| dc.description.abstract | Many countries consider the multiple barriers system as a feasible way for the final nuclear disposal. Bentonite has been preferred as a primary buffering material for the disposal of low-level nuclear waste. In order to realize the engineering properties of mixed buffer material, this study use laboratory tests to simulate the situation of the in-situ repository. The low-level nuclear waste disposal facility components are affected by hydrological (H) and mechanical (M) coupling processes simultaneously. Bentonite-sand mixtures as buffer material affected by H-M coupling effects, needs to keep the vault physically stable to avoid damage from displacements initiated by the weight of the vault and stresses exerted by other components of the multiple barriers system.Then, the finite element program ABAQUS was then employed to carry out the numerical simulation of the creep behavior.
MX-80 bentonite and K-V1 bentonite are used as raw clay materials in this study. These clays are mixed with Miaoli area silica sand to produce the buffer material. The buffer material was prepared by mixing 30 % of silica sand from 70 % of MX-80 bentonite and K-V1 bentonite. Modified proctor compaction test were conducted to determine the maximum dry unit weight and optimum moisture content of buffer material. Specimens of buffer material prepared according to the modified proctor compaction test were evaluated for swelling potential, hydraulic conductivity, and mechanical strength. Finally, using the constant stress direct shear test carried out for the saturated buffer material to obtains the creep parameters.
The first part of results from laboratory tests shows that (1) Mixtures with 30 % of silica sand and 70 % of MX-80 / K-V1 bentonites, satisfy safety requirements of SKB. (2) Results of the constant stress direct shear test shows that, the earlier the initial occurrence of creep happened as applied higher stress level. (3) One-dimensional compression tests shows that, the deformation behaiviors of MX-80 bentonite-sand mixtures at clay effective density of 1600 kg/m3 will be slightly expaned in anaphase as applied to normal stress 1.6 MPa. The second part is the numerical simulation results revealing the creep parameters obtained from constant stress direct shear tests. Parameters of A and α were the extrapolation of the relationship between (γ_0 ) ̇ and Dr, and the slope of the relationship between (γ_0 ) ̇ and Dr, respectively, and Dr is stress-dependent constant. The m values is a significantly factor in the evaluation of creep behaviors, and it can explian the phenomenon that the strain rates decreased with time and represent more prominent creep behavior as its vaules are much bigger. This study used the Drucker-Prager creep model in the numerical simulation. The instantaneous displacements caused by vault weight, and the results of simulation K-V1 bentonite-sand mixtures have higher displacements compared to MX-80. In the model tests, the mixed buffer material was predicted to be fully saturated in 20 years. The displacements due to creep behavior were found at the interface of buffer and backfill affected by the swelling pressure after saturation. | en_US |