Clays in Natural and Engineered Barriers for Radioactive Waste Confinement: an introduction

Abstract

The concept of engineered geological disposal has been developed for the safe long-term management of long-lived radioactive waste. This involves emplacement of radioactive waste in deep geological repositories that contain and isolate the waste and, consequently, protect humans and the environment. The ‘multiple barrier concept’ is the cornerstone of all proposed schemes for the geological disposal of radioactive waste. Based on the principle that uncertainties in performance can be minimized by conservatism in design, the concept invokes a series of complementary barriers, both engineered and natural (geological), between the waste and the surface environment. Each successive barrier represents an additional impediment to the movement of radionuclides. Regarding engineered barriers, for lowand intermediate-level waste the waste may be incorporated in a relatively stable and inert matrix such as cement, bitumen, lead-alloy or polymer resin (the choice varying depending on the waste management organization); glass may be used in the case of certain high-level reprocessing wastes. Owing to the very low leach-rate of glass in groundwater, vitrification is widely accepted to be one of the best methods of immobilizing the aqueous products from the reprocessing of spent fuel. Many waste containers will provide some form of physical barrier to groundwater. However, because of the relatively small volumes of waste involved, spent fuel, vitrified waste and other highly active wastes will be totally encapsulated in corrosion-resistant metal canisters that are designed to prevent groundwater entry for extended time periods in excess of 100 000 years. Depending on the disposal concept, engineered barriers may comprise the buffer/backfill medium enclosing the waste containers, the tunnel/borehole liner, and the backfill and high-integrity seals placed in the repository access ways or emplacement boreholes. The buffer/backfill medium enclosing the waste will often also provide both a physical and a chemical barrier to radionuclide migration. The functions of the engineered/chemical barriers are: † to reduce the rate of corrosion of the waste containers and thus extend their life; † to limit the rate of hydraulic transport; † to limit the release of radionuclides from the waste-form to the far-field (geosphere) after container failure; † to limit the migration of radionuclides along the pathway provided by the access tunnels and shafts of a repository or the boreholes in the case of a deep borehole emplacement.