A formulation of the net nuclear fuel flow for the long-term operation of a fission reactor is established. One distinguishing feature of this representation is the effective smoothing of the discrete refueling processes. The representation thus provides an equivalent continuous fuel trajectory of the net fuel requirements over long time intervals. The derived expressions are easily evaluated and found to be in excellent agreement with conventional extensive and detailed calculations. Additionally, some useful global reactor lifetime fuel cycle characterizations have been formulated and examined.