Comments on the application of bifurcation analysis in BWR stability analysis

Highlights: • It is impossible to understand real stability states of BWRs by system code analysis alone. • An advanced reduced order model is employed for BWR bifurcation analysis. • Two examples are presented where coexisting stability states spontaneously appear. • A limit point of cycles can cause a jump of the decay ratio (DR). • The dominance change of in-phase and out-of-phase oscillations is demonstrated. - Abstract: Currently, BWR stability analysis is most often performed by the application of system codes which provide the time evolution of the neutron flux or thermal power at a defined operational point (OP) after imposing a system parameter perturbation. However, in general it is impossible to understand the real stability state of the BWR at a specific OP by the application of system code analysis alone. Hence, we are exploring methods developed in the nonlinear dynamics field in order to reveal the nature of the BWR stability states when power oscillations are observed. A powerful method is bifurcation analysis. In order to motivate this "nonlinear thinking" versus "linear thinking", in this paper we will demonstrate some examples of phenomena which can only be understood in nonlinear terms by application of bifurcation theory and where linear interpretation leads to incorrect conclusions.