Symposium on Advanced Sensors and Modeling Techniques for Nuclear Reactor Safety

High Temperature Gas Flow and Heat Transfer Phenomena of Importance in Gen-IV High Temperature Gas Reactors

Gaseous coolants will be used in High Temperature Gas-cooled Reactors (HTGRs) and Very High Temperature Reactors (VHTRs), which are one of six Generation IV reactor types proposed by US DOE for the Next Generation Nuclear Plants. Although the HTGR and VHTR designs ensure passive cooling in case of loss of coolant accidents and loss of forced circulation, it is still not fully known how the reactors will behave under all conceivable accident scenarios. Several Phenomena Identification and Ranking Tables (PIRT) show that under normal steady-state, transient, and accident scenarios, the key phenomena leading to localized hot spots in the reactor core include flow laminarization, natural circulation, air ingress, water or steam ingress, degraded heat transfer in coolant channels, effects of bypass flow, and non-uniform heat generation across the core. This talk will cover our past and current experimental and numerical simulation studies of flow laminarization and air ingress in accident scenarios. The flow laminarization refers to a strongly heated turbulent gas flow exhibiting deteriorated turbulent heat transfer (DTHT) and even laminar heat transfer characteristics under forced convection conditions. It can occur even if the incoming flow in the heated flow channel is well above the critical Reynolds number. Our air ingress-related work studies the effect of nitrogen (simulating air) entering the lower plenum and subsequent transport through a hot reactor core, which could cause exothermic oxidation of graphite in the core and severe damages to the structural integrity of the reactor core.