Abstract: To study the two phase flow boiling phenomena under external reactor vessel cooling （ERVC）condition, the RELAP code was used to simulate the REPEC nonheating experiment. The simulation is mainly focused on the effect of air flow rate, injection distribution and inlet/outlet area on the natural circulation mass flow velocity and flow phenomena. A good agreement is achieved between the simulation results and experimental data. The natural circulation flow velocity increases with the increasing of air flow rate first and then decreases gradually. The mass flow velocity increases when enlarging the inlet/outlet areas. The flow velocity and flow stability are more sensitive to the outlet area.

Key words: external reactor vessel cooling(ERVC), large scale nuclear power plant, RELAP code, natural circulation