Analytical studies of the effects of power on flow instability in parallel channels with upward flow of coolant have predicted that the Ledinegg flow instability, encountered as flow is decreased for typical operating power levels, would not be experienced at low-power levels. For a system in which the flow of coolant is upward, the increased buoyancy enhances flow in the channel, so that as the void increases, the overall pressure loss decreases. Under this condition, flow instability does not occur. Testing was performed to confirm the predicted behavior and to provide data for benchmarking of computer codes used for predicting the performance of reactor fuel elements. The demand curves traced in these tests are part of the multidimensional demand surface for the test apparatus. The basic coordinates of this surface are flow rate, pressure drop, and power. A fourth significant independent variable is system pressure, so that the behavior of the system is represented by a family of Δp-flow-power surfaces for each pressure level. This testing confirmed that, at low power levels comparable to decay heat removal power, the buoyancy effects may become dominant so that the demand curve for the fuel assembly turns downward and flow instability will not occur.
