The Role of Engineered Barriers in the Disposal of Nuclear Fuel Waste - The Canadian Perspective

Abstract

The concept developed in Canada for disposing of nuclear fuel waste is disposal at a depth of 500-1000 m in plutonic rock of the Canadian Shield. The waste would be isolated from the biosphere by a multi-barrier system consisting of engineered components supplementing the natural containment potential of the host-rock. The key engineered barriers are the waste form itself (used Candu fuel), the container that holds the waste form, and sealing materials used to envelop the container and to backfill and seal the vault excavations.

During the past fourteen years, AECL has carried out a comprehensive research program on the design, materials selection, and performance assessment of engineered barriers. The research has recognized the generic nature of the concept development phase of the program whilst taking account of the key parameters characteristic of the plutonic rock disposal environment. The research approach has included studies of underlying processes, large-scale component testing and demonstration, conceptual engineering, model development and the study of natural analogs. On the basis of the research we have:

• developed a number of container designs that meet the primary structural requirements that will exist in a disposal vault;


• demonstrated that both copper and titanium can provide long-term containment of the waste; depending on material choice and container wall thickness, lifetimes between 10³ and 10⁶ years are considered, achievable;


• demonstrated that intact used fuel bundles are a highly durable waste form from which most of the radionuclides will be released at a slow rate controlled by the low solubility of the UO₂ fuel matrix;


• demonstrated that cement- and clay-based materials can provide effective sealing of excavated openings and a physico-chemical environment that ensures that radionuclide transport between the container and the host rock occurs primarily by diffusion rather than by groundwater flow.

The results have demonstrated that there is considerable flexibility in the selection of materials and designs for engineered barriers for a disposal system in plutonic rock that could meet potential constraints that might be imposed by conditions at an actual disposal site.