2012 Springer Science+Business Media, LLC. All rights reserved. The nuclear design of a reactor plant involves defining the nuclear environment that will exist inside the reactor core. This phase of design work has a great bearing on thermal-hydraulic and mechanical analyses; hence, close coordination with these activities must be maintained. Safety and control requirements are closely tied to the nuclear design effort and must be considered for all phases of the fuel cycle. Power distributions are needed in order to determine peak-to-average power factors for thermal-hydraulics analysis. Other neutronics calculations are needed to obtain the following kinds of information: required fissile fraction and inventories, fuel cycle data, shielding data, and transient response. The topics considered in this chapter include multigroup diffusion theory, geometrical considerations for obtaining input to the diffusion equation, and spatial power distributions and neutron flux spectra encountered in typical fast reactor designs.