To provide a convenient and reliable method for estimating the resonance heterogeneity effects, a multiregion equivalence relation will be derived for fast reactor lattice cells. It will be shown that the broad group cross sections of “each plate” in a multi-region problem can be calculated by using a cross-section set of the Bondarenko type. Moreover, for the special cases where the thickness of all the resonance plates is equal, the present formulation is shown to be equivalent to the two-sided E3 formulation of Meneghetti.