Breeding estimates for long-term reactor fuel logistics are pursued, specifically deriving an instantaneous or transitory fuel growth rate definition, γ(t), from the basic space- and time-dependent fuel cycle equations. The derivation is valid for either the discontinuous or continuous fuel cycle treatments. The resulting definition is applied to a uranium-plutonium fast reactor operating in the closed fuel cycle mode. Transitory growth rate results are calculated for various fuel isotopic weight-factor sets and initial fuel compositions. These results show γ(t) to be practically independent of the isotopic weight-factor sets, provided the γ(t) is calculated from the time-dependent variation of the fuel isotopes. The growth rate derivation automatically yields the fuel composition shift in the form of the reactor fuel time derivative. Investigations of the impact of this quantity on transitory breeding descriptions show that it is the erroneous neglect of the fuel composition-shift term that induces the previously observed strong dependence of the growth rate upon the fuel isotopic weight-factor sets. Accurate approximation of the instantaneous fuel growth rate using transitory static reaction rate information (fuel-shift term neglected) is shown possible with the substitutional critical mass (CM) worth weights, .