The Canadian program for sealing underground nuclear fuel waste vaults

Abstract

The general objective of the Canadian Nuclear Fuel Waste Management Program is the safety assessment of the disposal of nuclear reactor wastes in underground vaults mined in plutonic rock formations of the Canadian Shield. The isolation of waste forms from the surface environment is effected by a system of multiple additive barriers, each one engineered to a high degree of excellence. In successive order, these barriers are the container, the buffer, the backfill of vault openings, shaft and drift seals, and plugs in boreholes.The buffer is primarily a chemical barrier designed to retard the migration of chemical species that may corrode the containers, and also the outward migration of radionuclides in case of container failure. The buffer is an engineered mixture of high-purity expandable clays and sand or crushed rock. The backfill material in drifts and shafts is primarily a physical barrier, designed to stabilize the rock excavations and to retard the movement of radionuclides by advection.Specially designed drift and shaft seals are being considered, to enhance the function of the backfill. The seals are engineered barriers that may include reinforced concrete bulkheads, compacted clay and sand plugs, and grouting of the surrounding rock. Key words: nuclear fuel waste, underground disposal, engineered barriers, swelling clay, buffer, backfill, shaft seals.