Experimental Investigation of Natural Circulation During Air-Ingress Scenario in a Very High Temperature Reactor

In order to investigate air-ingress phenomena in a gas-cooled very high temperature reactor (VHTR), natural circulation experiments have been conducted in a helium flow loop after the injection of nitrogen into the lower plenum. A pair of helium analyzers were used to measure the nitrogen and helium concentrations in the lower plenum and upper plenum. The changes in the nitrogen concentration in the upper plenum were used to calculate the time required for the transport of nitrogen from the lower plenum to upper plenum through a riser flow channel made of graphite. The effect of system temperature and pressure on the rate of nitrogen transport has been studied extensively. Furthermore, a close examination of the graphite flow channel wall temperatures at different elevations showed small but sudden drops indicating the arrival of nitrogen at each elevation. From these data, the upward transport of nitrogen injected into the lower plenum under natural circulation conditions could be quantitatively investigated. The experimental findings indicate that the driving mechanisms for air transport through the reactor core of VHTR would result from both molecular diffusion and natural circulation. At low graphite temperatures in the riser, molecular diffusion is the dominating mechanism; however, as the riser temperature increases, natural circulation becomes dominant and the rate of nitrogen transport increases. Further, the time constants for these mechanisms have been calculated using a simplified species transport equation.