In AP1000 plant, the automatic depressurization system (ADS) works to discharge the high-temperature and high-pressure steam from the Reactor Coolant System (RCS) primary side to the In-containment Refueling Water Storage Tank (IRWST) in the LOCA conditions. In particular, for the AP1000 plant, both the IRWST and ADS spargers are specially designed, and the ADS spargers are located in one corner of the IRWST. All the factors lead to the special and complicated thermal and flow behavior in the IRWST, which in turn have great influences on the condensation effects of the ejected steam. In the present work, an overall scaled-down IRWST and ADS sparger models are built to study the condensation and mixing phenomena in the accidental depressurization events in AP1000. Thermocouples matrix with more than 200 T-type sheathed thermocouples are utilized to measure the three dimensional temperature in the large tank. The Particle Image Velocimetry (PIV) is employed for the measurement of the natural convection flow velocity. Based on the experimental data, the local spraying flow patterns, flow behavior, and thermal stratification phenomena in IRWST etc. are analyzed. The results indicate that the spraying steam condensation flow patterns are closely related to the subcooling degree in the IRWST. In addition, the stratification number is developed to evaluate the thermal stratification extent in the IRWST, which indicates that only part of the fluid are used efficiently for condensing the spraying steam directly.

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