Keywords: activation energy, clay, canister settlement, creep, microstructure, shearing, smectite.

Highly radioactive waste from nuclear reactors will be contained in heavy metal canisters embedded in dense clay and placed in large boreholes in the rock at depth. The clay provides tightness but may yield under the shear stresses induced by the canister weight, which can affect the tightness of the canister/clay contact and the homogeneity of the clay embedment. The important physical process is the strain rate of the settlement that the canisters will undergo is by creep under largely constant volume conditions. Prediction of the settlement can be made by applying a creep model of stochastic nature according to which strain is caused on the molecular level and accumulated by the successive activation of energy barriers of different heights. This model implies that ordinary smectite-poor clays with direct particle contacts will undergo attenuating creep according to a log-time strain rate as long as the shear stresses do not cause irrepairable microstructural distortions. For smectite-rich clays the model would give less retardation and a viscous behaviour similar to that of fluids.

For investigating this the creep strain rate was recorded using a ring shear box and it was found that a smectite-rich and an equally dense micaceous clay behaved differently and in agreement with the theoretical prdictions. Thus, for the lastmentioned clay the strain evolved in agreement with the creep theory, i.e. at low shear stresses the strain rate dropped quickly and the strain tended to reach a finite value, while at intermediate shear stresses the strain rate followed the log time rate. At higher shear stresses the strain rate dropped more slowly. For the smectite clay, on the other hand, the strain rate evolved quite differently. Hence, it was largely constant indicating that the clay behaves like a viscous fluid, which is in agreement with the assumption that the energy barrier spectrum has no real peak.

Applying the results to the canister case one finds that the generated shear stresses will lead to retarded and ultimately stagnant canister settlement in buffer clay of the micaceous clay, while in smectite-rich buffer clay, creep-controlled settlement will be less strongly retarded and may continue for ever. The estimated settlement may be several millimetres already after some hundred years and considerably more than 10 mm after E6 years. The creep rate may be even higher since the thermodynamic nature of the processes will speed it up by the relatively high temperature (80-90°C) in the first hundreds of years. However, retardation caused by chemically induced strengthening will tend to counteract this tendency.

This paper shows that even dense clays undergo time-dependent shear strain that affects their potential to maintain the canisters in the desired position.

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