Comparison of Power Pulses from Homogeneous and Time-Average-Equivalent Models

Abstract

The time-average-equivalent model is an "instantaneous" core model designed to reproduce the same three dimensional power distribution as that generated by a time-average model. However it has been found that the time average-equivalent model gives a full-core static void reactivity about 8% smaller than the time average or homogeneous models. To investigate the consequences of this difference in static void reactivity in time dependent calculations, simulations of the power pulse following a hypothetical large-loss-of-coolant accident were performed with a homogeneous model and compared with the power pulse from the time-average-equivalent model. The results show that there is a much smaller difference in peak dynamic reactivity than in static void reactivity between the two models. This is attributed to the fact that voiding is not complete, but also to the retardation effect of the delayed-neutron precursors on the dynamic flux shape. The difference in peak reactivity between the models is 0.06 milli-k. The power pulses are essentially the same in the two models, because the delayed-neutron fraction in the time average-equivalent model is lower than in the homogeneous model, which compensates for the lower void reactivity in the time-average equivalent model.