A Transient Modal Analysis of a BWR Instability Event

Abstract

To solve the time dependent neutron diffusion equation a modal method, based on the expansion of the neutronic flux in terms of the dominant Lambda modes of a static configuration of the reactor is presented. This method is used to analyse transients of a nuclear power reactor where an instability event can be developed. A simulation of a transient with the same conditions given for the case 9 of Ringhals stability benchmark has been analysed. It is shown that with these conditions an out of phase oscillation associated with the two first azimuthal modes can be developed. These results are corroborated using a power modal decomposition, using the local power distribution provided by RAMONA code. To complete the analysis, the modal feedback reactivities have been calculated to study the coupling mechanism among modes.

KEYWORDS:

• regional oscillations
• spatial harmonics
• three-dimensional time-domain code
• modal kinetic analysis
• Lambda modes
• modal reactivities
• modal power decomposition
• BWR type reactors
• instability
• transient modal analysis