Effect of power on Ledinegg flow instability in parallel upflow channels

As part of the design support effort for the proposed heavy-water new production reactor (NPR-HWR), the Savannah River Site Heat Transfer Laboratory conducted natural circulation tests to simulate reactor decay power conditions. A thermal-hydraulic analysis shows that a Ledinegg (parallel channel) flow instability cannot occur at these conditions. The results of this analysis agree with the results of the natural circulation tests. The effect of flow rate and power on flow instability can be illustrated using a demand curve. A demand curve gives the pressure drop across a heated channel as a function of flow rate at a constant power, inlet temperature, and exit pressure. The shape of the demand curve for a heated flow channel strongly depends on power. At high power levels, such as for NPR-HWR normal and transient accident conditions, there is a local minimum pressure drop on the demand curve. This minimum occurs because the pressure drop across the heated channel increases when vapor is generated inside the channel. When this happens, flow is diverted to adjoining parallel channels. This will cause the surface of the affected channel to overheat and fail