Temperature and Moisture Distributions in a Clay Buffer Material Due to Thermal Gradients
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TEMPERATURE AND MOISTURE DISTRIBUTIONS IN A CLAY BUFFER MATERIAL DUE TO THERMAL GRADIENTS A.M.O. Mohamed and R.N. Yong Geotechnical Research Centre, McGill University, 817, Sherbrooke St. West, Montreal, Quebec, Canada H3A 2K6 B. Kjartanson AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE ILO
ABSTRACT Several series of one-dimensional tests were used to investigate the nature of transient heat and moisture movements in a clay buffer under different imposed temperature gradients. The measured temperature and moisture profiles were used to calculate the diffusion parameters governing heat and moisture movement in the buffer material . The diffusion parameters are shown to depend on the moisture content, temperature and moisture equilibrium time.
INTRODUCTION The concept of disposing a nuclear fuel waste in a vault excavated at a depth of 500 to 1000 m below the ground surface in the plutonic rock of the Canadian Shield is being investigated as part of the Canadian Nuclear Fuel Waste Management Program. A number of engineered barriers such as the corrosion-resistant waste container, the buffer and the backfill will be needed in addition to the naturally low-permeability rock to limit radionuclide releases from the solidified waste. Containers enclosing the heat-emitting waste will be placed in boreholes drilled into the floor of the emplacement rooms, and will be separated from the host rock by a bentonite-sand buffer material. The remainder of the vault will be filled largely with a clay-based backfill. The performance of unsaturated buffer material under an imposed thermal gradient is being investigated. The thermal gradient, in turn, induces a coupled heat and water flow in the system. Significant movement of water can cause the buffer to shrink, which may lead to crack formation in the buffer and form a gap between the container and the buffer and/or between the buffer and the rock. This will impair the buffer's thermal performance. Some knowledge about the status of water in the buffer material is needed to enable the thermal performance of the buffer to be assessed before it becomes saturated. The buffer is likely to remain unsaturated for a long period of time while the water table returns slowly to its natural level after the vault has been sealed; this time period represents the critical period for the thermal performance of the buffer. The nature of the coupling of the heat and water flow processes will be influenced not only by initial conditions (volumetric water content and temperature), but also by the boundary conditions (temperature changes, water accessibility and external restraint). The methodologies used to predict the coupled heat and water flows in unsaturated clay-based materials generally involve the development of a transport model and an evaluation of its associated flow coefficients or diffusion parameters. A number of models have been developed with limited success [1-4]. Many of the difficulties are related to the way the driving forces are expressed and the way their
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