FEBEX Bentonite Research Articles

Soil-water characteristic curves of bentonites in isochoric conditions during wetting: measurement and prediction

The determination of the soil-water characteristic curve (SWCC) of compacted bentonites in the isochoric condition is a prerequisite for unsaturated flow simulations in several geoenvironmental applications. The SWCC data are, however, not readily available for many compacted bentonites over a wide suction range due to difficulties associated with the testing. In this work, wetting SWCCs of four Indian bentonites of different plasticity were established experimentally at compaction dry densities of 1.4, 1.6, and 1.8 Mg/m3 in isochoric conditions using two independent laboratory techniques in different suction ranges. The modified Kovács (MK) model was extended to simulate the SWCC data of the Indian bentonites. Empirical relationships between MK model parameters and compaction dry density for these bentonites were established. A theoretical procedure for the estimation of SWCCs of the compacted bentonites based on the basic bentonite properties and proposed correlations was proposed and validated on the available wetting SWCC data of compacted GaoMiaoZi (GMZ), MX80, and FEBEX bentonites from the literature studies. The proposed method predicts the wetting SWCCs of compacted powder bentonites, in volume-restrained condition, from the basic properties satisfactorily. The proposed model is an approximate method for predicting the SWCCs of bentonites in the absence of measured data and in understanding the influence of dry density and plasticity.

Bentonite Nanoparticle Stability and the Effect of Fulvic Acids: Experiments and Modelling

In this study, the critical coagulation concentration (CCC) for FEBEX bentonite colloids is determined by colloid coagulation studies under variation of pH, electrolyte concentration, and fulvic acid (GoHy-573FA) content. For CaCl2 electrolyte solution, a pH-independent Ca-CCC of 1 mmol L−1 is found. In the case of NaCl background electrolyte, a pH-dependent Na-CCC can be determined with 15 ± 5 mmol L−1 at pH 6, 20 ± 5 mmol L−1 at pH 7, 200 ± 50 mmol L−1 at pH 8, 250 ± 50 mmol L−1 at pH 9, and 350 ± 100 mmol L−1 at pH 10, respectively. The addition of 1 mg L−1 dissolved organic carbon in the form of fulvic acid (FA) increases the Ca-CCC to 2 mmol L−1. An association of FA with FEBEX bentonite colloids as surface coating can clearly be identified by scanning transmission X-ray microscopy (STXM). The experimental bentonite stability results are described by means of an extended DLVO (Derjaguin–Landau–Verwey–Overbeek) approach summing up hydration forces, short-range Born repulsion, van der Waals attraction, and electrical double layer repulsion. The measured zeta (ζ)-potential of the bentonite colloids is applied as platelet face electrokinetic potential and the edge electrokinetic potential is estimated by the combination of silica and alumina ζ-potential data in the ratio given by the FEBEX bentonite structural formula. Adjusting the montmorillonite face electrokinetic potential by a maximum of ±15.9 mV is sufficient to successfully reproduce the measured stability ratios. Due to the uncertainty in the ζ-potential measurement, only semiquantitative calculations of the stability ratio can be given.

A double-structure hydromechanical constitutive model for compacted bentonite

This paper presents a double-structure elastoplastic constitutive model that can describe the hydromechanical behavior of compacted bentonite. The model is formulated based on a new developed double-structure hydromechanical framework that consists of four key components, including the mechanical model and water retention model at the macrostructure scale and microstructure scale, respectively. Hydromechanical coupling at each structure scale is achieved by liking the effective stress with the corresponding water retention model. Independent coupled hydromechanical models are developed for each structure scale and the global response is obtained with an introduction of the structure parameter ξ. The elastoplastic deformation occurs at both the microstructure and macrostructure in the proposed model. Model responses under the different hydromechanical loading paths are finally discussed and the model performance is investigated and validated via simulations of comprehensive tests on FEBEX bentonite. The simulation results demonstrate that the proposed model is able to reproduce various swelling behavior of compacted bentonite, including the swelling deformation under different constant stresses and the swelling pressure under the constant volume. In particular, the dry density effects can also be reproduced.

A water retention curve model describing the effect of temperature

AbstractProperly simulating multiphase flow involving non‐isothermal processes in unsaturated soils requires the development of water retention curve (WRC) models with temperature effects. However, the majority of the existing WRC models are developed for isothermal conditions. This study aims to develop a new WRC model describing the effect of temperature on suction. Based on an assumption of an isotropic and homogeneous porous medium at the macroscopic scale, the formulations of non‐isothermal suction account for the temperature effects on the surface tension of the water–air interface and air bubble shape. The tension of the water–air interface is expressed as a function of temperature, and the air bubble shape becomes a function of temperature and saturation degree. The suction formulations are then used to extend the van Genuchten equation to non‐isothermal conditions. The final equation of the proposed model is a function of suction, and effective degree of saturation and temperature. A laboratory test was also designed to measure the WRCs of a loamy sand at different temperatures. The proposed model was validated against the measured results as well as experimental data of clayey‐silt sand, Boom clay and FEBEX bentonite reported in the literature. The very good agreements show the capability and feasibility of the model to predict the temperature dependence of water retention behaviour with a wide range of soils.Highlights A new WRC model was developed for non‐isothermal conditions. The model considers temperature effects on water surface tension and air bubble shape. We tested the model against measured and literature data, with very good agreements. The model is applicable to predict non‐isothermal water retention behaviour of unsaturated soils.

Eighteen years of steel–bentonite interaction in the FEBEX in situ test at the Grimsel Test Site in Switzerland

ABSTRACTCorrosion of steel canisters containing buried high-level radioactive waste is a relevant issue for the long-term integrity of repositories. The purpose of the present study was to evaluate this issue by examining two differently corroded blocks originating from a full-scale in situ test of the FEBEX bentonite site in Switzerland. The FEBEX experiment was designed initially as a feasibility test of an engineered clay barrier system and was recently dismantled after 18 years of activity. Samples were studied by ‘spatially resolved’ and ‘bulk’ experimental methods, including Scanning Electron Microscopy, Elemental Energy Dispersive Spectroscopy (SEM-EDX), μ-Raman spectroscopy, X-ray Fluorescence (XRF), X-ray Diffraction (XRD), and 57Fe Mössbauer spectrometry, with a focus on Fe-bearing phases. In one of the blocks, corrosion of the steel liner led to diffusion of Fe into the bentonite, resulting in the formation of large (width > 140 mm) red, orange, and blue colored halos. Goethite was identified as the main corrosion product in the red and orange zones while no excess Fe2+ (compared to the unaffected bentonite) was observed there. Excess Fe2+ was found to have diffused further into the clay (in the blue zones) but its speciation could not be unambiguously clarified. The results indicate the occurrence of newly formed octahedral Fe2+ either as Fe2+ sorbed on the clay or as structural Fe2+ inside the clay (following electron transfer from sorbed Fe2+). No other indications of clay transformation or newly formed clay phases were found. The overall pattern indicates that diffusion of Fe was initiated when oxidizing conditions were still prevailing inside the bentonite block, resulting in the accumulation of Fe3+ close to the interface (up to three times the original Fe content), and continued when reducing conditions were reached, allowing deeper diffusion of Fe2+ into the clay (inducing an increase of 10–12% of the Fe content).

Analysis of corrosion mechanisms of carbon steel in contact with FEBEX bentonite

Abstract Geological disposal is the most realistic option for high-level radioactive waste in Japan. In considering long-term stability for geological disposal, several types of materials have been studied as engineered barriers with a host rock. We focused our study on metal and bentonite as engineered barrier materials and investigated the long-term corrosion tendency of the metal exposed to bentonite. An electrochemical method for inducing accelerated corrosion was studied in a laboratory, and we analysed some field samples from a FEBEX dismantling project (FEBEX-DP) in Switzerland for comparison with our experimental results.

Performance of THM monitoring instrumentation in FEBEX bentonite barrier after 18 years of operation under repository-like conditions

The FEBEX (Full-scale Engineered Barrier Experiment in Crystalline Host Rock) was a research and demonstration project, which aimed to study the behaviour of near-field components in a repository for high-level radioactive waste in granite formations.Two electrical heaters were placed in a horizontal drift excavated in the granodiorite of the Grimsel Test Site in Switzerland, and surrounded by a barrier made of highly compacted bentonite blocks (FEBEX bentonite or Serrata type). A wide range of sensors were installed to track the most essential THM parameters: total pressure cells, pore pressure sensors, thermocouples, displacement sensors, moisture sensors of different types (capacitive, psychrometric and TDR) and others.The activities of the FEBEX “in situ” test started in 1994, the installation was completed in 1996 and the operational phase (heating and natural hydration) started in February 1997. A partial dismantling was carried out after 5 years of operation in 2002 and the final dismantling was performed in 2015 after more than 18 years.This paper reports the observations and analysis performed to the sensors recovered during the final dismantling operation.Qualitative and quantitative assessment of performance by type of sensor and measuring technique were done based on the results from data recorded during the operational lifetime, from the dismantling operation and from analysis carried out.As main conclusion, many sensors survived under harsh conditions and their performance was significantly better than initially expected and provided valuable information about the THM parameters for more than 18 years. The damage type/degree varied depending on sensor type/material (specific sensors failed quickly whereas others remained intact even with no need for re-calibration). This allows suggesting possible improvements. The re-calibration results for the surviving sensors indicated that their accuracy had hardly changed and that the obtained data was trustworthy.

Research on water retention and microstructure characteristics of compacted GMZ bentonite under free swelling conditions

In this study, water retention tests under free swelling conditions were performed to investigate the water intake (or loss) behaviour of compacted GMZ bentonite. First, the water retention characteristics were investigated, and then the microscopic pore structure was observed by environmental scanning electron microscope (ESEM). The results indicate that GMZ bentonite has a strong swelling (or a limited shrinkage ability) due to water intake (loss). The suction behaviour of GMZ bentonite is similar to MX80 bentonite and FEBEX bentonite. We also find that the confinement conditions can affect the suction behaviour of the material, especially at high relative humidity (RH). Additionally, a mathematic model can fit the mass change data very well. Microscopic tests show that the granular sensation of GMZ bentonite is obvious for a sample at low RH. With the increase in RH, the surface of GMZ bentonite becomes more smooth. The differences in the porosities calculated by the macroscopic and microscopic tests can be attributed to image resolution. The inter-laminar pores and intra-aggregate pores cannot be observed by the ESEM method. In addition, ESEM observation can provide an intuitive basis for the further research of the seepage property of GMZ bentonite.

Geochemical conditions for the formation of Mg silicates phases in bentonite and implications for radioactive waste disposal

The present study evaluates the formation of magnesium silicates phases as a result of the alkaline alteration of FEBEX bentonite in long-term experiments. The results are relevant in the context of radioactive waste disposal since the bentonite barrier will partly change its original mineralogy and condition its long-term geochemical behavior. Bentonite samples from an in situ experiment of interaction with a CEM-II-type concrete performed for 13 years in a rock gallery and a laboratory experiment of interaction with a CEM-I-type concrete performed for 10 years provide some experimental evidences on the mineralogical alteration. Results required multiple analytical techniques to resolve the nature of the Mg silicates. X-ray diffraction, thermogravimetric analyses, scanning electron microscopy, infrared spectroscopy and 27Al and 29Si nuclear magnetic resonance have been used. The mineralogical alteration is complex since several Mg silicates may coexist in the same region not only with themselves but also with carbonates and calcium (aluminium) silicate hydrates. Brucite intercalation in the interlayer of smectite, previously reported as a chlorite-like phase in a former study, is observed. In addition, a serpentine-type mineral phase was better observed in the in situ samples, at least by XRD, and a 2:1 trioctahedral phyllosilicates was better observed in the laboratory samples. Formation of Mg silicates in the bentonite barrier may buffer the Ca alkaline front originated in concrete and may decrease the porosity at the concrete-bentonite interface.

Coupled THMC models for bentonite in an argillite repository for nuclear waste: Illitization and its effect on swelling stress under high temperature

Subsurface manipulations such as those expected from the disposal of heat-emanating radioactive waste in deep repositories can induce strongly coupled Thermal (T), hydrological (H), mechanical (M) and chemical (C) processes. Adequate coupled THMC models are highly desirable or even indispensable for performance assessment of such repositories, for examples for the analysis of bentonite-based engineered barrier system (EBS) surrounding the emplaced waste. In this study, we present coupled THMC model simulations of a generic nuclear waste repository in an argillite with a bentonite-based buffer. The objective is to evaluate the chemical changes in the EBS bentonite and their effect on mechanical behavior under high temperature, attempting to shed light on whether EBS bentonite can sustain temperatures higher than 100°C without significant impact on barrier's performance.Two scenarios were simulated for comparison: a case in which the temperature near the waste canister peaks at 200°C and a case in which the temperature at the same spot culminate with about 100°C. Simulations for a generic case with Kunigel-VI bentonite as backfill and Opalinus Clay as host rock were conducted for 1000years and reported in the previous study (Zheng et al., 2015). In this paper, simulations for 100,000years have been done for two types of bentonite-based buffer materials: Kunigel-VI and FEBEX bentonite. This enables us to evaluate how different types of bentonite behave in terms of the illitization and its impact on swelling stress and whether we can generalize these results to support decision making. The simulations show the occurrence of illitization in the bentonite buffer and the enhancement of illitization under high temperature. However, FEBEX bentonite undergoes less illitization mainly due to the higher ion concentration in pore water and the lower content of K-feldspar in the bentonite mineral composition. Moreover, the reduction of swelling stress by chemical changes is more pronounced for Kunigel-VI bentonite than for FEBEX bentonite. Overall, the results of our model simulations suggest that an argillite repository with a bentonite-based EBS that is similar to FEBEX bentonite could sustain temperatures much higher than 100°C as far as illitization concerns. Model results also reveal that illitization is stabilized after about 2000years in bentonite near the waste package, but continues in bentonite near the bentonite-argillite interface, which manifests the strong effect of geochemical interaction between EBS bentonite and host rock on long term illitization in bentonite.

Size distribution of FEBEX bentonite colloids upon fast disaggregation in low-ionic strength water

AbstractBentonite colloids generated from the backfill barrier in nuclear waste repositories may act as radionuclide carriers, if they are stable and mobile. Repository scenarios with highly saline groundwater inhibit colloid stability as particles tend to aggregate but, in the time frame of repositories, groundwater conditions may evolve, promoting particle disaggregation and stabilization. The disaggregation of FEBEX bentonite colloids by fast dilution to lower ionic strength was analysed in this study. Time-resolved dynamic light-scattering experiments were carried out to evaluate the kinetics of bentonite colloid aggregation and disaggregation processes in Na+ and Na+-Ca2+ mixed electrolytes of low ionic strength. Attachment and detachment efficiencies were determined.Aggregation is promoted by increasing ionic strength, being more efficient in the presence of divalent cations. Once bentonite colloids are aggregated, a decrease in ionic strength facilitates disaggregation, but the process is not fully reversible as the initial size of the stable bentonite colloids at low ionic strength is not fully recovered. Particle-size distribution and concentration in suspension were analysed on disaggregated samples by single particle-counting measurements. Small colloids were measured in the disaggregated samples but their population was smaller than in the initial stable sample, especially in the presence of Ca2+.

Lime mortar-compacted bentonite–magnetite interfaces: An experimental study focused on the understanding of the EBS long-term performance for high-level nuclear waste isolation DGR concept

The aim of this study was to obtain evidences regarding the physical and geochemical processes occurring as a result of the combined effects of cementitious materials from the concrete degradation and magnetite from steel corrosion on the bentonite barrier during disposal of high-level radioactive waste.A series of six experiments were done that attempt to reproduce the repository conditions prevailing from 1000 to 3000years after emplacement of wastes. A lime mortar was used as the source of calcium and alkalinity as this is the presumed reactive product produced during concrete degradation at long-term. Magnetite powder was used to simulate the final corrosion product of cast iron and C-steel under anaerobic conditions. Either a natural FEBEX bentonite or a pretreated “aged” sample, depleted in exchangeable Mg and enriched in K, were used as the swelling clay component. Experiments, with both types of bentonite, were performed simultaneously in cylindrical specimens (50mm diameter, 25mm length), confined in a Teflon® sleeve/steel case cells. These specimens were composed of cement mortar in contact with compacted bentonite, which was in turn in contact with compressed magnetite powder. They were hydrated with an artificial Na+–Ca2+×SO42−type Spanish reference clayey formation water for 18months at 60°C and constant hydraulic pressure applied through the base of the mortar.After dismantling and sampling the specimens, distribution of soluble ions, exchangeable cations and mineralogy were studied in the bentonite by different instrumental techniques. Iron migration or any impact of the corrosion products in the bentonite was not noticeable in the clay. Both, mortar and magnetite acted as sinks of chloride and sulfate. Small quantities of Ca–Al-sulfates and carboaluminates, which can allocate chlorides, were determined near the mortar-bentonite interface. Portlandite dissolved near the bentonite interface and induced the formation of calcium silicates hydrates (C-S-H) phases cementing the clay interface characterizing a calcium front that was developed from the mortar towards the bentonite. Magnesium silicate hydrates (M-S-H) phases were also concentrated at the interface with mortar in the natural bentonite. It was also determined that natural bentonite has potentially higher buffering capacity attenuating the calcium alkaline front than the pretreated clay. In both cases, a low porosity bentonite–mortar zone was experimentally created at the interface. This type of material should be carefully studied in order to predict the potential for further development of a diffusive alkaline alteration, the radionuclides retention and the consequences in the hydration rate of the unaffected bentonite buffer.

Changes in the microstructure of compacted bentonite caused by heating and hydration

Two twin 40-cm long columns of compacted FEBEX bentonite were tested in Teflon cells; water was supplied through the top surface of the columns and in one of them a heater was placed at the base and set to 100°C. The purpose of these tests was to simulate the behaviour of an engineered barrier in a radioactive waste repository and investigate the effect of the thermal gradient on saturation. In particular, changes in the pore size distribution and interlayer size have been investigated in this work. The thermal gradient had a strong influence on the water intake and distribution. Water content and dry density gradients persisted in the two columns after 12 years of testing. These changes gave place to the modification of the bentonite microstructure, overall increasing the microstructural void ratio and the proportion of adsorbed, interlayer water.

Experimental investigation into temperature effect on hydro-mechanical behaviours of bentonite

The bentonite barrier of underground repositories for high-level radioactive waste will be hydrated by the groundwater while it is subjected to high temperatures due to the radioactive decay of the wastes. These changes of temperature affect the hydraulic and mechanical responses of bentonite, which has important effects on design and performance of repositories. The temperature influence on the hydro-mechanical behaviour of bentonite was studied in this paper by experiments, which were carried out with the Spanish FEBEX bentonite compacted at dry densities expected in the repository (from 1.5 to 1.8Mg/m3). The dependence of the swelling strains of bentonite on the temperature has been measured from 30 °C to 90 °C. At high temperatures the swelling capacity of clay slightly decreases. Also, a clear decrease of swelling pressure as a function of temperature was observed for the same dry densities. Nevertheless, the deformation of bentonite is more dependent on the stress than the temperature. An increase in the permeability of water saturated bentonite with temperature has also been detected. The water retention curves of bentonite compacted at different dry densities were determined under isochoric conditions and in the range of temperatures from 20 °C to 120 °C. For a given density and water content, the suction decreases as the temperature increases at a rate, which is larger than the one predicted on the basis of water surface tension changing with temperature. Mechanisms related to the physico-chemical interactions that take place at microscopic level, in particular the transfer of interlayer water to the macropores triggered by temperature, seem to explain qualitatively the experimental observations.

Impact of hydrothermal treatment of FEBEX and MX80 bentonites in water, HNO3 and Lu(NO3)3 media: Implications for radioactive waste control

Engineered barriers of deep geological repositories (DGR) are commonly constructed with bentonite. FEBEX and MX80 bentonites have been selected by different countries as reference materials for the sealing of repositories; however, their chemical reactivity with high-level long-lived radioactive wastes (HLRW) under subcritical conditions had not been explored before. The hydrothermal stability in neutral and acid media and chemical reactivity in contact with an actinide analogous compound were both studied. The long-range and short-range structural changes were analyzed by X-ray diffraction, nuclear magnetic resonance and scanning electron microscopy. Both bentonites have exhibited a good stability in neutral and acid media and have generated a new phase immobilizing the actinide analogous compound. The extent of the chemical reaction is higher in MX80 bentonite than in FEBEX bentonite.

Laboratory investigation of the role of desorption kinetics on americium transport associated with bentonite colloids

Understanding the parameters that control colloid-mediated transport of radionuclides is important for the safe disposal of used nuclear fuel. We report an experimental and reactive transport modeling examination of americium transport in a groundwater–bentonite–fracture fill material system. A series of batch sorption and column transport experiments were conducted to determine the role of desorption kinetics from bentonite colloids in the transport of americium through fracture materials. We used fracture fill material from a shear zone in altered granodiorite collected from the Grimsel Test Site (GTS) in Switzerland and colloidal suspensions generated from FEBEX bentonite, a potential repository backfill material. The colloidal suspension (100 mg L−1) was prepared in synthetic groundwater that matched the natural water chemistry at GTS and was spiked with 5.5 × 10−10 M 241Am. Batch characterizations indicated that 97% of the americium in the stock suspension was adsorbed to the colloids. Breakthrough experiments conducted by injecting the americium colloidal suspension through three identical columns in series, each with mean residence times of 6 h, show that more than 95% of the bentonite colloids were transported through each of the columns, with modeled colloid filtration rates (kf) of 0.01–0.02 h−1. Am recoveries in each column were 55–60%, and Am desorption rate constants from the colloids, determined from 1-D transport modeling, were 0.96, 0.98, and 0.91 h−1 in the three columns, respectively. The consistency in Am recoveries and desorption rate constants in each column indicates that the Am was not associated with binding sites of widely-varying strengths on the colloids, as one binding site with fast kinetics represented the system accurately for all three sequential columns. Our data suggest that colloid-mediated transport of Am in a bentonite-fracture fill material system is unlikely to result in transport over long distance scales because of the ability of the fracture materials to rapidly strip Am from the bentonite colloids and the apparent lack of a strong binding site that would keep a fraction of the Am strongly-associated with the colloids.

Uranium immobilization by FEBEX bentonite and steel barriers in hydrothermal conditions

FEBEX clay is considered a reference material in engineered barriers for safe storage of nuclear waste and uranium is a minor component of high-level radioactive waste (HLRW) and a main component of the spent nuclear fuel (SNF). Here, the kinetics of reaction of uranium with FEBEX was investigated in addition to the uranium immobilisation ability and the structural analysis of the reaction products. Hydrothermal treatments were accomplished with UO22+ and tetravalent actinide simulator ZrO2+, also present in HLRW. The quantification of the reaction was performed through gamma spectrometry of uranium. Two mechanisms for UO22+ retention by FEBEX were detected: adsorption and formation of stable and insoluble new phases. The structural analyses performed using ZrO2+, confirmed the uranium adsorption and the presence of new phases, ZrO2 and Zr(SiO4), that emphasise the existence of a chemical reaction with the bentonite. The analysis of the velocity of reaction uranium-clay minerals revealed temperature dependence. An exponential fitting suggested that the removal of uranium from solution at temperatures over 200°C could be completed in less than a year. For lower temperatures, several years are needed. Milliequivalents of UO22+ immobilised by the clay depended on temperature and time and were over cation exchange capacity (CEC) of FEBEX even at 100°C (reaching 600% of CEC). The reaction with steel, also temperature dependent, was finally analysed. At 200°C 40–70% of uranium reacted with steel. But only 30–15% reacted at 300°C and 100°C. The reactions provide a stable immobilisation mechanism for uranium even when its sorption and swelling capacities fail. Our experiments will be of particular interest for very deep borehole disposals were higher temperatures and pressures are expected.

A homogenization-based model for estimating effective thermal conductivity of unsaturated compacted bentonites

A homogenization-based effective thermal conductivity model was proposed for unsaturated compacted bentonites. The microstructure of soils was approximated with pores of spheroidal shape, diverse sizes and random orientation, and solid matrix of homogeneous properties (or soil particles of the same assumptions with pores for the self-consistent approach). With the consideration of preferential invasion of the wetting fluid (water) into pores of smaller sizes and by virtue of the analytical solution to Eshelby’s inclusion problem in heat conduction, the model was developed using homogenization techniques such as the dilute, Mori–Tanaka (MT), interaction direct derivative (IDD) and self-consistent (SC) schemes for different consideration of the interactions between pores and the solid phase. The proposed estimates are dependent on the thermal conductivities of the solid, liquid and gas phases, porosity, the degree of saturation, and the aspect ratios of pores and/or soil particles. The proposed model was validated against five sets of laboratory measurement data on the MX-80, FEBEX, Kunigel-V1 and GMZ01 bentonites. Compared to Chen’s series–parallel structural model recently developed for unsaturated compacted bentonites (Chen et al., 2014), the proposed model not only overall exhibits better performance, but as a great advantage, has much clearer physical mechanisms and significantly reduces the number of parameters by three or four, depending on the homogenization schemes adopted. It is demonstrated that the predictions by the MT, IDD and SC schemes strictly fall within the Weiner bounds and the Hashin–Shtrikman bounds over the full range of porosity and saturation, with the SC estimates overall having slightly better performance than the MT and IDD estimates.

Plutonium sorption and desorption behavior on bentonite

Understanding plutonium (Pu) sorption to, and desorption from, mineral phases is key to understanding its subsurface transport. In this work we study Pu(IV) sorption to industrial grade FEBEX bentonite over the concentration range 10−7–10−16 M to determine if sorption at typical environmental concentrations (≤10−12 M) is the same as sorption at Pu concentrations used in most laboratory experiments (10−7–10−11 M). Pu(IV) sorption was broadly linear over the 10−7–10−16 M concentration range during the 120 d experimental period; however, it took up to 100 d to reach sorption equilibrium. At concentrations ≥10−8 M, sorption was likely affected by additional Pu(IV) precipitation/polymerization reactions. The extent of sorption was similar to that previously reported for Pu(IV) sorption to SWy-1 Na-montmorillonite over a narrower range of Pu concentrations (10−11–10−7 M). Sorption experiments with FEBEX bentonite and Pu(V) were also performed across a concentration range of 10−11–10−7 M and over a 10 month period which allowed us to estimate the slow apparent rates of Pu(V) reduction on a smectite-rich clay. Finally, a flow cell experiment with Pu(IV) loaded on FEBEX bentonite demonstrated continued desorption of Pu over a 12 day flow period. Comparison with a desorption experiment performed with SWy-1 montmorillonite showed a strong similarity and suggested the importance of montorillonite phases in controlling Pu sorption/desorption reactions on FEBEX bentonite.

An effective thermal conductivity model for unsaturated compacted bentonites with consideration of bimodal shape of pore size distribution

An effective thermal conductivity model was proposed for unsaturated compacted bentonites with consideration of the bimodal shape of pore size distribution curves. The pores of soils were grouped into two dominant pore size modes corresponding to the intra- and inter-particle pores, and were simulated with randomly distributed spheroidal inclusions of different aspect ratios. With the assumption of preferential invasion of the wetting fluid (water) into pores of smaller sizes and by virtue of the analytical solution to the inhomogeneous inclusion problem in heat conduction, the model was developed using the Mori-Tanaka (MT), Ponte Castaneda-Willis (PCW) and self-consistent (SC) homogenization approaches for different considerations of the interactions between pores and the solid phase. The proposed model is functions of the thermal conductivities of the solid, liquid and gas phases, porosity, the degree of saturation, the aspect ratios of pores and/or soil particles, and the fraction of the smaller group of pores. The proposed model was validated against five sets of laboratory measurement data on the MX-80, FEBEX, Kunigel-V1 and GMZ01 bentonites, showing a good agreement between the model predictions and the laboratory measurements. The responses of the model with respect to the geometries of pores and solid particles were examined. Compared to series-parallel structural models, the proposed model may overall exhibit better performance if proper homogenization schemes are adopted, but as an advantage, the model has clearer physical mechanisms and a smaller number of parameters.