Numerical study on reverse flow in U-tubes of UTSG under natural circulation

Abstract

The inverted U-tube steam generator (UTSG) is the key hub between primary side and secondary side in nuclear power plants. However, it is found that reverse flow occurs in UTSG under natural circulation condition, which has a negative effect on the reactor safety. Additionally, in barge-mounted nuclear power plants, the UTSG will incline due to the sea wave. The characteristics of heat and mass transfer in UTSG under the inclined condition will also differ from that on land-based equipment. It is meaningful to do research on reverse flow in U-tubes of UTSG in inclined under natural circulation condition. In this paper, a three-dimensional computational fluid dynamic (CFD) model is carried out to study the reverse flow in UTSG. Three inclined directions at different angles of UTSG are taken into consideration. Through some cases, the temperature profiles and mass flow rates in the primary side of the UTSG are calculated. The influence of secondary side temperature on the reverse flow is studied. The results are obtained that reverse flow most easily occurs in shorter tubes of vertical UTSG under natural circulation condition. As the temperature on the secondary side increases, the pressure drop between inlet and outlet of UTSG is negative, and its absolute value decreases, resulting in a decrease in the total number of reverse flow tubes. Under the inclined conditions, the inclined angle of UTSG has a great effect on the reverse flow. Along the inclined direction, reverse flow occurs more easily in the U-tubes with lower inlet cross-section locations. The results are validated by the estimate code Reactor Excursion and Leak Analysis Program 5 (RELAP5/MOD3.3).