Abstract: The separate-effect overall scaled-down passive residual heat removal heat exchanger (PRHR HX) and in-containment refueling water storage tank (IRWST) experiment was conducted to investigate the secondary side heat transfer characteristics of the scaled C-shape bundle used in PRHR HX in both the single-phase natural convection stage and the two-phase boiling heat transfer stage. The experimental data were used to validate the traditional heat transfer correlations predicting the heat transfer effects of PRHR HX. The experimental results indicate that the thermal stratification phenomenon is very obvious in the heat removal process. For the vertical bundle of PRHR HX, the traditional correlations tend to be conservative in both the single-phase and two-phase heat transfer processes. In the aspect of the lower horizontal bundle, the Churchill correlation is recommended in natural convection stage, and the Rohsenow pool boiling correlation is appropriate in the boiling heat transfer stage. However, for the upper horizontal bundle, because of the additional influences of the buoyancy effects and agitation of the fluid and bubbles, the heat transfer effects are better than those of the lower horizontal bundle.