Assessing the Long-Term Behaviour of the Industrial Bentonites Employed in a Repository for Radioactive Wastes by Studying Natural Bentonites in the Field (original) (raw)
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2009
Results of a 3 years study regarding the changes of MX-80 bentonite after reaction with solutions of different ionic strength and pH occurring in repositories in granite, in clay and in salt formations are presented. Each solution reacted differently with the bentonite and affected differently the resulting mineralogy and swelling pressure (SWP). After 3 years of reaction SWP were highest in contact with water (over 4 MPa), significantly lower in contact with low ionic strength solutions (around 3 MPa) and lowest in contact with high saline brines (mostly under 1 MPa). Alteration and partial dissolution of montmorillonite was observed. The Mg, Al and Si contents in the solutions increased with reaction time. Concomitantly in the octahedral layers of montmorillonite Mg was substituted by Al and the interlayer charge decreased. The correlation between the alteration process, the charge reduction and SWP is presented. The key for the observed mineralogical alteration may be the acidity...
2008
In an effort to secure the comfort of all aspect of life and security through electricity production, research, nuclear weapons production and medical uses, radioactive waste is being generated throughout the world as a result. Without pro – active management and protection, the hazardous nature of radioactive materials can pose serious threats to human health and the environment. A waste containment or disposal facility is required to isolate the waste from man and the biosphere to avoid any undue radiation exposure. Deep geological repository or disposal facility is considered as an effective way to isolate high level radioactive waste, HLW from the human environment, for which a multi – barrier system plays a very significant role. In this paper, literature collection was carried out, aimed to overview the present knowledge about the concepts of repository for HLW and to identify technologies that are currently available in the nuclear industry worldwide and the current state of ...
Swiss Journal of Geosciences, 2015
The practice of utilising natural analogues in assessing the long-term behaviour of various components of geological repositories for radioactive waste is already well established in most disposal programmes. Numerous studies on bentonites, focussing on bentonite interaction with other components of the engineered barrier system and a range of host rock environments, are present in the literature. In this article, recent bentonite natural analogue studies are briefly reviewed, and gaps in the current literature identified, with the aim of (1) suggesting where relevant new information could be obtained by data mining published bentonite natural analogue studies with a new focus on current safety case requirements, (2) collecting relevant information by revisiting known bentonite analogue sites and conducting investigations with more appropriate analytical techniques, and (3) identifying novel study sites where, for example, bentonite longevity in very dilute to highly saline groundwater conditions can be studied. It must be noted that the use of natural analogues in safety case development is likely to be site and repository design-specific in nature and thus emphasis is placed on the appropriate use of relevant natural analogue data on bentonite longevity.
Environmental Earth Sciences, 2014
This study assesses the geotechnical performance of a compacted bentonite-sand mixture with a bentonite content ranging from 15 to 30 % by weight to be used as a material component in a waste sealing system. Geotechnical laboratory tests such as compaction, falling head permeability, swelling, unconfined compression and shear strength tests were conducted to select an optimum mixture which eventually led to a recommendation to select an optimum bentonite-sand mixture possessing a bentonite content of 30 % for the isolation of underground waste disposal facilities. Bentonite-sand seal design was performed for the optimum seal selected as a function of the axial stress applied to the seal and seal length-to-radius ratio.
American Mineralogist, 2014
Geologic disposal of spent nuclear fuel in high capacity metal canisters may reduce the repository footprint, but it may yield high thermal loads (up to 300°C). The focus of this experimental work is to expand our understanding of the hydrothermal stability of bentonite clay barriers interacting with metallic phases under different geochemical, mineralogical, and engineering conditions. The hydrothermal experiments were performed using flexible Au/Ti Dickson reaction cells mounted in an externally-heated pressure vessel at 150-160 bars and temperatures up to 300°C for five to six weeks. Unprocessed Wyoming bentonite, containing primarily montmorillonite with minor amount of clinoptilolite, was saturated with a K-Ca-Na-Clbearing water (~1,900 mg/L total dissolved solids) at a 9:1 water:rock mass ratio. The bentonite and solution combination contained either steel plates or Cu-foils and were buffered to low Eh using magnetite and metallic iron. During reactions, pH, K + , and Ca 2+ concentrations decreased, whereas SiO 2(aq) , Na + , and SO 4 2concentrations increased throughout the experiments. Pyrite decomposition was first observed at ~210°C, generating H 2 S (aq,g) that interacted with metal plates or evolves as a gas. The aqueous concentrations of alkali and alkaline earth cations appear to be buffered via montmorillonite and clinoptilolite exchange reactions. Illite or illite/smectite mixedlayer formation was significantly retarded in the closed system due to a limited K + supply along with high Na + and SiO 2(aq) concentrations. Precursor clinoptilolite underwent extensive recrystallization during the six week, 300°C experiments producing a Si-rich analcime in addition to authigenic silica phases (i.e., opal, cristobalite). Analcime and feldspar formation partially sequester aqueous Al 3+ , thereby potentially inhibiting illitization. Associated with the zeolite alteration is a ~ 17 % volume decrease (quartz formation) that translates into ~ 2% volume loss in the bulk bentonite. These results provide chemical information that can be utilized in extending the bentonite barriers' lifetime and thermal stability. Zeolite alteration mineralogy and illitization retardation under these experimental conditions is important for the evaluation of clay barrier long-term stability in a spent nuclear fuel repository.
The Use of Clay as an Engineered Barrier in Radioactive-Waste Management – A Review
Clays and Clay Minerals, 2013
Geological disposal is the preferred option for the final storage of high-level nuclear waste and spent nuclear fuel in most countries. The selected host rock may be different in individual national programs for radioactive-waste management and the engineered barrier systems that protect and isolate the waste may also differ, but almost all programs are considering an engineered barrier. Clay is used as a buffer that surrounds and protects the individual waste packages and/or as tunnel seal that seals off the disposal galleries from the shafts leading to the surface. Bentonite and bentonite/sand mixtures are selected primarily because of their low hydraulic permeability in a saturated state. This ensures that diffusion will be the dominant transport mechanism in the barrier. Another key advantage is the swelling pressure, which ensures a self-sealing ability and closes gaps in the installed barrier and the excavation-damaged zone around the emplacement tunnels. Bentonite is a natural geological material that has been stable over timescales of millions of years and this is important as the barriers need to retain their properties for up to 10 6 y. In order to be able to license a final repository for high-level radioactive waste, a solid understanding of how the barriers evolve with time is needed. This understanding is based on scientific knowledge about the processes and boundary conditions acting on the barriers in the repository. These are often divided into thermal, hydraulic, mechanical, and (bio)chemical processes. Examples of areas that need to be evaluated are the evolution of temperature in the repository during the early stage due to the decay heat in the waste, re-saturation of the bentonite blocks installed, build-up of swelling pressure on the containers and the surrounding rock, and degradation of the montmorillonite component in the bentonite. Another important area of development is the engineering aspects: how can the barriers be manufactured, subjected to quality control, and installed? Geological disposal programs for radioactive waste have generated a large body of information on the safety-relevant properties of clays used as engineered barriers. The major relevant findings of the past 35 y are reviewed here.
Physics and Chemistry of the Earth, Parts A/B/C, 2008
The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal were simulated in a series of tests performed in cylindrical cells (length 60 cm, diameter 7 cm). Inside the cells, six blocks of FEBEX bentonite compacted to dry density 1.65 g/cm 3 were piled up, giving rise to a total length similar to the thickness of the clay barrier in a repository according to the Spanish concept. The bottom surface of the material was heated at 100°C and the top surface was injected with granitic water. The duration of the tests was 6, 12, 24 and 92 months. The temperatures inside the clay and the water intake were measured during the tests and, at the end, the cells were dismounted and the dry density, water content and hydro-mechanical properties were measured at different positions. The injection of water provokes, near the hydration surface, a decrease of the dry density due to the increase of the water content and the clay swelling, while heating gives rise to an increase of the dry density and a reduction of the water content in the hottest areas.
Hydraulic and mechanical properties of compacted bentonite after 18 years in barrier conditions
Applied Clay Science, 2018
The FEBEX "in situ" test was performed at an underground laboratory in Grimsel (Switzerland) with the aim of studying the behaviour of components in the near-field of a nuclear waste repository. A gallery of 2.3 m in diameter was excavated through the granite and two heaters, simulating the thermal effect of the wastes, were placed inside, surrounded by a barrier of highly-compacted bentonite blocks. In 2015, after 18 years of operation, the experiment was dismantled. Some of the bentonite samples taken were tested in the laboratory to characterize, among others, their physical state and determine their permeability and swelling capacity. There were significant changes in water content and dry density across the bentonite barrier: their distribution was radial around the axis of the gallery, with the water content decreasing from the granite towards the axis of the gallery and the dry density following the inverse pattern. The swelling capacity of the samples was related to their position in the barrier. In the internal, drier part of the barrier an increase of the swelling capacity with respect to the reference bentonite was detected, whereas the samples from the external part swelled less than expected. This was attributed to the different salinity of the samples. The hydraulic conductivity was mainly related to the dry density of the samples and decreased with respect to the reference bentonite. This decrease was not related to the position of the samples and could be related to the microstructural reorganization of the bentonite during the 18-year operation-which brought about an average decrease in the pore size-and to the low hydraulic gradients applied to determine the permeability of the samples retrieved.