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Many countries intend to use compacted bentonite as a barrier in their deep geological repositories for nuclear waste. In order to describe and predict hydraulic conductivity or radionuclide transport through the bentonite barrier, fundamental understanding of the microstructure of compacted bentonite is needed. This study examined the interlayer swelling and overall microstructure of Wyoming Bentonite MX-80 and the corresponding homo-ionic Na(+) and Ca(2+) forms, using XRD with samples saturated under confined swelling conditions and free swelling conditions. For the samples saturated under confined conditions, the interparticle, or so-called free or external porosity was estimated by comparing the experimental interlayer distances obtained from one-dimensional XRD profile fitting against the maximum interlayer distances possible for the corresponding water content. The results showed that interlayer porosity dominated total porosity, irrespective of water content, and that the interparticle porosity was lower than previously reported in the literature. At compactions relevant for the saturated bentonite barrier (1.4-1.8 g/cm(3)), the interparticle porosity was estimated to ≤3%., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Abstract: Silica sol, i.e., colloidal SiO2, may be used as a low-pH injection grout for very fine fractures in the construction of deep geological repositories for radioactive waste in Sweden and in Finland. If the bentonite barrier encounters SiO2-colloid particles under conditions favorable for aggregation, there is concern that it will modify the bentonite barrier at the bentonite/bedrock interface. In this study qualitative experiments were performed with mixed dispersions of SiO2-colloids and bentonite or homo-ionic Na/Ca-montmorillonite. Samples were prepared at different colloid concentrations and treated under various conditions such as low and high ionic strength (0.3M NaCl), as well as dehydration and redispersing. Free swelling and settling experiments were performed in order to qualitatively compare the conditions in which SiO2-colloids affect the bulk/macro properties of bentonite. In order to study specific SiO2-colloid/montmorillonite interactions and preferred type of initial aggregation, dilute dispersions of homo-ionic montmorillonite dispersions mixed with varying concentrations of SiO2-colloids were prepared and selected samples were characterized by PCS, SEM/EDS, AFM and PXRD. The results from this study show that bentonite and montmorillonite particles can be modified by SiO2-colloids when mixed in comparable amounts, due to dehydration or high ionic strength. Some indications for increased colloidal stability for the SiO2-colloid modified clay particles were also found. From the AFM investigation it was found that initial attachment of the SiO2-colloids in Na+ dominated samples seemed to occur on the edges of the montmorillonite layers. In Ca2+ dominated samples not subjected to excess NaCl, SiO2-colloid sorption onto the faces of the montmorillonite layers was also found. In all, contact between the bentonite barrier and ungelled Silica sol should preferably be avoided. [Copyright &y& Elsevier]

Abstract: In many concepts for final storage of spent nuclear fuel bentonite will be used as an engineered barrier, mainly due to its inertness, plasticity and ability to retard transport of radionuclides by adsorption. In the event of water-bearing fractures making contact with the bentonite barrier, generation and transport of colloidal particles will strongly depend on groundwater composition and the surface properties of the colloidal particles. The bentonite barrier will unavoidably be exposed to ionizing radiation from the spent nuclear fuel but very little is known about effects of ionizing radiation on bentonite concerning colloidal stability. In this work we have studied the effect of γ-radiation on the stability of dilute colloidal Na+-montmorillonite dispersions using a Cs-137 γ-source (doses of 0–53.2 kGy). Aggregation kinetics and sedimentation experiments revealed significant radiation effects, evident as increased colloid stability. The only rationale for this is a γ-radiation induced increase in surface potential. The effects appeared to depend on the Na+-montmorillonite concentration in the irradiated dispersions, indicating that the change in surface potential is induced by aqueous radiolysis products. [Copyright &y& Elsevier]

Abstract: Through-diffusion experiments in bentonite with humic colloids in the size range of 1–10nm were carried out. Bentonite was compacted to 0.6–1.8g/cm3 dry density and equilibrated with 0.01 and 0.1M NaClO4 solutions. Experiments with Eu(III) diffusing in the absence and presence of humics were run in parallel, as well as Co(II) diffusing through compacted bentonite in the presence of humics. The humic colloid diffusion experiments were run for 60 days and the humic concentration in the outlet solutions measured at time intervals. The experimental breakthrough curves for humic substances (HS) as well as the HS, Co(II) and Eu(III) profiles in the bentonite were simulated using the finite difference based computer code ANADIFF. Regardless of the compaction and ionic strength of solutions, humic colloids diffused through the compacted bentonite. The effects of humic colloids on both Co(II) and Eu(III) sorption as well as on diffusion were significant. The apparent diffusivity (D a) increased significantly for both Co(II) and Eu(III) when humic colloids were present and the distribution coefficient (K d) values decreased. [Copyright &y& Elsevier]

Abstract: Compacted and water saturated bentonite will be used as an engineered barrier in deep geological repositories for radioactive waste in many countries. Due to the high dose rate of ionizing radiation outside the canisters holding the nuclear waste, radiolysis of the interlayer and pore water in the compacted bentonite is unavoidable. Upon reaction with the oxidizing and reducing species formed by water radiolysis (OH•, e− (aq), H•, H2O2, H2, HO2 •, H3O+), the overall redox properties in the bentonite barrier may change. In this study the influence of γ-radiation on the structural Fe(II)/FeTot ratio in montmorillonite and its reactivity towards hydrogen peroxide (H2O2) was investigated in parallel experiments. The results show that under anoxic conditions the structural Fe(II)/FeTot ratio of dispersed Montmorillonite increased from ≤3 to 25–30% after γ-doses comparable to repository conditions. Furthermore, a strong correlation between the structural Fe(II)/FeTot ratio and the H2O2 decomposition rate in montmorillonite dispersions was found. This correlation was further verified in experiments with consecutive H2O2 additions, since the structural Fe(II)/FeTot ratio was seen to decrease concordantly. This work shows that the structural iron in montmorillonite could be a sink for one of the major oxidants formed upon water radiolysis in the bentonite barrier, H2O2. [Copyright &y& Elsevier]

Abstract: The stability of natural bentonite suspensions has been investigated as a function of temperature at pH 9 and ionic strength 10−3 M. The sedimentation rate of the particles is directly related to their stability. The sedimentation kinetics was determined by examining the variation of particle concentration in solution with time. The observed kinetics for sedimentation is discussed quantitatively in terms of the potential energy between particles. The ζ-potential of the particles was measured and the DLVO theory was used to calculate attractive and repulsive potentials. Experimental observations are consistent with DLVO model predictions and show that the stability of bentonite colloids increases with temperature. Differences with other colloidal systems can be attributed to the temperature dependence of the surface charge of bentonite particles. [Copyright &y& Elsevier]

An experiment series at A¨spo¨ Hard Rock Laboratory, called “Long Term Test of Buffer Material”, LOT, are carried out at A¨spo¨ Hard Rock Laboratory to validate models of clay buffer performance at standard KBS-3 repository conditions and to quantify clay buffer alteration processes at adverse conditions. In conjunction with the tests of the bentonite, cation diffusion tests using caesium and cobalt are performed.Each test contains 38 cylindrical blocks of bentonite clay with a hole in the middle which are placed around a copper rod in a vertically drilled hole at a depth of 450 m. In each test, four identical cylindrical bentonite plugs, doped with 1 MBq 57Co and 134Cs, respectively, are inserted in the fifth block from the bottom. The system was left to be saturated with ground water before heating of the central copper rod started to simulate the thermal activity of radioactive waste.The experiments continued for 14 months before the bentonite blocks were drilled out using over-core drilling technique. The lowest blocks were cut up, sparsely in the outer layer, and in cubic centimeters, big samples closer to where the activity was inserted. All samples were analyzed using a gamma spectrometer to get a three-dimensional picture of the activity distribution. The system was then fitted to a diffusion model to obtain apparent diffusivities for the two cations.The apparent diffusivity for cobalt agrees well with those obtained in other in situ experiments and in laboratory studies, while the value for caesium is lower than expected. This can be due to that the clay was not fully water saturated during the experiment. [Copyright &y& Elsevier]

Bentonite is a strong radionuclide (RN) adsorbent. As a consequence, it is proposed as one of the engineered safety barriers in many nuclear waste disposal concepts in granite formations. Despite the many beneficial effects of bentonite, in contact with groundwater of low ionic strength montmorillonite colloids may be released from the bentonite buffer and transported towards the biosphere carrying the RNs bound to it. During the transport of colloids in bedrock fractures, size separation of clay colloids may occur, which may further affect RN mobility. In this work, RN adsorption (Th(IV), U(VI), Np(V), Tc(VII) and Pu(IV)) onto size fractionated montmorillonite colloids was studied in a synthetic, carbonated groundwater. Fractionation was done by simple settling procedures and sequential centrifugation. We combined batch adsorption experiments and geochemical modelling for the adsorption studies. U(VI), Np(V) and Tc(VII) did not adsorb to montmorillonite in the synthetic groundwater. Adsorption of Th(IV) and Pu(IV) is strong but, within experimental uncertainties, not significantly affected by the fractionation process. Montmorillonite colloids obtained by fractionation of the raw clay material but in the presence of organic matter during the initial separation step present significantly reduced uptake of Th and Pu. Based on the results, implementation of an “average log K D ” (i.e. average distribution coefficients) for all colloidal sizes in reactive transport modelling codes would be acceptable. [ABSTRACT FROM AUTHOR]

Bentonite is planned to be used as a technical barrier in the final storage of spent nuclear fuel and high level vitrified waste. In contact with ground water of low ionic strength, montmorillonite colloids may be released from the bentonite buffer and thereby enhance the transport of radionuclides (RNs) sorbed. In the present case, clay colloids represent aggregates of several clay mineral layers. It is of major importance to determine RN sorption properties for different sizes of montmorillonite aggregates, since size fractionation may occur during particle transport in natural media. In this study, a protocol for size fractionation of clay aggregates is developed, by sequential and direct centrifugation, in the presence and absence of organic matter. Seven colloidal fractions of different mean aggregate sizes are obtained ranging, when considering the mean equivalent hydrodynamic sphere diameter (ESD), from ~ 960 nm down to ~ 85 nm. Applying mathematical treatments (Jennings and Parslow, 1988) and approximating the clay aggregates to regular disc-shaped stacks of clay mineral sheets result in mean surface diameters varying from ~ 1.5 μm down to ~ 190 nm. All these colloidal fractions are characterized by XRD, IC and ICP-OES where they are found to have the same chemical composition. The number of edge sites (aluminol and silanol) is estimated (in mol/kg) for each colloidal fraction according to (Tournassat et al., 2003). It is calculated from the mean particle sizes obtained from AsFlFFF and PCS measurements, where the clay aggregates are approximated to regular disc-shaped stacks of clay mineral sheets. The estimated number of edge sites varies significantly for the different clay dispersions. In addition, stability studies using the various clay colloidal fractions are performed by the addition of NaCl, CaCl 2 or MgCl 2 , in the presence or absence of organic matter, where no difference in stability is found. [ABSTRACT FROM AUTHOR]

Abstract: Compacted bentonite is proposed as an engineered barrier in many concepts for disposal of high level nuclear waste. After the initial deposition however, the bentonite barrier will inevitably be exposed to ionizing radiation (mainly γ) under anoxic conditions. Because of this, the effects of γ-radiation on the apparent diffusivity values and sorption coefficients in bentonite for Cs+ and Co2+ were tested under different experimental conditions. Radiation induced effects on sorption were in general more noticeable for Co2+ than for Cs+, which generally showed no significant differences between irradiated and unirradiated clay samples. For Co2+ however, the sorption to irradiated MX80 was significantly lower than to the unirradiated clay samples regardless of the experimental conditions. This implies that γ-radiation may alter the surface characteristics contributing to surface complexation of Co2+. With the experimental conditions used, however, the effect of decreasing sorption was not large enough to be reflected on the obtained D a values. [Copyright &y& Elsevier]