Numerical Study on Cold Plume Behavior in the RPV of a Small Mobile Reactor During Safety Injection

Following a loss-of-coolant accident, the reactor safety injection system is activated, and a large amount of coolant is injected into the reactor pressure vessel (RPV). This induces cold plume phenomena and temperature nonuniformity inside the vessel, which may threaten the structural integrity of the RPV. Transient numerical simulations are performed to investigate the complex cold plumes that arise inside the pressure vessel of a small mobile reactor under safety-injection conditions. By evaluating the flow-field evolution under different injection paths and flow rates, and by innovatively adapting classical plume entrainment theory to define an equivalent coefficient for the nuclear engineering context, this study systematically analyzes the formation and development of the cold plumes and their entrainment–mixing mechanisms. The results indicate that, in such compact RPVs, the entrainment and mixing intensity of the cold plumes is relatively weak, resulting in ineffective thermal mixing during the development stage. Although increasing the injection flow rate enhances heat transfer and reduces thermal gradients, the improvement exhibits diminishing returns. A comparison of different injection paths reveals that a dual-line injection scheme leverages plume–plume interaction to effectively strengthen radial mixing, accelerate temperature-field homogenization, and enlarge the wall-cooling region, thereby alleviating local pressurized thermal shock (PTS).