Interface-associated microbial growth in anoxic pressure vessels containing compacted gapfill bentonite

Area of Science:

• Geomicrobiology
• Nuclear Waste Management
• Materials Science

Background:

• Deep geological repositories (DGR) utilize compacted bentonite as a barrier to contain nuclear waste.
• Microbial communities, including sulfate-reducing bacteria (SRB), can potentially compromise DGR integrity through microbiologically influenced corrosion.
• Understanding microbial behavior within bentonite under repository conditions is essential for long-term safety.

Purpose of the Study:

• To investigate the impact of bentonite dry density on microbial community abundance and composition.
• To assess the influence of incubation time (1, 4, and 12 months) on microbial growth in bentonite under anoxic conditions.
• To determine the threshold dry density of bentonite that inhibits microbial proliferation relevant to DGR.

Main Methods:

• Incubation of bentonite samples at varying dry densities (1.25-1.60 g cm-3) in pressure vessels.
• Culturing and enumeration of heterotrophic bacteria and SRB.
• Analysis of microbial community composition using sequence-based methods.
• Monitoring microbial populations over extended periods under simulated repository conditions.

Main Results:

• Culturable heterotrophs increased in lower-density bentonite (1.25-1.40 g cm-3) after 1 month.
• Higher abundances of culturable SRB were observed in outer bentonite layers compared to initial material.
• SRB-associated sequences were more prevalent on pressure vessel surfaces, indicating interface-associated growth.
• Microbial growth in inner bentonite layers was significantly limited at dry densities of 1.45 g cm-3 and above.

Conclusions:

• A minimum bentonite dry density of 1.45 g cm-3 is effective in limiting microbial growth within the repository's inner layers.
• While SRB showed some increase in outer layers, their overall abundance remained low and did not escalate significantly over time.
• The findings support the use of highly compacted bentonite as a robust barrier against microbial activity in DGRs.