Effect of Temperature on the Interface Shear Resistance Between Precompressed Bentonite Backfill-Blocks of a Nuclear Waste Repository in Finland

The effect of temperature on the interface shear resistance between bentonite backfill blocks of a typical deep-rock nuclear waste repository is studied. In Finland, the proposed nuclear waste repository at Olkiluoto incorporates the Swedish KBS-3V multi-barrier concept for the isolation of contained spent nuclear fuel. This multi-barrier comprised of the fuel canister, bentonite buffers around the fuel canisters, backfill materials in deposition tunnels and the host rock. One of the key aspects of the multi-barrier design in the KBS-3V is the buffer-backfill interaction. The saturated density of the buffer should be maintained within a narrow range of 1950 and 2050 kg/m3at all times, so that the integrity of the multi-barrier is not compromised. Interface shear resistance of tunnel backfills is a key parameter that controls the uplift (swelling) of the buffer into the tunnel backfill upon water saturation that causes loss of buffer density. In other words, the tunnel backfill should be able to sustain and provide adequate resistance against the swelling buffer. The present KBS-3V backfilling concept involves the use of pre-compacted bentonite clay blocks to fill the majority of the deposition tunnel volume. These individual clay blocks form interfaces which could act as potential planes of shear failure. Therefore, the shear resistance and behavior of the block interfaces under varying repository conditions such as temperature needs to be studied. In the present study, interface shear behavior between proposed bentonite blocks were tested under a range of temperatures between +20°C and +80°C. A modified temperature controlled simple direct shear box was used in the testing. Test results showed no significant variation in the interface friction with increasing temperature. The interface shear resistance data from this study could be incorporated into the detailed modelling of the buffer-backfill design of the KBS-3V.