(GRS-A-3844) Mechanisms of transformation of bentonite barriers – testing a new experimental concept
Bentonites are foreseen as a construction material for at least one of the engineered barriers buffer, backfill, sealing elements of boreholes, drifts and shafts in the currently developed or proposed geological repository concepts for high-level radioactive waste (HLW) and spent nuclear fuel (SF) worldwide. These barriers should either (i) retard the movement of external solutions into – and of internal pore solutions within – the repository area in general and most importantly in the near field of the emplaced HLW/SF, or (ii) exert support pressure on the surrounding rock masses or (iii) retard the migration of radionuclides from the repository area into the biosphere.
The clay mineral montmorillonite constituting the most of bentonites is able to absorb water in between its aluminosilicate layers, which can thereby be pushed up to four times farther apart as compared to the dehydrated state despite the external pressures of several megapascals at the repository relevant depths /MAD 89/. This swelling ability is responsible for the first two functions of the engineered barriers listed above, as the swelling montmorillonite particles, the most of which are characterized by the lateral dimensions below 2 μm, effectively (i) fill up the meso- and macropores in the bentonite material – reducing thus its permeability for solution flow – and (ii) exert a swelling pressure on the surrounding solid material upon the up to twofold increase of their thickness /LIK 06/. The knowledge of the swelling pressure and the permeability of bentonites is thus of high importance for their application in a repository system.