Nuclear Technology / Volume 175 / Number 3 / September 2011 / Pages 529-537
Technical Paper / NURETH-13 Special / Thermal Hydraulics
An experimental study on flooding in a large-diameter tube is being conducted. In a countercurrent, two-phase-flow system, flooding can be defined as the onset of flow reversal of the liquid component that results in cocurrent flow. Flooding can be perceived as a limit to two-phase countercurrent flow, meaning that pairs of liquid and gas flow rates exist that define the envelope for stable countercurrent flow for a given system. Flooding in the AP600 pressurizer surge line can affect the vessel refill rate following a small-break loss-of-coolant accident. Analysis of hypothetical severe accidents with current simplified flooding models shows that these models represent the largest uncertainty in steam generator tube creep rupture. During a hypothetical station blackout scenario without auxiliary feedwater recovery, should the hot leg become voided, the pressurizer liquid will drain to the hot leg, and flooding may occur in the surge line. Experiments were conducted using a 76.2-mm (3-in.)-diam tube along with subcooled water and superheated steam as the working fluids at atmospheric pressure. Water flows down the inside of the tube as an annulus while the steam flows upward in the middle. Water flow rates vary from 0.00022 to 0.00076 m3 /s (3.5 to 12 gal/min), and the water inlet temperature is [approximately]70°C. The steam inlet temperature is [approximately]110°C. It was found that a larger steam flow rate was needed to achieve flooding for a lower water flow rate and for a higher water flow rate. These unique data for flooding in steam-water systems in large-diameter tubes will reduce uncertainty in flooding models currently utilized in reactor safety codes.