By combining a modified version of the so-called “adiabatic” method for reactor dynamic calculations with a simplified flow redistribution scheme, an efficient method for predicting three-dimensional core behavior has been developed for pressurized water reactor transients. Both the simplified core reactivity and the flow redistribution calculations are shown to yield close approximations of the results obtained by more rigorous approaches. A modification of this technique is shown to be applicable to some boiling water reactor transients. The procedure is found to be substantially more rapid than those most commonly used for predicting three-dimensional light water reactor core behavior during transients in which thermal-hydraulic feedback is significant.