Mixing and sulphate‐reducing activity of bacteria in swelling, compacted bentonite clay under high‐level radioactive waste repository conditions

Aim: The fate of micro‐organisms in the bentonite clay surrounding high‐level radioactive waste (HLW)‐containing copper canisters in a future Swedish underground (500 m) repository were investigated. Methods and Results: Laboratory experiments were designed in which the mixing of various bacterial species with swelling bentonite was studied. A clear trend of fewer cultivable bacteria at depth was seen in the clay. This trend was consistent as the incubation time was increased from 8 h to 28 weeks. Sulphate‐reducing bacteria were found to be active, reducing sulphate at the lowest density studied, 1·5 g cm−3, but sulphate reduction activity ceased at higher densities. Conclusions: The number of viable micro‐organisms in an HLW repository bentonite clay buffer will decrease rapidly during swelling and very few viable cells will be present at full compaction. Significance and Impact of the Study: Sulphate‐reducing bacteria will most probably not be able to induce corrosion of HLW‐containing copper canisters.