Porewater in Compacted Water-Saturated MX-80 Bentonite
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Porewater in Compacted Water-Saturated MX-80 Bentonite Torbjörn Carlsson1, Arto Muurinen1, 0LFKDáMatusewicz1 and Andrew Root2 1 VTT Technical Research Centre of Finland, Espoo, P.O. Box 1000, FI-02044 VTT, Finland. 2 MagSol, Tuhkanummenkuja 2, 00970 Helsinki, Finland. ABSTRACT Bentonite is planned to be used in many countries as a buffer material in repositories for spent nuclear fuel. The proper understanding and modelling of the functioning of the watersaturated bentonite requires knowledge about the bentonite microstructure and also the way water is distributed between different phases. This paper presents experimental results from our studies of water in compacted, water-saturated MX-80 bentonite at dry densities in the range 0.71.6 g/cm3. Three techniques, Cl-porosity, SAXS and proton NMR measurements, were applied to samples kept at room temperature, while TEM imaging was applied to high pressure frozen samples. The combined results of these techniques strongly indicate that the two major water phases in the compacted MX-80 bentonite samples are ‘interlayer’ and ‘non-interlayer’ water. The results of the relative amounts of different water types by SAXS and NMR are very similar. The results by Cl-porosity measurement indicate that only part of the non-interlayer water is available for anions. Those observations are discussed in comparison to TEM micrographs. Our study provides solid experimental evidence for the presence of two major water phases in watersaturated bentonite and estimates their relative proportions and pore sizes. INTRODUCTION High-level nuclear waste (HLW) produced by Swedish and Finnish nuclear power plants is planned to be disposed of in accordance with the KBS-3 repository concept [1, 2]. This means, briefly, that HLW is embedded in iron/copper canisters, which are placed in deposition holes in crystalline rock at a depth of 400-700 m. Each canister is planned to be completely surrounded by a layer of compacted bentonite with a thickness of about 0.35 m. In case that one or more canisters are damaged, the two remaining safety barriers between the HLW and the biosphere are the bentonite buffer and the bedrock. According to [1], “…the buffer shall prevent and retard the dispersion of radioactive substances from the canister to the bedrock.” The efficiency of the bentonite buffer in this respect is, however, a complex matter, closely coupled to the porewater chemistry, the bentonite dry density, and the bentonite microstructure. Assessment and modeling of the bentonite’s efficiency as a buffer require proper quantitative data concerning the bentonite microstructure and the way porewater is distributed between various phases under different conditions. This was, for example, recently pointed out by Holmboe (2011) [3], who noted that that results from several studies [4-11] indicate that the non-interlayer porosity in water-saturated bentonite can constitute a significant fraction of the total porosity, for dry density values
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