M.V. Villar

Pore waters extracted from compacted bentonite subjected to simultaneous heating and hydration

A compacted clay block of montmorillonite clay has been simultaneously subjected to heating and hydration by parallel and opposite fronts, in order to determine the physico-chemical effects of the thermohydraulic process on the clay and on its pore water. The pore waters of the clay have been extracted at ambient temperature after the thermal-hydration treatment by a compaction at high pressure (60 MPa) technique. They have been analyzed, and the chemistry has been evaluated by using the speciation program EQ3NR. The movement of salts towards hot areas has been observed as a result of the thermo-hydraulic gradients, but it is conditioned by anionic retention processes, which mainly affect the mobility of SO42− anions. The distributrion of the exchangeable cations is modified by the thermal gradient.

Investigation of the behaviour of bentonite by means of suction-controlled oedometer tests

In the context of an investigation of the near-field as a repository of high-level radioactive waste, the FEBEX Project, a set of laboratory tests has been designed to give a better understanding of the thermo-hydro-mechanical behaviour of unsaturated, expansive clays and to achieve an experimental corroboration of the constitutive models for these clays. Among these tests, several suction-controlled oedometer tests have been performed on a compacted, highly-expansive clay. Seven of them, in which combined paths of loading, wetting and drying have been followed, are presented in this paper. Suctions employed range from 0 to 500 MPa, and vertical loads from 0.1 to 9 MPa. The point of air entry has been identified for suctions around 120 MPa, beyond which the sample becomes very stiff. The influence of the stress path on the subsequent reversibility of the deformations has been highlighted. The swelling properties seem to be kept after intense drying. The behaviour observed, including collapse, agrees with models that relate the effects of the deformations in the microstructural level to those in the macrostructure.

Long-term geochemical evolution of the near field repository: Insights from reactive transport modelling and experimental evidences

The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of the repository. The bentonite barrier has three main safety functions: to ensure the physical stability of the canister, to retard the intrusion of groundwater to the canisters, and in case of canister failure, to retard the migration of radionuclides to the geosphere. Laboratory experiments (< 10 years long) have provided evidence of the control exerted by accessory minerals and clay surfaces on the pore water chemistry. The evolution of the pore water chemistry will be a primordial factor on the long-term stability of the bentonite barrier, which is a key issue in the safety assessments of the KBS-3 concept. In this work we aim to study the long-term geochemical evolution of bentonite and its pore water in the evolving geochemical environment due to climate change. In order to do this, reactive transport simulations are used to predict the interaction between groundwater and bentonite which is simulated following two different pathways: (1) groundwater flow through the backfill in the deposition tunnels, eventually reaching the top of the deposition hole, and (2) direct connection between groundwater and bentonite rings through fractures in the granite crosscutting the deposition hole. The influence of changes in climate has been tested using three different waters interacting with the bentonite: present-day groundwater, water derived from ice melting, and deep-seated brine. Two commercial bentonites have been considered as buffer material, MX-80 and Deponit CA-N, and one natural clay (Friedland type) for the backfill. They show differences in the composition of the exchangeable cations and in the accessory mineral content. Results from the simulations indicate that pore water chemistry is controlled by the equilibrium with the accessory minerals, especially carbonates. pH is buffered by precipitation/dissolution of calcite and dolomite, when present. The equilibrium of these minerals is deeply influenced by gypsum dissolution and cation exchange reactions in the smectite interlayer. If carbonate minerals are initially absent in bentonite, pH is then controlled by surface acidity reactions in the hydroxyl groups at the edge sites of the clay fraction, although its buffering capacity is not as strong as the equilibrium with carbonate minerals. The redox capacity of the bentonite pore water system is mainly controlled by Fe(II)-bearing minerals (pyrite and siderite). Changes in the groundwater composition lead to variations in the cation exchange occupancy, and dissolution-precipitation of carbonate minerals and gypsum. The most significant changes in the evolution of the system are predicted when ice-melting water, which is highly diluted and alkaline, enters into the system. In this case, the dissolution of carbonate minerals is enhanced, increasing pH in the bentonite pore water. Moreover, a rapid change in the population of exchange sites in the smectite is expected due to the replacement of Na for Ca.

Effects of heat/water flow interaction on compacted bentonite: Preliminary results

Abstract Compacted bentonite blocks have been heated and hydrated in a stainless steel cell in order to simulate, in the laboratory, the conditions of the clay barrier in a high-level radioactive waste repository. Temperature distributions at different times, rate of hydration, final water content and dry density have been measured. Some chemical parameters, as electrical conductivity in an aqueous extractable amorphous silica, have also been obtained. For the periods of time considered (up to 2500 h), the hydration process is not affected by the thermal gradient, the high suction of the bentonite being the critical factor in the initial water uptake of the clay barrier. A remarkable saline environment has been detected near the heater, due to salt migration towards dried areas. This phenomenon should be taken into account in further investigations of the mechanical and geochemical behaviour of the clay barrier.

Hydraulic properties of montmorillonite-quartz and saponite-quartz mixtures

Abstract The work presented in this paper is part of a project of characterization of Spanish clays to be used as backfill and sealing materials in high-level radioactive waste repositories. Hydraulic conductivity and swelling pressure tests have been carried out in montmorillonite-quartz and saponite-quartz mixtures with a quartz weight percentage between 0 and 60%. The linear dependence between the logarithm of the hydraulic conductivity and the clay dry density has been shown, with the pure clay samples having the same behaviour as the clay-quartz ones, provided that the percentage of quartz is below a certain quantity. A threshold value of clay dry density has been found, being the trend of the hydraulic conductivity different above and below that value. Swelling pressure seems to account for this fact and for the slightly different behaviour of both clays.

Thermo-hydraulic gradients on bentonite: distribution of soluble salts, microstructure and modification of the hydraulic and mechanical behaviour

Abstract In the context of an investigation of the near field for a repository of high-level radioactive waste (HLW), the FEBEX Project, a set of laboratory tests has been designed to give a better understanding of the thermo-hydro-mechanical and geochemical behaviour of the compacted bentonite. Small compacted samples of bentonite were heated during variable periods of time of up to 80 days under different thermal gradients. The hydration water was either granitic, simulating the conditions of the outer part of the barrier, or saline, which simulates the chemistry of the pore waters inside the bentonite barrier. At the end of the thermo-hydraulic (TH) treatment, a geochemical characterisation was performed in different sections, both of the soluble components and of the solid phase. The microstructure was analysed by means of optical microscopy and determination of the BET surface and pore size distribution. The swelling capacity of the whole sample and its permeability were also checked. The results of the permeability and swelling tests were compared to those obtained in experiments performed under the same conditions with nontreated samples. As a result of hydration, there exists a rapid movement of chloride towards the heater. This ion is progressively excluded of the bentonite after saturation that makes the bulk salinity of the clay decrease. Anion exclusion should prevent the anion transport once the bentonite is saturated, but the existence of preferential passages is postulated to explain the continuous salinity decrease. This phenomenon makes the salt content in the compacted bentonite very limited, with localised anomalies prior to saturation. All the physico-chemical parameters that have been determined are virtually unchanged during the TH treatment. An increase of the hydraulic conductivity after the TH treatment with saline water has been observed, while the swelling capacity of the samples treated with granitic water slightly increases after treatment.

Retention Curves of Two Bentonites at High Temperature

Two methods were developed to determine the retention curve for a fixed dry density and at high temperature. The materials used are the MX–80 and the FEBEX bentonites. The retention capacity of the FEBEX bentonite is higher than that of MX–80, i.e. for a given suction and dry density, the water content of the FEBEX bentonite is higher. However, the difference between both bentonites becomes smaller towards the lower suctions. In both bentonites the retention capacity has decreased as temperature increased, and the difference in the retention capacity of the two bentonites became less important at high temperature.

Gas-breakthrough pressure of FEBEX bentonite

Abstract The gas-breakthrough pressure values in saturated FEBEX bentonite were determined for different dry densities and sample sizes. They increased clearly with dry density and were always higher than the swelling pressure of the bentonite. In high-density samples, gas flow tended to stop abruptly after breakthrough, whereas, in lower density samples, gas flow decreased gradually until a given pressure gradient was reached. The permeabilities computed after breakthrough (which usually did not stabilize) were inversely related to dry density. This would indicate that, despite the fact that flow took place through preferential pathways, the bentonite matrix and its swelling conditioned the ease of pathway formation. These paths sometimes closed quickly after breakthrough and others remained open, allowing a gradual decrease in gas flow. After resaturation of the bentonite, the same breakthrough pressures and permeabilities were found, pointing to the perfect healing of these preferential pathways. A sealed interface along the bentonite did not seem to affect the breakthrough pressure or permeability values.

Porosity Changes due to Hydration of Compacted Bentonite

As a contribution to the understanding of bentonite hydration, the porosity of two different compacted bentonites with different degrees of saturation was analysed by mercury intrusion porosimetry and X-ray diffraction. The percentage of macropores, as well as the macropore mode, decreased with dry density both in wet and dry samples. Wet samples showed a predominance of microporosity. The basal spacing increased with water content. The porosity distribution for a given water content was not the same for samples hydrated before compaction than for samples saturated under constant volume. In the latter case it clearly evolved over time, the interlayer distance becoming higher.

Gas and water permeability of concrete

Abstract Concrete is used as a barrier on surface or near-surface facilities for the final disposal of low- and intermediate-level radioactive waste, where gas can be generated and affect the hydraulic properties and the processes taking place in concrete. In this framework, gas-transport properties of concrete samples were investigated using two different laboratory test set-ups: a non-steady-state equipment working under low injection pressures; and a newly fine-tuned steady-state set-up working under different pressures. Permeability decreased with water content increase but was also greatly affected by the hydraulic history of concrete (i.e. if it had been previously dried or wetted). The intrinsic permeability determined with gas flow was about two orders of magnitude higher than that determined with liquid water (10−16 v. 10−18 m2), probably due to the chemical reactions taking place during saturation (carbonation). The relative gas permeability of concrete increased sharply for water degrees of saturation smaller than 50%. The boundary conditions also affected the gas permeability, which seemed to be mostly conditioned by the back pressure and the confining pressure, on the whole decreasing as the effective pressure increased. It is considered that the Klinkenberg effect was not relevant in the range of pressures applied.