The present special issue of Clay Minerals contains 19 full papers of scientific studies presented at the 5th conference on “Clays in natural and engineered barriers for radioactive waste confinement” held at Montpellier 2012. Since 2002, ANDRA, the French National Radioactive Waste Management Agency, developed this conference into the most important event for all kinds of scientists from all over the world dealing with the disposal of highly and longlived radioactive waste (HLRW). With these conferences, ANDRA contributed significantly to the outstanding scientific level of European research in this field. Therefore, Clay Minerals is happy to be able to provide the present compilation of recent HLRW disposal research articles in clay mineralogy. The 19 papers published in this special Montpellier 2012 issue were classified according to different areas within the field of HLRW disposal research.

In situ migration experiments using different radiotracers have been performed in the HADES Underground Research Facility (URF), built at a depth of 225 m in the Boom Clay formation below the SCK–CEN nuclear site at Mol (Belgium). Small-scale experiments, mimicking laboratory experiments, were carried out with strongly retarded tracers (strontium, caesium, europium, americium and technetium). Contrary to europium, americium and technetium which are subjected to colloid mediated transport, the transport of strontium and caesium can be described by the classic diffusion retardation formalism. For these last two tracers, the transport parameters derived from the in situ experiments can be compared with the laboratory-derived values. For both tracers, the apparent diffusion coefficients measured in the in situ experiments agree well with the laboratory-derived values.

In the large-scale experiments (of the order of metres) performed in the URF, non-retarded or slightly retarded tracers (HTO, iodide and H14CO3–) were used. The migration behaviour of these tracers was predicted based on models applied in performance assessment calculations (classic diffusion retardation) using migration parameter values measured in laboratory experiments. These blind predictions of large-scale experiments agree well in general with the experimental measurements. Fitting the experimental in situ data leads to apparent diffusion coefficients close to those determined by the laboratory experiments. The iodide and H14CO3– data were fitted with a simple analytical expression, and the HTO data were additionally fitted numerically with COMSOL multiphysics, leading to about the same optimal values.

In the post-closure period of a geological disposal facility for radioactive waste, leaching of cement components is likely to give rise to an alkaline plume which will be in chemical disequilibrium with the host rock (which is clay in some concepts) and other engineered barrier system materials used in the facility, such as bentonite. An industrial analogue for cement-clay interaction can be found at Tournemire, southern France, where boreholes filled with concrete and cement remained in contact with the natural mudstone for 15–20 years. The boreholes have been overcored, extracted and mineralogical characterization has been performed. In this study, a reactive-transport model of the Tournemire system has been set up using the general-purpose modelling tool QPAC. Previous modelling work has been built upon by using the most up-to-date data and modelling techniques, and by adding both ion exchange and surface complexation processes in the mudstone. The main features observed at Tournemire were replicated by the model, including porosity variations and precipitation of carbonates, K-feldspar, ettringite and calcite. It was found that ion exchange needed to be included in order for C-S-H minerals to precipitate in the mudstone, providing a better match with the mineralogical characterization. The additional inclusion of surface complexation, however, led to limited calcite growth at the concrete-mudstone interface unlike samples taken from the Tournemire site that have a visible line of crusty carbonates along the interface.

Highly alkaline environments induced by cement-based materials are likely to cause the physical and/or chemical properties of the bentonite buffer materials in radioactive waste repositories to deteriorate. Assessing long-term alteration of concrete/clay systems requires physicochemical models and a number of input parameters. In order to provide reliability in the assessment of the long-term performance of bentonite buffers under disposal conditions, it is necessary to develop and verify reactive transport codes for concrete/clay systems. In this study, a PHREEQC-based, reactive transport analysis code (MC-CEMENT ver. 2) was developed and was verified by comparing results of the calculations with in situ observations of the mineralogical evolution at the concrete/argillite interface. The calculation reproduced the observations such as the mineralogical changes in the argillite limited to within 1 cm in thickness from the interface, formation of CaCO3 and CSH, dissolution of quartz, decrease of porosity in the argillite and an increase in the concrete. These agreements indicate a possibility that models based on lab-scale (∼1 year) experiments can be applied to longer time scales although confidence in the models is necessary for much longer timescales. The fact that the calculations did not reproduce the dissolution of clays and the formation of gypsum indicates that there is still room for improvement in our model.

Bentonite, which is envisaged as a promising engineered barrier material for the safe disposal of highly radioactive waste, was and is investigated in different large scale tests. The main focus was and is on the stability (or durability) of the bentonite. However, most countries concentrated on one or a few different bentonites only, regardless of the fact that bentonite performance in different applications is highly variable. Therefore, SKB (Svensk Kärnbränslehantering) set up the first large scale test which aimed at a direct comparison of different bentonites. This test was termed the ‘alternative buffer material test’ and considers eleven different clays which were either compacted (blocks) or put into cages to keep the material together. One so-called package consisted of thirty different blocks placed on top of each other. These blocks surrounded a heated iron tube 10 cm in diameter. Altogether three packages were installed in the underground test laboratory Äspö, Sweden. The first package was terminated 28 months after installation and the bentonite had been exposed for the maximum temperature (130°C) for about one year.

Almost all geochemical and mineralogical alterations of the different bentonites (apart from exchangeable cations) were restricted to the contact between iron and bentonite. The increase of the Fe2O3 content was attributed to corrosion of the tube. However, the typical 7 or 14 Å smectite alteration product was not found. At the contact of one sample, siderite was precipitated. Some samples showed anhydrite and organic carbon accumulation and some showed dissolution of clinoptilolite and cristobalite. IR spectroscopy, XRD, and XRF data indicated the formation of trioctahedral minerals/domains in the case of some bentonites. Even more data has to be collected before unambiguous conclusions concerning both alteration mechanisms and bentonite differences can be drawn.

Bentonites are candidate materials for the encapsulation of radioactive waste. In the ‘Alternative Buffer Material test’ (ABM), compacted ring-shaped blocks of eleven different buffer materials (mainly bentonites) were packed vertically on top of each other with an iron tube as heater in the centre. These buffer materials started with various exchangeable cation populations (ECpopulation). The first ‘ABM package’ was terminated 28 months after installation and the bentonites had been exposed to the maximum temperature (130°C) for about one year. The aim of the present study is first: to describe modification of the cation exchange population, and second to understand the influence of the groundwater on cation exchange at different scales. No significant horizontal variation of any exchangeable cation (EC) was detected between 1 and 7 cm distance from contact with the iron tube. Large total differences of the ECpopulations, however, were observed for the individual blocks after the field experiment (n = 21 blocks) with respect to the composition of the reference materials. The average cation exchange capacity (CEC) values of the analysed bentonites (n = 9 blocks) decreased by 5.5 meq/100 (1.1 – 8.8 meq/100 g) after the experiment. Exchangeable Na+ and Mg2+ decreased on average, whereas Ca2+ increased. This trend was pronounced in the top region of the parcel (upper seven blocks). Although most changes occurred on the large scale of the whole test parcel, small but important changes were also recorded in the vertical direction on the centimetre scale. The observed differences cannot be explained assuming simply that a bentonite reacts only with neighbouring blocks, which would mean that the system was more or less closed. The differences are much larger and the only conclusion from this observation is that the whole package seems to be influenced by the groundwater which was added from a water tank at the experiment site, enabling at least partial equilibration between the different blocks.

Bentonite is one of the more safety-critical components of the engineered barrier system in the disposal concepts developed for many types of radioactive waste. It is used due to its favourable properties (including plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides) and its stability in relevant geological environments. However, bentonite is unstable under alkaline conditions and this has driven interest in low-alkali cements (leachate pH of 10–11). To build a robust safety case, it is important to have supporting natural analogue data to confirm understanding of the likely long-term performance of bentonite. In Cyprus, the presence of natural bentonite in close proximity to natural alkaline groundwaters permits the zones of potential bentonite/alkaline water reaction to be studied as an analogy of the potential reaction zones in the repository. Here, the results indicate minimal volumetric reaction of bentonite, with production of a palygorskite secondary phase.

Water-saturated bentonite is planned to be used in many countries as an important barrier component in high-level nuclear waste (HLW) repositories. Knowledge about the microstructure of the bentonite and the distribution of water between interlayer (IL) and non-interlayer (non-IL) pores is important for modelling of long-term processes. In this work the microstructure of water-saturated samples prepared from MX-80 bentonite was studied with nuclear magnetic resonance (NMR) and small-angle X-ray scattering spectroscopy (SAXS) coupled with chloride exclusion modelling. The sample dry densities ranged between 0.7 and 1.6 g/cm3. The NMR technique was used to get information about the relative amounts of different water types. Water in smaller volume domains has a shorter relaxation time than that in larger domains due to the average closer proximity of the water to the paramagnetic Fe at the layer surfaces. The results were obtained using 1H NMR T1ρ relaxation time measurements with the short inter-pulse CPMG method. The interpretation of the NMR results was made by fitting a sum of discrete exponentials to the observed decay curves. The SAXS measurement on bentonite samples was used to get information about the size distribution of the IL distance of montmorillonite. The chloride porosity measurements and Donnan exclusion calculations were used together with the SAXS results to evaluate the bentonite microstructure. In the model, the montmorillonite layers were organized in stacks having IL water between the layers and non-IL water between the stacks. In the modelling, the number of layers in the stacks was used as fitting parameters which determined the IL and non-IL surface areas. The fitting parameters were adjusted so that the modelled chloride concentration was equal to the measured one. The NMR studies and SAXS studies coupled with the Cl porosity measurements provided very similar pictures of how the porewater is divided in two phases in bentonite.

Bentonite clay is planned to form a part of deep-geological repositories of spent nuclear fuel in several countries. The extremely long operation time of the repository requires an indepth understanding of the structure and properties of used materials. In this work the microstructure of a simplified system of Ca-montmorillonite is investigated using a set of complementary methods: X-ray diffraction, small angle X-ray scattering, nuclear magnetic resonance, transmission electron microscopy and ion exclusion. The paper presents experimental results obtained from compacted, water saturated samples in the dry density range 0.6–1.5 g/cm3. It can be observed that different methods yield similar quantification of water present in the interlamellar space. Combined results support the multiple porosity concept of the bentonite structure.

Disposal of Finnish spent nuclear fuel is planned to be based on the KBS-3 repository concept. The role of the bentonite buffer in this concept is essential, and thus the behaviour of the bentonite has to be known. The experiments in this paper concentrated on providing information about the effects of carbon dioxide CO2(g) partial pressure on compacted sodium bentonite, giving an insight into the buffering capacity. The experimental setup consisted of a hermetic box which had a CO2-adjusted atmosphere, and the bentonite was in contact with this atmosphere through water reservoirs. The results indicated that it is possible to measure online the changing pH in the porewater inside compacted bentonite using IrOx electrodes. It was found that the pH fell if the CO2 partial pressure increased above atmospheric conditions. The experimental results indicated a greater fall in pH than in our model in the test cases where CO2 was present. The pH in the experiment with 0 PCO2 remained nearly constant throughout the 5 month period. On the other hand, the pH dropped to near 6 with 0.3 PCO2 and to 5.5 with 1 PCO2.

In situ measurements were carried out to quantify montmorillonite dissolution rates at a compaction pressure ranging from 0.04 to 10.00 MPa and temperature of 70°C in 0.3 M NaOH solution (pH 12.1 at 70°C) using vertical scanning interferometry (VSI) and an auto-compaction cell. Ex situ measurements of the reacted samples using atomic force microscopy (AFM) were performed to quantify the ratio of edge surface area (ESA) to total surface area (TSA) (XESA = ESA/TSA). Accordingly, the actual ESA for the montmorillonite examined by in situ VSI could be estimated. The XESA value increases as a function of run duration or compaction pressure. At atmospheric pressure, XESA is approximately 0.0054 and converges to ∼0.0107 at 10 MPa, An expression that relates reactive surface area and montmorillonite compaction (XESA/XESA initial = kXESA, k: variable factor) is kXESA = 1.0 + 0.64628 P0.1527 where P is in MPa. Using the calculated XESA, dissolution rates from the in situ VSI measurements are obtained. The early dissolution (<1500 min) at less compaction pressure tends to show faster rates (>1.0 × 10-11 mol/m2/s) than that at higher compaction pressure. The rates after >1500 min are slower, with values of less than 3×6 10-12 mol/m2/s, but there is no significant dependency on the density in the range from 1.0 to 1.7 Mg/m3. These observed rates for compacted montmorillonite are two-orders of magnitude slower (2.63×10-13 mol/m2/s) than dissolution rates in the suspended state.

The effects of simulant groundwater composition, pH and temperature on the dissolution and alteration of Na- and Ca-montmorillonite have been studied. Prior to the experiments, Wyoming type Na-montmorillonite, Swy-2, was purified to decrease the amount of accessory minerals. For Ca-montmorillonite experiments, the interlayer cation Na+ of purified Swy-2 was exchanged with Ca2+. The batch experiments were conducted with the purified montmorillonites in simulated fresh and saline waters at 25°C and 60°C under anaerobic conditions in an Ar atmosphere. The concentrations of Si, Al, Fe and Mg were analysed from ultra-filtered solution samples with High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS) as a function of dissolution time. The pH evolution was also measured. The solid smectite phases were analysed with X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). XRD analyses indicated that the nature of the smectite mineral did not change over 140 days. However, the experimental conditions, more or less, modified the structure (e.g. the layer stacking of montmorilllonite; the partial dissolution of the smectite), which cannot be detected by XRD but was evidenced by chemical data, and can be considered as a possible contributor to the stacking faults of the montmorillonite. The log rates (mol g–1 s–1), based on the dissolved amount of Si, varied between –10.64 and –12.13 depending on the experimental conditions.

The interaction of two different bulk bentonites (Na- and Ca-types) with three types of simulated cement waters (pH 9.7, 11.3 and 12.0) and one saline groundwater simulate (pH 8.3) as a reference, was studied in batch reactors at 25°C. The solution pH was monitored in order to keep the pH as steady as possible by replacing the leaching solution with fresh one when needed. After 554 days, one set of parallel samples was removed from the experiment in order to investigate the possible changes in the bentonite materials.

The buffering capacity of bentonite was clearly observed, especially at the beginning of the high-pH experiments, as the pH of the leaching solutions decreased quite dramatically due to interaction with bentonite. The solution chemistry results showed a decrease of Ca content in all leachate samples, but especially in pH 12.0 experiments. Small amounts of silica were released throughout the experiment. Both bentonites in pH 12.0 experiments also released detectable amounts of Al, while in the lower pH experiments the levels were below detection limit. These observations were also supported by chemical analyses of the bentonite materials. Only minor changes were detected in the mineralogy, and they were mainly concentrated on experiments at pH 11.3 and pH 12.0. The measured swelling pressure showed an increase in pH 12.0 experiments. The results obtained in this research may facilitate modelling of bentonite interaction with high-pH solutions.

In order to evaluate the long-term behaviour of the engineered barriers in geological disposal sites for transuranic element-bearing (TRU) waste, an evaluation by numerical analysis is required. Although chemical and hydraulic/mechanical analyses have been conducted independently until now, essentially both type of phenomena occur simultaneously and produce synergistic effects. Therefore, we focused attention on the buffer (bentonite) engineered barrier and conducted a study of which involved incorporating hydraulic/mechanical phenomena into the chemical analysis of bentonite alteration. The simulations employed weakly-coupled chemical and hydraulic/mechanical effects to study the behaviour in one dimension.

The results showed that the dissolution of the montmorillonite is suppressed in the buffer section nearest the cement material. Moreover, in order to achieve a fully coupled analysis in future, the present study also identifies issues that need to be resolved.

Bentonite is a potential material for use in the engineered barrier of radioactive waste repositories because of its low hydraulic permeability, self-sealing capability and retention capacity. It is expected that bentonite would react at the elevated temperatures accompanying the radioactive decay in the nuclear waste. The presented study was started in order to improve understanding of the coupled influence of temperature and (pore) water on the physicochemical and mineralogical properties of bentonite during thermal treatment under near-field relevant conditions. Granular Na-bentonite MX-80 was differently saturated (Sr = 1–0.05) and stored at different temperatures (50–150°C) in a closed system. Upon dismantling after different periods of time (3 to 18 months), mineralogical characteristics, cation exchange capacity and content of leachable cations, as well as physicochemical properties such as surface area and water adsorption were investigated.

The results showed a high mineralogical stability. A slight conversion from the sodium to an earth alkali form of the bentonite was observed. However, considerable changes in the physicochemical properties of the bentonite were observed, particularly by treatment above the critical temperature of 120°C. The cation exchange capacity decreased during heating at 150°C by approximately. 10%. The specific surface area dropped by more than 50%. The water uptake capacity under free swelling conditions showed a slight tendency to lower values especially for samples heated for more than 12 months. The water vapour adsorption ability in contrast drops by 25% already within three months at T = 120°C. These changes are mostly related to the variations in the interlayer cation composition and to smectite aggregation processes. The observed alterations are rather subtle. However, temperatures ⩾ 120°C had a remarkable negative influence on different properties of MX-80.

During the installation of the buffer in a deposition hole of an HLW repository, it is necessary to control water flow from the fractured rock into the deposition hole. Water flow with inflow rate greater than 0.001 l/min may cause piping and erosion of the buffer, and may trigger mass redistribution of the buffer, sedimentation and material separation of bentonite materials. This paper describes the condition of parameters which cause piping and erosion; revised conditions which keep advection, inflow rate, buffer component, gap between buffer materials, gap between outside wall and buffer block, and type of water. The results from the experiment show the condition of the self-sealing function of bentonite materials, formation of piping, allowable limit of inflow rate in the case of an Na type bentonite block of 70 wt.% Kunigel V1 and 30 wt.% silica sand, or a pellet of 100 wt.% Kunigel V1. Piping and erosion continue until the engineered barrier (EB) is filled with water, and then the hydraulic gradient becomes small. Piping may lead to erosion and redistribution of material which needs to be taken into account in the long-term performance assessment.

The geological disposal of radioactive waste, based on a multi-barrier concept wherein the first barrier consists of the metal waste container and the final barrier the host rock, is widely considered the only viable solution to this issue. The bentonite-based seal around the canister forms one of the barriers. The unique swelling and sealing capabilities of bentonite play a major role in repository safety concepts in that they allow the bentonite barrier to withstand serious mechanical damage without its function being compromised.

This paper presents experimental research focusing on the dynamics and mechanics of the sealing of cracks and joints using bentonite-based materials. Physical models were used to simulate the contact point of bentonite-based sealants with cracks in the rock mass. The models examined the ability of the tested material to fill the crack thus preventing the creation of a preferential water pathway. The results show that in most cases total bentonite advance (for the same material) into fissures is, primarily, linearly dependent on fissure width. The absolute value of advance could be related to the overall swelling ability of the material characterized by its swell index or swelling pressure.

An experimental study of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock under hydration is presented. The investigation, which combines environmental scanning electron microscopy (ESEM) and digital image correlation techniques, has been carried out at the micrometric scale of the composite microstructure of the rock. Specimens were hydrated in the ESEM over a wide range of relative humidity and observations conducted on two planes: plane 1 parallel to the bedding plane, and plane 2 perpendicular to it. The observations reveal that the local swelling (which can be quantified at a local gauge length of about 5 μm) is strongly anisotropic in both planes. The global swelling, measured over areas of about 500 μm in width, is also clearly anisotropic in plane 2 (with major swelling direction normal to the bedding plane), but not in plane 1. The global isotropy in plane 1 arises from the uniform distribution of the orientation of anisotropic local strains, while the anisotropic swelling in plane 2 is due to a preferred local orientation.

Highly alkaline environments induced by cementitious materials in radioactive waste repositories are likely to alter montmorillonite, the main constituent of bentonite buffer materials, and are likely to cause the physical and/or chemical properties of the buffer materials to deteriorate. The deterioration may cause variation in hydraulic conductivity of the buffer. However, empirical data on the variation of hydraulic conductivity are scarce, mainly because the alteration of compacted buffer materials, sand-bentonite mixtures, is extremely slow. In this study, laboratory experiments were performed to observe changes in hydraulic conductivity of sand-bentonite mixtures, accompanied by their alkaline alteration, using NaOH-based solutions at 80–90°C. Series-1 multi-step alteration/water conduction experiments resulted in an increase in the hydraulic conductivity by one order of magnitude over a 200 day period. Series-2 single step alteration/water conduction experiments revealed a decrease in the montmorillonite contents with time and a resulting increase in the hydraulic conductivity by 30 times over the 67 day period. Series-3 simultaneous alteration/water conduction experiment also demonstrated an increase in the hydraulic conductivity by 30 times over the 150 day period. The results proved that the alkaline alteration of the bentonite buffer can increase the hydraulic conductivity. The data obtained in this study are useful for verification of the code that will be used for assessing the alteration.

We studied the diffusive transport of Cs, Np, Am and Co in compacted sandbentonite mixtures by using the through-diffusion method. The experiments for Cs were performed under various aqueous compositions. Effective diffusivity (De) values of 4.7×10–10 to 5.9×10–9 m2 s–1 were obtained with a somewhat large variation. Apparent diffusivity (Da) values, on the other hand, showed less variation, ranging from 2.0×10–12 to 6.2×10–12 m2 s–1. The results indicated that diffusive flux was proportional to the concentration gradient on the basis of the amount of Cs in the unit volume of the compacted sand-bentonite mixtures rather than the Cs concentration gradient in pore water. Because the former concentration gradient in the mixtures was nearly equal to that of adsorbed Cs, the diffusion of Cs in the mixtures was probably dominated by the concentration gradient of the Cs adsorbed on the mixtures. In addition, the effective/apparent diffusivity of 237Np(IV) and apparent diffusivity of 241Am(III) and 60Co(II) in the mixtures were determined in 0.3/0.03 mol l–1 (NH4)2CO3/Na2S2O4 solution.