Stratified flow-induced air-ingress accident assessment of the GAMMA code in HTGRs

An air-ingress accident is a major safety issue pertaining to high-temperature gas-cooled reactors. To see the effect of a stratified flow, which is a multi-dimensional phenomenon that occurs in large broken pipes, we perform 1-D and 2-D air-ingress simulations in the guillotine break of the main coaxial pipe of a 600 MWth GT-MHR with the GAs multicomponent mixture transient analysis (GAMMA) code. We used a 2-D fluid volume to build the coaxial inlet pipe, the lower plenum of the reactor core, and the cavity and simplified the other components as 1-D fluid blocks. After the guillotine break of the main coaxial pipe, the air in the reactor cavity flows into the reactor core in four phases: the blow-down phase, the stratified flow phase, the molecular diffusion phase, and the natural convection phase. In the early stage of a broken pipe, the lower plenum region of the reactor is filled with air within 30 s by a density-driven airflow. In a 1-D simulation, the process of filling the lower plenum with air ingressed from a cavity caused by the diffusion process takes 30 min. However, after 30 s, the flow velocity of air ingressed into the broken pipe decreases and the diffusion phase eventually begins. The natural circulation in this scenario starts after more than 360 h for the 1-D simulation but fails to commence after more than 500 h for the 2-D simulation. The belated natural circulation in the 2-D simulation is mainly attributed to the slower diffusion process in the core region. In turn, the slower diffusion occurs because the temperature of the air in the lower plenum is lower in the 2-D simulation than in the 1-D simulation. The maximum core temperature in the 2-D simulation was by 60 °C lower than that in the 1-D simulation.