A fuel cycle analysis system is presented for performing fast reactor fuel cycle calculations. The REBUS system, an acronym for REactor BUrnup System, solves for the infinite time (equilibrium) operating conditions of a recycle system under fixed conditions. REBUS obtains the feed enrichments, the burn step (operating) time, and the control requirements which satisfy the constraint on the fuel discharge burnup, give the desired unpoisoned multiplication constant at some specified time during reactor operation, and maintain criticality. REBUS includes models of both the in-reactor fuel management and the external cycle. The in-reactor fuel management model permits any physically realizable fuel management scheme. In the external cycle, reprocessing and sale of the discharged fuel and refabrication with charge fuel makeup from reprocessing plants and/or external feed supplies can be studied. The isotope chain matrix may contain β−, β+, and α-decay terms as well as (n, γ), (n, p), (n, α), (n, 2n), and (n, f) reactions. The REBUS system is comprised of a neutronics model and a fuel cycle model. The fuel cycle model contains no geometric information so that any neutronics solution can be used (zero to three dimensional, diffusion or transport theory, direct or synthesis). REBUS has been operated with one- and two-dimensional diffusion theory neutronics models up to the present time.