where N 3 / N8 is the ratio of moderator and U nuclei in the homogeneous mixture, σ S is the microscopic scattering cross section of the moderator. As can be seen from the formula, the effective resonance integral decreases with increasing U concentration. The more U nuclei in the mixture, the less likely absorption by a single nucleus of the moderating neutrons will take place. The effect of absorption in some U nuclei on absorption in others is called resonance level shielding. It increases with increasing concentration of resonance absorbers.