[en] The sensitivity of fast reactor design parameters to nuclear data uncertainties is investigated. Based on a review of the available cross section literature, the uncertainties in the cross sections of ²⁴⁰Pu, ²⁴¹Pu, and ²⁴²Pu, as well as ²³⁹Pu and ²³⁸U, are evaluated. The effects of these cross section uncertainties upon beginning-of-life physics parameters and long-term fuel cycle characteristics are assessed by making uniform cross section adjustments within the range of uncertainties. The reactor model selected for this study is representative of current designs of a 1000 MW(e) mixed-(Pu,U) oxide-fueled fast breeder reactor. ENDF/B Version III data are used for a data reference. A series of one-dimensional multigroup diffusion-perturbation calculations is performed to study the sensitivity of beginning-of-life reactor design parameters, including eigenvalue, Doppler and sodium void reactivity coefficients, breeding ratio, material worths, reaction rates, power fractions and fissile mass inventory. The results of these sensitivity calculations are compared with target accuracies for calculation of reactor parameters. These parameter sensitivities are further used to estimate the nuclear data uncertainties which would be allowable to meet the target accuracies. A series of reactor burnup calculations evaluate the sensitivity of fuel cycle characteristics during the approach to equilibrium conditions. The effects of nuclear data uncertainties upon the time-variation of the breeding ratio, doubling time, power distribution, fissile loading, fissile mass discharge rates, control requirements, and reactivity swing are evaluated. The reference fuel cycle calculations are also used to assess the plutonium isotopic composition discharged from the equilibrium cycle of a first-generation fast breeder reactor. (U.S.)