Instability Analysis in Peach Bottom NPP Using a Whole Core Thermalhydraulic-Neutronic Model with RELAP5/PARCS v2.7

Abstract

In this paper we present the results from a BWR stability analysis performed on an operating point, called PT_UPV, of Peach Bottom NPP. This point is inside the exclusion region and it is achieved departing from test point 3 by a control rod movement as it is usually performed in Nuclear Power Plants. The simulation has been made with the coupled code RELAP5-MOD3.3/PARCS v2.7. The thermalhydraulic model includes all the reactor vessel components: jet pumps, recirculation pumps, downcomer, reactor core and also the separator and the dryer. The reactor core has been modeled with 72 thermalhydraulic channels, 71 represent the active core and 1 represents the core bypass. The reactor core thermalhydraulic-to-neutronic representation (mapping) has been divided in four quadrants according to the first and second power harmonics (Lambda modes) obtained previously with the VALKIN code. This mapping was chosen in order not to condition the oscillation pattern. The purpose of this study is to qualify this coupled code against this kind of 3D complex accidents that take place inside the core. The calculated results show that point PT_UPV is an unstable point and the obtained relative axial power distribution shows a bottom-peaked profile, which is characteristic of unstable cores.