Bentonite as a containment barrier for the disposal of highly radioactive wastes

doi:10.1016/0169-1317(89)90006-9

Applied Clay Science

Volume 4, Issue 2, June 1989, Pages 157-177

https://doi.org/10.1016/0169-1317(89)90006-9 Get rights and content

Abstract

The project for the storage of highly radioactive waste in Switzerland is characterized by a system of successive safety barriers. One of these barriers is the highly compacted bentonite backfill. The functions of this bentonite backfill are described and some investigations of the important properties as swelling and water uptake are presented. In order for the radioactive material to leak out into the biosphere, water from the surrounding rock has to reach the steel canisters, and these have to be broken through by corrosion. Conservative estimates call for a time of at least 1000 years. After this period the activity of the waste comes mostly from actinides. Breakthrough time for the actinides is estimated at 10⁴–10⁶ years. The long-term stability of montmorillonite under elevated temperature and pressure has been examined by the investigation of K-bentonites of Kinnekulle (Sweden).

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Integrity of engineered barrier system (EBS) needs to be secured for the safe isolation of high–level radioactive waste. Mixed clay bentonite, which composes the buffer of EBS, is known to experience the clay synthesis while interacting with heated pore fluid during radioactive decay. This clay synthesis includes the illitization of smectite, where expandable smectite transforms into non–expandable illite. In this study, we experimentally evaluated the changing characteristics of Gyeongju bentonite relevant to the swelling, hydrologic, and chemical behavior, which is used for the Korean disposal systems for high–level radioactive wastes. Hydrothermal reaction experiments of Gyeongju bentonite with 0.01 M, 0.1 M, and 1 M KCl were conducted to observe the illitization of smectite, followed by the analyses of mineral and chemical compositions with X–Ray Diffraction and X–Ray Fluorescence (XRF). Increasing Young's modulus and decreasing swelling pressure were subsequently measured, along with the changing specific surface area, average particle radius, average pore radius, and total pre volume. Inverse modeling of reaction rate constant, activation energy, and specific surface area of dissolving Ca–montmorillonite and precipitating illite was conducted based on the experimental results, as providing the input data for the conceptual forward modeling of THC behavior of Gyeongju bentonite. From the forward modeling, 26.2%–reduction of swelling pressure of bentonite buffer after 10,000 years was forecasted. Forward modeling coupled local sensitivity analysis was subsequently conducted, which provided the relative impacts of reaction parameters on the swelling behavior. They quantitatively predicted the impact of hydrothermal reaction of smectite on swelling behavior, which is expected to be beneficial for the predictive estimation of long–term integrity of bentonite buffer and EBS.
Effect of thermal ageing on physical properties of MX80 bentonite under high-temperature conditions
2022, Engineering Geology
Citation Excerpt :
Disposal of high-level radioactive waste (HLW) in deep geological repositories (DGRs) is one of the most effective and applicable methods, which has received the worldwide attention that involves isolation of these wastes from the biosphere (Dixon et al., 1985; Bucher and Müller-Vonmoos, 1989; Pusch, 2008; SKB, 2006; Wang et al., 2006; NAGRA, 2009; Chapman and Hooper, 2012; Kale and Ravi, 2018; Wang, 2019).
In a deep geological repository (DGR) system, the buffer layer is indispensable to ensuring the safe disposal of high-level radioactive nuclear waste (HLW). Because the heat generated by the decay of the spent nuclear fuel in a canister is released to the surrounding buffer layers, the bentonite buffer material experiences long-term high-temperature conditions. Therefore, the variations in physical properties of bentonite buffer material under high-temperature conditions are one of the important parameters in the DGRs for HLW. A series of tests on the specific gravity, specific surface area (SSA), Atterberg's limits, and free swelling ratio of MX80 bentonite after heating for different times at a high temperature of 200 °C were conducted, to investigate the influence of thermal ageing time on its physical properties. Then, the microscopic investigations, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), were conducted to explain the aforementioned variations from the microscopic point of view. Based on the crystal layer structure characteristics of montmorillonite, the quantitative relationships between the physical properties of bentonite and its mineral composition or bound water content were established to further explain the mechanism by which thermal ageing affects the physical properties of MX80 bentonite. The results indicate that with increasing heating time, the specific gravity, SSA, liquid limit, plastic limit, plastic index and free swelling ratio decrease sharply by 2.5%, 4.5%, 3.1%, 5.5%, 2.8% and 30.2%, respectively, within 15–30 days of heating. After 30 days of heating, their variations are negligible. Under high-temperature conditions, the transformation of mineral composition, desorption of bound water, and changes in the micro-morphology are the fundamental reasons for the variations in the physical properties of bentonite, and they influence and interact with each other.
Water and ion diffusion in partially-water saturated compacted kaolinite: Role played by vapor-phase diffusion in water mobility
2022, Journal of Contaminant Hydrology
Diffusion is the main transport process of water and solutes in clay-rich porous media owing to their very low permeability, so they are widely used as barriers against contaminant spreading. However, the prediction of contaminant mobility can be very complicated when these media are partially water-saturated. We conducted diffusion experiments for water (HTO and HDO) and ions (²²Na⁺ and ¹²⁵I⁻) through partially water saturated compacted kaolinite, a weakly charged clay material, to quantify the distinct diffusive behavior of these species. The osmosis method was used to set kaolinite samples at 67, 86 and 100% saturation. The results showed that desaturation led to a sharp decrease in diffusive rates by factors of 6.5, 18 and 35 for HTO, ¹²⁵I⁻ and ²²Na⁺, respectively, from 100 to 67% of the degree of saturation. Thus, to interpret water diffusivities, we proposed a model taking into account the diffusion of water in both gas and liquid phases, using diffusion data obtained for ions, considered as inert species. This model was capable of properly predicting water diffusive flux, especially at a low degree of saturation (67% saturation), for which the assumption made for the occurrence of air phase continuity throughout the sample appears to be more relevant than at 86% saturation.
Influence of thermal gradient and canister corrosion on the hydration of compacted Barmer bentonite
2022, Annals of Nuclear Energy
Compacted bentonite is being used as a buffer material for the safe dumping of high-level radioactive wastes (HLW) in the deep geological repository (DGR) due to its favorable physicochemical properties. However, the temperature variation, probably because of continuous heat emitting canister (initially −150 °C to 250 °C) and geothermal gradient, affect its performance. There would be bentonite iron interface reactions due to the corrosion of steel canister, which might cause reactive transport through the compacted bentonite buffer. Therefore, it is essential to know the influence of a combination of thermal as well as a chemical gradient on the reactive transport through the compacted bentonite buffer in terms of its physicochemical, thermomechanical, mineralogical stability. The purpose of this particular study is to establish the field conditions in the lab by fabricating a small scale set up which would create a thermal gradient on the compacted Barmer bentonite (B₁ and B₂) samples (compacted at 1.5 Mg/m³, 1.75 Mg/m³ and 2 Mg/m³ densities) by heating its one side at 60 °C, 110 °C, 150 °C and 200 °C temperatures for 28 days each and the system was hydrated by distilled water from the other side and was kept in touch with the metallic iron powder. It was observed that the degree of saturation of both bentonites decreased with an increase in temperature as well as density. However, the cation exchange capacity at lower density reduced more than that of higher density. The microstructural observation revealed the presence of impurities between the lamella of bentonite after 28 days that might be hematite.
Oedometric compression and thermal volume behavior of compacted Gaomiaozi bentonite saturated with salt solution
2021, Geomechanics for Energy and the Environment
Citation Excerpt :
After their placement, the bentonite blocks are gradually hydrated with groundwater, and they provide essential buffer functions such as sealing the gap, supporting the canister, and retarding the nuclide migration. When implementing the conceptual design of the deep geological repository, the decay heat of radioactive waste and the chemistry of groundwater are the two major concerns.1–4 The temperature gradient initiated by the high decay heat causes differential hydration swelling and thermal expansion of the buffer material.
Saline solution has considerable influence on the thermo-hydro-mechanical behavior of compacted bentonite. In this study, the effects of NaCl solution on the swelling deformation, oedometric compression, and thermal volume behavior of compacted Gaomiaozi bentonite were investigated. Results show that compressibility of the specimens decreases with increasing NaCl concentration of the presaturation solution. At an overconsolidation ratio OCR $=$ 1 with a vertical pressure $σ_{v} = 0.1$ MPa and at OCR $=$ 3 with $σ_{v} = 0.1$ MPa, the specimens exhibited contraction during heating below the transition temperature, followed by dilation. A larger vertical pressure led to a higher transition temperature (at which the thermal strain turns from contraction to dilation). However, the specimens exhibited only thermal dilation at OCR $=$ 10 with $σ_{v} = 0.1$ MPa and at OCR $=$ 2 with $σ_{v} = 0.5 MPa$ . Additionally, for a given vertical pressure and presaturation solution, the specimens with a higher OCR exhibited a larger irreversible dilation after a thermal cycle. Moreover, the influence of the presaturation solution on the thermal volume change is non-monotonic. A dilute solution (NaCl solution of 0.1 mol/L) and high salinity solution (NaCl solution of 1.0 mol/L) markedly promote the thermal expansion. These experimental trends can be attributed to the fact that attenuation of the diffused double layer repulsion and electrostatic interaction as well as the reduction of the free water at high saline conditions harden the soil.
Failure modes for hydrated bentonite plugs used in well decommissioning operations
2020, Applied Clay Science
This paper aims to theoretically investigate and characterize the failure mechanism of hydrated bentonite plugs, when used as a sealing material in wellbores. Using the proposed theoretical models, it was determined that when hydrated bentonite plugs failed due to friction at the walls, created by internal swelling pressure or through internal shear failure, the resulting failure pressure was linear with the plug height (H), coefficient of friction or internal shear strength, and inversely proportional to plug diameter (D). On the other hand, if the friction at the walls was created by plug weight then the failure pressure was a function of H²/D. However, in this case, the frictional strength increased parabolically with height, and, at reasonable plug heights, the frictional strength became greater than shear strength, and resulted in failure due to internal shear failure, which also made the failure pressure a function of H/D. This suggests that plug strength is usually a linear function of H/D. Moreover, the coefficient of friction and internal shear strength are dependent on final moisture content of the hydrated plug, the salinity of the hydration water, pressure, temperature and hydration time. The strength of a bentonite plug is inversely proportional the total volume by which it expands during hydration and takes in excess of 185 days to achieve this state. Plug strength decreased in saline water, although even at elevated concentrations sufficient strength remained to produce and adequate seal.From very limited data, shown here, Queensland bentonite, from the 5D zone of Amcol's Gurulmundi mine, appeared to have similar plugging strength as Wyoming bentonite.

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Bentonite as a containment barrier for the disposal of highly radioactive wastes

Abstract

Geochim. Cosmochim. Acta

Chemical and physical properties of paleozoic potassium bentonites from Kinnekulle, Sweden

Clays Clay Miner.

Quelldruck von hochverdichteten Bentoniten

Nagra Tech. Ber.

Herstellung und Homogenität hochverdichteter Bentonitproben

Nagra Tech. Ber.

The Mathematics of Diffusion

Ionendiffusion in hochverdichtetem Bentonit

Nagra Tech. Ber.