As one of the six proposed designs for Generation IV nuclear reactors, the high-temperature gas reactor (HTGR) is being designed to have various passive safety features. Its system safety performance has been investigated both experimentally and numerically, particularly under depressurization scenarios that may occur during postulated accident conditions. In this study, we consider a pipe break accident in the main loop, in which high-temperature and high-pressure helium is discharged into the reactor cavity, resulting in complex flow phenomena involving helium filling, gas mixing, and natural circulation within the cavity.

To investigate the jet discharging behavior near the break and the resulting gas mixing in the reactor cavity, a scaled HTGR reactor cavity test facility has been constructed at the City College of New York in which relevant experimental investigations are being carried out. In parallel, unsteady Reynolds-averaged Navier-Stokes (URANS) models have been developed based on geometry and operating conditions of the experimental setup. Numerical simulations have been conducted to reproduce representative test cases, including a mild-buoyant case and a strong-buoyant case with the injection of 75°C nitrogen and 300°C helium, respectively, into the cavity, which was initially filled with room temperature air.

Due to the nature of the flow, which becomes quasi-steady during the long transient, a relatively large Courant-Friedrichs-Lewy number of up to 30 is used to accelerate the simulations, ensuring that the long transient process can be captured at a reasonable computational cost. Overall, the URANS predictions show good agreement with the experimental data in terms of time evolution of local gas temperature and oxygen concentration at various sensor locations within the cavity.