•Pool-boiling mechanism and HTCs for the specific C-shape PRHR HX are investigated.•A revised heat transfer correlation with q″ and H/D for vertical bundle is proposed.•Typical pool-boiling correlation is valid to predict the HTCs for lower horizontal bundle.•Forced convective flow boiling correlations are assessed for upper horizontal bundle. In advanced third generation reactor AP1000, the C-shape Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) is the key equipment in the passive safety system. The mutual influences of the C-shape bundle and the irregular IRWST lead to the complicated heat transfer characteristics. It is still unclear which heat transfer correlations are appropriate for the vertical and horizontal bundle sections, respectively. In the present work, an overall scaled-down IRWST and PRHR HX experimental bench has been built up to simulate the pool boiling performance of the C-shape PRHR HX bundle. The key thermal parameters are measured to evaluate the heat transfer mechanisms and heat transfer coefficients (HTCs) for different bundle sections. The traditional and the revised correlations are further compared with the experimental data of the C-shape PRHR HX bundle. The validated correlations provide applicable and effective references for the reliable operation and system analysis of the passive safety system.