Abstract: Passive residual heat removal heat exchanger (PRHR HX) is one of the key equipments of a passive core residual heat removal system of advanced nuclear reactors, which is placed in a in-containment refueling water storage tank (IRWST). The water level of the tank decreases due to the water evaporation under the accident condition or the water leakage by accident, and then the heat transfer area of PRHR HX and water outside the tubes and the external flow field of the tubes change, resulting in the decrease of the passive residual heat removal ability of the core. The experimental device and numerical model of half water level PRHR HX were established to analysis the heat transfer performance of PRHR HX in half water level condition. And the heat transfer rate, temperature distribution and flow field characteristics were obtained both in transient and steady states. The experimental results show that the heat transfer rate is reduced only 12%-22% from that in a full tank to that in half water level tank, the passive residual heat removal system still has higher heat transfer capacity. During the transient heating process, the heat transfer mode outside the tube starts from the pure single phase convective flow heat transfer, gradually appears local subcooled boiling and local saturated boiling. Finally, the heat transfer mode rapidly changes from local to the whole tube length saturated boiling. Moreover, a significant thermal stratification phenomenon still exists in the process of gradually heating up in the water tank. The local flow circulation at different regions with different heights is found in numerical simulation results.