A MODAL-BASED REDUCED-ORDER MODEL OF BWR OUT-OF-PHASE INSTABILITIES

A low-order model of BWR out-of-phase instabilities is developed via modal analysis. The modal point kinetics equations are derived using the fundamental and first harmonic modes of the neutron diffusion equation. Parameter estimation in the frequency domain is used to determine the coefficients of the transfer functions (or differential equations) representing the necessary BWR feedback characteristics. Combined with well-known nonlinear reactor kinetics. the resulting simple structure of the model provides physical insight into the mechanisms behind the out-of-phase coupled thermalhydraulic/ neutronic BWR instability. This investigation is specifically oriented toward the LaSalle Unit II BWR. An out-of-phase simulation of the LaSalle BWR illustrates the ability of these equations to effectively reproduce the phenomenon