Investigation of rotating mode behavior in BWR out-of-phase limit cycle oscillations – Part 1: Reduced order model. (December 2018)

• Record Type: Journal Article
• Authors: Wysocki, A.; Manera, A.; Downar, T.; March-Leuba, J.
• Is Part Of: Annals of nuclear energy. Volume 122(2018)
• Page Start: 393
• Page End: 407
• Publication Date: 2018-12
• Subjects: BWR stability -- Limit cycle -- Rotating mode -- Out-of-phase oscillations -- Reduced-order model; Nuclear energy -- Periodicals; Nuclear engineering -- Periodicals; 621.4805
• DOI: 10.1016/j.anucene.2018.08.032
• Languages: English
• ISSNs: 0306-4549

Abstract

Highlights: Out-of-phase oscillations were simulated in a 4-channel reduced-order model. A novel approach was used to model thermal hydraulic channel coupling in the plena. With little or no neutronic feedback, a rotating mode limit cycle was observed. With strong neutronic feedback, a side-to-side limit cycle behavior was observed. Abstract: Previous 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, i.e. constant phase shift of approximately 90° between the first two azimuthal modes) in out-of-phase limit cycle oscillations, regardless of initial conditions and even if the first two azimuthal modes have different natural frequencies. This suggests a nonlinear coupling between these modes; otherwise, the phase shift between these modes would change at a constant rate during the limit cycle. The goal of the present work is to gain further insights on the rotating mode behavior using a simplified mathematical model which contains all of the 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 stand-alone 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. Additional analyses using higher-fidelity numerical modeling, as well as a physical explanation for the rotating behavior seen in both sets of analyses, will be provided in a companion paper ("Part 2").