The purpose of this study was to demonstrate the neutronic feasibility and competitive fuel cycle economics of sodium fast reactors operating with uranium metal  (UZr) fuel on a once-through fuel cycle. Uranium startup fast reactors (USFRs) decouple their deployment from that of expensive reprocessing and recycle facilities. This could facilitate and speed up the deployment of conventional fast reactors, which, in their traditional designs, heavily depend on the availability of reprocessing facilities for transuranic fuel production. The uranium requirement and fuel cycle cost of studied USFR core designs are calculated to be comparable to those of typical light water reactors. The main design constraint is the fast neutron fluence imposed on the cladding material, which is required to be below 5.0×1023 n/cm2 even for advanced oxide dispersion strengthened steels. Therefore, moderators need to be inserted in the fuel assemblies to lower the fast neutron flux so that the fuel residence time limited by neutron fluence can be extended to match the reactivity limited fuel residence time. In this study, magnesium oxide is used for reflectors as well as for the moderator.