| Popis: |
Bentonite is an important buffer material between the copper canister and bedrock in geological disposal of spent nuclear fuel in Finland. The long-term experiment with compacted Na-bentonite with low saline ground-water simulant and copper canister was initiated in 1997. The aim of the experiment was to evaluate changes in chemical, mineralogical and microstructural parameters in bentonite in oxic and anoxic conditions. The experiment was planned to be finished after ten months, but two samples were stored and dismantled in 2012. Although the experimental setup was not initially designed for microbiological studies, they were performed the end of the experiment. Changes in the bentonite mineralogy included the observation of secondary copper minerals formation in the middle part of the bentonite matrix in copper cylinder in oxic conditions. Copper content in pyrite increased when moving from the middle of the bentonite towards the copper cylinder surface in both oxic and anoxic experiments. Microstructural studies on bentonite did not show any significant differences in bentonite structure between samples taken from anoxic and oxic experiments. Chemical changes were typical dissolution of gypsum and calcite, which releases sodium, sulphate and carbonates into external water, while calcium exchanges sodium in the interlamellar spaces of bentonite. The presence of living microbes on bentonite and on copper surface could not be demonstrated in this study but microscopical studies revealed living microbial cells in the external water surrounding copper cylinders. According to the sequencing, sulphate (SRB) and iron reducing bacteria (IRB) were detected in bentonite, water and copper surface. SRBs can produce corrosive sulphide and IRBs can be related to processes that could be linked to the loss of swelling properties in bentonite. Fungal conidia and hyphae were detected by SEM in water and several groups of Ascomycetes and Basidiomycetes were identified from bentonite samples. |
