A direct vessel vertical injection (DVVI) system, which is an improved variation of the current direct vessel injection (DVI) system for the APR1400 (Advanced Power Reactor 1400 MWe), is being developed by PHILOSOPHIA, Inc. and the Seoul National University in collaboration with the Korea Atomic Energy Research Institute (KAERI). The DVVI aims to sizably enhance penetration of the emergency core coolant (ECC) into the downcomer. This industry-academia-laboratory trilateral project is being carried out at the THETA (Transient Hydrodynamic Engineering Test Apparatus) apparatus. Once the engineering design is verified and validated, the system is slated to be employed in the more advanced reactor D2R2 (Digital R&D Reactor) to cope with loss-of-coolant accidents (LOCAs). While the ECC injection through the DVI system is known to cause multidimensional thermal hydraulic phenomena such as the impingement and breakup in the reactor downcomer, the DVVI injection system can deliver more ECC into the lower plenum and the core because of the increased downward momentum. CFX, which is a multidimensional fluid dynamics code, was used to analyze the flow pattern for the water from the DVVI versus DVI nozzles and the air from the cold legs. Results indicated that the two-phase flow distributions and downward velocities between the DVVI and DVI systems were considerably different and that the ECC penetration path into the core was clearer in the DVVI system. Experiments were performed in the 1:10 scaled-down test apparatus to measure the water-air two-phase direct bypass ratio for the DVVI system, which was less than that for DVI. The direct bypass was visualized and measured. Once the nozzle manufacturing and assembly procedures are engineered, results of this work may well contribute to improving the ECC penetration capability in the advanced reactors like D2R2.

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