| Popis: |
The chemical erosion of bentonite buffer material, during periods of glaciation, is a point of concern in the approval of safety cases for disposal of radioactive waste in crystalline rock. Currently, quantitative mass loss assessments are based on a 100% montmorillonite buffer and lead to unacceptable mass loss predictions. In practice however, the buffer will be comprised of ≈80% montmorillonite and ≈20% accessory minerals. A better understanding therefore of the role accessory minerals play in the erosion process is required to inform more realistic mass loss assessments. As indeed is the presence of fracture aperture variability on the extrusion/erosion process, as almost exclusively, research carried out to date has been in uniform aperture fracture flow cells. Four month duration erosion experiments were undertaken, designed to mimic groundwater flow through, and buffer extrusion/erosion into, a fracture intersecting a deposition hole in the KBS-3V disposal concept upon contact with dilute water. Purified montmorillonite in a planar aperture fracture was examined, as was the reference bentonite buffer material for the KBS-3V concept, MX80 (with accessory minerals), in both planar and naturally variable aperture fractures. In line with work by other researchers, the purified montmorillonite exhibited a steady mass loss throughout the experiment, with no mechanism in place to attenuate mass loss. Data correlated for the experiments with MX80 to reveal an erosion mechanism in which the accessory minerals serve to inhibit erosion when deposited at the extrusion/groundwater interface at a critical thickness. Irrespective of whether a variable or planar aperture fracture was used. Aperture variability plays a role in that, after deposition of the minerals at a critical thickness and subsequent decrease in mass loss from the system, force chain development in the accessory minerals against the variable aperture wall facilitated a rise in swelling pressure, resulting in breach of the accessory mineral barrier. An erosive period ensues, during which the breach is healed and the barrier regains its integrity. Simplified assessments, scaling the experiments to a repository scenario by incorporating the mitigating effect of the presence of accessory minerals into current quantitative mass loss assessments, predict mass loss over the course of a repository assessment period to be within acceptable limits. |
