An expression for the charge life of a reactor is derived in terms of a model which assumes that enough uniformly distributed poison is always present to keep the reactor critical with control rods withdrawn. The burnout distribution is assumed to be constant and to be the same as at the end of the run, or to follow the calculated thermal flux distribution. Two group perturbation theory expressions for reactivity changes due to fuel burnout and uniform poison removal may then be equated and integrated under plausible simplifying restrictions to yield an expression for charge life in terms of calculable, or measurable quantities.