A mechanism for bentonite buffer erosion in a fracture with a naturally varying aperture

In the deep geological disposal of nuclear waste in crystalline rock, erosion of the bentonite buffer may occur during periods of glaciation. Previous researchers have examined the mechanism and rates of extrusion and erosion for purified montmorillonite samples in smooth planar fractures. In this paper, we investigate the influence of using as delivered MX-80 material (i.e. including accessory minerals) and a naturally varying aperture on bentonite erosion. A bespoke fracture flow cell was constructed for this purpose and flow through conducted with deionised water. Throughout the experiment, gravimetric analysis was undertaken on the effluent and the swelling pressure of the bentonite monitored. Quantitative image analysis of the extrusion process was also undertaken. When the swelling pressure data was analysed, alongside both the oscillations in erosion rate and the area of the accessory mineral ring, a two-stage mechanism governing the erosion process became apparent. Once an accessory mineral ring had formed at the edge of the extruded material, further increases in swelling pressure resulted in a breach in the accessory mineral ring, triggering an erosive period. During which, the mineral ring was supplemented with additional minerals. The cycle repeated until the ring was sufficiently strong that it remained intact. This observed process results in erosion rates one order of magnitude less than those currently used in long-term safety case calculations.