Abstract: The invention provides fuel assemblies and reactor cores assembled from them which offer improved reactor performance and/or burnup rates and/or ease of manufacture. In particular, the invention provides a mixed oxide fuel assembly for a nuclear reactor in which the fuel assembly comprises a plurality of fuel rods, a largest diameter fuel rod type, a smallest diameter fuel rod type, and one or more intermediate size fuel rod types being provided, between 25% and 100% of the peripheral fuel rods being provided of the intermediate size or sizes fuel rod type, the corner rods of the fuel assembly being of the smallest size fuel rod type.