Abstract: The flow structure in plenum of steam generator has an important influence on the reverse flow phenomenon, and the inlet and outlet nozzle arrangements can change the flow structure in the plenum. In this paper, the three-dimensional models of steam generators with different nozzle arrangements were established. The flow structure and heat transfer in those steam generators were simulated under low flow rate condition. The effects of nozzle arrangements on reverse flow phenomenon were analyzed. The results show that the nozzle arrangement has significant influence on velocity field in plenum, and then affects the pressure drop and flow distribution of U-tube. If the nozzle arrangement is changed properly, the flow distribution will be more uniform and the negative pressure drop will increase, thus the critical flow rate of reverse flow and the reverse flow rate will decrease.