Hydraulic Modeling of Air-Entraining Vortex Formation During Flow Withdrawal from Water Storage Tanks

In response to recent U.S. Nuclear Regulatory Commission (NRC) Component Design Bases Inspection (CBDI) issues, many US nuclear power stations have been required to demonstrate that minimum submergence requirements were properly determined for flow withdrawal from various safety related storage tanks. In many cases, the licensees failed to consider a vortex allowance, or applied an inappropriate vortex methodology. For Duke Energy’s McGuire Station, a Refueling Water Storage Tank (RWST) model was constructed using a geometric scale of 1:4.073. Testing included transient water level conditions simulating the field for selected flows (corresponding to prototype flows of 1,600 to 19,700 gpm) and water levels giving submergences of 1 to 5 ft above the suction nozzle in the model (prototype submergences of 4 to 20.3 ft). Results showed that with no return flow, the submergence at the onset of air entrainment ranged from 0.049 to 0.705 ft prototype for flows ranging from 1,600 to 19,700 gpm prototype, respectively. Based on the test results, it was determined that a vortex suppression device was not required for the McGuire RWST, as the expected water levels during operation would be higher than those indicated for onset of air entrainment for a given flow. The scale model testing showed that the critical submergences for initiation of air-entraining vortices were much lower than those predicted by Hydraulic Institute guidelines.