Thermal Behaviour of Backfill Material for a Nuclear Fuel Waste Disposal Vault
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THERMAL BEHAVIOUR OF BACKFILL MATERIAL FOR A NUCLEAR FUEL WASTE DISPOSAL VAULT
R.N.YONG and A.M.O.MOHAMED Geotechnical Research Centre, McGill University, 817 Sherbrooke St., W., Montreal, Quebec, Canada H3A 2K6 S.C.H.CHEUNG
Atomic Energy Of Canada Limited, Whiteshell Research Establishment Pinawa, Manitoba, Canada, ROE iLO
ABSTRACT The concept of disposing of used nuclear fuel in engineered rock formations is being studied in the Canadian Nuclear Fuel Waste Management Program. After the used fuel is emplaced in the vault, the vault would be backfilled. The backfill has to satisfy a number of engineering requirements. A reference backfill with satisfactory hydraulic, thermal and mechanical properties has already been selected. As the used fuel in the waste containers decays, heat will be generated and this heat will raise the temperature of the backfill material. The performance of the reference backfill material was evaluated over the temperature range 20-1000 C. This paper addresses the results of experiments on thermal response and pressure development in the backfill for the period shortly after vault closure. INTRODUCTION
The Canadian Nuclear Fuel Waste Management Program is assessing the concept of disposing of waste in a vault excavated at a depth of 500 to 1000 m below the ground surface in plutonic rock of the Canadian Shield [1l. Besides the natural low permeability rock, a number of engineered barriers are used to limit radionuclide release from the solidified waste. These are the corrosion-resistant container, the buffer and the backfill. The containers will be placed in boreholes drilled into the floors of emplacement rooms and separated from the host rock by bentonite-sand buffer material. The remainder of the vault will be filled with an earthen,claybased backfill. Various backfill materials have been studied (21. The reference material is a mixture of crushed granite aggregates and Lake Agassiz clay. It has been found that a well-graded mixture with a maximum aggregate size of 19 mm and 25% clay content compacted to a dry density of 2.2 Mg/M 3 has satisfac- tory engineering properties, such as low hydraulic conductivity, some swelling capacity, sufficient load bearing capacity and acceptable thermal conductivity (2,31. In the vault, the backfill will be heated by the waste containers up to a maximum temperature of 100°C. This paper describes laboratory studies of thermal response and pressure development in backfill subjected to various temperature gradients in a closed system, i.e. a no-moisture-flux boundary at the external boundary of the backfill. The closed system is used to simulate the hydraulic condition of the rock for the period soon after vault closure. During this period, hydraulic flow from the surrounding rock to the backfill is likely to be low due to slow re-establishment of the groundwater table to its natural condition. The backfill could undergo thermal drying, with the potential for shrinkage and cracking that may impair its thermal perfomance. The development of backfill pre
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