The intermediate resonance approximation of resonance absorption is applied to a heterogeneous fast reactor assembly to see how each resonance of 238U deviates from the narrow resonance approximation. The resonance integral is calculated for the 50 resonances of 238 U in ENDF/B-II below 1.9 keV. The averaged deviation of these resonances from the narrow resonance extreme was found to be ∼4%. It is concluded that the effective group cross sections in heterogeneous fast systems can be estimated reasonably well by the narrow resonance approximation, even though this approximation tends to underestimate the resonance integrals noticeably for a handful of resonances with extremely large neutron widths.