Investigation on Single Phase Water Reverse Flow in Inverted U-Tubes of Steam Generator under Condition of Natural Circulation

Using the code RELAP5/MOD3.3 to model and calculate the natural circulation of single-phase flow for the PWR,this paper provides the distribution of forward flow and reverse flow in the inverted U-tubes of the steam generator（SG） under some typical operating conditions in the natural circulation case,and analyzes the cause,occurrence condition and judgment principle of reverse flow phenomenon.The calculation results show that the occurrence conditions for reverse flow phenomenon are that the steam generator outlet pressure is higher than the inlet pressure and gravitational pressure drop is lower than the total of frictional pressure drop and area change pressure drop.As to the nuclear power plant described in this paper,if the water level of steam generator keeps normal,the mass flux of the shorter U-tubes will drop more quickly and reverse flow will occur.The temperature distributes uniformly in U-tubes with reverse flow and it is almost identical with that of SG in secondary side.