The Effects of Dry Density and Porewater Salinity on the Physical and Microbiological Characteristics of Compacted 100%
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0985-NN13-02
The Effects of Dry Density and Porewater Salinity on the Physical and Microbiological Characteristics of Compacted 100% Bentonite S. Stroes-Gascoyne1, C.J. Hamon1, D.A. Dixon1, C.L. Kohle1, and P. Maak2 1 Atomic Energy of Canada Limited, Whiteshell Laboratories, Pinawa, MB, R0E 1L0, Canada 2 Ontario Power Generation, 700 University Avenue, Toronto ON, M5G 1X6, Canada ABSTRACT This study examined the conditions required to suppress microbial activity in compacted bentonite, such that microbially influenced corrosion (MIC) of copper waste containers, surrounded by compacted bentonite in a future deep geologic repository, would become insignificant. Experiments were carried out to determine the effects of dry density and porewater salinity on swelling pressure, water activity (aw) and the culturable microbial community in compacted bentonite. A dry density ≥ 1.6 g/cm3 ensures that aw is < 0.96 and the swelling pressure > 2 MPa. Both conditions suppress microbial culturability below background levels (2.1 x 102 Colony-Forming Units/g) in as-purchased bentonite. Under such conditions, cells likely survive as dormant cells or inactive spores, which greatly reduces the possibility of significant MIC. Observations in natural clay-rich environments support these findings. INTRODUCTION Many countries are in the process of developing strategies for long-term management of highlevel nuclear waste within stable geological formations. In many concepts, densely compacted bentonite-based materials would be employed to surround the corrosion-resistant metal containers holding the waste. This would provide physical, chemical and hydrological protection of the waste package and restrict radionuclide migration. One process that may potentially affect the service life of copper waste containers is microbially influenced corrosion (MIC) [1]. Under certain repository environments the bentonite directly surrounding the containers may be required to suppress active microbial metabolism and thus MIC. Previous studies have suggested that in bentonite-based materials most microbes cease to actively metabolize at a water activity (aw) value of 2 MPa eliminates microbial activity in bentonite [4]. Physical factors that contribute to the observed isolation capacity of clays are the very small average pore throat diameters and the low percentage of interconnected porosity found in clays, which greatly restricts the mobility and metabolism of microbes [5]. Sub-surface bacteria require interconnected pore throats > 0.2 µm diameter for sustained activity although viable but dormant bacteria can survive in such sediments for long (possibly geological) times [6]. Another factor affecting microbial activity in clay-rich environments is the availability of water, which is reduced by interactions with solute molecules and by adsorption on solid surfaces [6]. The objective of this study is to determine the conditions required, in compacted bentonite in contact with copper waste containers, to suppress microbial activity as much as possible, s
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