A Physical mechanism for rotating lines of symmetry in BWR OIJT-of-phase limit cycle oscillations

Recent neutronic thermal-hydraulic (TH) 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 understand why the rotating mexk is specifically preferred, and to determine under what conditions it might occur. This was accomplished using a multi-channel, multi-modal reduced-order model, using a special modification of the fixed pressure drop boundary condition to destabilize the oul of-phase mode over the in-phase mode. The four channel model showed a clear preference for rotating mode behavior under standalone TH conditions and for conditions with weak neutronic feedback. When neutronic feedback was strengthened, the side-to-side mode (stationary symmetry line) was favored instead. A physical explanation has been put forth to explain why the routing symmetry line behavior is preferred from a TH standpoint demonstrating that the system is most unstable when the variation in the total inlet flow rate is minimized, and that the rotating mode is the most successful in minimizing this variation as compared to the side-to-side case or any other oscillation pattern. An explanation for why the neutronic channel coupling appears to favor a stationary symmetry line rather than a routing symmetry line is left as a topic of future investigation.