Engineered barriers for geological disposal of radioactive waste : $$b microbial interactions and their limits with thermo-hydro-mechanical-chemical processes at the waste canister/bentonite interface

Abstract

Nuclear waste is a global problem, with many countries in the process of creating deep geological repositories to store this waste. In proposed repository designs MX80 bentonite clay has been selected as the buffer and backfill. Extensive studies have been carried out on the geomechanical properties of the clay; however, the role of microbes has not been fully investigated. Specifically, in the UK, iron-reducing bacteria are a concern as carbon steel waste canisters will contribute iron oxides and rust products to the immediate environment. Iron-reducing bacteria can reduce Fe(III) to Fe(II) and some species are adapted to high temperatures and low water availability, in keeping with conditions within the repository. Iron-interacting bacteria were found to be indigenous to MX80 bentonite and microbiallyinfluenced iron-reduction was observed up to groundwater salinities of 0.45 M NaCl. The limits of this community at different temperatures and pressure were investigated through a series of batch experiments and subsequent enrichments, where necessary. Fe(II), Fe(Total) and pH were measured throughout the respective experiments and substrates were collected and analysed by XRD, SEM and EDX. The indigenous iron-reducing community and various iron substrates were used to investigate indirect interactions through a series of agar plate experiments. The potential bacterial production of H2 and silica-solubilising ability was also investigated. In some experiments steel was included to represent the waste canister, results from these experiments suggest that bacteria play a role in passive protection of steel against corrosion. Significant differences in plasticity and mineralogy of MX80 were seen in all biotic experiments. Additionally, silica release coupled to metal / microbe interactions was observed. Transformation of clay minerals through iron reduction or release of silica to groundwater could significantly impact the geomechanical properties of MX80, and thus negatively affect the function of the barrier.