A Study of extrusion behaviour of buffer material into fractures

Tanai, Kenji ; Matsumoto, Kazuhiro*

As a candidate buffer material for geological disposal of high level radioactive waste, densely compacted bentonite has a number of favourable properties, such as its low permeability and high capacity for radionuclide sorption. Furthermore, as the bentonite will be resaturated and swelled gradually after repository closure, not only will any gaps within the bentonite be sealed, but the bentonite may also be excluded into fractures in the surrounding rock, diverting a part of the water flow away from the repository. This situation is potentially advantageous to repository safety. However, if loss of bentonite into fractures due to extrusion and subsequent erosion of the extruding front by flowing groundwater is too pronounced, then the decrease in density of the bentonite within the repository may lead to reduce its favourable properties. Therefore, the extrusion and erosion of bentonite buffer from the disposal pit or tunnel should be quantitatively understood to ensure long-term performance of the engineered barriers for the geological disposal system. In this study, an attempt was made to use X-ray CT technique to measure, non-destructively, the density distribution of bentonite buffer extruded into the artificial fractures.