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High density uranium dioxide (UO2) pellets with grain sizes between 0.9μm and 9μm were produced by spark plasma sintering (SPS). A systematic study was performed by varying the sintering temperature between 750°C and 1450°C and hold time between 0.5min and 20min to obtain UO2 pellets with a range of theoretical densities (TD) and grain sizes. The microstructure development in terms of grain size, density and porosity distribution was investigated. The oxygen/uranium (O/U) ratio of the resulting pellets was found to decrease after SPS. The thermal conductivity of UO2 pellets increased with the theoretical density but the grain size in the investigated range had no significant influence. The measured thermal conductivity values up to 900°C were consistent with the reported literature for conventionally sintered UO2 pellets. The benefits of using SPS over the conventional sintering of UO2 are summarized. [ABSTRACT FROM AUTHOR]