Effect of U-tube length on reverse flow in UTSG primary side under natural circulation

Highlights: ► A model is proposed for the analysis of reverse flow in U-tubes. ► The mechanism of reverse flow is studied by the linear micro-disturbance theory. ► The relations between U-tube length and characteristic parameters are derived. ► Reverse flow in UTSG may occur in shorter and/or longer tubes. - Abstract: For natural circulation, it is shown that parallel flow in the tubes of inverted U-tube steam generators may be non-uniform, and reverse flow occurs within some U-tubes. However the current researches on the mechanism and space distribution of reverse flow in the U-tubes gave different results. In present work, a model of one-dimension and steady state flow is proposed, which can be used to analyze the effect of U-tube length on reverse flow in UTSG primary side under natural circulation. According to the linear micro-disturbance theory, the mechanism of reverse flow is studied. The relationships between U-tube length and critical pressure drop and critical velocity are nonlinear, at which the flow instability happens. There exists a critical U-tube length which has the maximal absolute value of critical pressure drop. The results are validated by the best estimate code RELAP5/MOD3.3