[en] Fuel cycle parameters including end-of-cycle reactivity, end-of-batch nuclide concentrations, and fuel cycle costs are sensitive to certain nuclear data. Sensitivity analyses for cross sections, fission yields, decay constants and other nuclear data indicate directions for worthwhile improvements in data and methods, and yield information important for selection of proper design margins. Considerable success has been achieved in understanding the relationships among various data uncertainties and water reactor fuel cycle costs. These relationships have been shown to be dependent on fuel cycle option and reactor type. Sensitivities depend in an intrinsic way on propagating and compensating effects that take place during the fuel cycle. Fuel cycle parameters and costs are found to be significantly dependent on particular thermal and resonance cross sections and flux disadvantage factors, on fission yields of neutrons and energy, and on certain other data. The results bring out the importance for power reactor sensitivity analysis of dealing with the full fuel cycle