Abstract: Thermal-hydraulics characteristics of AP1000 passive residual heat removal heat exchanger (PRHR-HX) at initial operating stage were analyzed based on the porous media model. The data obtained from RELAP5 under the condition of the station blackout was employed as the inlet flow rate and temperature boundary of CFD calculation. The distribution resistances were added into the C-type tube bundle region which was treated as porous zone. Three-dimensional distributions of velocity and temperature in the in-containment refueling water storage tank (IRWST) were calculated by the commercial CFD code FLUENT. The heat transfer between the primary loop side and the IRWST side was calculated by user self-defined function UDF. The results show that the heat transfer capability decreases with the increase of water temperature and a thermal stratification phenomenon in the tank occurs. The present results indicate that the method containing coupled heat transfer from the primary loop side fluid to IRWST side fluid and porous media model is a suitable approach to study the transient thermal-hydraulics of PRHR/IRWST system.