| Summary: | The Pebble Bed Modular Reactor (PBMR) is a 4th generation nuclear reactor
based on the HTR-Modul of Siemens currently being developed by Eskom in
South Africa. The major safety characteristics of the PBMR are the fuel
design and physical dimensions that make it an inherently safe reactor. This
means that the reactor will not melt down like a typical Light Water Reactor
(LWR) when cooling of the reactor is lost.
The thermo-hydraulic analysis of the Used Fuel Tank (UFT) is of great
importance in the safety analysis of the PBMR. The UFT is one of two types
of tanks that will be used to store fuel that has been in the reactor for a finite
time. The fuel would therefore contain fission products and would generate
decay heat. This decay heat should be removed to limit the temperature of
the fuel.
The temperature of the fuel should be limited to prevent the release of fission
products to the environment. The temperature limit on the fuel during storage
is required to ensure that the graphite in the fuel does not oxidize in the
presence of oxygen. The fuel is normally kept in a helium environment, but it
must be shown that the fuel is safe when there is air ingress into the system.
The purpose of this study is therefore to determine the temperature
distribution in the fuel and the components of the used fuel tank for different
scenarios. This includes the forced cooling of the tanks and the possibility of
cooling the tanks with natural convection.
Computational Fluid Dynamics (CFD) was used to model the various heat
transfer mechanisms present. This includes convection heat transfer between
the gases and the solids, conduction through the solids and thermal radiation
between most of the surfaces. The effect of natural convection was also
included, as the pipes through the tank cause result in high mass flow through
these pipes due to the buoyancy effect.
The results show that the fuel temperature will not exceed the allowable limit
during forced cooling if the Heating, Ventilation and Air-conditioning (HVAC) is
supplied at 6 kg/s. The possibility of cooling the tanks with passive means
during upset events looks promising, but it is dependant on the design of the
chimneys. The chimney cross-flow area was the most significant factor
influencing the air mass flow through the system.
The chimney design and the rest of the system not included in this study
should be analysed in detail before the passive operation of the system can
be guaranteed. === Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2004.
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