Abstract: Small break loss of coolant accidents (SBLOCAs) in the cold leg represent fundamental design-basis scenarios. They remain pivotal for the safety assessment and licensing of generation Ⅲ and advanced nuclear power plants. The behavior of the primary system under such accidents dictates the efficacy of passive mitigation strategies. Therefore, this research aims to elucidate the complex thermal-hydraulic interactions within the HPR1000 reactor design. It seeks to validate the operational robustness of passive safety systems against a spectrum of SBLOCAs. To achieve these objectives, a systematic experimental campaign was designed and executed. Researchers at the Nuclear Power Institute of China designed and constructed a scaled, full-height integral test facility. This facility accurately modeled the passive safety systems of the HPR1000 reactor. A comprehensive matrix of experiments was subsequently conducted. These simulations replicated cold leg SBLOCAs with varying orifice diameters. High-resolution instrumentation captured transient parameters throughout the short-term injection phase. The experimental data reveal distinct behavioral patterns. The primary system response follows a four-stage evolution. Initially, the blowdown stage dominates. Rapid coolant discharge occurs due to critical flow. This causes an immediate pressure plunge. Subsequently, the natural circulation stage ensues. Here, the primary pressure stabilizes, forming a plateau. Later, the automatic depressurize system (RPPS) depressurization stage activates. RPPS action accelerates the injection of accumulator (ACC). This effectively arrests the vessel liquid level decline. The minimum level consistently appears in this interval. Finally, the refueling water storage tank (RWST) injection stage commences. A steady recovery of the vessel inventory is observed. The analysis further clarifies scaling effects. As the break size increases from 2 cm to 17.2 cm, the depressurization rate accelerates sharply. The overall transient duration contracts significantly, dropping from 5446 s to 1863 s. Passive injection initiation occurs much faster, reducing from 685 s to merely 56 s. Furthermore, the high-pressure makeup (HPMT) transition time decreases drastically from 2 913 s to 110 s. The distinction between water-injection and steam-injection phases becomes increasingly blurred. Concurrently, the duration of the passive residual heat removal system with a flow rate above 0.6 t/h shortens by 2 847 s. In conclusion, the passive safety systems demonstrate exceptional reliability. It effectively mitigates consequences for breaks up to 17.2 cm. The system ensures core coverage and maintains long-term cooling safety. These findings confirm that the passive safety system provides a robust capability against SBLOCA events. As the largest and most functionally comprehensive integral test facility in China, this apparatus continues to serve as a critical platform for conducting integral effect tests related to advanced nuclear reactors. It further explores the potential of passive safety systems in mitigating accident consequences. The valuable data obtained will also provide important support for the verification of independently developed nuclear power software.