Application of modal neutron kinetics to boiling water reactor oscillation problems

A modal neutron kinetics method was developed and applied to new problems with boiling water reactor oscillations. The modal method is uniquely suited for such problems because the oscillation components, in-phase and out-of-phase, correspond directly to separate expansion functions. One problem is understanding the origin and predicting the magnitude of the in-phase component that is always present during out-of-phase power oscillations. Another exercise of the method was the calculation of the relative critical power ratio (CPR) response to in-phase and out-of-phase oscillations, known as the DI-VOM curve, using a fast single hydraulic channel model. The new calculations confirm the BWR owners group results and similar calculations using the full three-dimensional neutronics and multichannel models of the RAMONA-3 code. In addition, the origin of the large difference between the in-phase and out-of-phase CPR responses could be explained Modal analysis of the reactivity biases associated with oscillating reactivity insertions for the two known modes could explain the out-of-phase mode higher propensity to growth compared with the in-phase mode of oscillation.