In an earlier work, the author presented a theory of the diffusion coefficient in a reactor lattice, leading to expressions valid in full generality. However, for practical purposes it was necessary to admit simplifying assumptions. But now, with the help of modern computers, weaker approximations appear possible. Assuming only two hypotheses, (a) zero-order approximation in , and (b) cylindricalization of the cell, a diffusion coefficient calculation can be transformed into a one-dimensional problem, the solution of which is practically as simple as the calculation of the classical fine structure. The difficulty concerning the reflection of neutrons from the boundary is overcome here; moreover, handling of angular fluxes is avoided, without any approximation. Formulas for the calculation of the diffusion coefficients in the framework of integral transport theory are presented.