Large-scale in situ experiments are being conducted at Atomic Energy of Canada Limiteds Underground Research Laboratory. Excavations at depths of 240-m and 420-m below the ground surface provide access to the largely unfractured granite within the Lac du Bonnet batholith in the Canadian Precambrian Shield. Sealing experiments are being conducted to provide supporting information for acceptance of the Canadian concept for nuclear fuel waste disposal and for the disposal facility preliminary design. Experiments which investigate the in situ performance of clay- and cement-based materials used for sealing, and the influence of near-field damage in the rock on sealing capabilities, include the Buffer/Container Experiment, the Mine-by Experiment, the Connected Permeability Experiment, the Isothermal Buffer/Rock/Concrete Plug Interaction Test and tunnel sealing experiments. These experiments are briefly described in this paper.
The Canadian concept for nuclear fuel waste disposal calls for the construction of a vault within a plutonic rock mass of the Precambrian Shield at a depth of between 500 and 1000 m. Atomic Energy of Canada Limited (AECL) has developed two conceptual designs. The first involves isolation of waste containers in vertical 5-m-deep by 1.24-m-diameter emplacement boreholes, and the second requires placement of the waste containers within horizontal rooms. In both designs, a highly compacted sand/clay barrier material (or buffer) surrounds the waste container and completely fills the excavation. Issues relating to the performance of each design include the performance of the buffer and backfill materials, the potential for a near-field zone of excavation induced damage to act as a potential pathway for fluid flow, and the ability of the far-field rock to act as a barrier to radionuclide transport. A performance factor of a nuclear fuel waste repository is the ability of the engineered and natural barriers to inhibit the transport of radionuclides from the waste form to the earth's surface. The primary transport mechanisms include diffusion and advective transport within the water of the rock or backfill. The buffer and backfill between the container and the rock provide a low permeability material encapsulating the waste container. The buffer material will swell as it takes on water, and where it is in close contact with the rock, it will exert a swelling pressure on the rock walls. Small increments in this confining pressure are sufficient to enhance the long-term stability of the near- field rock and will decrease the potential for any extension of the excavation damage zone with time (Read and Martin 1996). The ability of the rock mass to act as an effective barrier will depend largely upon the degree of fracturing in the rock, the distance between tunnels and major conductive pathways, and the extent of excavation induced fracturing adjacent to the tunnels. An important element of the repository design process is the development of sealing materials and strategies, and the testing of their effectiveness in well-controlled in situ experiments. For the past 15 years, AECL has been investigating technologies for the construction of engineered barriers within a nuclear fuel waste repository.
