Inherent safety is claimed for gas-cooled pebble bed reactors, such as the South African
Pebble Bed Modular Reactor (PBMR), as a result of its design characteristics, materials used,
fuel type and physics involved. Therefore, a proper understanding of the mechanisms of heat
transfer, fluid flow and pressure drop through a packed bed of spheres is of utmost
importance in the design of a high temperature Pebble Bed Reactor (PBR). In this study,
correlations describing the effective thermal conductivity through packed pebble beds are
examined. The effective thermal conductivity is a term defined as representative of the
overall radial heat transfer through such a packed bed of spheres, and is a summation of
various components of the overall heat transfer.
This phenomenon is of importance because it forms an intricate part of the self-acting decay
heat removal chain, which is directly related to the PBR safety case. In this study standard
correlations generally employed by the thermal fluid design community for PBRs are
investigated, giving particular attention to the applicability of the correlations when simulating
the effective thermal conductivity in the near-wall region. Seven distinct components of heat
transfer are examined namely: conduction through the solid, conduction through the contact
area between spheres, conduction through the gas phase, radiation between solid surfaces,
conduction between pebble and wall, conduction through the gas phase in the wall region,
and radiation between the pebble and wall surface.
The effective thermal conductivity models are typically a function of porosity in order to
account for the pebble bed packing structure. However, it is demonstrated in this study that
porosity alone is insufficient to quantify the porous structure in a randomly packed bed. A new
Multi-sphere Unit Cell Model is therefore developed, which accounts more accurately for the
porous structure, especially in the near-wall region. Conclusions on the applicability of the
model are derived by comparing the simulation results with measurements obtained from
various experimental test facilities. This includes the PBMRs High Temperature Test Unit
(HTTU) situated on the campus of the North-West University in Potchefstroom in South Africa.
The Multi-sphere Unit Cell Model proves to encapsulate the impact of the packing structure in
a more fundamental way and can therefore serve as the basis for further refinement of
models to simulate the effective thermal conductivity. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/9628 |
| Date | January 2009 |
| Creators | Van Antwerpen, Werner |
| Publisher | North-West University |
| Source Sets | North-West University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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