A Study of Mechanical Effect of Simulated Fault Movement on Engineered Barrier System
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A Study of Mechanical Effect of Simulated Fault Movement on Engineered Barrier System Mayuka Nishimura1, Takashi Hirai2, Kenji Tanai1, and Mikazu Yui1 Waste Isolation Research Division, Waste Management and Fuel Cycle Research Center, 1 Japan Nuclear Cycle Development Institute, Ibaraki, 319-1194, Japan 2 Takenaka Civil Engineering & Construction Co., LTD, Tokyo, 136-8570, Japan ABSTRACT The objective of this study is to clarify the mechanical effect on the engineered barrier system (EBS) of a fault movement, presupposed to occur in a high-level radioactive waste repository. The plan of this study is; 1) to understand the mechanical behavior of the buffer material when shearing takes place during experimental tests, and 2) to make progress in numerical analysis techniques in order to estimate the effect of fault movement on the EBS at a disposal site. Accordingly, the first part of this paper reports tests, which were carried out on a 1:20 model of the EBS. The experimental results indicate that the increased total pressure is due, in part, to the increase of pore water pressure. The second part of this paper reports the results of finite element simulations of the experiments. The calculation results show that the permeability of the near-field rock, which influences the amount of water draining from the buffer material, affects the pressure increase in the buffer material. With appropriately set parameters, the calculation shows agreement with the experimental results. INTRODUCTION It is important to take into account the effects of fault movement in designing a high-level radioactive waste disposal in Japan. If fault movement occurs at a repository site, the EBS can be sheared, resulting in a pressure increase in the buffer material. This is expected to result in damage to the overpack or to the near-field rock. Therefore it is necessary to examine the behavior and robustness of an EBS when affected by a fault movement. These studies contribute to the reliability of radioactive waste disposal. To aid in achieving this objective, it is necessary to develop a technique for estimating the behavior of the EBS affected by fault movement using an analysis method, such as finite element analysis (FEA). As a first step, experimental tests are needed for understanding the behavior of the buffer material. Comparing the results of the numerical simulations to the experimental data can then be used to verify the reliability of the analysis method. Earlier studies have been carried out using experimental data and numerical analysis of the Swedish design of the EBS [1,2,3]. In those studies, the influence of shear rate on the experimental models was examined. Simulation analyses of experimental models were also carried out assuming two types of material models for the buffer material; a total stress model that did not consider the pore water pressure and an effective stress (total stress minus the pore water pressure) model. The total stress model was chosen in consideration that the total stress concept is applicable in fast
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