This study takes Inner Mongolia GMZ bentonite as the research object. Samples with different water contents were obtained via the previous water retention test. Further, a gas permeability test of GMZ bentonite was carried out. The effects of confining pressure, water content and gas injection pressure on the gas permeability were studied, and the pore structure evolution was observed by microscopy. The results show that the water absorption (or loss) of bentonite can affect its water content, saturation, dry density and porosity, further affecting its gas permeability characteristics. The evolution of the gas permeability is influenced by the coupling of these factors and the confining pressure, which is between 10-19 and 10-15 m2. During the first cycle of low confining pressure, the results show that the test results of the gas permeability are very close, despite the large difference in water contents. Further analysis found that in addition to the water content, the change in pore structure also affects the gas permeability. The effects of the two basically offset each other, and as a result, the change in gas permeability is not very clear. After undergoing a cyclic loading and unloading, the gas permeability gradually decreases as the water content increases. In this process, the change in water content is the dominant factor. In addition, the microscopic image (after permeability tests) shows that the bentonite sample possesses a dense pore structure at high water content, that graininess is strong at low water content, and that a large number of pores exist. Regarding the effect of the gas pressure, it is found that the effect of gas pressure on the gas permeability is more evident at low water content.

1. Introduction

For the final disposal of high-level radioactive waste (HLW), a feasible solution generally accepted by the international community is to store the cured and packaged HLW in a stable formation of 500-1000 m deep underground (i.e., high-level radioactive waste repository (HLWR)). With the design concept of “multi-barrier system” (including surrounding rock geological barrier, artificial barrier based on bentonite and waste storage containers), HLW is effectively isolated from human living environment, as shown in (Chen et al. 2014; Liu et al. 2018; Wang et al., 2013). During the construction of the repository, the bentonite as a buffer/backfill material is pre-compressed into a block of a regular shape. Over time, water will gradually erode the buffer material of the repository, and sealing will be obtained. In this process, bentonite blocks will extrude each other due to expansion. Meanwhile, due to complex physical and chemical reactions, gases will gradually be produced and the pressure will gradually increase. The accumulation and escape of gases further affect the hydration and expansion characteristics of bentonite, thus the stress state around bentonite is constantly changing. Therefore, it is necessary to clarify the evolution law of gas permeability of the bentonite barrier and the evolution characteristics of its pore structure in this process.

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