Modelling the density homogenisation of a block and granular bentonite buffer upon non-isothermal saturation

Abstract

This paper presents a numerical analysis of the mechanical performance of a bentonite clay buffer for the containment of nuclear waste in the context of deep geological disposal. The design of the buffer is based on the Swiss concept where the waste canisters are emplaced on pedestals of compacted bentonite blocks and the remaining space between the tunnel and the canister is backfilled with grains of highly compacted bentonite. A complete analysis of the long-term performance of the repository requires a good understanding of the mechanical evolution of the bentonite upon heating from the radioactive waste and hydration from the host rock. Despite its importance, the implications of the initial heterogeneous bentonite layout, characterised by blocks and grains, on the final dry density at the repository scale in the steady state have not been previously studied. The present study aims to shed light into these processes by means of finite element modelling using an advanced constitutive model for the bentonite behaviour that considers several thermo-hydro-mechanical couplings. The constitutive model is shown to be able to reproduce several laboratory tests involving saturation of block and pellets at different dry densities. The model predictions, extended up to 100,000 years, indicate that the bentonite blocks and grains tend to homogenise in terms of dry density as the buffer reaches full saturation. Due to the different swelling potential of the block pedestal and the granular backfill, the canister is subjected to movements, although these remain relatively small. The impact of initial segregation of the granular bentonite is also studied and it is seen to not to affect substantially the mechanical evolution of the buffer, although it might reduce canister displacements.