THE CANADIAN APPROACH TO SAFE, PERMANENT DISPOSAL OF NUCLEAR-FUEL WASTE

The assessment of nuclear fuel waste disposal deep in plutonic rock of the Canadian Precambrian Shield is now well advanced. A comprehensive understanding has been developed of the chemical and physical processes controlling the containment of radionuclides in used fuel. The following conclusions have been reached: - Containers with outer shells of titanium or copper can be expected to isolate used fuel from contact with groundwater for at least 500 years, the period during which the hazard is greatest. - Uranium oxide fuel can be expected to dissolve at a rate less than 10(-8) per day, resulting in very low rates of radionuclide release. This is consistent with observations of uranium oxide deposits in the earth's crust. - Transport of radionuclides away from the containers can be significantly delayed by placing a compacted bentonite-clay based layer between the container and the rock mass. - The granite plutons of interest consist of relatively large rock volumes of low permeability separated by relatively thin fracture zones. The low permeability volumes are sufficiently large to accommodate a vault design that will ensure radionuclides do not reach the surface in unacceptable concentrations. Our field and laboratory investigations, together with assessments of conceptual disposal vault designs, give us confidence that the combination of engineered barriers and a technically suitable plutonic rock site will meet the requirements for safe disposal of nuclear fuel wastes in Canada.