Modeling of shutdown cooling reactivity effects with simulate

During plant outages and subsequent return to power operations, the fuel isotopic inventory, relative to that of steady-state operation, changes as nuclides go through radioactive decay. Accurate predictions of essential core neutronic parameters require that the reactivity impact of these isotopic changes be taken into account. Extended outages, as experienced in Japan following the Fukushima accident, as well as consecutive outages, which are close in time such that the residual effects from the first one influence the reactivity of the subsequent one, may challenge the reactivity models implemented in three-dimensional (3D) core simulators. This paper studies the modeling issues with the core simulators for analyzing nominal and extended refueling shutdown outages as well as combination of such with mid-cycle outages. The shutdown cooling (SDC) reactivity models that are implemented based on macroscopic (lumped) depletion and explicit isotope tracking are described for the Studsvik 3D core simulators, SIMULATE-3 and SIMULATE5. Using relevant numerical test cases, it is demonstrated that the explicit isotope-tracking model offers robust and accurate prediction capabilities for computing shutdown-cooling reactivity, regardless of the outage time. (author)