The Role of Highly Compacted Bentonite in Localized Suppression of Microbial Activity in a Nuclear Fuel Waste Repository

Abstract

Highly compacted bentonite (HCB), through its inherent physical properties, is able to suppress microbial activity if dry density remains sufficiently high (i.e., preferably ≥ 1600 kg/m³). At certain locations in a repository the HCB dry density may not be sufficiently high, which could result in increased microbial activity in these locations. An additional concern is the potential migration of microbes through desiccation-induced fractures in the HCB that have not healed rapidly or sufficiently enough upon re-saturation. If microbes are able to rapidly travel through such fractures, there is a risk that they could populate the area immediately around the containers and (temporarily at least) increase the potential for microbiologically influenced corrosion of the containers. Therefore, a series of experiments were carried out to address this possibility through the examination of the movement of fluorescent polystyrene microspheres of microbial sizes (0.2 to 0.6 μm) along interfaces and through fractures in HCB plugs. Results confirmed that these microspheres were able to travel along the interfaces between the HCB clay plugs and pressure cells, but not through the intact clay matrix. In those plugs containing an induced fracture at the start of the experiment, some microspheres were found inside the healed fractures in about 50% of the experiments, suggesting that microbial intrusion into healed or healing fractures is possible, but not massive. Further work would be required with actual microorganisms to confirm these results.