Higher Order Effects in Calculating Boiling Water Reactor Doppler and Void Reactivity Feedback

The effect of steam voids and control rods on the Doppler feedback and of bypass voids on the total void feedback has been calculated for a gadolinia-shimmed boiling water reactor. Calculations were done using a point model, i.e., feedback was expressed in terms of reactivity coefficients determined for individual four-bundle configurations and then appropriately combined to yield reactor results. For overpower transients, the inclusion of the void effect will make the Doppler feedback stronger. The effect of control rods is to reduce Doppler feedback. For overpressurization transients, the inclusion of the effect of bypass void will increase the reactivity due to void collapse.