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The Alternative Buffer Material ABM5 experiment is an in situ medium-scale experiment performed at Äspö Hard Rock Laboratory (HRL) conducted by SKB in Sweden with the aim of analysing the long-term stability of bentonites used as an engineering barrier for a high-level radioactive waste repository (HLWR). In this work, four different ring-shaped Ca- and Na-bentonite blocks, which were piled around a carbon steel cylindrical heater, subjected to a maximum temperature of 250 °C and hydrated with saline Na-Ca-Cl Äspö groundwater (0.91 ionic strength), were characterized after dismantling. This work allowed us to identify the main geochemical processes involved, as well as the modifications in the physico-chemical properties and pore water composition after 4.4 years of treatment. No significant modifications in mineralogy were observed in samples close to the heater contact, except an increase in Fe content due to C-steel corrosion, carbonate dissolution/precipitation (mainly calcite and siderite) and Mg increase. No magnetite and a low amount of Fe(II) inside the clay mineral structure were detected. No modifications were observed in the smectite structure, except a slight increase in total and tetrahedral charge. A decrease in external surface area and cation exchange capacity (CEC) was found in all samples, with lower values being detected at the heater contact. As a consequence of the diffusion of the infiltrating groundwater, a modification of the composition at clay mineral exchange sites occurred. Ca-bentonites increased their Na content at exchange sites, whereas Na-bentonite increased their Ca content. Exchangeable Mg content decreased in all bentonites, except in MX-80 located at the bottom part of the package. A salinity gradient is observed through the bentonite blocks from the granite to the heater contact due to anions are controlled by diffusion and anion exclusion. The pore water chemistry of bentonites evolved as a function of the diffusion transport of the groundwater, the chemical equilibrium of cations at exchange sites and mineral dissolution/precipitation processes. These reactions are in turn dependent on temperature and water vapor fluxes. [ABSTRACT FROM AUTHOR]

Abstract: The Long Term Test of Buffer Material (LOT) project at the Äspö Hard Rock Laboratory, Sweden, is a series of medium-scale field experiments focused on validating models and hypotheses concerning long term processes in the bentonite buffer of a repository for high-level radioactive waste. The test parcels emplaced in crystalline bedrock consist of blocks of compacted MX80 bentonite embedding a Cu-tube equipped with a heater to simulate the heat generation from radionuclide decay. The A2 test parcel had been subjected to elevated temperature (up to 130°C) and hydration by a Na–Ca–Cl type groundwater for almost 6years when it was retrieved to be analysed. The analyses included determinations of chemical composition, cation exchange capacity (CEC), exchangeable cations and mineralogy. Both the bulk bentonite and dialysed, homo-ionic Na-clay (<2μm and <0.2μm fractions) were analysed when relevant. Sulphate was redistributed in the heated part of the buffer under the thermal and hydration gradients that prevailed during the test period. Anhydrite accumulated in the warmer parts, whereas gypsum was dissolved in the peripheral parts of the buffer where water was supplied. Carbonate dissolution increased with temperature in the warmest parts, whereas chloride behaved conservatively in all blocks. Cu was incorporated in the bentonite matrix at the surface of the Cu-tube indicating some corrosion, which may be explained by reactions in an early stage of the test when trapped oxygen existed in the system. Along with the dissolution/precipitation reactions the porewater composition changed, which resulted in replacement of exchangeable sodium by calcium and magnesium in the warmest zone. Also Mg in the clay (<2μm and <0.2μm fractions) displays a clear gradient with peak values at the heater. Because several of the alternative sinks for Mg were eliminated in the sample preparation prior to the chemical analysis (purified clay fractions, removal of carbonates, Na-saturation) the smectite is suggested a candidate sink for Mg. Parallel with the increase in Mg, a loss in Si is indicated and CEC tends to increase in the clay that had been heated at 130°C. A loss in tetrahedral Si that is balanced by Al, and replacement of Al by Mg in the octahedral sheet would imply that the layer charge of the smectite increased, which would be consistent with the higher CEC values of these samples. The changes in CEC are, however, close to the analytical resolution of the CEC method and no effect of the changes in chemical composition can be detected in the XRD-characteristics of the clay. Therefore, supplementary high-resolution analyses are required to verify whether the structure of montmorillonite has altered in the test period. [Copyright &y& Elsevier]

Regular wood ash recycling has been suggested as a precautionary measure for preventing acidification and development of under-balanced nutrient budgets in Swedish forest ecosystems, particularly in areas where whole-tree harvest is being practised. Granulation can be used as a means of reducing the reactivity of the ash. In two experiments on podzolic soils, the humus layer and the upper 15 cm of the mineral soil were sampled 2 years after applying a well-hardened, granulated wood ash at rates of 1 to 6 ton ha-1. The timing potentialof the ash corresponded to 28% CaO. Cation exchange capacity, pH andbase saturation were positively correlated with ash dose, and exchangeable acidity was negatively correlated with ash dose, in the upper part of the humus layer at both sites. The total amount of exchangeable base cations (kmolc ha-1) had, on average, increased by about 2 x (square root of) dose (ton ha-1) in the sampled soil compartment. A comparison with previous studies suggests that granulated ash should generally cause a less drastic pH increase compared with loose ash of the same origin. [ABSTRACT FROM AUTHOR]