Performance assessment of a bentonite–sand mixture for nuclear waste isolation at the potential Akkuyu Nuclear Waste Disposal Site, southern Turkey (original) (raw)
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Evaluation of a compacted bentonite/sand seal for underground waste repository isolation
Environmental Geology, 2006
This study investigates the performance of an optimum compacted bentonite/sand mixture seal for the isolation of underground waste repositories. Engineering geological tests such as compaction, flow, swelling, mechanical and shear strength tests have been conducted to select an optimum mixture and to recommend a stable bentonite/sand seal length-to-radius ratio (L/a) as far as the factor of safety (F) is concerned. The results of the compaction permeameter tests led to a recommendation to select an optimum compacted bentonite/sand mixture possessing a bentonite content of about 20% to satisfy the minimum regulatory hydraulic conductivity requirement. Engineering geological analysis of the seal/rock mechanical interaction with regard to reduce the possibility of seal slip led to a recommendation to utilize a seal L/a of at least 25.
Evaluation of a sand bentonite mixture as a shaft/borehole sealing material
Applied Clay Science, 2018
The mechanical and hydrological characteristics of compacted sand bentonite mixtures with bentonite contents ranging from 5 to 40% were investigated in the laboratory in order to assess their use as a waste isolation material and to select an optimum sand bentonite mixture. Laboratory tests included compaction, compaction permeability, unconfined compression and direct shear tests which led to a recommendation to select a mixture with a bentonite content of 30% for the isolation of underground geological waste disposal repositories. This study complements the previous studies of the authors of this manuscript by determining the mechanical and hydrological properties of sand bentonite mixtures that possess bentonite contents > 30% to determine the geotechnical properties (i.e., unconfined compressive strength, Young's modulus, cohesion and angle of internal friction) and the mechanical behavior of these relatively high levels of bentonite mixtures for the first time in the literature.
Analysis of the hydration of a bentonite seal in a deep radioactive waste repository
Engineering Geology, 2005
A deep geological repository for nuclear waste requires the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. The paper presents the coupled hydromechanical analyses of an in situ test carried out in the HADES underground laboratory in Mol, Belgium. The test examines the effectiveness of an expansive clay seal in a horizontal borehole specifically drilled for this purpose. The analysis covers the phase of seal hydration up to saturation and subsequent pore pressure equilibration. Hydraulic and mechanical constitutive laws suited to expansive clay materials have been chosen for the analyses with all the parameters determined independently of the in situ test. A quite good agreement has been found between test observations and computed results suggesting that the numerical formulation employed is able to reproduce the main features of a real sealing system. D
Simulation of hydromechanical behaviour of bentonite seals for containment of radioactive wastes
Canadian Geotechnical Journal, 2017
Geological disposal of radioactive wastes relies on a multiple barrier system to provide long-term containment and isolation of the wastes. The excavation of the repository creates openings and disturbed zones in the host rock formations that need to be properly sealed. Bentonite-based materials are being considered worldwide as a preferred type of sealing material, since they possess desirable characteristics such as low permeability, high sorption capability, and swelling potential allowing them to close internal cracks and gaps at interfaces with other materials. The French Institute for Radiation Protection and Nuclear Safety (IRSN) has led an experimental program consisting of a series of laboratory and large in situ experiments to assess the hydromechanical behaviour of bentonite seals. The experiments consisted of the forced re-saturation of pre-fabricated blocks of bentonite–sand mixture, with technological voids between bentonite seals and the walls of the steel cell (in th...
Compacted sand–bentonite mixtures for the confinement of waste landfills
Acta geotechnica, 2024
This paper illustrates the results of an experimental study on sand-bentonite mixtures for their use as confinement barriers for solid waste landfills. The mixtures have been prepared parametrically varying the percentage of bentonite. The sample preparation method was established willing to simulate the compaction processes on site. In fact, the compacted samples were tested following two different stress-wetting paths representative of the possible stress and imbibition sequences occurring on a landfill confinement barrier. In the first case, the barrier comes into contact with rainwater before being subjected to the overloading stress induced by waste disposal, while, in the second case, the barrier is overloaded by the waste before being wetted by the leachate. The compressibility and permeability of the sand-bentonite mixtures were determined, in both cases, by oedometric compression tests. The experimental results are analysed and compared in order to evaluate the influence of the bentonite content on the mechanical and hydraulic behaviour of the mixture. Interpretation of the results is also accomplished with a micro-mechanical investigation of the mixtures fabric. Suitable compositions of sand and bentonite are finally proposed for the design of effective confinement barriers. Keywords Bentonite compaction Á Bentonite swelling Á Bentonite hydraulic conductivity Á Compaction of sand-bentonite mixtures Á Hydraulic conductivity of sand-bentonite mixtures Á Oedometer test on bentonite Á Oedometer test on sandbentonite mixtures Á Sand-bentonite mixtures Á Swelling of sand-bentonite mixtures Á Waste landfills confinement & Erminio Salvatore
2016
In this study, natural bentonite was used as natural<br> clay material and samples were taken from the Kalecik district in<br> Ankara. In this research, bentonite is the subject of an analysis from<br> standpoint of assessing the basic properties of engineered barriers<br> with respect to the buffer material. Bentonite and sand mixtures were<br> prepared for tests. Some of clay minerals give relatively higher<br> hydraulic conductivity and lower swelling pressure. Generally,<br> hydraulic conductivity of these type clays is lower than <10-12 m/s.<br> The hydraulic properties of clay-sand mixtures are evaluated to<br> design engineered barrier specifications. Hydraulic conductivities of<br> bentonite-sand mixture were found in the range of 1.2x10-10 to<br> 9.3x10-10 m/s. Optimum B/S mixture ratio was determined as 35% in<br> terms of hydraulic conductivity and mechanical stability. At the<br> second stag...
Efficiency of a borehole seal by means of pre-compacted bentonite blocks
Physics and Chemistry of the Earth, Parts A/B/C, 2007
The backfilling and sealing of shafts and galleries is an essential part of the design of underground repositories for high-level radioactive waste. Part of the EC funded project RESEAL studied the feasibility of sealing off a borehole in plastic Boom Clay by means of pre-compacted bentonite blocks. Two bentonites, namely the FoCa and Serrata clay, have been used. Based on laboratory tests, the bentonite blocks had an initial dry density of about 1.8 g/cm 3 to obtain a swelling pressure of about 4.4 MPa, corresponding to the in situ lithostatic stress, at full saturation. The set-up was equipped with several sensors to follow-up the behaviour of the seal and the surrounding host rock during hydration. Full saturation was reached after five months and was mainly reached by natural hydration. Swelling pressure was lower than originally foreseen due to the slow reconsolidation of the host rock. Later on, the efficiency of the seal with respect to water, gas and radionuclide migration was tested. The in situ measured permeability of the seals was about 5 · 10 À13 m/s. A gas breakthrough experiment did not show any preferential gas migration through the seal. No evidences of a preferential pathway could be detected from 125 I tracer test results.
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.
Geotechnical Characteristic of Compacted Sand Bentonite Mixture
International Journal for Research in Applied Science and Engineering Technology
Sand and bentonite mixture is generally used as a liner or backfill material at waste disposal sites and construction of hydraulic barriers. Bentonite is added to sand to achieve a higher compaction density and lower shrinkage. Permeability and strength are important properties of sand-bentonite mixtures and are often required for the design of the liner/barrier of the containments. A review of the literature showed that most of the past research work has focused on studying the influence of bentonite on the various geotechnical properties of sand-bentonite mixtures. In this study, the permeability, and strength characteristics of sand-bentonite mixture is investigated to support recommendation for a cost-effective liner material with locally available soil. Locally available Narmada sand of Jabalpur, Madhya Pradesh (India) was mixed with different proportions of sodium bentonite. A series of standard tests such as Modified Compaction test, Unconfined Compression Strength (UCS) test and Permeability test for strength and hydraulic conductivity characteristics, were conducted on different of sandbentonite mixtures. The mixtures were formed by mixing local sand, with (0%, 5%, 10%, 15%, 20% and 25%) by dry of sodium bentonite. The investigations are presented to show the influence of bentonite clay on compaction, Atterberg limits, FSI, UCS, and permeability of Narmada sand by increasing the bentonite content 5% by weight each time. It was found that there is significant improvement in MDD and OMC of sand-bentonite mixture. This study focuses on the effect of bentonite content on strength and permeability, which are two important requirements of hydraulic liner material, of sand-bentonite mixtures.
2014
During the excavation of a shaft or drift, the rock properties around the underground openings could be altered due to the level of the ratio in situ stresses over strength of the rock. And even in some case, the rock mass could be fractured. One concern regarding waste disposal is that the associated disturbance and damage created in this area around these excavations might change the favorable properties of such formations and thus negatively impact the repository performance. To limit radionuclides migration along drifts and through the EDZ, seals will be implemented in drifts and shafts. These seals will be composed of swelling clay (mainly bentonite) core in between two concrete plugs. After natural hydration from the surrounding rock mass, the bentonite will swell and apply radial pressure against the drift wall. In this context, ANDRA designs a large scale sealing experiment which is called NSC (French acronym for Noyau de SCellement) The main objective is to back analyze the...