Computational fluid dynamics analysis of the initial stages of a VHTR air ingress accident using a scaled down model
Abstract An air ingress accident is considered to be one of the design basis accidents of a very high temperature gas cooled reactor (VHTR) The air ingress accident is initiated, in its worst case scenario, by a complete break of the hot duct in what is referred to as a double ended guillotine break. This leads to an initial loss of the primary helium coolant via depressurization. Following the depressurization process, the air helium mixture in the reactor cavity could enter the reactor core via the hot duct and hot exit plenum. In the event that air ingresses into the reactor vessel, the high temperature graphite structures in the reactor core and hot plenum will chemically react with the air, which could lead to damage of in core graphite structures and fuel, release of carbon monoxide and carbon dioxide, core heat up, failure of the structural integrity of the system, and eventually the release of radionuclides to the environment. Studies in the available literature focus on the phenomena of the air ingress accident that occur after the termination of the depressurization, such as density driven stratified flow, molecular diffusion, and natural circulation. However, a recent study shows that flow reversal could occur due to Taylor wave expansion near the end of the depressurization, which could affect subsequent stages of the air ingress accident scenario. Therefore, to properly understand and evaluate the depressurization effects, numerical simulations are performed for the double ended guillotine break of the Gas Turbine Modular Helium Reactor (GT MHR) cross vessel with a computational fluid dynamics (CFD) tool, ANSYS FLUENT. A benchmark and error quantification study of the depressurization shows that the ANSYS FLUENT model can predict the depressurization problem with relatively low uncertainty. In addition, the computational results show that the depressurization of a double ended guillotine break behaves as an isentropic process. The observed flow oscillations near the end of the depressurization promote mixing of helium gas and air near the break. The results of the CFD analyses also show that the density driven stratified flow, which is postulated to be the next stage of the air ingress accident scenario, is strongly dependent on the density difference between the air helium mixture in the containment and the helium in the reactor vessel. Therefore, the flow oscillations near the end of the depressurization stage may have a minor, yet notable, effect to slow down the air ingress due to density driven stratified flow by decreasing the bulk density of the gas mixture in the containment through the addition of helium and increasing the bulk density in the reactor vessel through the addition of air.