Investigation of local BWR instabilities with a four heated-channel Reduced Order Model

Abstract

This paper deals with the modeling of Boiling Water Reactor (BWR) local instabilities via so-called Reduced Order Models (ROMs). More specifically, a four-heated channels ROM, which was earlier developed (Dykin et al., submitted for publication), was modified in such a way that the effect of local perturbations could also be accounted for.This model was thereafter used to analyze a local instability event that took place at the Swedish Forsmark-1 BWR in 1996/1997. Such a local instability was driven by unseated fuel assemblies. Comparisons between the results of ROM simulations and actual measurement data demonstrated that the developed ROM was able to correctly reproduce the main features of the event. The ROM has also the ability to give some further physical insights into the phenomena taking place in case of instabilities. For the particular instability event investigated, it was for instance demonstrated that the global and regional oscillation modes were stable, but were excited by the local oscillation acting as an external perturbation. When performing a modal decomposition of the measured neutron flux in case of an instability event driven by a local oscillation, each mode will apparently be excited, whereas in reality such modes might be stable. Such an apparent contradictory behavior is due to the inability of a modal decomposition to catch with only a few modes the spatial dependence of the neutron flux in case of a local oscillation.