Investigation on reverse flow phenomena in UTSGs under two-phase natural circulation with low steam quality

The reverse flow phenomenon is disadvantageous for the normal operation and the reactor safety, which can occur in some tubes of an inverted U-tube steam generator (UTSG) under both single and two-phase natural circulations (NCs). Despite the rich studies of reverse flow behavior under the single-phase NC, special and in-depth investigations concerning that under the two-phase NC are far from enough. Using the system analysis code RELAP5/MOD3.3, the reverse flow phenomenon in a UTSG under the two-phase NC with low steam quality is simulated and analyzed in this paper. The effects of secondary side temperature and inlet mass quality on this phenomenon are also studied. Based on the theoretical model for calculating the two-phase flow and heat transfer characteristics in the U-tubes, the mechanism explanations of the simulation results are given. The conclusions show that the reverse flow firstly occurs in the longest U-tube. The mass flow rates in the normal flow U-tubes will increase and the inlet temperatures of these U-tubes will decrease when the reverse flow appears. With the increase of secondary side temperature or the decrease of inlet mass quality, both the critical mass flow rate (CMFR) of steam generators (SGs) and the CMFR of U-tubes will decrease, thus the reverse flow can be effectively reduced. The reverse flow phenomenon under the two-phase NC will be more easily to occur compared with that under the single-phase NC.