Abstract: A nuclear fuel pin for a fast breeder nuclear reactor has a central fissile region containing fissile material in the form of vibro-compacted microspheres and axial breeder regions above and below the central region containing fertile material in the form of pellets or packed microspheres. The microspheres containing fissile material are of two sizes with the smaller microspheres packed into the interstices between the larger microspheres. To prevent intermixing of the material of the fissile and fertile regions a barrier layer is located between the fissile region and each breeder region which comprises a layer of microspheres having a diameter intermediate the diameter of the larger and smaller diameter microspheres of the fissile region.