Experimental study on air ingression and residence time of a small-scale decay tank for a research reactor

Highlights: • The air ingression-and-trap phenomenon was observed in small-scale experiments. • The air invading PCS reduces the core's water flow under the required flow rate. • In a scaled-down decay tank (single-phase flow), the shortest residence time was measured. • Residence time in a real-scale decay tank is estimated by a dimensional analysis. - Abstract: An air ingression phenomenon in a pool-type research reactor was observed in small-scale experiments, assuming that a pipe on the upstream of a decay tank was ruptured. The air and water formed a two-phase flow on the upstream of the decay tank, and the air was trapped in the decay tank, occupying its upper volume. The trapped air could be detected by measuring the differential pressure between the inlet and outlet of the decay tank. Also, the shortest residence time of water during the normal operation was measured to verify the decay tank's design adequateness. The measured residence time in the small-scale experimental facility is used to estimate the residence time in a real-scale decay tank by a dimensional analysis with a dimensionless kinetic pressure. The estimated residence time is in good agreement with the CFD result from the particle-tracking and unsteady UDS methods.