The economic analysis of a large fast breeder is dependent on fuel-cycle parameters, such as fuel-discharge rate and breeding ratio. In this work, the variation of fuel-cycle parameters with several burnup-model characteristics was studied. These characteristics are the amount of region detail used in describing the reactor, the initial fissile content of the reactor, the maintenance of criticality during the burnup step, the distribution of the control poison during the burnup step, and the flux or power shift over the reactor lifetime. Each of these model characteristics was studied in detail for its effect on the burnup history of the reactor. The mass balances obtained from several of the burnup studies were input to a reactor economics code to determine the economic effects of changes in the model characteristics. The greatest effect on the fuel-cycle analysis was produced by the treatment of the relative flux shift between burnup intervals.