Bentonite-sand Buffer Research Articles

Impact of pore fluid salinity on the mechanical behavior of unsaturated bentonite–sand mixture

High salinity of local groundwater and the thermal gradient present within the deep geological repository environment make it necessary to investigate the effect of the pore fluid salinity on the hydro-mechanical behavior engineered barriers at different degrees of saturation. This study examines the impact of CaCl2 salt solution at two concentrations, i.e., 100 and 250 g/L on the compressibility and shear strength behavior of the bentonite–sand buffer (BSB) within a range of saturations. Both as-compacted and desiccated BSB specimens were studied. Desiccated specimens were prepared in a constant-suction environment using the vapor equilibrium technique, and high-pressure cells were used to carry out the triaxial shear experiments. Experimental results were compared with the available data from the literature for BSB materials prepared with distilled water in order to get a better understanding of the impact of the salinity. The comparison shows that the existence of salt in the specimens has notable effect on the total suction levels, compressibility and shear strength of the material.

Temperature effects on unsaturated hydraulic property of bentonite-sand buffer backfilling mixtures

The influence of temperature on the engineered properties of bentonite-sand mixtures (B/S) is of major concern in the design of engineered barriers in underground repositories for high-level radioactive waste disposal. We experimentally studied the influence of temperature on soil unsaturated hydraulic properties related to water holding capacity and permeability of GMZ B/S in China. The vapor equilibrium method and water infiltration apparatus were used to measure the soil water characteristic curve (SWCC) and unsaturated hydraulic conductivity (k u). The results show that the SWCC under different temperatures from 20 °C to 60 °C tends to be the same. Temperature influence on unsaturated permeability is more relevant at low suctions, no clear effect is detected below a degree of saturation of 74%, and experimental data show that temperature dependence on unsaturated permeability is small.

Salt content impact on the unsaturated property of bentonite–sand buffer backfilling materials

Bentonite mixed with sand is often considered as possible engineered barrier in deep high-level radioactive waste disposal in China. In the present work, the vapor transfer technique and water infiltration apparatus were used to measure the soil water characteristic curve (SWCC) and unsaturated hydraulic conductivity (ku) of bentonite–sand mixtures (B/S) effected by salt content. Results show, the water-holding capacity and ku increase slightly with the concentration of Na+ in pore liquid increasing from 0g/L to 12g/L, similar with the solution concentration of Beishan groundwater in China. Salt content in the laboratory produced only one order of magnitude increase in ku, which is the “safe” value. The different pore liquid concentrations used in this study led to small differences in thickness of diffuse double layer of bentonite in mixtures, this might explain why some differences have been found in final values of ku.

Thermohydromechanical modelling of the buffer/container experiment

In order to investigate how heat affects the performance of a dense bentonite–sand buffer material, a full-scale in situ experiment, the Buffer/Container Experiment (BCE), was conducted by Atomic Energy of Canada Limited (AECL). In the BCE, an electric heater simulating the heat output of a disposal container of used fuel was installed vertically in a 1.24-m-diameter by 5-m-deep borehole surrounded by bentonite–sand buffer. The borehole was drilled in the floor of a room excavated in Canadian Shield granite on the 240 Level of AECL's Underground Research Laboratory (URL). The thermal, hydraulic and mechanical responses were monitored in the bentonite–sand buffer and in the surrounding rock. Power was provided to the heater for approximately 896 days. In order to get a better understanding of the coupled thermohydromechanical (T-H-M) process in the BCE, numerical simulations were conducted using CODE_BRIGHT to model the evolution of the temperature, thermally induced pore water pressure and mechanical response in the bentonite–sand buffer and the surrounding rock. The thermal response in the experiment was successfully modelled. The simulated water seepage from the rock to the borehole was compared with measured results for the BCE. The thermally induced moisture movement in the buffer was also reasonably well simulated. The comparison of the simulated pore water pressure in the rock with the measurement indicates that the influence of temperatures on the hydraulic parameters of rock should be further studied. The modelled vertical displacements in the rock are also compared with the measured data in the BCE and the analysis of the rock displacement suggested that the in situ thermal expansion coefficient of the rock could be much higher than the coefficient determined in the laboratory.

Experimental study on the performance of light and dense backfills

Numerical modeling is a cost-effective and important approach to examine the long-term performance of engineered materials. However, to identify the appropriate constitutive model for a particular material it is necessary to measure physical properties in a laboratory. Laboratory experiments provide the data from which the input parameters for the selected model can be interpreted. This paper studies the mechanical behaviour of two clay-based sealing materials — light backfill and dense backfill — recommended for use in disposal of nuclear waste in a proposed Canadian repository. Test specimens of light and dense backfills were saturated in a triaxial cell and then subjected to specified isotropic and shearing stress paths to measure consolidation and shear characteristics. The triaxial results are interpreted in a critical state context. The light backfill results suggest that it has similar stress–strain behaviour to a bentonite–sand buffer, which has a similar composition, but different preparation procedures and design requirements. Results from testing of dense backfill indicate that it is a much stiffer and stronger material than light backfill. Dense backfill is expected to provide mechanical support to the used-fuel container and other sealing components of the Canadian repository, and the mechanical behaviour of dense backfill satisfies these requirements.

Use of numerical simulations to assess hydraulic and mechanical measurements of bentonite–sand buffer in an in-floor borehole

As part of the process of developing concepts for isolation of used nuclear fuel in a deep geological environment, a large in-ground experiment, the Isothermal Test (ITT), was conducted at Atomic Energy of Canada Limited's Underground Research Laboratory. This installation was designed and constructed to examine how a large mass of dense unsaturated bentonite–sand buffer would take on water from the surrounding rock. The ITT was installed in a 1.24-m-diameter by 5-m-deep emplacement borehole drilled in the granite 240 m below the ground surface. The lowermost 2 m of this borehole was filled with an in situ compacted bentonite–sand buffer material. This buffer material was capped by a 1.25-m-thick plug of high-performance concrete and the installation was left undisturbed for 6.5 years. During this time, the pore pressures, total pressures, water uptake and temperature in the buffer and the surrounding rock were continuously monitored. Numerical simulations were conducted using CODE_BRIGHT to model the evolution of the pore pressure in the buffer and the surrounding rock as well as mechanical response of the buffer. The simulated water seepage from the rock to the emplacement borehole was compared with measured results. The simulated pore water pressure in the rock, the pore water suction in the buffer and the earth pressure in the buffer were compared with data measured during the test and inferred from end-of-test water saturation measurements. Graphs of the simulated displacements in the buffer are presented.

Hydro-thermal performance of unsaturated bentonite-sand buffer material

Several series of one-dimensional heat and moisture flow tests were performed to examine the moisture and temperature distributions in the buffer material compacted to a dry density of 1.67 Mg m −3 and water content of 17.7%. In all tests, water was allowed to infiltrate into a horizontal soil column from one end under a constant hydrostatic head of 276 kPa. Also the specimens were heated from the other end by the heater to a constant temperature. It is experimentally demonstrated that the moisture moves from both ends toward the mid part of the soil column due to both thermal gradient from one end and hydraulic gradient from the other end. It was observed that, in spite of no overall volume change, local volume change occurs in the system. The measured temperatures along the length of the specimen indicate that temperature distributions stabilize within a short period of time. The time required for the temperature to stabilize decreases as the heater skin temperature increases. The diffusivity parameters are calculated using the measured moisture and temperature profiles combined with the finite difference method. Powell's optimization algorithm was used to determine the material parameters. Good agreements between experimentally measured and calibrated volumetric water content shows that the diffusion parameters can be expressed in a linear function of the volumetric water content and temperature.

Numerical modelling of heat and moisture transport through bentonite-sand buffer : Radhakrisha, H S; Crawford, A M; Kjartanson, B; Lau, K C Can Geotech JV29, N6, Dec 1992, P1044–1059

Numerical modelling of heat and moisture transport through bentonite-sand buffer : Radhakrisha, H S; Crawford, A M; Kjartanson, B; Lau, K C Can Geotech JV29, N6, Dec 1992, P1044–1059

Numerical modelling of heat and moisture transport through bentonite–sand buffer

A computer code TRUCHAM developed to model the coupled heat and moisture flow through a porous medium is used to assess the thermohydraulic performance of the clay-based engineered barrier encapsulating nuclear waste containers in a deep geological disposal vault in the borehole emplacement concept. This paper contains an overview of the development of the numerical model and its application to the buffer–container experiment at the Underground Research Laboratory of Atomic Energy of Canada Limited Research. The thermohydraulic transport parameters required for the analysis were determined by specially designed laboratory experiments. The needs for further development of the model and the material properties are identified. Key words : buffer, clay barrier, coupled heat and moisture, heater experiment, numerical model, nuclear waste disposal, thermal diffusivity, unsaturated soil.

Experimental modelling of the near-field thermal regime in a nuclear-fuel waste disposal vault

Experiments have been performed to study the thermal performance of a backfilled disposal vault in which the waste containers are emplaced in boreholes and surrounded by a bentonitesand buffer. The near-field thermal regime in a buffer-backfill disposal vault in which the nuclear fuel waste is emplaced in canisters, is studied by one-eight scale experimental models. The thermal performance of the buffer and backfill are examined in a thermal drying environment prior to the ground water recharge of the disposal vault. The formation of shrinkage cracks in the buffer due to drying and the response of these shrinkage cracks to the rewetting condition and their influence on the near-field thermal regime are assessed.

Heat-induced moisture movement within a clay-based sealing material

The bentonite-sand buffer material, which isolates the nuclear-fuel waste containers from the host rock mass, is a vital component of the multiple engineered barrier scheme advocated by the Canadian Nuclear Fuel Waste Management Program. This paper presents results of laboratory investigations which examine the influence of the container surface temperatures on the moisture migration within a compacted buffer in a borehole environment.

Modelling the in Situ Performance of Bentonite-Sand Buffer

ABSTRACTIn the Canadian nuclear fuel waste management concept, a number of engineered barriers, such as the bentonite-sand buffer which surrounds the waste container in the emplacement boreholes, are used to inhibit the transport of radionuclides. The buffer material is also required to effectively conduct heat from the fuel-waste containers to the surrounding rock. To a large extent, in situ buffer performance will depend on the degree of moisture within the buffer. The moisture content will in turn depend on temperature, temperature gradients, and buffer initial and moisture flux boundary conditions. Modelling of coupled heat and moisture transport in the buffer before resaturation is necessary to assess in situ buffer performance. This paper describes the results of a parametric study using the Philip and de Vries coupled heat and moisture transport model to assess the effects of variations in the moisture diffusivity parameters and the boundary conditions on buffer performance.The results show that the thermal performance of the buffer is affected by heat-induced moisture movement. In particular, the thermal vapour diffusivity, DTvap, has the most significant effect on thermal drying in a closed system. Work is currently underway to improve our capability to model coupled heat and moisture transport in buffer. Laboratory experiments are in progress to more accurately define the moisture diffusivity parameters and the model is being modified to include the effects of boundary moisture fluxes and pressure potentials so that the resaturation process may be modelled. A full scale buffer/container experiment is currently being planned for conduct in AECL's Underground Research Laboratory to assess further the effects of scale and in situ boundary conditions on buffer performance and to qualify the model.

Near-Field Thermal Analysis of the Nuclear Waste Disposal Vault

AbstractThis paper describes an experimental and theoretical analysis of the near-field thermal performance of a backfilled disposal vault in which the waste containers are emplaced in boreholes and surrounded by a bentonite-sand buffer. Effects of heat and moisture diffusion on the thermal performance of clay-based buffer and backfills were studied by means of laboratory scale experiments. The bentonite-sand buffer exhibited cracking due to desiccation. The impact of such cracking on the perforamnce of buffer as an engineered barrier is being investigated.An integrated finite difference computer code ‘TRUCHAM’ was written to analyze the transient heat and moisture flow regime in a disposal vault and was validated by a scale model heater experiment. It is hoped that with proper characterization of the material properties ‘TRUCHAM’ can be employed as a useful tool in assessing the thermal response of the prototype disposal vault. Further work is needed to integrate the effects of heat and moisture on the shrinkage and expansion of clay-based buffer and backfills.