Investigation of Limit Cycle Behavior in BWRs with Time-Domain Analysis.

Abstract

This thesis investigates the use of time-domain codes for boiling water reactor (BWR) stability analysis, with emphasis on out-of-phase limit cycle behavior. A detailed validation of the TRACE/PARCS coupled thermal hydraulic (TH)/neutronic code system was performed for both in- and out-of-phase instabilities using operating BWR data. Additional studies under hypothetical operating conditions indicated the possibility of a “rotating mode” limit cycle behavior, in which the line of symmetry exhibited a steadily-rotating behavior. This occurred even when the first two azimuthal neutronic modes had different (linear) natural frequencies, indicating that a nonlinear coupling mechanism was causing the steady rotation over time. The principal original contribution of this thesis is the characterization of this rotating mode behavior, prediction of the conditions under which it is expected to occur, and an explanation for this behavior based on physical principles governing BWR dynamics. This was achieved through the use of two simplified models: a four-channel TRACE model with a fixed total flow rate, and a multi-channel, multi-modal reduced-order model. Attention was given to the TH boundary conditions used for these models, which were found to play a critical role in determining the in- or out-of-phase behavior as well as the behavior of the out-of-phase limit cycle line of symmetry. For all standalone TH cases performed, a preference for rotating behavior was observed; however, for coupled TH/neutronic cases, it was found that strengthening the TH coupling between channels favored the rotating mode, while strengthening the neutronic coupling between channels favored the side-to-side mode with a stationary symmetry line. A physical explanation was put forth to explain why the rotating symmetry line behavior is preferred from a thermal hydraulic standpoint. This explanation examines the time-dependent variation in total flow rate for general (nonlinear) oscillations, and demonstrates that (1) this variation is typically minimized under a rotating mode pattern and (2) this yields the most unstable configuration for out-of-phase unstable cases. Additionally, it was found that larger-amplitude limit cycles converged to the rotating behavior more quickly than smaller-amplitude limit cycles under similar conditions.