Highly radioactive waste from nuclear reactors will be contained in heavy metal canisters embedded in dense clay and placed in large boreholes in rock at large depth. The clay provides tightness but may yield under the shear stresses induced by the canister weight. This paper shows that even dense clays undergo time-dependent shear strain that affects their potential to maintain the canisters in the desired position. Smectite-rich clay with one dominant interparticle bond type tends to behave like viscous fluids with nearly constant rate of settlement, while micaceous clay with a spectrum of barriers to microstructural strain shows attenuating and ultimately stagnant strain rate. Keywords: activation energy, clay, canister settlement, creep, microstructure, shearing, smectite.

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