The occurrence of Steam-Water Co-Current Flow (SWCOCF) in the AP1000 Pressurizer Surge Line (PZR SL) is an important phenomenon during the depressurization stage of the Small Break Loss-of-Coolant Accidents (SB-LOCAs), and the predictions of SWCOCF pressure drop in PZR SL are important issues in the AP1000 reactor safety analyses. To investigate the pressure drop characteristics of SWCOCF in the PZR SL assembly of AP1000, a PZR SL test model made of stainless steel is designed with scaling ratio of 1:9.15, and a test loop is also established to supply SWCOCF conditions for the experiments. The test section is mainly composed of a long inclined tube with a short acrylic visual section, a whole surge line tube, and a vertical section. The SWCOCF pressure drop experiments are conducted with pressure ranging from 0.1MPa to 0.24MPa and vapor mass fraction from 0.02 to 0.5. The research results show that: the whole surge line two-phase friction multiplier increases with the increasing of steam fraction, and decreases with the increasing of pressure. A prediction model of two-phase friction multiplier for the PZR SL is developed based on the Martinelli-Nelson method, which can predict the tests data very well.

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