(489b) Natural Circulation in a VERY High Temperature Reactor (VHTR)

The generation-IV nuclear reactors, Very High Temperature gas-cooled Reactors (VHTRs), are proposed by the US Department of Energy to produce Green House Gas emission-free electricity and hydrogen. VHTRs are designed with passive safety features to withstand hypothetical accidental scenarios such as loss of coolant accidents, earth quakes and tsunamis, etc. But it is not well understood how the VHTR’s passive safety designs will react to the aforementioned accident scenarios. Knowledge about the natural circulation flow and heat transfer in Pressurized Conduction Cooldown (PCC) and Depressurized Conduction Cooldown (DCC), and the effects of bypass flow in forced circulation flows are essential for safety of VHTRs. Further, an air ingress scenario in which ambient air would enter the core through a break in the coolant flow loop piping could cause oxidation of a hot graphite core and severely damage the core. In the present study, (1) the natural circulation flow and heat transfer, (2) the effects of air ingress at different pressures and temperatures (3) forced convection and the effect of bypass-coolant flow phenomenon have been investigated experimentally.

To understand the natural circulation flow phenomena in a VHTR and study the effects of air-ingress, natural circulation experiments have been performed by injecting nitrogen (instead of air to avoid graphite oxidation) into the lower plenum of a helium-filled flow loop consisting of a riser and downcomer connected by a lower plenum at the bottom and upper plenum at the top. The riser was made of a graphite test section containing a single flow channel and electrical heater rods and instrumented with thermocouples. The transport of nitrogen through the graphite flow channel has been quantified by measuring the volumetric concentrations of nitrogen in lower and upper plena. The transport of nitrogen through the graphite test section could be determined by detecting small but sudden changes in the local graphite temperatures at different axial elevations. The steady state gas circulation rates of the helium-nitrogen mixture between the hot riser and cold downcomer have also been measured. The experimental findings indicate that the driving mechanism for nitrogen transport in the air-ingress scenario results from both molecular diffusion and natural circulation. At low graphite temperatures in the riser, molecular diffusion is the dominant mechanism for nitrogen transport, but as the graphite temperature increases, the natural circulation becomes more dominant, and the nitrogen transport rate increases.