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Investigation of Rotating Mode Behavior In Bwr Out-of-phase Limit Cycle Oscillations – Part 1: Reduced Order Model

by Aaron J Wysocki, Annalisa Manera, Thomas Downar, Jose March-leuba
Publication Type
Journal Name
Annals of Nuclear Energy
Publication Date
Page Numbers
393 to 407

Previous neutronic/thermal-hydraulic coupled numerical simulations using full-core TRACE/PARCS and SIMULATE-3K BWR models have shown evidence of a specific “rotating mode” behavior (steady rotation of the symmetry line) in out-of-phase limit cycle oscillations, regardless of initial conditions and even if the first two azimuthal modes have different natural frequencies. The goal of the present work is to gain further insights on the rotating mode behavior using a simplified mathematical model which contains all important physics for this application while providing sufficient flexibility and simplicity to allow for in-depth understanding of the underlying phenomena. This was accomplished using a multi-channel, multi-modal reduced-order model, using a modification of the fixed pressure drop boundary condition to simulate channel coupling via the inlet and outlet plena, in order to destabilize the out-of-phase mode over the in-phase mode. Examination of the time-dependent solution of the nonlinear system showed a clear preference for rotating mode behavior in the four-channel model under standalone TH conditions and for conditions with weak neutronic feedback. When neutronic feedback was strengthened (i.e. larger reactivity feedback coefficients), the side-to-side mode (stationary symmetry line) was favored instead.