Predictive BWR core stability using feedback reactivity coefficients projected on neutronic eigenmodes

The determination of the stability properties of Boiling Water Reactors usually rely on performing many time-dependent calculations for various combinations of values for the core power and the core flow. The aim of such calculations is to estimate the variation of the Decay Ratio in the core power/flow operating map, from which possible exclusion areas are defined. This paper demonstrates using a Reduced Order Model that the stability properties of a core with respect to global and regional oscillations are entirely determined by the projection of the feedback reactivity coefficients onto pairs of neutronic eigenmodes and their adjoint functions. This means that such projections inherently contain all information about the stability properties and their examination is sufficient to characterize the stability of a core. Most notably, the relative contributions of each fuel assembly to the core-wise projections give an indication to the core designer about the fuel assemblies possibly destabilizing the core. The core designer could thereafter improve core stability by either moving such assemblies to other locations or use another fuel assembly design. Although the method could be used independently of detailed stability calculations, the approach detailed in this study provides a more qualitative than quantitative core stability evaluation. This means that the method is most efficient if the stability features of a reference core are known.