Experimental and Theoretical Research on Heat Transfer Characteristics of the Scaled-Down C-type PRHR HX Model under Different Cooling Water Levels

Passive Residual Heat Removal heat exchanger (PRHR HX) is the key equipment of the passive residual heat removal system in AP1000 reactor. It is designed in special C-type heat exchange tube bundle and locates in the In-containment Refueling Water Storage Tank (IRWST). In some accidental events, the cooling water in the IRWST may leak, evaporate, or passively inject into the primary loop, leading to the drop of water level. The corresponding temperature, flow, and heat transfer effects may change under different water level conditions. In the present work, the heat transfer performance of PRHR HX model under different cooling water levels conditions are studied based on the modified scaled-down PRHR HX&IRWST experimental facility. The temperature distributions in both primary and secondary side distributions are obtained. Meanwhile, key parameters including the heat transfer quantity, heat flux, and heat transfer coefficient are analyzed in depth. The experimental results indicate that though the total heat transfer quantity decreases as the water level of the water tank drops, but due to the increase of average heat flux, the residual heat removal effect can also be maintained at a relatively high level. Furthermore, combined correlations under both nucleate boiling and natural convection are employed to depict the heat transfer mechanisms and predict the heat transfer coefficient of different PRHR HX tube bundle sections. Both the experimental and theoretical results provide important references for the operation and safe analysis of the prototype PRHR HX.