Macro- and Microflux, I: Periodic Lattices

In order to shorten the time of reactor core calculations, the actual core structure is often replaced by a simpler structure, such as a periodic lattice whose neutron flux is determined through some periodic microfluxes and through an overall macroflux. In the framework of the well-known perturbation formalism, it is shown that the macroflux is obtained from a two-group diffusion equation in which the coefficients are determined from transport cross sections and microfluxes. The relationships between microfluxes are given. It is shown that in a finite core the flux is described by an asymptotic and a transient term. A simple problem is solved by means of the presented theory, showing that it is capable of providing a truncated series expansion of the exact results. The theory presented is applied to the evaluation of measurements.