Experimental orthogonal functions for the qualification of BWR stability events. Application to Peach Bottom NPP

In this work, BWR stability analysis was performed on an operating point (PT_UPV) of Peach Bottom NPP which is inside the exclusion region. The simulation was 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 transient starts with this control rod movement. 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. After control rod movement, a limit cycle in-phase oscillation on the total reactor power evolution is obtained, together with a coupled out-of-phase oscillation in the whole 3D power evolution. In order to complete the stability analysis, the local power range monitors (LPRMs) simulated signals obtained from the neutronic code has been analyzed using the singular system analysis. The results confirm the out-of-phase power oscillation in the core.