Microbial occurrence in bentonite-based buffer, backfill and sealing materials from large-scale experiments at AECL's Underground Research Laboratory

Abstract

The Canadian deep geological repository (DGR) concept involves placement of used fuel containers (UFC's) at a depth of 500–1000 m in a stable rock mass, surrounded by compacted bentonite-based barriers that fulfill multiple specific roles including hydraulic, mechanical, thermal, and chemical protection of UFC's and control of radionuclide migration. Microbial characteristics of bentonite-based buffer materials used in a number of large-scale experiments in AECL's Underground Research Laboratory (URL) were studied in order to address the potential for microbial activity and its possible consequences in a DGR. Buffer and sealing materials consisted of 50 to 70% bentonite with the remainder silica sand. Backfill materials consisted of 25% clay and 75% crushed, graded rock or 10% bentonite and 90% sand. Microbial analysis included bulk buffer and backfills samples as well as samples taken from interface environments. Results showed that culturable populations of heterotrophic aerobes, anaerobes and sulphate-reducing bacteria (SRB) were present at all locations examined, more abundant at interface locations, and absent only in those samples affected by heat and desiccation. Compaction upon placement reduced the culturable aerobic population by several orders of magnitude but culturable anaerobic microbes were not significantly affected by compaction. The viable population was considerably larger than the culturable population, suggesting potential for future increased activity, if conditions became more favourable. The buffer materials studied here appear not able to reduce microbial activity sufficiently to the point of being insignificant with respect to possible consequences in a DGR. Therefore, the current Canadian DGR design now employs compacted 100% bentonite buffer directly in contact with UFC's, in order to attempt to reduce microbial activity in this barrier to insignificant levels.