Effect of condenser tube inclination on the flow dynamics and instabilities in a passive containment cooling system (PCCS) for nuclear safety

The release of steam into the containment of a nuclear plant during accidents (e.g. Fukushima) impose tremendous thermal and pressure load on the structure. Passive Containment Cooling Systems (PCCS) are heat exchangers designed to remove and transfer heat using natural forces (e.g. buoyancy, gravity) from the containment to protect its structural integrity. Depending on the design, the heat sink is usually a water pool or ambient air circulating around the steel containment structure. The presented work deals with the external condenser PCCS design where steam condenses on the external surface of a condense tubes in the containment. The heat sink is an overhead water pool and flow circulation occurs via the downcomer and riser tubes connected to either side of the condenser tube. The first part of this work provides an overview of the existing literature on external condenser PCCS highlighting a major shortcoming – the influence of the condenser tube inclination on the resulting flow dynamics within the downcomer and riser tubes has not yet been investigated. The second part of this work discusses the INTRAVIT (Investigation of passive heat Transfer in a Variably Inclined Tube) test facility setup at the University of Luxembourg to address the aforementioned shortcoming. The condenser tube inclination in the INTRAVIT facility could be adjusted between 0° (horizontal) and 90° (vertical) making it possible to quantitatively assess the different flow phenomena (e.g. mass flow excursions, boiling, flashing, geysering etc.) taking place in the downcomer, condenser and riser tubes at different inclinations. An important feature of the facility is the easy adjustment of pool temperature to perform tests at different starting temperatures. In the final part of this work, results from 6 tests conducted using a combination of two different pool temperatures (50 °C and 85 °C) and three condenser tube inclinations (5°, 60°, and 90°) are discussed. Temperature, local void fraction, and flow rate data from thermocouples, needle probes, and mass flow sensors provide a detailed description of the stable single phase and highly unstable two-phase flow behavior observed during the tests.