Microstructural features of compacted MX-80 bentonite after the long-time experiment
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Microstructural features of compacted MX-80 bentonite after the long-time experiment Michał Matusewicz, Joonas Järvinen, Markus Olin and Arto Muurinen VTT Technical Research Centre of Finland Ltd., Otakaari 3K, P.O. Box 1000, FI-02044 VTT, Finland ABSTRACT Bentonite clay is proposed to be a buffer material in the KBS-3 method of deep geological disposal of spent nuclear fuel in Sweden and Finland. It is essential to know the long term behaviour of the material when evaluating the safety performance of the barrier system. Therefore, various experiments and modelling have been carried out with bentonite in different conditions during the last decades. However, most of the laboratory experiments in controlled conditions last less than few years only. In this study 15 years old compacted MX-80 bentonite samples were studied. They stayed closed in a copper canister in contact with low salinity groundwater both in aerobic and anaerobic conditions. After dismantling the experiment a thorough characterization was carried out, including analysis of chemical composition of bentonite and external solution, clay microstructure, mineralogy and microbial activity. In this presentation the focus is on the microstructural features of the samples. Smallangle X-ray scattering (SAXS), nuclear magnetic resonance (NMR) and anion exclusion measurement were used to investigate the nano and micro porosity features of the material. The results are compared with similar analysis of MX-80 samples prepared in the laboratory and with short time equilibration with external solution. Calculations based on SAXS data give similar results but there are differences for NMR based results. This could be caused by dissimilarities of between the samples at the mesoscale level. No differences were observed between the oxic and anoxic conditions suggesting no influence of redox conditions on the bentonite microstructure. INTRODUCTION Bentonite clay is considered in many countries to form a part of the high-level nuclear waste disposal facilities. Finland and Sweden plan to use the KBS-3 method with MX-80 bentonite as the buffer material protecting the copper canisters containing spent nuclear fuel. Due to the long life time of the repository, in the range of thousands of years, it is crucial to know the materials used and understand their behaviour. It is widely accepted that the microstructure of the bentonite clay is chiefly created by the montmorillonite layers stacked together [1]. A significant part of the total porosity of the highdensity bentonite consists of the slit-like nanopores between montmorillonite layers. It often referred to as the interlamellar porosity (IL porosity). Between clay aggregates and so-called accessory minerals (mainly quartz grains, plagioclase, feldspars) [2] there are larger pore structures. Those mesopores are referred to as the non-interlamellar porosity (non-IL porosity). In many cases the investigation of the clay structure is carried out on the samples that had a relatively short time for equilibration. The clay prepa
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