Multidimensional ECC behaviors of air-water flow in a downcomer annulus with direct vessel injection mode during the LBLOCA reflood phase

The direct vessel injection (DVI) technology has been increasingly adopted in new pressurized water reactors due to its advantages in simplifying the safety injection system design and enhancing economic benefits. This study focuses on the modified DVI safety injection system of the HPR1000. An improved linear scaling method was employed to model the prototype, and relevant experiments were conducted on a 1:8.5 scale visualization test section. During the refilling and reflooding stages of a large break loss of coolant accident (LBLOCA), phenomena such as counter current flow limitation (CCFL) and bypass flow near the break were observed within the annular cavity. The research investigated the impacts of various break locations and the presence or absence of guiding structures on the bypass effect in the DVI safety injection system. Additionally, comparisons were made between DVI safety injection and cold - leg injection. The findings reveal that direct bypass flow dominates the safety injection bypass during the refilling and reflooding stages of an LBLOCA. The closer the cold - leg break location is to the DVI nozzle, the more significant the increase in bypass flow at the break location. The installation of guiding devices can effectively reduce the proportion of safety injection bypass flow. The data results of this study provide crucial insights for the optimization and innovation of the modified safety injection system in HPR1000.