Peach bottom instability analysis with a RELAP5/PARCSv2.7 detailed thermal-hydraulic-neutronic model

Abstract

In this work, BWR stability analysis has been performed on an operating point (PT_UPV) of Peach Bottom NPP which is inside the exclusion region in the operating power-flow map. The simulation has been made with the coupled code RELAP5-MOD3.3/PARCSv2.7. This point is achieved departing from test point 3 by a control rod movement as it is usually performed in Nuclear Power Plants.

The thermal-hydraulic model is a detailed model that 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 thermal-hydraulic channels, 71 represent the active core and 1 represents the core bypass. The reactor core thermal-hydraulic 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 transient starts with the control rod movement. 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.