A model of Passive Residual Heat Removal Heat Exchanger (PRHR HX) and In Containment Refueling Water Storage Tank (IRWST) in AP1000 had been scaled by power-volume theory. FLUENT code analysis for the characteristic of thermal stratification phenomenon and natural convective of the scaled model was performed for 2000s of transients to investigate the temperature field distributions and flow field distributions in IRWST. As for the numerical experiment, the distributions had been achieved at different times. The centralized layout of PRHR HX, which looks like a circle in a section, makes the temperature of fluid outside tubes higher at the location that closer to the upper of the circle in the horizontal tube section and closer to the center of the circle in the vertical tube section, and there may appear nuclear boiling. Then temperature difference between the tube side and fluid side is smaller, causing the heat exchanger efficiency is worse at these locations. In order to improve the heat transfer effect of PRHR HX, two schemes had been proposed. First, increase horizontal section length and shorten the vertical section length. Second, Flat layout replaces centralized layout. These models were established and simulated by FLUENT. Heat transfer coefficients of "C-tubes" were comparative analyzed to study which kind of the heat transfer effect was better. It contributes to provide some references for improving the heat transfer effect of PRHR HX.
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