[en] A pellet construction for a fast breeder reactor fuel pellet is provided that reduces swelling with a minimum of density penalty. Ninety percent or more of the fissile material is contained in a powder having a stable microstructure and a density less than about 85 percent of theoretical. This allows the fission product gases to escape. At least half of the fertile material is contained in particles having diameters greater than 50 microns and individual densities averaging more than 95 percent of theoretical. The pellet is a pressed mixture of the particles and powder. In the preferred embodiment, the powder consists essentially of uranium-plutonium carbide and the particles consist of uranium carbide