| dc.description.abstract | For the construction of a repository for spent nuclear fuel, the deep geological disposal method is currently accepted internationally. The multi-barrier design is used by combining engineered barriers and natural barriers for isolation of the wastes from the human living environment. In the complex environment of hundreds or even 100,000 years after the closure of the disposal facility, the creep characteristics of the buffer material will significantly affect the long-term stability of the engineered barrier. During the process of saturation, due to construction, expansion and erosion, reduced density of buffer may occur in local area. In this study, MX80 bentonite from Wyoming, USA was used as the test material, and buffer material was made by static loading. Through measurement of hydro-mechanical parameters including swelling pressure and hydraulic conductivity, and shear experiments including direct shear test and constant-stress shear test, the creep parameters were finally obtained and can be used in the creep model of the buffer material at varying densities. In addition, numerical simulation was used to explore the long-term creep behavior of buffer in a deposition hole by the finite element method.
The experimental results show that the performance of the buffer material is degraded at low dry densities. With the decrease of dry density, the maximum swelling pressure decreases exponentially while the hydraulic conductivity increases exponentially, and the shear strength of both unsaturated and saturated specimens decreases significantly. Thus, it is necessary to limit the magnitude of the reduction in the density in the long-term consideration. Meanwhile, in the pre-swell mode the performance of buffer is found to be different from that in the constant volume mode. In the former, due to the swelling process experienced by the bentonite, the time-history curve presents a three-stage characteristic, which is different from the bimodal curve exhibited in the later mode. The measurement of hydraulic conductivity shows that the increasing trend is also more obvious for the pre-swelled specimens, indicating reduced water resistance. Moreover, the shear strength in the pre-swell mode is greatly reduced. The creep in the low-density area will take place earlier and continue to develop progressively, which should draw special attention in the long-term safety assessment for the disposal.
The numerical simulation results show that the creep behavior of the deposition hole can be predicted using the creep parameters obtained and the creep model established. The displacement is related to the saturation history with similar trends but different details for different parts. The saturation process will develop rapidly in the first 50 years, then slow down and reach full saturation at around 100 years. At the interface of the buffer material and the backfill material, the vertical displacement increases continuously and reaches its maximum value at the 100,000th year. The saturation time history of the low-density buffer material is very similar to that of the constant density buffer of 1600 kg/m³, with slightly smaller maximum vertical displacement at the interface of buffer and backfill. | en_US |