AbstractA semi-empirical equation is used to predict excess reactivity in the ETR as a function of power-time in terms of reactivity parameters which are routinely measured in a mockup of each ETR core in the ETR Critical Facility. Constants in the equation were evaluated by fitting the equation to plots of excess reactivity as a function of power-time obtained for several cores in the ETR. The equation and reactivity measurements fulfill the need for frequent and reliable predictions of core charge life as well as control-rod positions during reactor operation.