Reactor experiments on type-I and type-ll BWR stability

Investigations on Type-I (hydrostatic head) and Type-11 (density wave) BWR stability characteristics are reported. Experimental data are obtained from the Dodewaard natural-circulation BWR at conditions far away from nominal operating points. The Type-I stability boundary was reached, the Type-II stability boundary was crossed. At this latter unstable condition the reactor power showed peculiar oscillation modulations. Experimentally obtained decay ratios and resonance frequencies are put into context using a physical model. Flashing is shown to be an important phenomenon for driving the coolant flow at start-up conditions and for Type-1 stability features. From tow-pressure experiments, it appears that the decay ratio is an unsale measure for Type-11 stability close to the stability boundary.