A model is presented which enables us to find the distribution of fuel enrichment that minimizes the fuel cycle cost of a fast reactor, subject to constraints on the enrichment, power, and power density. The reactor is described by a discontinuous one-group diffusion model in slab geometry.Making use of Pontryagin’s Maximum Principle, as extended by Gossez and by Vincent and Mason, the optimal sequence of control (enrichment) zones is found a priori. The latter consists of a central constant power density zone, a maximum enrichment zone, a minimum enrichment zone, and a reflector.The numerical solution of the problem is based on an automatic double iteration search procedure requiring no input trial function.Under the economic conditions considered, it seems preferable to start up the first fast breeder demonstration plants with a core surrounded by reflector elements; radial blanket subassemblies should be inserted only later, and progressively, when fabrication costs decrease and the operational knowledge improves.