A one-dimensional single-channel thermal-hydraulics model has been developed to investigate possible occurrence of density-wave instability in two U. S. Gen-IV reactors cooled by supercritical fluids, i. e., the Supercritical Water-cooled Reactor (SCWR) and Gas-cooled Fast Reactor (GFR). Water density in the SCWR core changes from 780 kg/m3, to 90 kg/m3, whereas the density of supercritical carbon-dioxide in the reference GFR changes from 155 kg/m3 to around 110 kg/m3. The standard frequency domain approach with a decay ratio of induced velocity amplitude of 0.5 has been used to determine the onset of flow instability. With suitable inlet orificing, the hot channel of SCWR has been found to be stable. Sensitivity studies show that the hot channel decay ratio reaches the critical value of 0.5 when either the reactor power is raised to 118% of full power or the core flow rate is reduced to 86% of nominal flow rate. System pressure has only a moderate effect. Detailed 3-D studies, preferably with neutronic feedback, should be carried out for the SCWR design because of its sensitivity to various important parameters. The GFR reference design has been found to be very stable since the density change in the GFR core is rather small compared to that in the SCWR design.

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