An experimental study has been performed to investigate thermal-hydraulic phenomena in the downcomer during LBLOCA with Direct Vessel Injection (DVI), which is a new Safety Injection System (SIS) of the Advanced Power Reactor 1400 MW (APR1400). In order to understand the downcomer boiling phenomena experimentally, the KAIST Downcomer Boiling experiment has been performed with the same as the thickness of the reactor vessel of APR1400 (21 cm) and a height of 150 cm to observe downcomer boiling phenomena and to measure local parameters such as local phase velocities, local void fraction, collapsed water level and heat flux from the heated wall. From the KAIST Downcomer Boiling experiment, we visually observed strong liquid recirculation and vapor jetting near the heated wall due to the axial migration of voids only in the thin layer (about 4 cm) of the heated wall but little bubble migration out of the bubble region. Local phase velocities and local void fraction were measured to estimate the drift-flux parameters in the downcomer channel. Heat flux from the heated side was back-calculated to find the CHF and to estimate nucleate boiling heat transfer coefficients.

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