(489b) Natural Circulation in a VERY High Temperature Reactor (VHTR)
AIChE Annual Meeting
Wednesday, November 13, 2019 - 8:22am to 8:44am
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.