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

Investigations on Type-I (hydrostatic head) and Type-II (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-I stability features. From low-pressure experiments, it appears that the decay ratio is an unsafe measure for Type-II stability close to the stability boundary.