Harmonic modes of the neutron diffusion equation: application to BWR stability

U.S. utilities are pursuing solutions to the Boiling Water Reactor (BWR ) coupled nuclear/thermal-hydraulic stability issue which involve the detection of oscillations using in-core neutron flux instrumentation. To provide reliable indication of oscillations and avoid unnecessary reactor trips, the solutions require knowledge concerning the spatial distribution of oscillations within the core. The modal method is used to evaluate this spatial distribution and aid in predicting the expected oscillation modes for varying reactor conditions. In summary: The expected modes of oscillation for BWR coupled nuclear/thermalhydraulic instabilities are an important aspect of the design of systems to detect and suppress the oscillations prior to exceeding fuel design limits. Prediction of the Eigenvalue separation and neutron flux distributions for harmonics of the neutron diffusion equations have been successfully performed using a modified BWR simulator. The harmonic neutron flux distributions have been shown to provide an accurate representation of the spatial distribution of oscillation magnitudes during a BWR instability and can be used to developed detailed simulations of the oscillations for use in evaluating detection systems. (authors)