An evaluation of fission product heat effect is required to protecta nuclear reactor from overheating after shutdown or to make proper use of the available decay heat. It may, therefore, prove convenient and practical to have the fission product heating included continuously in the reactor dynamics model like the delayed neutrons, following any changes in the reactor power level. This would be particularly useful in a multiple start-stop operation. In the method presented here, a modified Way-Wigner formulation is used to introduce the fission product decay heat in the reactor heat balance equation, following continuously any changes in operating conditions. A reference graph has been prepared showing computed and normalized fission product power decay after different operating times, with the decay curves arranged in time sequence. Their starting points show the fission product power buildup during reactor operation. Following the delayed-neutron pattern, several decay groups are used to make the equations fit the graph. The number of decay groups and the amount of detail in the reference graph depend on the desired accuracy. In the results of an analog simulation study, shown here, satisfactory agreement was reached between the analog plot and the reference decay and buildup curves. The method can be applied to any reactor type and nuclear fission process when a desired dynamics model as well as fission product decay data are available. In addition, xenon poisoning equations are shown adapted to the normalized reactor model.