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Investigation of Rotating Mode Behavior In Bwr Out-of-phase Limit Cycle Oscillations – Part 2: Trace/parcs Model and Physical Explanation

by Aaron J Wysocki, Annalisa Manera, Thomas Downar, Jose March-leuba
Publication Type
Journal
Journal Name
Annals of Nuclear Energy
Publication Date
Page Numbers
378 to 392
Volume
122

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 study is to provide additional in-depth analyses of the predicted “rotating-mode” behavior in BWR out-of-phase limit cycle oscillations, as well as a physical explanation for why this mode is favored over side-to-side or other oscillatory behaviors from a thermal hydraulics perspective. Results are presented using TRACE and TRACE/PARCS for a small number of parallel channels, which confirmed that the conclusions developed from the reduced order model remain applicable when applying a full two-fluid, six-equation, finite-volume modeling approach. From these results, a physical explanation has been put forth to explain why the rotating symmetry line behavior is preferred from a TH standpoint, demonstrating that predominantly out-of-phase unstable systems are most unstable when the variation in the total inlet flow rate is minimized (which minimizes the effective single-phase to two-phase pressure drop ratio), and that the rotating mode is the most successful in minimizing this total flow rate variation as compared to the side-to-side case or any other oscillation pattern. The conclusion is that the rotating mode will be favored for any out-of-phase unstable system of parallel channels with no neutronic feedback or relatively weak neutronic feedback. Previous analyses have indicated that systems with sufficiently strong neutronic coupling may favor the side-to-side oscillation mode over the rotating mode; this topic is left as a subject of future investigation.