APPLICATION OF MODAL NEUTRON KINETICS TO BOILING WATER-REACTOR OSCILLATION PROBLEMS

A modal neutron kinetics method was developed and applied to new probl ems with boiling water reactor oscillations. The modal method is uniqu ely suited for such problems because the oscillation components, in-ph ase and out-of-phase, correspond directly to separate expansion functi ons. One problem is understanding the origin and predicting the magnit ude of the in-phase component that is always present during out-of-pha se power oscillations. Another exercise of the method was the calculat ion of the relative critical power ratio (CPR) response to in-phase an d out-of-phase oscillations, known as the DI-VOM curve, using a fast s ingle hydraulic channel model. The new calculations confirm the BWR ow ners group results and similar calculations using the full three-dimen sional neutronics and multichannel models of the RAMONA-3 code. In add ition, the origin of the large difference between the in-phase and out -of-phase CPR responses could be explained Modal analysis of the react ivity 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.