Nonlinear analysis of a natural circulation boiling water reactor

A dynamic model of natural circulation boiling water reactors (BWRs) is ana lyzed using a bifurcation code and numerical simulations. The two fundament al bifurcation types relevant to BWRs, the supercritical and the subcritica l Hopf bifurcations, are first studied in natural circulation systems witho ut nuclear feedback. The effect of nodalization approximation in the riser on stability and bifurcation characteristics of the system is determined. T he strong effect of the nuclear-thermohydraulic interaction on the nonlinea r characteristics of a natural circulation BWR is then explored in a parame tric study. Supercritical bifurcations become dominant in the thigh-power) Type-II region for small values of the subcooling number and a strong nucle ar-thermohydraulic coupling. A cascade of period-doubling pitchfork bifurca tions (deep in the unstable region) is also predicted by the model under th ese conditions. Subcritical bifurcations in the Type-II instability region were found for larger values of the subcooling number Both Hopf-bifurcation modes were also encountered in the Type-II instability region (low power o r high power/high subcooling). Finally, the nonlinear reactor model was val idated successfully compared with nonlinear power oscillations measured in a natural circulation BWR.