An analytical function for the fuel cycle cost (FCC) of a fast reactor has been developed, that is sufficiently simple to be used as an objective function in applications of optimal control theory, such as Pontryagin’s maximum principle, but also sufficiently accurate, even for fuel cycle parametric studies. It is shown that under definite economic conditions the fuel enrichment distribution u(r) that minimizes the FCC will also minimize, in cylindrical geometry, an integral of the form

,

where Φ(r) is the neutron flux and lk, and Vk are parameters that are constant in each region k.