A semi-analytical/empirical relationship has been developed to describe the axial flux variation in one dimension in a nuclear reactor when the local reactivity indicator k∞ is an explicit function of the neutron flux. This relationship is valid over a wide range and correlates well with the results produced by other methods. This relationship expresses in a single explicit equation the excess reactivity as a function of the fuel cycle time, axial flux form factor, and neutron leakage.