| Summary: | As a result of manufacturing and temperature constraints of the reactor core components,
leakage flow is an inevitable, and generally undesirable, occurrence within the PBMR
reactor. Leakage flow occurs between the narrow gaps of the graphite blocks within the
Core Structures as a result of the large pressure gradient over the pebble bed.
The PBMR utilizes computational fluid dynamics (CFD) codes for the simulation of flow and
heat transfer through the reactor. Due to hardware limitations, it is not yet possible to model
the leakage paths between the graphite blocks of the reactor CFD model in detail since, in
some locations the leakage paths are in the order of 175-micron in width and would require a
very fine mesh structure. It is therefore required to simplify some of the more complex
leakage flow paths with the use of a porous medium sub-model.
In order to calibrate the porous medium sub-model to produce similar flow resistance as the
detail leakage path, it is necessary to separately model the complex leakage path in detail,
using CFD to determine the actual flow resistance characteristics as function of leak flow rate
and helium density. There was a wide spread in the calculated Reynolds numbers
throughout the flow path of the detail leakage paths, and it was uncertain whether the
leakage flow was laminar, in the transition zone or turbulent. This raised uncertainty with
regards to the accuracy of the CFD models of the detail leakage paths.
An experiment was devised that contained all the flow phenomena of the actual detail
leakage paths within the reactor, and was used to validate the numerical CFD modelling of
the helium flow through the side reflector leakage paths. Three leakage gap sizes, 175, 280
and 380-micron were experimentally tested. The experiments were simulated with CFD and
it was found that there was a good correlation between the laminar CFD results in both the
175 and 280-micron gap sizes.
It was finally concluded that the detail leakage path CFD models were correctly modelled as
laminar. === Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2005.
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