Helium-air mixing in simulated reactor cavities of high temperature gas reactors

•Gas mixing in High Temperature Gas Reactor cavities has been investigated.•Experiments were performed by injecting helium gas into a 5-cavity system.•Oxygen concentrations and temperature profiles were measured continuously.•Injection into the lower part of the cavity showed good mixing of helium with air.•Injection into the upper part of the cavity showed poor mixing and stratification. The spatial and temporal variations in air and helium concentration and temperature fields in simulated reactor cavities of High Temperature Gas Reactor (HTGR) were investigated experimentally. This accident scenario follows a hypothetical small pipe break in the Reactor Pressure Vessel and discharge of high temperature helium into the surrounding cavity. A scaled multi-compartment experimental facility based on the General Atomics Modular High Temperature Gas Reactor (GA-MHTGR) design was constructed and used in the gas mixing experiments. Oxygen sensors and thermocouple probes were installed in all five cavities to measure the oxygen (or helium) concentrations and gas mixture temperature distributions. The experimental results indicated the significance of the pipe break location and cavity venting configuration in gas mixing phenomena for the HTGR design. If air ingress occurs following Depressurized Conduction Cooling (DCC) of the reactor core, the oxygen concentration near the pipe break could be different depending on the elevation of the break location and the degree of mixing and stratification of helium and air in the reactor building cavities.