Stability of single-phase natural-circulation flows in steam generator U tubes

The stability of single-phase natural-circulation flows in steam generator U tubes is studied analytically. Momentum and energy equations are solved simultaneously for U tubes with different length, and the stability condition is obtained for each U tube. Stability parameters are evaluated for the natural circulation tests conducted with the Large Scale Test Facility. It is shown that the stability parameters are in the stable region for the test with normal flows, while in the unstable region for the test with normal and reverse flows. The normal flows in shorter U tubes are found to become stable if the flows in several longer U tubes are stalled under the unstable condition.