A cylindrical system of finite height is being considered, consisting of a series of homogeneous, concentric annular zones surrounded by moderator in the radial direction. For this system, a complete solution of the monoenergetic, stationary neutron transport equation is found in the PN spherical harmonics approximation. In the moderator, the scalar flux converges asymptotically towards the solution of the diffusion equation. When coupled with the heterogeneous (source-sink) theory, the method yields the scalar flux in a reactor and, after minor modifications, also applies to the exponential assembly.