<p>The bentonite buffer, the engineered barrier of geological repositories for disposal of high-level waste, will enter into fractures as a result of the action of groundwater, affecting the long-term safety function of high-level waste disposal. In this study, expansion-erosion tests on GMZ bentonite were carried out under simulated fracture conditions using a specially constructed device. When the bentonite comes into contact with water, it immediately begins to swell and extrude into the fracture; this extrusion ceases after a certain period of time. The extrusion distance of bentonite in the fracture and the amount of bentonite erosion are proportional to the fracture aperture. The fracture dip angle has a pronounced effect on erosion. When the fracture dip angle is 0°, the main reason for bentonite erosion is migration of colloidal particles with the flowing water. Non-clay impurity mineral layers are formed at the edge of the sample during erosion: these layers can prevent erosion from continuing. When fracture dip angle is high, the swelling bentonite will be structurally unstable as a result of erosion, and continuous deposition in the direction of gravity will occur. Under these conditions, the bentonite is unable to form a closed impurity minerals layer. This study provides a scientific basis for the design, construction and safe operation of repositories in China.</p>