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Several countries intend to use a multi-barrier concept for the long-termdisposal of spent nuclear fuel. Many of these concepts, including the Swedish KBS-3 concept, involve an engineered barrier consisting of compacted bentonite which will be positioned around canisters holding the nuclear waste. Due to the compact microstructure, high swelling capacity as well as the large sorption capacity for most cationic radionuclides and fission products,highly compacted and water-saturated bentoniteis favored. During the mandatory lifetime of the deep geological repository (≥100000 years), the bentonite barrier will inevitably be subjected to changing groundwater composition and flow, temperatureand ionizing irradiation. If low ionic strength groundwater intersects the bentonite barrier, the stability of the barriercan be endangereddue to erosion. In this thesis, three different topics are embracedregarding the barrier, namely i)characterization of the microstructure and its properties of saturated compacted bentonite; ii)Effects of γ-radiation on bentonite dispersions, as well as on compacted bentonitein terms of radionuclide retention;iii)The potential effects of irreversible interactions between the proposed injection grout Silica sol (SiO2-colloids) and bentoniteparticles.The microstructure was investigated bycolloid filtration experiments, as well as through a basal spacing analysis using XRD. The colloid filtration experiments, usingsmall gold colloids of different sizes (2, 5 and 15 nm) as tracers, showed that the microstructural constraints of compacted bentonite effectively filter even extremely small inorganic colloids. Colloid transport was only observed at the lowest dry density, i.e. at the highest water content used,where the average interlayer distance in the montmorillonite particles exceeded thesize of the tracer colloids. From the XRD experiments theso-called free porosity of saturated compacted bentonite was determined by comparing the interlayer distances (basal spacings –montmorillonite layer thickness) with the theoretical interlayer distances corresponding tono free porosity. Irrespective of water content for the Na+dominated clays samples investigated at low ionic strength, the free porosity proved to be very low. When highly compacted, as in the bentonite barrier, the free porosity was ≤3%meaning thatthe interlayer voids comprise the major part of the total porosity in the bentonite barrier, in line with the colloid filtration experiments. The γ-radiation experiments showed a radiation-induced increase in colloid stability ofmontmorillonite dispersions. This radiation-induced effect also changed the settlingbehavior of irradiated montmorillonite and bentonite dispersions. Furthermore, γ-radiation was also found to increase the structural Fe(II)/FeTotratio in montmorillonite,resulting in an increased reactivity towards H2O2, one of the major anticipated oxidants under repository conditions. However, although both effects were attributed to reactions with the short-lived radicals formed upon water radiolysis, the change in thestructural Fe(II)/FeTotratio could not explain the radiation-induced increase in colloid stability.The effect of γ-radiation on radionuclide retention in compacted bentonite was examined in diffusion and batch sorption experiments,usingboth137Cs+and 60Co2+. In the batch sorption experiments a general decrease in the radionuclide sorption coefficient Kdwas found for Co2+upon γ-irradiation, but not for Cs+.As expected no difference in apparent diffusivity (Da) of Cs+between irradiated and unirradiated samples were found. As for Co2+, the effect of decreasing sorption to the irradiated clays was unfortunately not large enough to be reflected under these experimental conditions.Effects of SiO2-colloids on bentonite particles were studied in free swelling and settling experiments,as a function SiO2/montmorillonite ratio,after different types of treatmentsrepresentative for different repository scenarios. Results showed that the size and colloidal stability of bentonite and montmorillonite particles can be modified by SiO2-colloids, but only when mixed in comparable amounts (by weight) under high ionic strength (0.3 M NaCl) and especially from dehydration. From an AFM investigation using Na-montmorillonite, the sorption of the SiO2-colloids was found to preferentially occur along the edges of the layers, whereas for Ca-montmorillonite sorption also occurredon the faces. Upon aggregation with the SiO2-colloids, the interlayer distances of montmorillonite were left unchanged as measured with XRD, suggesting that the SiO2-colloids only interact with the extralamellar surface of montmorillonite particles.