Transverse Flow in High-Temperature Gas-Cooled Reactor Graphite Elements

In a high-temperature gas-cooled reactor, there is a narrow gap between the fuel stick and the graphite septum separating it from the rapidly flowing helium coolant. Porosity of the graphite permits a small part of the helium to flow through the gap. This parasitic “transverse flow” may be significant in fission product transport. The extent and pattern of this flow is calculated in terms of the resistivities to gas flow: axially for the gap and radially for the septum. The latter was measured for a number of Fort St. Vrain elements, and the former was obtained from specifications. The calculated flow in a fuel hole amounts to a fraction of 1 cm³/s on the average and may reach slightly over 1 cm³/s.