|dc.description.abstract||The world is currently experiencing an energy crisis. To cope with the rising demand in
South Africa, nuclear power was identified as a clean, safe and reliable source of electricity.
The Pebble Bed Modular Reactor (PBMR) is an inherently safe, next-generation nuclear
power plant that uses pebble fuel. In the event of a depressurized loss of coolant (DLOFC)
accident, the reactor will passively cool itself, and remain within safe limits.
The main purpose of this dissertation was to perform an uncertainty study on the PBMR
reactor during a DLOFC accident to demonstrate this safety feature. An extensive literature
survey was carried out to research the concept of uncertainty, methods for addressing
uncertainty and to gather the required input data to set up a model of the PBMR reactor.
The model requirements were established by use of a systematic PIRT process. A detailed
model of the reactor was set up in Flownex after making the necessary assumptions and
simplifications. A sensitivity and Monte Carlo sampling platform was set up in conjunction
with Flownex in order to perform the uncertainty study.
During the DLOFC transient, the best-estimate maximum fuel, core-barrel and RPV
temperatures reached 1529, 621 and 490’C respectively. Sensitivity studies showed that
the parameters that most strongly influence the results are the power profile, decay heat,
pebble bed effective conductivity and the properties of the graphite reflector. Variations in
fluid properties had a negligible influence on the DLOFC results.
Statistical processing of the Monte Carlo simulation results provided uncertainty bands for
each output. The conclusion was that with 95% confidence, there is a 5% probability of
exceeding maximum fuel, core-barrel and RPV temperatures of 1582, 638 and 503 CC
respectively. All three of these temperatures are below the maximum allowable temperature
for each respective component. Thus all three components will stay within their code cases
during the unlikely event of a DLOFC.
The final effort in this study went to verification and validation (V&V) of the results. This
process included V&V of the input data, software, the calculation and the model
development. These processes included: a detailed internal review; comparison with
analytical solutions; comparison with alternative independent calculations; and comparison
with experiment. The effective pebble bed thermal conductivity is currently being validated
via construction of the Heat Transfer Test Facility (HTTF). The large extent of V&V activities
that have been carried out provides a high level of confidence that the results produced in
this dissertation are satisfactory, if not slightly conservative.||