Interpretation of in-phase and out-of-phase BWR oscillations using an extended reduced order model and semi-analytical bifurcation analysis

An in-depth investigation of the properties of the elements of the eigenvectors associated with these two modes of oscillation is carried out. Results show that these eigenvectors provide information as regards the corresponding oscillation mode (in-phase or out-of-phase) without solving the set of system ODEs. The analysis clearly brings out the fact that in-phase and out-of-phase oscillations are whole-system mechanisms. A clear distinction is thereby made between these oscillation modes, on the one hand, and the fundamental and first azimuthal neutronics modes on the other hand.