Abstract
A two-channel nuclear-coupled thermal-hydraulic model is introduced to simulate both in-phase and out-of phase oscillation modes in a BWR. This model comprises three parts : a spatial mode neutron kinetics with the fundamental and first azimuthal modes; fuel heat conduction dynamics, and a thermal-hydraulics model. The present model is an extension of the Karve et al. model, i.e., a drift flux model (DFM) is used instead of the homogeneous equilibrium model (HEM) for two-phase flow, and lambda modes are used instead of omega modes for the neutron kinetics. The two-channel model is employed in stability and bifurcation analyses, carried out using the bifurcation code BIFDD. The stability boundary (SB) and the nature of the Poincare-Andronov-Hopf bifurcation (PAH-B) are determined and visualized in a suitable two-dimensional parameter/state space. The first and the second stability boundaries are presented for different values of the reactivity feedback of the first mode. The first SB corresponds to the points on the parameter space at which the real part of the first largest pair of eigenvalues is equal to zero, while, the second SB is associated to the second largest eigenvalue. A detailed investigation on these two SBs has been carried out to associate each SB with the in-phase and out-of-phase modes by analyzing the shape of their corresponding eigenvectors. Furthermore, numerical integrations of the set of ODEs have been carried out in the MATLAB environment using the Gear's method. Results confirm the predictions of the bifurcation analyses.
