Canister displacement in KBS-3V. A theoretical study

The vertical displacement of the canister in the KBS-3V concept has been studied in a number of consolidation and creep calculations using the FE-program ABAQUS. The creep model used for the calculations is based on Singh-Mitchell's creep theory, which has been adapted to and verified for the buffer material MX-80 in earlier tests. A porous elastic model with Drucker-Prager plasticity has been used for the consolidation calculations. For simplicity the buffer has been assumed to be water saturated from start. In one set of calculations only the consolidation and creep in the buffer without considering the interaction with the backfill was studied. In the other set of calculations the interaction with the backfill was included for a backfill consisting of an in situ compacted mixture of 30% bentonite and 70% crushed rock. The motivation to also study the behaviour of the buffer alone was that the final choice of backfill material and backfilling technique is not made yet so that set of calculations simulates a backfill that has identical properties with the buffer. The two cases represent two extreme cases, one with a backfill that has a low stiffness and the lowest allowable swelling pressure and one that has the highest possible swelling pressure and stiffness. The base cases in the calculations correspond to the final average density at saturation of 2,000 kg/m3 with the expected swelling pressure of 7 MPa in a buffer. In order to study the sensitivity of the system to loss in bentonite mass and swelling pressure seven additional calculations were done with reduced swelling pressure down to 80 kPa corresponding to a density at water saturation of about 1,500 kg/m3. The calculations included two stages, where the first stage models the swelling and consolidation that takes place in order for the buffer to reach force equilibrium. This stage takes place during the saturation phase and the subsequent consolidation/swelling phase. The second stage models the deviatoric creep in the buffer during 100,000 years. The volumetric creep is not modelled, which thus may cause a slight underestimation of the canister displacement. The motive for excluding volumetric creep is that a canister settlement caused by volumetric creep will not change the total mass of bentonite under the canister but will only increase the density and is thus not judged to be a problem. Moreover, the volumetric creep is of the same order of magnitude as the deviatoric, which means that the canister displacement caused by this creep will be as insignificant as the deviatoric creep. The calculations show that the canister settlement is very small even at low swelling pressure and density. The base case corresponding to the expected final swelling pressure of the buffer 7,000 kPa yields a total settlement of the canister of only 0.35 mm for the fixed boundary case, while there is a heave of the canister of about 4.5 mm at the other case with 30/70 backfill due to the upwards swelling of the buffer. At reduced swelling pressure the settlement increases but is not more than about 23 mm at the very low swelling pressure 80 kPa for both cases. Another phenomenon that may affect the buffer is a reduction in strength or friction angle with time, which affects the degree of mobilized shear strength, which is a critical creep parameter. A reduced friction angle will reduce the strength and thus affect the degree of mobilized strength in a similar way as a reduced swelling pressure. So the reduction in shear strength can for the creep also correspond to a reduction in friction angle with kept initial density and swelling pressure. At the swelling pressure 80 kPa the deviatoric stress at failure is only 70 kPa, which for the swelling pressure 7,000 kPa corresponds to a friction angle of only 0.29 deg. This shows that not even a strong reduction in friction angle is a threat to the canister integrity. The conclusion is thus that the expected displacement of the canister from consolidation and creep during 100,000 years is very small and for the case of 30/70 backfill actually will result in a heave of the canister. The sensitivity analyses with reduced swelling pressure corresponding to reduced density or reduced friction angle also show that the canister displacement is very insensitive to such phenomena since the total settlement will be less than a few cm even at a buffer density of 1,500 kg/m3 or at a friction angle of 0.3 deg