Dissertations and Theses

Date of Award


Document Type



Chemical Engineering

First Advisor

Masahiro Kawaji

Second Advisor

Sanjoy Banerjee


Air ingress, flow relaminarization, VHTR


Very High Temperature Reactors (VHTRs) are one of six Generation IV reactors that have been proposed for DOE’s Next Generation Nuclear Plant. In addition to using gaseous coolants, VHTRs also display benefits of passive safety systems including intra-core natural circulation for heat removal in accident scenarios. However, a number of substantial engineering challenges are expected in VHTRs, and due to the high temperatures of the coolant involved, material behavior in various components needs to be better understood. Our work focuses mainly on two key phenomena that could lead to localized hot spots in the VHTR reactor core if not addressed properly.

The first phenomenon is the issue of air ingress scenario under Depressurized Conduction Cooldown involving natural circulation of the gaseous coolant wherein a crack or breakage might occur in the reactor system due to an externally or environmentally initiated event leading to depressurization. Natural circulation experiments have been performed using pure He, pure N2, and binary gas mixtures (He-N2) representing a helium-air mixture. Helium analyzers were used to measure v 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 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. In order to measure the natural circulation flow rate, a mass flow measurement system was designed and constructed as well. The natural circulation flow and heat transfer data along with the density and viscosity effects are analyzed to identify possible changes in heat transfer rates from the graphite to the coolant in the riser.

The second phenomenon studied is the issue of heat-driven flow relaminarization (forced convection), in which a strongly heated turbulent gas flow, exhibits heat transfer characteristics of laminar flows in the downstream part. As the gas is heated, a reduction in the gas density causes the bulk flow to accelerate upward, but the viscosity also increases leading to reduced Reynolds numbers. Numerical simulations of turbulent forced convection are performed in a two-step process to fundamentally understand the nature of the experiments using a massively parallel, spectral element code called Nek5000. The first step incorporated a replication method along with recycled periodicity to successfully sustain turbulence throughout a very long pipe (L/D ratio = 234). Once turbulence is found to be sustained, the next vi and final step involved application of experimental conditions to this strongly heated pipe setup. Finally, the effect of heat driven flow relaminarization on heat transfer characteristics and turbulence parameters is studied. These experiments and simulations provide useful data for validation of VHTR design and safety analysis codes.



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