Sealing ability of Wyoming bentonite pellets foreseen as buffer material – Laboratory results

The NAGRA concept for a high level waste repository includes bentonite pellets for backfilling around the spent nuclear fuel canisters. Pellets have therefore been produced from granulated Wyoming bentonite (Am. Colloid., brand name MX-80), which have been tested as part of NAGRA’s research program with respect to its sealing properties. Two types of pellet material were used, one containing only bentonite and the other containing a mixture of 70% bentonite and 30% sand. As reference, standard granulated MX-80 bentonite material was also used. The pellets were industrially crushed in order to give better grain size distribution with respect to field compaction. In all tests a 0.2 M NaCl test solution was used, simulating the groundwater composition of Opalinus Clay host rock. Swelling pressure and hydraulic conductivity were measured for all three materials at five different samples densities, ranging from 1500 kg/m3 to 2050 kg/m3 after full water saturation.The results unambiguously show that there is no significant difference at any density between the MX-80 pellet material and the original granulated MX-80 material with respect to swelling pressure or hydraulic conductivity. Thus, the basic sealing properties in the pellet materials are not affected by the drying or compaction treatment.