In a homogeneous environment, all U nuclei are in the same conditions with respect to the resonant neutron flux. In a heterogeneous environment uranium is separated from the moderator, which significantly affects the resonant neutron absorption. Firstly, some of the resonant neutrons become thermal neutrons in the moderator without colliding with uranium nuclei; secondly, resonant neutrons hitting the surface of the fuel elements are almost all absorbed by the thin surface layer. The inner U nuclei are shielded by the surface nuclei and participate less in the resonant neutron absorption, and the shielding increases with the increase of the fuel element diameter d. Therefore, the effective U resonance integral in a heterogeneous reactor depends on the fuel element diameter d :