Investigation of Performance of a Large-Scale Water-Based RCCS Under Varying Decay Heat Loads

The objective of the present study is to investigate the effects of varying decay heat loads and tank inlet elevations on the performance of a large-scale water-based reactor cavity cooling system (RCCS) utilizing the water-based Natural convection Shutdown heat removal Test Facility (NSTF) at Argonne National Laboratory. A power parametric series with prototypic decay heat loads of 1.4 MWt, 1.75 MWt, 2.1 MWt, and 2.4 MWt) (34.4 kWt, 43.0 kWt, 51.6 kWt, and 58.5 kWt scaled) with a low tank inlet configuration was tested.The system performance metrics at the two-phase quasi-steady state were compared, mainly the system flow, steam generation rate, system pressure, and liquid and structure temperatures. The effect of the decay heat power level on the boiling front progression was also investigated.It was found that lower decay heat loads caused delayed boiling front progression. The results were then compared to previous data sets collected at identical testing conditions but with chimney piping configured at the mid tank elevation configuration. Overall, the tank inlet elevation did not significantly influence the system two-phase quasi-steady-state performance at the studied decay heat levels. However, the tank inlet elevation had a significant impact on the two-phase flow instability characteristics and the boiling front propagation.A static flashing model identified from the literature predicted the flow oscillation period well but would result in high flashing superheating. Additional analysis also identified the presence of Type-I density wave oscillations (DWOs) in the present study. By plotting all the test results on the subcooling and Zuber plane, a chimney exit quality of ~0.3% was identified as a trigger for the onset of flow oscillations.