Effects of break geometry and orientation on helium-air mixing in simulated reactor cavities of high temperature gas reactors

Abstract

The goal of this study was to experimentally examine the spatial and temporal variations of air and helium concentrations and temperature fields within simulated reactor cavities of a High Temperature Gas-cooled Reactor (HTGR) following helium discharge into an initially air-filled reactor cavity system. The research scenario involved a hypothetical small pipe break in the Reactor Pressure Vessel (RPV), resulting in the release of pressurized high-temperature helium into the surrounding cavity. A scaled five-compartment experimental facility, modeled after the General Atomics Modular High Temperature Gas-cooled Reactor (GA-MHTGR) design, was constructed for helium and air mixing experiments. Oxygen sensors and thermocouple probes were installed in all five cavities to measure the oxygen concentrations and temperature distributions of the gas mixture. The helium concentration could then be determined from the measured oxygen concentration in the helium-air mixture. Custom helium injection nozzles were fabricated enabling the investigation of circular and rectangular break geometries as well as horizontal and upward or downward helium injection directions. Detailed transient temperature maps were generated using a combination of a fiber optics temperature sensor and multiple thermocouple probes. The experimental findings highlighted the significant impact of the injected helium jet velocity and direction on the gas mixing process. Lower gas injection velocities resulted in a higher buoyancy effect on the gas jet leading to helium mixing mostly with air in the upper regions of the cavity. Conversely, higher injection velocities exhibited a lower buoyancy effect and uniform mixing of helium and air throughout the cavity. The findings also demonstrated how the direction of the injected helium jet influences the air-helium temperature profiles within the cavities.