The projected prismatic and pebble-bed preconceptual designs for the Very High Temperature Reactor (VHTR) both have large diameter, horizontally-oriented, exit ducts that lead hot helium from the core exit through the lower plenum to the balance-of-plant. In the event a significant leak is present in the exit duct the system will depressurize and the possibility of air ingress into first the lower plenum and then the core must be considered. The intrusion of air against helium flow is driven by the weight difference between the two fluids. Thus the potential for air intrusion always exists. Whether such a phenomenon will actually occur is not as clear. This paper proposes a simple criterion for the onset of air intrusion. The criterion is expressed in terms of a densimetric Froude number (the inverse of the bulk Richardson number) and a scaled pressure difference between the duct exit and the containment. The pressure difference is rendered dimensionless by dividing it with the product of the duct radius and the difference of specific weight of the two fluids. The dimensionless variables in the criterion enable its validity be verified by experiments using fluids other than helium and air. This paper presents the development of this criterion and experimental validation using water and biodiesel as the testing fluids. The relevance and application of this criterion to VHTR air ingress scenarios are described.

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