Application of Elasto-Plastic Model to Mechanical and Hydraulic Behavior of Buffer Material Under Water Uptake in a Repository

1990 ◽  
Vol 212 ◽  
Author(s):  
T. Fujita ◽  
K. Hara ◽  
Y. Yusa ◽  
N. Sasaki
Keyword(s):  
Water Uptake ◽  
Mechanical Stability ◽  
Near Field ◽  
Swelling Pressure ◽  
Plastic Model ◽  
Hydraulic Behavior ◽  
Compacted Bentonite ◽  
Buffer Material ◽  
Swelling Coefficient ◽  
Engineered Barrier

ABSTRACTMechanical and hydraulic behavior of buffer material during water uptake in a repository is a major issue from the viewpoint of mechanical stability of engineered barriers and near-field conditions for performance assessment. This paper presents the results of hydraulic-mechanical modeling of buffer material and the simulations carried out on an engineered barrier system under water uptake.Hydraulic behavior of compacted bentonite of buffer material was modeled as moisture diffusion. An elasto-plastic model was applied to the deformation behavior of compacted bentonite, of which swelling pressure was described by swelling coefficient under restraint condition. The hydraulic diffusivity and swelling coefficient were given based on the result of swelling tests of KUNIGEL-V1 bentonite which contains about 50 % montmorillonite. Being used this model, simulations on re-saturation behavior of an engineered barrier system were carried out for the cases of water uptake from the whole surface of both crystalline and sedimentary rock and from partial surface of opening. The results are : (1) The hydraulic and mechanical behavior of compacted bentonite can be described by a swelling-elasto-plastic model. (2) The distribution of the water content depends on the water uptake conditions. (3) The deformation of compacted bentonite and the displacement of the overpack under water uptake are negligibly small.

Coupling Swelling and Water Retention Processes in Compacted Bentonite

2011 ◽  
Vol 6 (1) ◽  
Author(s):  
Antti Lempinen
Keyword(s):  
Water Retention ◽  
Spent Nuclear Fuel ◽  
Effective Strain ◽  
Swelling Pressure ◽  
Model Parameters ◽  
Confined Water ◽  
Swelling Properties ◽  
Compacted Bentonite ◽  
Buffer Material ◽  
Swelling Factor

Compacted bentonite is the main candidate for buffer material in several plans for spent nuclear fuel repositories. One of its important properties is high swelling capacity, which is caused by interaction between water molecules and exchangeable cations. This interaction makes bentonite behave differently from capillary materials. In this article, a model for thermo-hydro-mechanical state of partially water saturated bentonite is presented. It couples the water retention and swelling properties with introduction of the swelling factor in effective strain. The Helmholz energy density determines the state with a relatively small set of independent parameters: swelling pressure, swelling factor, maximum confined water content and the reference state. The model parameters are determined from experimental data for FEBEX bentonite, and as a simple consistency check, confined suction curves are calculated and compared to test results. Consistency of the model with observations on nano- and microscale of bentonite is also discussed.

A Study on Gas Migration Behavior in Buffer Material using X-ray CT Method

2006 ◽  
Vol 932 ◽  
Author(s):  
K. Tanai ◽  
M. Yui
Keyword(s):  
Water Saturation ◽  
Gas Injection ◽  
Swelling Pressure ◽  
Digital Data ◽  
Migration Behavior ◽  
Gas Migration ◽  
Migration Test ◽  
X Ray ◽  
Compacted Bentonite ◽  
Buffer Material

ABSTRACTThis paper presents a study on gas migration behavior in a bentonite specimen with the aid of X-ray computer tomography (CT) scan data. The laboratory experiment was carried out to clarify gas migration behavior through saturated, compacted bentonite. X-ray CT was used to estimate the spatial distribution of gas and water saturation during gas migration test in the bentonite. For the gas migration test, the controlled flow rate of gas injection was adopted for pre-compacted samples of Kunigel V1 bentonite using helium gas, which is safer than hydrogen gas.A specimen was isotropically consolidated and saturated by synthetic seawater, simultaneously, by applying a backpressure. This was followed by injecting the gas using a syringe pump. Inlet and outlet gas fluxes were monitored. This test exhibited a significant threshold pressure for breakthrough, somewhat larger than the sum of the swelling pressure and the backpressure.The procedure of the X-ray CT measurement is as follows; i) measurement of the initial condition (saturated condition) of the compacted bentonite, ii) measurement of the gas injection condition as a function of time. The digital data obtained from the X-ray CT usually includes some noise. The stacking method can reduce the noise in CT values and enables to identify the gas migration area. The results indicate that gas is transported through preferential pathways in compacted bentonite, and is not homogenous.

New equations for swelling characteristics of bentonite-based buffer materials

2003 ◽  
Vol 40 (2) ◽  
pp. 460-475 ◽  
Author(s):  
Hideo Komine ◽  
Nobuhide Ogata
Keyword(s):  
Double Layer ◽  
Nuclear Waste ◽  
Van Der Waals ◽  
Swelling Pressure ◽  
Van Der Waals Force ◽  
Diffuse Double Layer ◽  
Mass Ratio ◽  
Compacted Bentonite ◽  
Buffer Material ◽  
Swelling Characteristics

Compacted bentonite and sand–bentonite mixtures are attracting greater attention as buffer material for repositories of high-level nuclear waste. This buffer material is expected to fill up the space between the canisters containing the waste and the surrounding ground by swelling. To produce the specifications, such as dry density, sand–bentonite mass ratio, and dimensions, of the buffer material, the swelling characteristics of compacted bentonite and sand–bentonite mixtures must be evaluated quantitatively. New equations for evaluating the swelling behavior of compacted bentonite and sand–bentonite mixtures are presented that can accommodate the influences of the sand–bentonite mass ratio and the exchangeable-cation composition of bentonite. The new method for predicting swelling characteristics is presented by combining the new equations with the theoretical equations of the Gouy–Chapman diffuse double layer theory and of the van der Waals force, which can evaluate the repulsive and attractive forces of montmorillonite mineral (i.e., the swelling clay mineral in bentonite). Furthermore, the applicability of the new prediction method has been confirmed by comparing the predicted results with laboratory test results on the swelling deformation and swelling pressure of compacted bentonites and sand–bentonite mixtures.Key words: bentonite, diffuse double layer theory, van der Waals force, nuclear waste disposal, swelling deformation, swelling pressure.

Modelling water uptake in highly compacted bentonite in environmental sealing barriers

Clay Minerals ◽  
2001 ◽  
Vol 36 (3) ◽  
pp. 435-446 ◽  
Author(s):  
J. Gattermann ◽  
W. Wittke ◽  
C. Erichsen
Keyword(s):  
Water Uptake ◽  
Large Scale ◽  
Swelling Pressure ◽  
Theoretical Models ◽  
Model Tests ◽  
Scale Model ◽  
Compacted Bentonite ◽  
Strain Behaviour ◽  
Large Scale Model ◽  
Computer Codes

AbstractPlans to close a German radioactive waste repository in rock salt include as one alternative the construction of a ‘Cross Section Closure (CSC)’ sealing barrier. The proposed material for the sealing barrier is highly compacted bentonite. To investigate the swelling behaviour of a highly compacted bentonite, a large number of laboratory tests were performed. In addition, large scale model tests were carried out to demonstrate the development of a nearly homogeneous and isotropic swelling pressure. The results of the large scale model tests were interpreted numerically based on the models for stress-strain behaviour and water uptake which are implemented in the finite element computer codes FEST03 and HYDOPO. The investigations show good agreement between the results of the model tests and the corresponding analyses and show that the theoretical models are capable of describing the behaviour of sealing structures based on highly compacted bentonite.

A Study of Mechanical Effect of Simulated Fault Movement on Engineered Barrier System

2006 ◽  
Vol 932 ◽  
Author(s):  
Mayuka Nishimura ◽  
Takashi Hirai ◽  
Kenji Tanai ◽  
Mikazu Yui
Keyword(s):  
Pore Water Pressure ◽  
Water Pressure ◽  
Near Field ◽  
Experimental Tests ◽  
Mechanical Effect ◽  
Experimental Results ◽  
Disposal Site ◽  
Fault Movement ◽  
Buffer Material ◽  
Engineered Barrier

ABSTRACTThe objective of this study is to clarify the mechanical effect on the engineered barrier system (EBS) of a fault movement, presupposed to occur in a high-level radioactive waste repository. The plan of this study is; 1) to understand the mechanical behavior of the buffer material when shearing takes place during experimental tests, and 2) to make progress in numerical analysis techniques in order to estimate the effect of fault movement on the EBS at a disposal site.Accordingly, the first part of this paper reports tests, which were carried out on a 1:20 model of the EBS. The experimental results indicate that the increased total pressure is due, in part, to the increase of pore water pressure. The second part of this paper reports the results of finite element simulations of the experiments. The calculation results show that the permeability of the near-field rock, which influences the amount of water draining from the buffer material, affects the pressure increase in the buffer material. With appropriately set parameters, the calculation shows agreement with the experimental results.

Fundamental Properties of Monolithic Bentonite Buffer Material Formed by Cold Isostatic Pressing for High-Level Radioactive Waste Repository

1999 ◽  
Vol 556 ◽  
Author(s):  
S. Kawakami ◽  
Y Yamanaka ◽  
K. Kato ◽  
H. Asano ◽  
H. Ueda
Keyword(s):  
Hydraulic Conductivity ◽  
Radioactive Waste ◽  
Swelling Pressure ◽  
Cold Isostatic Pressing ◽  
Small Samples ◽  
Isostatic Pressing ◽  
High Level Radioactive Waste ◽  
Compacted Bentonite ◽  
Buffer Material ◽  
High Level

AbstractThe methods of fabrication, handling, and emplacement of engineered barriers used in a deep geological repository for high level radioactive waste should be planned as simply as possible from the engineering and economic viewpoints. Therefore, a new concept of a monolithic buffer material around a waste package have been proposed instead of the conventional concept with the use of small blocks, which would decrease the cost for buffer material. The monolithic buffer material is composed of two parts of highly compacted bentonite, a cup type body and a cover. As the forming method of the monolithic buffer material, compaction by the cold isostatic pressing process (CIP) has been employed.In this study, monolithic bentonite bodies with the diameter of about 333 mm and the height of about 455 mm (corresponding to the approx. 1/5 scale for the Japanese reference concept) were made by the CIP of bentonite powder. The dry densities: pd of the bodies as a whole were measured and the small samples were cut from several locations to investigate the density distribution. The swelling pressure and hydraulic conductivity as function of the monolithic body density for CIP-formed specimens were also measured.High density ( ρd: 1.4–2.0 Mg/m3) and homogeneous monolithic bodies were formed by the CIP. The measured results of the swelling pressure (3–15 MPa) and hydraulic conductivity (0.5–1.4×10−3 m/s) of the specimens were almost the same as those for the uniaxial compacted bentonite in the literature. It is shown that the vacuum hoist system is an applicable the handling method for emplacement of the monolithic bentonite.

Water uptake and swelling properties of unsaturated bentonite buffer materials

1992 ◽  
Vol 29 (6) ◽  
pp. 1102-1107 ◽  
Author(s):  
T. Kanno ◽  
H. Wakamatsu
Keyword(s):  
Radioactive Waste ◽  
Water Uptake ◽  
Swelling Pressure ◽  
Radioactive Waste Disposal ◽  
Degree Of Saturation ◽  
Small Element ◽  
Elastic Model ◽  
Swelling Properties ◽  
Water Diffusivity ◽  
Buffer Material

The water diffusivity and the development of swelling pressure are investigated in buffer materials to be used for the geologic disposal of high-level radioactive waste during the stage of unsaturated water uptake. Highly compacted blocks of Japanese Na bentonite and a bentonite-sand mixture are used as the buffer material. The water diffusivity of the blocks has turned out to be approximately equal to that of Wyoming bentonite MX-80. Assuming that the local swelling pressure in a small element of a confined bentonite mass is proportional to the degree of saturation of the local area, an elastic model with an apparent Young's modulus is developed for the first step. According to this model, the swelling pressure of the bentonite mass as a whole is proportional to the avarage degree of saturation of the mass. For the development of the swelling pressure in the blocks, the calculated curve by this model is in good agreement with the experimental results except during the early parts of the process. Key words : bentonite, water uptake, water diffusivity, swelling pressure, elastic model, radioactive waste disposal.

Clay mineralogical investigations related to nuclear waste disposal

Clay Minerals ◽  
1998 ◽  
Vol 33 (1) ◽  
pp. 109-129 ◽  
Author(s):  
F. T. Madsen
Keyword(s):  
Nuclear Waste ◽  
Swelling Pressure ◽  
Nuclear Waste Repository ◽  
Apparent Diffusion Coefficients ◽  
Compacted Bentonite ◽  
Geotechnical Investigations ◽  
Buffer Material ◽  
Swelling Capacity ◽  
Apparent Diffusion ◽  
Swelling Strain

AbstractMineralogical and geotechnical investigations on the possible use of compacted bentonite as a buffer material in nuclear waste repositories are reported. The swelling capacity is highly dependent on the density of the compacted bentonite. Swelling pressures >30 MPa were measured for dry densities of ~2.0 g/cm3. Added iron or magnetite powder up to 20 wt% had no influence on the swelling capacity. Compacted mixtures of 20 wt% ground set cement and bentonite showed higher swelling pressures but lower swelling strain capability than compacted bentonite alone. Steam lowered the swelling pressure of compacted bentonite to ~60% of the original value. The influence was, however, reversible by ultrasonic treatment. The thermal conductivity of saturated compacted bentonite at a density of 2.0-2.1 g/cm3is ~1.35-1.45 W/m°K The volumetric heat capacity ranges from 3.1 x 106to 3.4 x 106j/m3°C The saturated hydraulic conductivity of the compacted bentonite is <10-12m/s. The apparent diffusion coefficients for various ions in compacted bentonite for water contents in the range of 20 to 25 wt% are: K+: 5 x 10-11, Cs+: 6 x 10-12, Sr2+: 3 x 10-11, UO22+: <10-13, Th4+: <10-13, Fe2+: 4 x 10-11, Fe3+: 4 x 10-11, Cl-: 1 x 10-10and I-: 1 x 10-10m2/s. The 'breakthrough time' for an apparent diffusion coefficient of 10-11m2/s in compacted bentonite 1 m thick was estimated to be ~3000 years. The mineralogical longevity was investigated on natural K-bentonites from Kinnekulle, Sweden, and Montana, USA. Although these materials have undergone considerable changes during diagenesis and contain various amounts of mixed-layer illite-smectite, they still have a substantial swelling and adsorption capacity. The investigations demonstrate that although the properties of bentonite are negatively influenced to a certain extent by heat, hot steam, iron and cement, compacted bentonite is still the best choice to act as a buffer material in a nuclear waste repository.

scholarly journals Swelling Pressure and Permeability of Compacted Bentonite from 10th Khutor Deposit (Russia)

Minerals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 742
Author(s):  
Artur Yu. Meleshyn ◽  
Sergey V. Zakusin ◽  
Victoria V. Krupskaya
Keyword(s):  
Swelling Pressure ◽  
Crystalline Rocks ◽  
Hydraulic Gradient ◽  
Deep Geological Repository ◽  
Compacted Bentonite ◽  
Buffer Material ◽  
Geological Repository ◽  
Hydraulic Conditions ◽  
Hydraulic Gradients

Bentonites from the 10th Khutor deposit (Republic of Khakassia, Russia) are considered a potential buffer material for isolation of radioactive waste in the crystalline rocks of Yeniseyskiy site (Krasnoyarskiy region). This study presents the results of a series of permeameter experiments with bentonite compacted to dry densities of 1.4, 1.6, and 1.8 g/cm3, saturated and permeated by the artificial groundwater from Yeniseyskiy Site. Permeation was conducted at hydraulic gradients of 180–80,000 m/m to simulate potential hydraulic conditions in the early post-closure phase of a deep geological repository (DGR). The respective swelling pressures of 0.8 ± 0.3, 2.2 ± 0.6, and 6.3 ± 0.3 MPa and permeabilities of (27 ± 15) × 10−20, (3.4 ± 0.8) × 10−20, and (0.96 ± 0.26) × 10−20 m2 were observed for the hydraulic gradient of 2000 m/m, which is recommended for the determination of undisturbed swelling pressures and permeabilities in permeameter experiments. Upon incremental increases in the hydraulic gradient, swelling pressures at all densities and permeability at the density of 1.8 g/cm3 remained unchanged, whereas permeabilities at 1.4 and 1.6 g/cm3 decreased overall by a factor of approximately 5 and 1.7, respectively. Seepage-induced consolidation and/or reorganisation of bentonite microstructure are considered possible reasons for these decreases.

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