[en] A liquid metal cooled fast breeder nuclear reactor has a fuel assembly or core comprising a plurality of closely spaced elongate fuel sub-assemblies upstanding side-by-side from a support structure. The sub-assemblies each comprise a plurality of spaced fuel pins within a tubular wrapper of cross-section generally hexagonal. The corners of the wrapper are replaced by flat sides or by sides which are inwardly convex which have the effect of increasing the resistance of the wrapper to dilation under coolant pressure. Reduced dilation enables the sub-assemblies to be more closely spaced so that vibration is reduced and the fuel inventory of the reactor can be increased even though the number of fuel pins in each sub-assembly is reduced, (UK)