MX-80 Bentonite Research Articles

Influence of Ca–Na–Cl physicochemical solution properties on the adsorption of Se(-II) onto granite and MX-80 bentonite

The adsorption behaviour of Se(-II) onto granite and MX-80 bentonite in Ca–Na–Cl solutions has been studied utilizing adsorption experiments and surface complexation modelling. Adsorption kinetic experiments allude to steady-state adsorption periods after 7 days for granite and 14 days for MX-80 bentonite. Batch adsorption experiments were carried out to determine the influence that the physicochemical solution properties would have on Se(-II) adsorption behaviour. Adsorption of Se(-II) onto granite and MX-80 bentonite follows the trend of anionic adsorption, with a decrease in Rd values as the solution pH increased. There is also an ionic strength influence on the adsorption of Se(-II) onto granite with a decrease in the Rd value as the ionic strength increased. This effect is not found when observing Se(-II) adsorption onto MX-80 bentonite. Final experiments with a representative groundwater, determined that the adsorption of Se(-II) onto granite and MX-80 bentonite returned Rd values of (1.80 ± 0.10) m3∙kg−1 and (0.47 ± 0.38) m3∙kg−1, respectively. In support of the experiments, a surface complexation modelling approach has been employed to simulate the adsorption of Se(-II) onto granite and MX-80 bentonite, where it was determined that two different surface complexes, ≡S_Se−and ≡SOH2+_H2Se were capable of simulating Se(-II) adsorption behaviour.

Insights into the thermo-hydraulic properties of compacted MX80 bentonite during hydration under elevated temperature

In high-level radioactive waste geological repositories, compacted bentonite undergoes coupled thermo-hydraulic processes due to heat released from a central waste canister and groundwater imbibition from the surrounding host rock. An understanding of these processes is essential for long-term simulations of radionuclide migration and canister corrosion, which requires an understanding of temperature effects on the coupled thermo-hydraulic properties governing these processes. In this study, a tank-scale radial infiltration test was used to investigate water imbibition processes in compacted bentonite under a central heater temperature of 200 °C that simulates high thermal gradients in a repository. Interpretation of this test focuses on evaluation of the liquid water wetting front during hydration and the interpretation of the temperature-dependent transient soil water retention curve (SWRC), thermal conductivity function (TCF), and hydraulic conductivity function (HCF). The SWRC during imbibition follows a temperature-dependent wetting path. Temperature effects on the hydraulic conductivity of bentonite in saturated conditions had the greatest effect on the shape of the HCF, with minimal temperature effects at higher suctions. The transient thermal conductivity data matched well with a new TCF linked with the SWRC shape. Although the bentonite layer was restrained, local deformations during hydration may have affected the shapes of the TCF and HCF.

Influence of the water content on the complex conductivity of bentonite

Compacted bentonite is considered as a potential buffer material for deep geological disposals of high-level nuclear wastes. Methodologies to non-intrusively monitor the water content of such sealing materials are important in the context of the safety of these storage facilities and for various engineering applications as well. Induced polarization is a non-intrusive geophysical method sensitive to the water content of porous media. We investigated the complex conductivity spectra of 69 samples made of 2 distinct MX80 bentonites, one in the form of powder from crushed pellets (Type I) and the other in the form of a granulated bentonite mixture (GBM, type II). The samples are prepared at different compaction states and saturations. The pore water conductivity of the porous samples is estimated to be ∼2.5 S m−1 (25 °C) by two different methods. The complex conductivity spectra were obtained at room temperature (25 ± 2 °C) in the frequency range 1 Hz-45 kHz. In-phase and quadrature conductivities reflect conduction (electromigration) and polarization processes, respectively. At a given frequency, both the in-phase and quadrature conductivities increase with the water content along a trend that is independent of the compaction state. An induced polarization model based on the dynamic Stern layer model is used to explain these results. The first Archie's exponent m is inferred from the formation factor F using the in-phase conductivity data versus the pore water conductivity data at different salinities (NaCl, 25 °C). The dynamic Stern layer model applied to polarization correctly predicts the dependence of the in-phase conductivity, quadrature conductivity, and normalized chargeability with the water content and the Cation Exchange Capacity (CEC). This petrophysical work can easily be applied to time-domain induced polarization data collected in field conditions to monitor hydraulic barriers.

Measured swelling, hydraulic and thermal properties of MX-80 bentonite: Distinguishing between material variability and measurement limitations

The Canadian Nuclear Waste Management Organization (NWMO) is investigating Sedimentary and Crystalline media as potential hosts for a Deep Geological Repository (DGR) for permanent isolation of used nuclear fuel and fuel wastes. In NWMO's concept, densely compacted bentonite clay is used to surround the used fuel containers and to backfill the placement rooms.Development of sealing concepts and performance prediction models requires many materials properties inputs. These inputs are important to the evaluation of hydraulic conductivity (k), swelling pressure (Ps) and heat transfer. Evaluation of these and other properties is complicated by factors such as unidentified variability in material composition (mineralogy) and differences in test methodologies. In order to eliminate the effects of technique, data obtained using similar methods are used in this study. This should result in data scatter being more related to materials variability and limitations than measurement method.In an effort to assess the variability in the reported Ps, k and thermal properties and determine representative values or relationships for input into performance models, sufficiently large databases are needed to allow for statistical evaluation for a full range of potential repository conditions. Unfortunately, in many cases these databases are not extensive, particularly for materials in a highly saline environment.This paper presents statistical evaluations of the relationships between density, k, Ps and thermal properties using a database generated from literature sources for a well-known bentonite (MX-80) under freshwater and saline porefluid conditions. Also included is previously unpublished work by NWMO where repository-relevant groundwaters were used in testing. To limit uncertainty, the data used were obtained from sources reporting use of MX-80 bentonite and providing at least basic mineralogical information. From these data, best-fit, confidence and prediction bands and intervals were generated.

Microstructural investigation into the damage behaviour of MX80 bentonite pellet under wetting/drying path using Mercury Intrusion Porosimetry (MIP) and Micro-Computed Tomography (μCT)

MX80 bentonite pellet has been considered as a promising sealing/backfilling material in geological disposal facilities for high-level and intermediate-level radioactive wastes. As the sealing/backfilling material which is subjected to hydration by pore water or vapour from host rock can be damaged by wetting-induced cracking, it is essential to clarify the suction effect on the damage behaviour of the material. In this study, a combined Mercury Intrusion Porosimetry/Micro-Computed Tomography (MIP/μCT) method was applied to investigate the microstructural damage of the MX80 bentonite cylinder-shaped 32-mm pellets equilibrated under suctions s from 113 to 1.5 MPa for wetting and from 113 to 262 MPa for drying. The enlarged pore size distribution (PSD) curves by MIP/μCT revealed an insignificant drying-induced closure of cracks, as opposed to massively developed wetting-induced cracks, with the size of the largest cracks reaching several millimetres. This is confirmed by the μCT observation. Indeed, the μCT images showed a negligible effect of drying on cracking, contrary to the effect of wetting characterised by significant cracking from the marginal area to the central part of pellet, indicating significant damage of pellet by wetting. Additionally, only horizontal cracks were identified at s ≥ 82 MPa. Vertical cracks emerged and propagated only at s ≤ 57 MPa, together with further expanding of horizontal cracks, due to the preferentially horizontal orientation of clay particles resulting from the one-dimensional compaction in the pellet fabrication. A damage coefficient D was defined as the ratio of the volume of macro-pores to the total volume of pellet. D increased slightly upon drying from suction 113 to 262 MPa and significantly upon wetting from suction 113 to 3.2 MPa, but kept almost unchanged when wetting furthered from suction 3.2 to 1.5 MPa because the clogging of macro-pores by the exfoliation effect slowed down the cracking rate.

Geochemical interactions at the steel-bentonite interface caused by a hydrothermal gradient

Bentonites are used in deep geological disposal facilities as an engineered barrier to isolate high level radioactive waste, contained in metallic canisters. The present study, performed at laboratory scale, evaluated the behaviour of MX-80 (Na-bentonite) and FEBEX (Ca-Mg-Na-bentonite) in contact with carbon steel, subjected to a hydrothermal gradient. A dominant Na-Cl-SO4 saline solution was injected towards the compacted bentonite from the top, while a heater, located at the bottom in contact with the steel disc, maintained a constant temperature of 100 °C. The cells were studied after one and six months of interaction. Changes in the physical (water content and specific surface area) and chemical (cation exchange capacity and element distribution) properties of the bentonite were observed, as well as the formation of a corrosion layer on the steel, at the interface with bentonite, mainly composed of magnetite, maghemite and hematite. The bentonites were mainly altered at the mm scale, being enriched in iron content, and changing their ion distribution to Ca-dominant smectite (in MX-80 bentonite).

Investigation into the mercury intrusion porosimetry (MIP) and micro-computed tomography (μCT) methods for determining the pore size distribution of MX80 bentonite pellet

Investigation into the mercury intrusion porosimetry (MIP) and micro-computed tomography (μCT) methods for determining the pore size distribution of MX80 bentonite pellet

Scaling effect on cesium diffusion in compacted MX-80 bentonite for buffer materials in HLW repository

Abstract In this study, radionuclide behavior in high-level radioactive waste (HLW) disposal repositories is complicated because of the spatial heterogeneity of porous media, coupled flow-transport mechanisms, and multiple chemical reaction processes. Discrepancies in the diffusion behavior of a non-sorbing tracer (HTO) and a reactive tracer (137Cs) in porous media have long been recognized but are not yet fully understood, which hinders effective assessment of the capabilities of buffer materials. This paper was dedicated to exploring and explaining the discrepancies in the transport behavior of non-sorbing and reactive tracers through laboratory experiments and an investigation of contributing mechanisms. Our results showed that for a bentonite sample of the same thickness, 137Cs has smaller apparent and less effective diffusion coefficients than those for HTO. These discrepancies can be attributed to the negative surface electric effects, atomic properties, and chemical reactions. In the case of bentonite samples with different thicknesses (0.5, 0.75, 2.0, 2.5 cm), the apparent and effective diffusion coefficients show an increasing trend with bentonite thickness. According to the experimental data and fitting results, the apparent and effective diffusion coefficients are highly related to bentonite thickness. Thus, scaling effects on transport parameters were proposed to explain the results, which were attributed to the nonuniform distribution of the pore space in the bentonite sample. The scale effect behavior of radionuclide was quantified through a regression analysis. The results can be used to improve buffer designs for radionuclides diffusion.

Effect of the alkaline solution concentration on swelling performance of MX80 granular bentonite

The influence of alkaline solution on swelling performance of granular bentonite is one of the significant factors that must be considered in the design of deep geological repository for disposing the high-level nuclear waste (HLW). In this paper, the effect of alkaline solution concentration on the swelling performance of compacted granular and powdered MX80 bentonite specimens were investigated by the swelling strain and swelling pressure tests, together with X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. Tests results showed that the swelling strain and swelling pressure of compacted granular MX80 bentonite increased by 2.6% and 7.6%, respectively, when the alkaline solution concentration changed from 0 to 0.1 mol/L. With increasing the concentration from 0.1 mol/L, the swelling strain and swelling pressure decreased significantly. When the alkaline solution concentration changed from 0 to 1.0 mol/L, the swelling strain and swelling pressure of the granular bentonite decreased by 35.1% and 41.6%. Observed XRD and SEM results showed that effect of the alkaline solution on bentonites led to a decline in montmorillonite content, and the large layered structure was dissolved into small layered structures and particles, which caused the decline in the swelling performance of granular MX80 bentonite.

Simple modelling of fast hydration processes in bentonite pellet fills

A novel hydro-mechanical formulation is presented to simulate the fast wetting of bentonite pellet fills. The model is based on a triple porosity concept, but its formulation uses a double porosity structure for simplicity. It is assumed that the fast wetting process results in a liquid pressure in the inter-pellet voids equal to the boundary liquid pressure. This enables the system of equations to be solved to have the same structure than that used to analyse the behaviour of the bentonite blocks with numerical tools based on double porosity approaches. Thus, it is relatively simple to extend such tools to incorporate the triple porosity conceptual model that is presented in this paper. To assess the scope of the proposed formulation, a set of tests in which the inter-pellet void space is rapidly flooded and a test in which the inter-pellet void space is circulated with progressively more humid air are simulated. The results obtained for pellets fills of both FEBEX and MX-80 bentonites are very satisfactory, despite the simplicity of the formulation presented.

Sealing materials for a deep geological repository: evaluation of swelling pressure and hydraulic conductivity data for bentonite-based sealing materials proposed for use in placement rooms

Several countries are planning to safely contain and isolate used nuclear fuel in a deep geologic repository. Similar to other countries, Canada's Nuclear Waste Management Organization (NWMO) proposes to place used fuel in corrosion-resistant containers deep underground as part of a multiple barrier approach. Key components of the sealing systems are bentonite-based and must maintain low hydraulic conductivity and an ability to swell when in contact with free groundwater. Swelling pressure ( Ps) and hydraulic conductivity ( k) data for the specific bentonite product known as MX80 bentonite under low (<15 g/L) through high (>325 g/L) total dissolved solids’ (TDS) pore fluids that simulate groundwaters that could be encountered in sedimentary and crystalline rock geospheres are reviewed, and information from ongoing NWMO studies of behaviour is presented. These data are statistically evaluated to establish the best-fit and prediction limit relationships between compacted dry density and Ps and k as they are influenced by porefluid composition. They also allow identification of dry density and(or) TDS (salinity) conditions where Ps and k may not meet defined performance. From these analyses, it is expected that bentonite can be placed such that on achieving fluid saturation and density equilibration, it meets the requirements set for placement room fill.

Investigation of water-retention and uniaxial compression behaviour of MX80 bentonite pellet at various suctions

MX80 bentonite pellet/powder mixture has been regarded as a candidate sealing material in deep geological high-level radioactive waste disposal. As single pellet plays an important role in the global hydromechanical behaviour of the mixture, a series of single pellet compression tests were conducted on cylindershape MX80 bentonite 32-mm pellet after reaching suction equilibrium. The changes in volume and water content during the suction equilibration with the vapour equilibrium technique allowed the water-retention properties to be investigated. X-ray μCT observations were also made on pellets after reaching the target suctions. Results showed that upon drying the water content, void ratio and volumetric strain decreased slightly, but increased significantly upon wetting. Moreover, upon drying the network of cracks presented insignificant change and the cracks were mainly located in the marginal areas of pellet, while on wetting, cracks substantially propagated from the marginal areas to the core of pellet. The uniaxial compression tests on single pellets showed that the axial strain εaxial contributed to the contractancy, while the lateral strain εlateral to the dilatancy. The uniaxial shear strength σa, max, the crack closure (CC) stress σcc, the crack initiation (CI) stress σci, the crack damage (CD) stress σcd and the Young’s modulus E were found to decrease with suction decreasing, evidencing the suction effect through softening and cracking. Because of the combined effect of suction and wetting-induced cracks, the Poisson’s ratios v increased until the suction decreased to 38-25 MPa, and then decreased with further suction decrease. Uniaxial loading closed the horizontal wetting-induced cracks at suctions ≤ 9 MPa, leading to a drop of εlateral before CC.

Numerical modelling of unsaturated MX-80 bentonite subjected to two different hydration paths and subsequent loading to high-pressures

MX-80 bentonite has been considered as a suitable material for the construction of engineered barriers employed in deep geological radioactive waste repositories. These barriers are generally formed of compacted unsaturated bentonite, the latter experiencing a slow saturation due to its low permeability whileinteracting with the surrounding groundwater. In order to verify the long-term safety requirements of engineered barriers, their response to hydration has to be carefully assessed. As part of the recent European project BEACON (Bentonite Mechanical Evolution), the behaviour of MX-80 bentonite subjected to different hydration paths was investigated in a number of laboratory and field experiments and numerical studies. This paper is concerned with numerical simulations of two laboratory experiments performed during the project, with the objective of examining the predictive capabilities of the proposed numerical modelling approach. The experiments were selected due to the granular state of bentonite at its placement in the testingapparatus, which differed from the large number of previous experiments conducted on specimens of compacted bentonite blocks. The paper provides a brief introduction to the adopted modelling framework, a summary of calibrated parameters for the hydro-mechanical constitutive modelling and the results of numerical simulations, concluding that a satisfactory numerical simulation of the experiments was achieved.

Modelling a hydration and heating column test on unsaturated bentonite using a double-porosity approach

A controlled re-saturation of a heated 50 cm long column made of MX-80 bentonite pellets in the CIEMAT laboratory facilities (Madrid, Spain) has been running for 10 years (2011-2021). The experimental data on the saturation state and the swelling response of the bentonite material at high temperatures provided useful information for the verification of a double-porosity approach that considers the hydro-mechanical coupling between the micro- and macro-pore levels and the definition of retention curves for each structural domain. The numerical modelling with this double-porosity model has shown to be able to reproduce the main behaviour features observed in the lab test.

Large scale Vertical SEALing (VSEAL) test: Impact of gas migration on bentonite based vertical seals

The Institute for Radiological protection and Nuclear Safety (IRSN) has launched the VSEAL project to investigate the impact of gas migration on the long-term performance of bentonite based vertical sealing systems. This project relies on in-situ experiments emplaced in IRSN´s Underground ResearchLaboratory (URL) in Tournemire (South France) and small-scale tests conducted in laboratory. The studied material is mixture of MX80 bentonite high density pellets and powder which are being evaluated as possible sealing materials in deep geological repositories. The test is still in progress, but the collected set of data provides already valuable information of the hydro mechanical behavior of Vertical sealing systems during hydration.

Elastic and cracking behaviour of MX80 bentonite pellet at various suctions in uniaxial compression

MX80 bentonite pellet/powder mixture has been considered as a candidate sealing/filling material in geological high-level radioactive waste disposal. As a single pellet can play an important role in the global hydro-mechanical behaviour of the mixture, the compression behaviour of a single MX80 pellet was investigated by performing uniaxial compression tests on cylinder-shaped pellets at different initial suctions, imposed by the vapour equilibrium technique. Large cracks were observed at suctions ≤ 9 MPa. Results from the compression tests showed that the axial strain εaxial contributed to the contractancy, whereas the lateral strain εlateral contributed to the dilatancy. The uniaxial shear strength σa, max, the crack closure (CC) stress σcc, the crack initiation (CI) stress σci, the crack damage (CD) stress σcd and the Young's modulus E were found to decrease with decreasing suction, providing evidence of the suction effect through softening and cracking. CI was considered as the inflection point of Δεlateral and the onset of dilatancy. Because of the combined effects of suction and wetting-induced cracks, the Poisson's ratios v increased until the suction decreased to 38–25 MPa, and then decreased with further suction decrease. Loading closed the wetting-induced horizontal cracks at suctions ≤ 9 MPa, leading to a drop of εlateral before CC.

Effect of heating time on hydro-mechanical behaviour of MX80 bentonite

Effect of heating time on hydro-mechanical behaviour of MX80 bentonite

Physical Modeling of Coupled Thermohydraulic Behavior of Compacted MX80 Bentonite during Heating

Physical Modeling of Coupled Thermohydraulic Behavior of Compacted MX80 Bentonite during Heating

Investigation of Suction Effects Due to Stress Release with Compacted MX80 Bentonite

Investigation of Suction Effects Due to Stress Release with Compacted MX80 Bentonite

Effect of thermal ageing on physical properties of MX80 bentonite under high-temperature conditions

In a deep geological repository (DGR) system, the buffer layer is indispensable to ensuring the safe disposal of high-level radioactive nuclear waste (HLW). Because the heat generated by the decay of the spent nuclear fuel in a canister is released to the surrounding buffer layers, the bentonite buffer material experiences long-term high-temperature conditions. Therefore, the variations in physical properties of bentonite buffer material under high-temperature conditions are one of the important parameters in the DGRs for HLW. A series of tests on the specific gravity, specific surface area (SSA), Atterberg's limits, and free swelling ratio of MX80 bentonite after heating for different times at a high temperature of 200 °C were conducted, to investigate the influence of thermal ageing time on its physical properties. Then, the microscopic investigations, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), were conducted to explain the aforementioned variations from the microscopic point of view. Based on the crystal layer structure characteristics of montmorillonite, the quantitative relationships between the physical properties of bentonite and its mineral composition or bound water content were established to further explain the mechanism by which thermal ageing affects the physical properties of MX80 bentonite. The results indicate that with increasing heating time, the specific gravity, SSA, liquid limit, plastic limit, plastic index and free swelling ratio decrease sharply by 2.5%, 4.5%, 3.1%, 5.5%, 2.8% and 30.2%, respectively, within 15–30 days of heating. After 30 days of heating, their variations are negligible. Under high-temperature conditions, the transformation of mineral composition, desorption of bound water, and changes in the micro-morphology are the fundamental reasons for the variations in the physical properties of bentonite, and they influence and interact with each other.