Experimental Design and Flow Visualization for the Upper Plenum of a Very High Temperature Gas Cooled for Computer Fluid Dynamics Validation

The Very High Temperature Reactor (VHTR) is a Generation IV nuclear reactor that is currently under design. It modifies the current high temperature gas reactor (HTGR) design to have a 1000 ^(0)C coolant outlet. This increases fuel efficiency and allows for other industrial applications. During the design process several studies are performed to develop safety codes for the reactor. One major accident of interest is the Pressurized Conduction Cooldown (PCC) scenario. The PCC scenario involves loss of forced coolant to the core but the loop stays pressurized. This results in a large buoyancy force that through natural convection reverses the flow of the core coolant loop to circulate into the upper plenum of the VHTR. Computer codes may be developed to simulate the phenomenon that occurs in a PCC scenario, but benchmark data is needed to validate the simulations. There are currently no experimental models to provide benchmark data for the PCC scenario. This study will cover the design, construction, and testing of a 1/16th scaled model of a VHTR that uses Particle Image Velocimetry (PIV) for flow visualization in the upper plenum. Three tests were run for a partially heated core at statistically steady state, and PIV was used to generate the velocity field of three naturally convective adjacent jets. Recirculation between the jets occurred until the jets reached the mixing point three cm from the outlet where turbulent mixing was observed. A sensitivity analysis was performed to confirm 1000 image pairs was sufficient to correctly represent the flow. The results were then validated by comparing the PIV results with experimental data and calculated values.