Bentonite as a Backfill Material in a High-Level Waste Repository

Abstract

The concept for the disposal of high-level radioactive waste (HLW) in Switzerland is illustrated in Figure 1: the waste, solidified in a borosilicate glass matrix, is encapsulated in a 25-cm-thick steel canister which has a minimum life expectancy of 1,000 years. After this time, the heat produced by 90Sr and 137Cs will have dissipated and the repository will have adjusted to the host rock ambient temperature of around 60°C.The steel canisters will be placed in horizontal tunnels, 3.7 m in diameter, at a depth of around 1,000 m in the granite of northern Switzerland, and the tunnels then backfilled. The backfill material, an important link in the chain of engineered and natural safety barriers, satisfies both physical/mechanical and chemical requirements. The most important of these are:• Low hydraulic conductivity compared to the host rock;• Good swelling properties to seal construction-caused joints and rock fractures;• Plasticity to absorb rock movements and to distribute pressure homogeneously;• Good retention of radionuclides; and• Stability over a period of at least 106 years.Economic aspects and availability should also be considered: each canister requires 88 tons of backfill material, or of the order of 250,000 metric tons for the planned repository.