A high-breeding fast reactor with fission product gas purge/tube-in-shell metallic fuel assemblies is proposed. Its reactor doubling time is <10 yr because fast reactors must have economical fuel breeding as well as economical electricity generation. The core has a high volume fraction of fuel (>50%) and realizes a very hard neutron spectrum. Thus, a fast-breeding 670-MW(electric) reactor with a high breeding ratio (1.84) and a short reactor doubling time (6.7 yr) is proposed. The structure of the fuel assembly, its fabrication, and the fission product gas purging mechanism are assessed and the new fuel assembly concept is determined to be feasible. Purging of the fission product gas does not affect the shielding requirement and can be managed by a small-scale cover gas treatment system because of the good fission product retention characteristics of the sodium in the fuel assemblies. The entire reactor configuration, including the intermediate heat exchangers, was assessed utilizing conventional reactor components.