Experimental Study on CCI Phenomena Following High-pressure Water Injection into Liquid Lead-bismuth during SGTR Accident in Lead-cooled Fast Reactor

The lead-cooled fast reactor (LFR), as one of the generation Ⅳ reactor types, employs lead-bismuth eutectic (LBE) as a primary coolant. Investigation of the coolant-coolant interaction (CCI) phenomenon during a steam generator tube rupture (SGTR) accident is of great significance for the structural integrity of the reactor pressure vessel. During such an accident scenario, the sudden ingress of high-pressure water into heavy liquid metal may generate rapid phase transition, intense thermal gradients, and pressure transients, potentially imposing complex dynamic loads on surrounding structures. A clear understanding of these coupled thermal-hydraulic processes is therefore essential for reliable safety evaluation of LFR systems. Currently, there is a lack of systematic research on the multiphase, multicomponent CCI behavior involving liquid metal, water, and steam under high-pressure water injection during SGTR accidents. Therefore, this study independently designed and constructed a water-jet-liquid-metal CCI experimental facility upon which systematic experimental investigations were conducted. The experimental setup mainly consists of four subsystems: a water pressurization and pre-heating system, an LBE heating and delivery system, a reaction system, and a pressure-relief and purification system. The reaction vessel was designed for 10 MPa and 500 ℃, and was equipped with multiple thermocouples and pressure transducers. High-pressure subcooled water was injected into the LBE pool from a fast-response solenoid valve (response time < 20 ms) through a nozzle with a 6 mm inner diameter, the tip of which was submerged 30 mm below the free surface. The experiments reveal transient intense fluctuations in the temperature field during jet penetration, clarifying the migration path of steam/water and the local heat-transfer mechanisms. It is found that a significant temperature drop occurs in the region near the nozzle. Furthermore, a higher initial temperature of the liquid LBE results in a larger temperature difference upon injection. This larger driving difference in turn leads to temperature oscillations with greater amplitude and faster frequency. Two types of gas pressurization are observed: One is a localized steam-explosion type with a sharp pressure peak, presumably caused by rapid phase change due to vapor-film instability within a small region; The other is a relatively smooth two-stage pressurization type. It consists of two distinct stages: a rapid pressurization stage, dominated by jet injection; followed by a slow pressurization stage, resulting from the continuous evaporation of residual water. Furthermore, the pressure variations measured inside the LBE pool and in the overlying gas space exhibit a high degree of synchrony. This study provides key experimental data and mechanistic understanding for the safety analysis of SGTR accidents in lead-cooled fast reactors, offering valuable reference for enhancing reactor safety design.