A fast efficient perturbation model is developed for generating four-group cross sections and flux spectrum and performing fuel depletion in a light water reactor unit cell configuration. In the thermal energy region, an approximate solution of the neutron spectrum is determined based on the Wigner-Wilkins free-gas hydrogen scattering model In the fast energy region, a combination of analytical and empirical techniques is used to determine resonance cross sections. These models are combined in a perturbation scheme and incorporated as the SIFAS code. Reaction rates of important nuclides in reactor cores can be estimated by the code to within 0.5% for fuel depletion studies with fuel burnup of up to 30 000 MWd/tonne.