AbstractThe hydrodynamics of coolant flow in a natural circulation, nuclear-heated boiler are dependent upon interactions of the generated heat, the available driving head of vapor in the two-phase mixture, and flow of the coolant. Where at steady operating conditions a slight increase in heat generation will induce unstable flow, circulation hydrodynamics can be investigated by small-signal techniques of control system theory. The flow-pressure interaction can be described in terms of the hydraulic impedance which is the frequency-transformed ratio of two perturbed quantities, differential pressure over flow rate. The hydraulic impedance is analogous to acoustic impedance (acoustic pressure/particle velocity) of compressible media and to mechanical impedance (force applied to structure/resulting velocity) of rigid body mechanics. Measurements of the flow-vapor interaction and of the flow-pressure interaction (hydraulic impedance) are compared to a simplified theory, to demonstrate how the impedance approach aids understanding of complex two-phase phenomena. As a practical application, the flow stability of a boiling loop is predicted by measured hydraulic impedances.