Reactor core isolation cooling system (RCIC) is designed to operate using saturated steam extracted from a main steam line. On the other hand, RCIC in Unit-2 of Fukushima Daiichi Nuclear Power Plant was activated before the tsunami hit and continued to operate under two phase flow conditions during the accident. When RCIC continuously operates without control by electric power, reactor water level would reach the main steam line resulting in turbine efficiency degradation and subsequent RCIC stop. The mechanism for RCIC of Unit-2 to have continuously operated under such two phase flow conditions is still unclear. Currently, a project is progressing to understand the true operating limitations of RCIC system under beyond design basis event. The Institute of Applied Energy is developing the new RCIC system model for the SAMPSON code in the project. The present paper proposes a trial model for RCIC system under two phase flow conditions. The model takes the effect of quality and pressure on turbine efficiency into consideration. SAMPSON calculation based on the model could reproduce qualitatively RPV pressure behavior of Fukushima Daiichi Unit-2 accident. However there are many uncertainties, such as water temperature of S/P, isentropic efficiency, and so on, so the model will be improved based on experiment and CFD results planned in the project.