Experimental investigation on hydrodynamics and mass transfer of a novel fission gas removal equipment for molten salt reactor

Highlights: • A novel design of tubular fission gas removal equipment with a serpentine pipe as the gas-liquid reactor. • The two-phase hydrodynamics in the vertical serpentine tube. • The gas removal performance of the novel device under various operating conditions. • A dimensionless correlation of modified Sherwood number. The separation of fission gases like 135-Xe and 85-Ke from the liquid nuclear fuel is a focused topic in the molten salt reactor (MSR) community. In this paper, a compact fission gas removal equipment with a serpentine tube as the gas-liquid contactor is introduced. The novel design is free of any moving or rotating components, bringing great convenience in the real-time fuel treatment. As crucial factors influencing the gas removal performance, the two-phase hydrodynamics and interfacial mass transfer characteristics in the vertical serpentine tube are investigated under a water-nitrogen-oxygen system. Both highspeed photographing and conductivity probe are employed to capture the evolution of bubbly flow inside the tubular contactor. The oxygen concentration differences between the inlet and outlet of the device are measured under various operating conditions, and the overall volumetric mass transfer coefficient $k_{l}a$ is derived to evaluate the gas removal efficiency. Experiment results imply a periodic transition of volume fraction profile between the off-center distribution in the downward flow and the near-wall distribution in the upward flow due to the centripetal effect in the U-bends. Bimodal distributions of bubble equivalent diameter are observed because of the intensive bubble coalescence inside the elbow. The overall $k_{l}a$ is found to be linearly proportional to the liquid Reynolds number and gas volume fraction. The dimensionless mass transfer correlation proposed in this work predicts well the overall volumetric mass transfer coefficient.