research is necessary to understand the coupled impacts of these two factors. In this study, the coupled effects of saline solution and vertical stress on the swelling deformation of compacted GMZ bentonite/sand mixtures were investigated. The one-dimensional (1D) swelling strains of bentonite/sand mixtures (30% sand by mass) were measured under vertical stresses of 0.1, 0.2, and 0.3 MPa in deionized water and 0.1, 0.5, and 1.0 mol/L NaCl or CaCl2 solutions. The results demonstrate that the final swelling strain decreased exponentially with increasing salinity under a given load and decreased with increasing vertical stress at a given concentration; CaCl2 had a greater effect than did NaCl on the final swelling strain. Furthermore, the test durations were accurately obtained by the log-time method, the salinity-test duration curves, and load-test duration curves did not always exhibit monotonic behavior. The relationship between the montmorillonite void ratio and vertical stress was still linear in double logarithmic coordinates for bentonite/sand mixtures immersed in NaCl or CaCl2 solutions. A modified prediction model was proposed to predict the final swelling strain of bentonite/sand mixtures at a low sand content with different dry densities and water content infiltrated with different concentrations of NaCl or CaCl2 solutions under different vertical stresses. [ABSTRACT FROM AUTHOR]

To ensure the long-term safety and performance efficiency of a deep geological repository for disposal of high-level radioactive waste, understanding and assessment of gas migration behaviour in its engineered barrier system are of significant importance. In this study, a coupled gas transport model is utilised to simulate and to analyse a series of gas injection/breakthrough experiments on saturated bentonite under rigid boundary or constant volume conditions. To explain the laboratory gas migration and breakthrough results, a diffusion and solubility-controlled gas transport mechanism, instead of controversial visco-capillary flow or dilatancy-controlled flow mechanisms, is implemented in the model. The aim is to examine the extent to which this mechanism can describe helium migration and breakthrough behaviours in rigidly confined, saturated bentonite specimens. The predicted results are found to be in good agreement, both qualitatively and quantitatively, with the observed experimental results indicating the adequacy of this mechanism to describe the transport processes. The model represents the gas breakthrough phenomenon as a function of gas solubility. When the dissolved concentration of the injected gas reaches the maximum soluble concentration in the entire porewater domain, gas breakthrough occurs. Since, the system reaches a steady state and no further gas can be dissolved in the rigidly confined specimens, any injected/dissolved gas must be equated by the amount dissipated to comply with the principle of mass conservation. The maximum solution concentrations of helium are predicted to be 2.01 × 10–5, 7.75 × 10–5, 1.07 × 10–4 mol/L for GMZ bentonite specimens with dry densities of 1.3, 1.5 and 1.7 g/cm3, respectively. The analysis of the injection pressure effects on gas migration behaviour revealed that, if sufficient time is permitted, gas breakthrough may occur at pressures lower than the laboratory observed injection pressures. [ABSTRACT FROM AUTHOR]

Gaomiaozi (GMZ) bentonite is the most extensively used buffer/backfill material in deep geological repositories used for the disposal of high-level nuclear waste in China. Highly alkaline ground water solutions derived from the cement dissolution may cause changes in the mineralogical composition and in the adsorption capacity of bentonite, and may affect the long-term performance of the engineering barrier system. In the present study, the alteration of montmorillonite and its influence on the adsorptive properties of GMZ bentonite were investigated. The X-ray diffraction patterns of the mineral composition showed that the main compound of the GMZ bentonite is montmorillonite. By contrast, its hydration with a solution of KOH at 80 °C produced cementitious materials, and transformation could likely lead to kaolinisation. Thermodynamic results show that the adsorption of Sr (II) on GMZ bentonite is exothermic and follows quasi-secondary adsorption kinetics. For an initial Sr (II) concentration C0 = 100 mg/L, the maximum amount of Sr (II) adsorbed (qe) on GMZ bentonite was 9.72 mg/g. The adsorption capacity of GMZ bentonite on Sr (II) decreased gradually and as a function of the mineral transformation time in a mixture of GMZ bentonite and KOH. The Freundlich isotherm adsorption model can describe in a satisfactory manner the adsorption of Sr (II) on GMZ bentonite or the transformation of bentonite at different degrees of mineral transformations. [ABSTRACT FROM AUTHOR]

Deep geological disposal has been adopted by many countries around the world as a method for disposal of high-level radioactive waste. During the construction and long-term operation of the repository, working as buffer/backfill materials, bentonite will hydrate and swell to fill up various voids in the repository including artificial gaps between canister and compacted bentonite blocks, blocks and blocks, blocks and natural barrier (host rock), etc. More importantly, this closure process will be inevitably affected by groundwater and its chemistry infiltrated. In this study, hydration tests with infiltration of solutions were conducted on GMZ bentonite specimens with artificial annular gaps. The hydration process was monitored by a camera, and the digital images taken were analyzed. Results show that the hydration process for highly compacted GMZ bentonite specimens with an annular gap can be divided into three phases, which can be identified by the development of cracks on the surface of the specimen. In the first phase, bentonite swells and gel generates upon hydration with gradually filling up of the annular gap. While in the second phase, cracks are observed in the center part on the specimen surface, and in the last phase, cracks gradually close up, and eventually disappear accompanied by macroscopic homogenization of the specimen. Different from that for the specimens infiltrated with deionized water, a more rapidly filling up of the artificial annular gap is observed for the specimens injected with salt solutions. Furthermore, with increasing concentration, both of the quantity and width of the cracks formed obviously decrease due to the reduction of both the DDL thickness and electrostatic repulsion forces. Meanwhile, with comparison to that infiltrated with NaCl solutions, a smaller scale of cracks is observed in the dry central part of the specimen infiltrated with CaCl2 solution. Finally, time consumed to reach the macroscopic homogeneous state for the specimen infiltrated with salt solutions is much shorter than that infiltrated with deionized water. Meanwhile, the time required for the specimen infiltrated with CaCl2 solution is shorter than that injected with NaCl solution. Results of the investigation on pore fluid chemistry effects on the swelling behavior of compacted GMZ bentonite with an artificial annular gap could be used for estimation of the sealing effects and time of the technological gaps in the disposal repository, as well as to provide reference for the design and even the final performance evaluation of the engineering barrier system. [ABSTRACT FROM AUTHOR]

Bentonite has been considered as a potential buffer/backfill material for construction of engineering barriers in deep geological repository for disposal of high-level nuclear waste. Anisotropy will be inevitably generated during static compaction during preparation of bentonite blocks, leading to induce possible influences on buffering function formation of the engineering barrier system and even the safety of the whole repository. In the present work, swelling deformation tests along the directions both perpendicular and parallel to the compaction plane were conducted on unsaturated compacted GMZ bentonite specimens using a self-developed lateral confined swelling test setup. Influences of initial dry density and vertical pressure on the final swelling strain and its anisotropy, as well as the mechanism of evolution of anisotropy coefficient on hydration, were analysed. Results demonstrate that the swelling time, average strain rate and final swelling strain increase with increasing initial dry density and decrease with increasing vertical pressure. Meanwhile, the swelling strain along the perpendicular direction is much higher than that along the parallel direction, exhibiting an obvious anisotropic swelling characteristic. The anisotropy coefficient is relatively large at the initial swelling stage, and then gradually decreases to be stabilised. The final anisotropy coefficient increases with initial dry density and decreases with vertical pressure. [ABSTRACT FROM AUTHOR]

Summary: In this study, a solid–liquid–gas coupled equation was established to simulate water retention characteristics of highly compacted GMZ bentonite. Then, modelling results were compared with laboratory test results. Results indicate that GMZ bentonite has a strong moisture expansion (or a limit drying shrinkage) characteristic. The control equation can simulate the water absorption and deformation characteristics very well at high relative humidity (or low suction). Environmental scanning electron microscope (ESEM) observation reveals the course grain soil texture of the surface under low relative humidity (RH), while the surface of GMZ bentonite becomes smooth (more fine‐grained soil texture) as RH increases. Differences were found between the porosities calculated by macroexperiment results and microscopic observations with ESEM method. This is because only the interaggregate pores can be observed by ESEM photographs. Additionally, we find that the simulated effective porosities are close to the results calculated by microscopic tests, while the effective porosity is considered as the main flow channel of flow. Further, the intrinsic permeability, the effective water and gas permeability are calculated based on the proposed model. The modelling results coincide well with the laboratory experimental results and support the reliability of the proposed model. [ABSTRACT FROM AUTHOR]

In the Chinese high-level radioactive waste geological disposal program, Gaomiaozi (GMZ) bentonite has been selected as the potential buffer/backfill material. After the closure of the repository, the Ca-OH-type alkaline solution (evolved cement water) released by cement degradation may last for more than 100,000 years. The bentonite will undergo the corrosion of evolved cement water (ECW) for a long period. This work focuses on the sorption property of GMZ bentonite altered by ECW. Firstly, the corrosion experiments on compacted GMZ specimens with the dry density of 1.70 Mg/m3 were carried out under constant volume conditions at two temperatures. Then, the sorption of europium (Eu (III)) onto the corroded GMZ bentonite was studied by batch experiments. The results of batch sorption tests indicate that the altered GMZ bentonite keeps an effective removal property with the uptake of Eu (III) more than 99%. The effect of high-temperature conditions of the repository on the sorption property of bentonite is not significant. The results also suggest that the evolved cement water presents no detrimental effect on the long-term adsorption performance of bentonite even under higher temperature conditions. [ABSTRACT FROM AUTHOR]

Gaomiaozi (GMZ) bentonite has been considered as a potential material for construction of engineering barrier in Chinese deep geological repository for disposal of high-level nuclear wastes. In the present work, thermal conductivity tests were conducted on statically compacted GMZ bentonite specimens using the needle probe method. Cubic specimens with different initial saturation degrees and dry densities were prepared and the thermal conductivity tests were conducted along the directions both parallel and perpendicular to the bedding plane of the specimen. Results show that the thermal conductivity nonlinearly increases with increasing saturation degree with a transition point that proximately corresponding to the air entry value. Meanwhile, the thermal conductivity along the parallel direction is much higher than that along the perpendicular direction. The anisotropy coefficient decreases with increasing saturation. However, the anisotropy coefficient increases with increasing dry density and the influence depends on saturation degree. [ABSTRACT FROM AUTHOR]

A series of wetting tests on unsaturated compacted GMZ07 sodium bentonite, taken from the Gaomiaozi area, Inner Mongolia, China, was performed to investigate the swelling characteristics of the GMZ07 sodium bentonite at temperatures of 5, 20, 60 and 75 °C. The influences of temperature and initial dry density on swelling deformation and swelling pressure were studied by performing a series of constant-stress and constant-volume tests under the wetting condition. The test results showed that the swelling deformation and swelling pressure due to saturation decreased slightly with increasing temperature. A linear relationship was present between the vertical pressure and corresponding saturated void ratio in the semi-logarithmic coordinate at a given temperature; this line is referred to as a state line. Swelling or collapse due to saturation was found to be dependent on the relative location of the initial state between the state line and initial state. The void ratios after saturation at different initial dry densities were close at the same temperature and vertical pressure. [ABSTRACT FROM AUTHOR]

The gas permeability of buffer/backfill materials is one of the key scientific issues in deep geological disposal. It is closely related to the pore structure. The current study investigates the pore structure and its effect on gas permeability by combining gas permeability tests and NMR tests on GMZ bentonite. It was found that the gas permeability decreases with the confining pressure. The variation in gas permeability is the result of changes in the pore structure. The initial pore size distribution of the compacted GMZ bentonite sample with a dry density of 1.70 g/cm3 is dominated by micron pores. After being subjected to certain confining pressures, the pores become composed of two parts: nanoscale and micron-scale pores, with nanoscale pores predominating, pores develop more discretely and connectivity becomes poor. As the confining pressure increases, the pore space tends to develop towards a uniform distribution, while the pore structure undergoes irreversible changes. The response of the pore structure to stress has a certain hysteresis, when the stress is fixed, the internal pore structure would undergo a certain time adjustment to reach a stable state. [ABSTRACT FROM AUTHOR]

Gas permeability is a crucial parameter for assessing the long-term gas tightness of bentonite in deep geological repositories. Although extensive experimental work has been conducted, there remains a deficiency in models capable of describing the variations in its gas permeability, particularly when considering the impacts of stress and temperature. This study presents a gas permeability model that directly addresses the initial relative humidity (RH), stress, and temperature conditions of unsaturated bentonite, utilizing a perspective grounded in the double pore structure. Validation of the model encompasses three key aspects: water retention characteristics, alterations in gas permeability under varying stress levels, and gas permeability fluctuations corresponding to temperature changes under constant stress conditions. The findings indicate that the model adeptly captures the variations attributable to each factor, enabling the quantitative assessment of gas permeability across multiple scales. The model parameters closely correlate with the material's hydraulic history, facilitating a consistent description through establishing a relationship with suction. This approach achieves a standardized quantification of gas permeability. Notably, the model can potentially simulate the evolution of gas permeability in compacted bentonite in deep geological disposal of high-level waste (HLW). • A gas permeability model considering the stress and temperature effects was proposed. • The validity of the model was verified by water retention and gas permeability tests. • The suction uniformity of the model parameters was discussed. [ABSTRACT FROM AUTHOR]

The mineral alteration and microstructure evolution of compacted GMZ bentonite was investigated during the long-term in the Beishan area, a candidate for a high-level radioactive waste repository in China. The compacted GMZ bentonite samples at initial dry densities of 1.5 and 1.7 Mg/m3 were infiltrated by synthetic Beishan Site Water (BSW) and two synthetic cement solutions (Young Cement Water -YCW and Evolved Cement Water - ECW) under constant volume conditions for about 2 months in this study. X-Ray diffractometry (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM-EDS) tests were carried out to observe the mineralogy and microstructure of the specimens after experiencing the three types of pore solutions. There is evidence of cation exchange reactions, some zeolites and C-S-H in the GMZ bentonite after the infiltration of alkaline solutions. The hydration of alkaline solutions and mineralogy alteration result in changes to the microstructure of compacted GMZ bentonite. The generated secondary mineral on the surface of montmorillonite leads to the clogging of inter-aggregate pores. In the case of high dry density, this resulted in pore occlusion. [ABSTRACT FROM AUTHOR]

Based on the results of one-dimensional free swelling tests, the coupled effects of NaCl-Na 2 SO 4 solutions and temperature on the swelling deformation property of compacted GMZ bentonite-sand mixtures were analyzed. The concentrations of the NaCl-Na 2 SO 4 solutions were 0, 0.008, 0.04, 0.08 and 0.16 mol/L, the temperatures were 20 °C, 40 °C, 60 °C, 80 °C and 90 °C and the sand contents were 0, 30 and 50%. X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) tests were also performed to find the influences of the temperature, solution concentration and sand content on the mineral composition and microstructure. The results showed that the maximum swelling strain of the specimens increased with an elevated temperature, while it decreased with increases in the solution concentration and sand content. The swelling process was accelerated by the rising temperature. The XRD tests indicated that the combined effect of a high temperature and a high solution concentration may have caused more Na+ from the saline solutions to enter the interlayer, partly displacing the divalent cations. The MIP tests demonstrated that adding sand to the bentonite could reduce the amount of inter-granular pores and increase the intra-aggregate pores. These findings are advantageous to the transfer of heat and pour fluid in the specimens; and thus, the swelling deformation capacity of the bentonite-sand mixtures was higher than that of pure bentonite under the same conditions. [ABSTRACT FROM AUTHOR]

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• Temperature-controlled steady state water permeability tests and gas injection tests were conducted. • Intrinsic (water) permeability, gas permeability and breakthrough pressures at temperatures were obtained. • Intrinsic permeability of GMZ bentonite ranges between 3.2 E-20 and 5.83 E-20 m2, and increases as temperature increases. • Measured effective gas permeability slightly fluctuates with time and increases with temperature increases. • Measured gas breakthrough pressures are much higher than the snap-off pressures. Determination of parameters for description of gas migration in saturated bentonite is of great importance for the design and construction of artificial barriers in the geological repository for the disposal of high-level radioactive nuclear waste. In this paper, temperature-controlled gas injection tests were conducted on initially water-saturated bentonite specimens using the residual capillary pressure (RCP) technique. Effective gas permeabilities in low injection pressures and the gas breakthrough pressures at temperatures 20, 40 and 60 °C were obtained. Results show that: (i) for all the temperatures tested, the intrinsic water permeabilities (k i n) range between 3.2 × 10−20 and 5.72 × 10−20 m2, furthermore, those values increase with rising temperature. Meanwhile, the intrinsic water permeabilities derived from the steady state tests are lower than those obtained from the non-steady state tests; (ii) for all the temperatures tested, the effective gas permeabilities (k e f f) corresponding to the viscous gas flow before gas breakthrough range between 4.81 × 10 - 24 and 2.74 × 10 - 22 m2, with slight fluctuations with time. The maximum effective gas permeabilities measured at the occurrence of gas breakthrough on the initially water-saturated bentonite specimens extend from 2.27 × 10 - 18 up to 3.32 × 10 - 17 m2 and increase as temperature increases, while the time required for gas breakthrough decreases as temperature increases; (iii) for all the temperatures tested, the gas breakthrough pressures measured on the initially water-saturated bentonite specimens vary from 2.74 to 4.08 MPa, while the residual capillary pressure differences, also denoted as snap-off pressures (P snap-off), range from 0.2 to 0.38 MPa. Additionally, the gas breakthrough pressures and the residual capillary pressures decrease as temperature increases. [ABSTRACT FROM AUTHOR]

The effect of Bacillus spp. on Re(VII) and Se(IV) diffusion in compacted GMZ bentonite was investigated by a through-diffusion method. Bacillus spp. had no significance effect on the effective diffusion coefficient (De) and the accessible porosity (εacc) of Re(VII), whereas a decrease of De and an increase of distribution coefficient (Kd) were observed for Se(IV). Moreover, Archie's law and an exponential decay equation were employed to fit the De value of Re(VII) and Se(IV). [ABSTRACT FROM AUTHOR]

Abstract Knowledge of the peak shear strength of compacted bentonites is of fundamental importance to high-level nuclear waste repositories, as it forms the basis of the analysis of repository stability. The shear strength properties of GMZ bentonites are throughout investigated by undrained direct shear tests with different concentrations of NaCl solutions, and are explained in the light of the modified effective stress concept, which takes into consideration of the osmotic suction. The modified effective stress concept is expressed as p ' = p + π (p / π) D s-2, incorporated with the normal stress (p) and osmotic suction (π) based on the bentonite surface fractal model. The peak shear strength behavior of GMZ bentonites in NaCl solutions appears to be explained satisfactorily by the modified effective stress. According to the Mohr-Coulomb criterion, the peak shear strength of bentonites saturated by distilled water and NaCl solutions is expressed by the same unique line in the τ - p ' plane. GMZ bentonite has the same values of the peak friction angle and the peak cohesion in distilled water and NaCl solutions of different concentrations. Highlights • Peak shear strength of bentonite expressed by modified effective stress according to the Mohr-Coulomb criteria • A modified effective stress expressed as p ′ = p + (p / π) D s ‐2 π , incorporating with osmotic suction (π) • The surface fractal dimension (D s) of GMZ bentonite measured using the nitrogen adsorption isotherm. [ABSTRACT FROM AUTHOR]

• The complex interactions among the hydration-induced cracks were revealed. • Existing cracks provided space to initiate and propagate the succeeding ones. • Closed cracks were prone to re-cracking once encountering stress/suction change. • Boundary effect on cracking degree was linked to friction in the swelling process. Bentonite is widely used as a buffer/backfill material to dispose of the high-level radioactive wastes (HLWs). The precast bentonite blocks can cause the technological voids to be widespread in the repositories, acting as the primary channels for water migration. Luckily, bentonite will swell considerably when exposed to water, making it possible to seal the voids. The self-sealing process intensifies the uneven expansive deformation of bentonite and induces the cracking phenomenon. To be close to the actual situation, various boundary conditions were considered in this paper to carry out a series of hydration tests on compacted Gaomiaozi (GMZ) bentonite. The dynamic evolution of hydration cracks was investigated in both qualitative and quantitative way. Experimental results showed that the cracks basically followed a tendency of emerging first, then extending and finally closing. The bentonite sample with radial and vertical technological voids was characterized with a greater cracking degree than the case where only a radial void existed. According to the time history curves of geometric parameters, there was a close correspondence between the increase and the decrease stages from different cracks, indicating that the existing cracks provided deformation space for the initiation and propagation of subsequent cracks by sacrificing themselves. Due to stress or suction change, a re-cracking phenomenon was reflected on some bimodal curves, which demonstrated a significant memorability of the hydration cracks. Generally, the discrepancy in cracking behavior under different boundary conditions was related to the friction effect. The existence of vertical technological voids decreased the swelling pressure, thus reducing the friction force on the boundary. The suppression effect on the crack propagation was consequently weakened, which was conducive to forming wide cracks and strengthening the mutual connection between cracks. [ABSTRACT FROM AUTHOR]

Abstract During the long-term operation of a deep geological repository, the buffer/backfill properties of compacted bentonite could be affected by the chemistry of the infiltrated porewater. In this study, swelling tests and microstructural analyses were conducted on the compacted GMZ bentonite specimens with an initial dry density of 1.5 or 1.7 Mg/m3 with infiltration of distilled water, as well as, KCl and KOH solutions (0.1 and 1.0 M). Influences of K+ solutions on the swelling behavior of compacted GMZ bentonite were investigated. Results show that swelling properties of compacted GMZ bentonite specimens could be significantly influenced by concentration of K+ solutions and dry density of specimens. Swelling pressure of compacted GMZ bentonite specimens was significantly attenuated by infiltration of K+ salt/alkaline solutions. For swelling strain tests with infiltration of KCl or KOH solutions, instead of a secondary swelling stage, a volumetric collapse stage was observed. Micro-structural test results demonstrate that the attenuation of swelling behavior could be attributed to the K+ exchange or silica dissolution. Highlights • Influences of K+ solutions on the swelling behavior of compacted GMZ bentonite were investigated • Swelling pressures of compacted GMZ bentonite were significantly attenuated by K+ salt/alkaline solutions • Swelling attenuation could be attributed to the K+ fixation, ion exchange, smectite dissolution and mineral transformation. [ABSTRACT FROM AUTHOR]

Abstract Gaomiaozi (GMZ) bentonite is a promising buffer material for Chinese high-level radioactive waste (HLW) disposal, and will be either compacted in situ or used as dense prefabricated material when constructing engineered barriers. Uniaxial compaction can induce anisotropy of the bentonite structure, resulting in anisotropic swelling. In this work, a series of swelling pressure tests were performed on compacted GMZ bentonite to investigate the anisotropy of its property and highlight the dry density effect. Two kinds of technological gaps were simulated to analyze the change in anisotropy of properties with limited swelling deformation. The relationship between swelling pressure and dry density was found to be logarithmic, and axial pressure increased more rapidly than radial pressure with dry density. The ratio of radial to axial pressure decreased significantly in the dry density range 1.395 Mg/m3–1.706 Mg/m3; beyond this, it remained at about 0.76. For the swelling volume ratio of less than 16.7%, the stable swelling pressure was controlled by the final dry density. For a larger swelling volume ratio, the axial gap induced a smaller swelling pressure compared with the case of no gap at the same final dry density. Highlights • The ratio of radial to axial swelling pressure decreases with dry density. • Larger gap induces smaller swelling pressure for the same final dry density. • Smaller gap induces density inhomogeneity. [ABSTRACT FROM AUTHOR]