Nodal analysis of density-wave oscillations in boiling water nuclear reactors

This work presents a model for density-wave oscillations in boiling water nuclear reactors (BWRs). A nodal approach is adopted to describe the power generation in the core. The nodal equations are derived within the framework of Avery's coupled-core kinetics theory. This method enables us to evaluate easily the internodal coupling coefficients using steady-state flux and importance distributions. Particular care is taken in treating the heater wall dynamics by using a distributed parameter model for the fuel rod. The flow is described by the homogeneous equilibrium conservation equations.A steady-state calculation is performed to determine the flow distribution and the neutronic coupling parameters in a core constituted by three hydrodynamic channel types and two neutronic radial nodes. The dynamics solution is based on linearization and use of the Nyquist stability criterion. The stability margin is found to be significantly affected by the coupling effect. The importance of a detailed representation of the neutronic behavior in BWR stability studies is demonstrated.