The RELAP5 code is one of the extensively used thermal hydraulic codes in nuclear reactor safety analysis. However, there exists some controversy on its capability under low pressure natural circulation. To verify its ability to predict the transient behavior of natural circulation under low pressure conditions, the RELAP5/MOD3.2 code is applied for the simulation of a PCS principle test facility (PCS-PTF) of ACP1000 in China. The coolants in PCS-PTF are heated by the condensation process with steam or steam-air-He mixture gas outside the heating section and then get cooled in the upper water tank. In the present study, experiments were carried out under different mixture gas pressure including 0.2 MPa, 0.3 MPa and 0.6 MPa. Calculated results predicted by RELAP5/MOD3.2 are compared with experimental data both in single and two-phase flow conditions. The results indicate that the RELAP5/MOD3.2 code could provide reasonable simulation on main phenomenon occurred in system. For the case of condensation with pure steam, the phenomena of single-phase flow and flashing induced two-phase flow could be well predicted. For the case of condensation with the presence of non-condensable gas, the geyser oscillation frequency from calculation shows good agreement with experimental results, while the mean flow rate in Conditions 3 and 4 are lower than that of experiment by 55.9% and 33.2%, respectively, which are resulted from the underestimate of outer-tube condensation heat transfer coefficient predicted using Colburn-Hougen model in RELAP5 code.
