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 m odal analysis. The modal point kinetics equations are derived using th e fundamental and first harmonic modes of the neutron diffusion equati on. 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 wit h well-known nonlinear reactor kinetics, the resulting simple structur e of the model provides physical insight into the mechanisms behind th e out-of-phase coupled thermal-hydraulic/neutronic BWR instability. Th is investigation is specifically oriented toward the LaSalle Unit II B WR. An out-of-phase simulation of the LaSalle BWR illustrates the abil ity of these equations to effectively reproduce the phenomenon. (C) 19 97 Elsevier Science Ltd.