Delayed fission energy effect on LWR normal operation and transients

Abstract

In reactor core analysis, equilibrium delayed fission energy is often assumed except for estimating the decay heat during reactor shutdown or post-accident scenarios. In this paper, we investigated a few slow and fast transient operational conditions in which the assumption of equilibrium delayed energy could be questionable. A model of lumping the delayed heat source into groups has been adopted in this study to calculate the time-dependent delayed power in the CASL neutronics code MPACT. For the normal operational depletion, the time-integrated effect of the delayed heat shows a small impact in calculating the flux normalization factor, and thus eigenvalue and depletion isotopics. The effect of delayed energy for the reactivity-initiated accident (RIA) problems is more interesting. For a PWR 3-D full core simulation of rod ejection, the peak core power is reduced by about 3% and the decay heat is significantly higher after SCRAM, with an overall energy release increased by 7%. Therefore, the time-dependent delayed model is more beneficial for fast transient analysis. In addition to the temporal effect, the spatial distribution of energy deposited from delayed gamma rays are compared with that from prompt gamma rays for a few typical LWR mini-lattices, and the results affirm that using the scaling approach for spatial energy deposition of delayed gamma rays has good accuracy.