Abstract: Passive residual heat removal is critical for nuclear reactor inherent safety after shutdown. Open natural circulation (ONC) systems show great potential in this field due to their simple structure and adaptability to marine heat sinks, but low driving force and low-pressure operation make them prone to two-phase flow instability caused by violent boiling and direct contact condensation. Traditional heat exchangers fail to achieve collaborative optimization of efficient heat transfer and stable operation. This study proposes a novel coupling scheme replacing conventional shell-and-tube heat exchangers with stainless steel-water heat pipe heat exchangers, aiming to achieve complete physical separation between hot- and cold-sides, reduce cold-side wall temperature, and fundamentally suppress flow oscillation while maintaining high heat transfer efficiency. An experimental platform was constructed comprising a heat source system, a heat pipe heat exchanger test section, a cold source system, and a measurement system. The test section contained 19 stainless steel-water heat pipes arranged in a triangular pitch, each with 500 mm evaporation and condensation lengths. The cold source system included an elevated cooling water tank simulating atmospheric-pressure ocean boundary conditions. Flow pattern visualization was performed via high-speed camera at 1 000 fps. Experiments were conducted using a stepwise heating method at three hot-side inlet flow rates (1.1, 1.6, and 2.0 t/h) and six temperature levels (140 to 290 ℃). CFD numerical simulation was performed using Ansys Fluent with a Realizable k-ε turbulence model, and a thermal resistance network model was established to compare cold-side wall temperatures between heat pipe and shell-and-tube heat exchangers. The coupled system rapidly establishes stable open natural circulation without any temperature or flow oscillation throughout the entire operating range. The system exhibits a sequential startup characteristic: hot-side heating first, followed by heat pipe phase-change heat transfer, and finally natural circulation buildup with increasing density difference. At a hot-side inlet temperature of 290 ℃ and flow rate of 1.56 t/h, the system achieves a maximum heat transfer capacity of 114 kW, corresponding to an average heat flux of 152.8 kW/m² and a cold-side natural circulation flow rate of 1.45 t/h. Both heat transfer capacity and natural circulation flow rate increase monotonically with rising hot-side inlet temperature and flow rate. A notable inflection point appears above 230 ℃ at the 1.1 t/h flow rate, where the power increase rate accelerates due to flow pattern transition. Visualization studies reveal four stable flow patterns in the outlet pipe: single-phase flow, intermittent bubbly flow, emulsion-like flow, and dispersed bubbly flow. No unstable flow patterns such as slug flow or intermittent jet flow are observed. Thermal resistance analysis demonstrates that under the 114 kW condition, the cold-side wall average temperature of the heat pipe heat exchanger is 38.9 ℃ lower than that of an equivalent shell-and-tube unit. The hot-cold separation characteristic of heat pipes physically isolates high-temperature hot fluid from low-pressure cold fluid, eliminating the direct contact that causes violent flash evaporation. The phase-change heat transfer mechanism enables highly efficient heat transfer with only a small temperature difference between the cold-side wall and working fluid, fundamentally suppressing the root cause of flow instability. This coupling scheme achieves the collaborative optimization of high-efficiency heat transfer and stable operation in low-pressure ONC systems, providing important experimental data and theoretical guidance for advanced passive residual heat removal system design.