Abstract
The Materials Corrosion Test (MaCoTe) project at the Grimsel Test Site (Switzerland) is being used to provide experimental data for modeling the effect of the bentonite buffer on microbially-influenced corrosion (MIC) in deep geological repositories for spent nuclear fuel. Using culture-based and molecular culture-independent methods, we evaluated bacterial characteristics in bentonites compacted to bulk densities of 1.25 g/cm3 and 1.5 g/cm3 after one-year in-situ incubation in MaCoTe borehole modules.
Abundance of aerobic and anaerobic microorganisms in the uncompacted MX-80 bentonite used in MaCoTe did not exceed 102 Colony-Forming Units per gram dry weight. Numbers of sulfide-producing microorganisms inside the highly-compacted bentonite (HCB) was <102 Most Probable Numbers per gram dry weight. Diversity was limited mostly to Gram-positive bacteria of the phyla Firmicutes and Actinobacteria that are able to survive the stresses found in carbon-scarce and arid clays. Aerobic spore-formers of the families Bacillaceae and Streptomycetaceae predominated in bacterial 16S rRNA gene sequences from friable MX-80, though Gram-negative Proteobacterial families, i.e. Xanthomonadaceae and Pseudomonadaceae, were also detected, indicating their ability to survive in the uncompacted clay. Compaction of clays to a density of 1.5 g/cm3 worsened conditions and almost completely-inhibited bacterial activity and spreading inside the HCB, consequently minimizing risks of biodeterioration and MIC inside HCB, even though environmental conditions outside of HCB were favorable for bacterial growth.
