Diffusion theory overestimates neutron transport in voided or low-density regions of a reactor when the diffusion coefficient is defined as 1/3Σtr. Alternative definitions of the diffusion coefficient for such regions have been proposed. The present paper summarizes some definitions of axial diffusion coefficient for cylindrical channels and proposes a modification to an earlier formula. The results of calculations for a channel in a fast reactor supercell model using different formulas are compared and the limitations of this method, which involves changing only the channel diffusion coefficient, are discussed.