The increasing demand for energy and the effect on climate change are some of the big drivers in
support of the nuclear renaissance. A great amount of energy is spent on studies to determine the
contribution of nuclear power to the future energy supply. Many countries are investing in
generation III and IV reactors such as the Westinghouse AP1000 because of its passive cooling
system, which makes it attractive for its safety. The pebble bed high temperature gas cooled
reactors are designed to be intrinsically safe, which is one of the main drivers for developing
these reactors.
A pebble bed reactor is a high temperature reactor which is helium–cooled and graphitemoderated
using spherical fuel elements that contain triple–coated isotropic fuel particles
(TRISO). The success of its intrinsic safety lies in the design of the fuel elements that remain
intact at very high temperatures. When temperatures significantly higher than 1600 °C are
reached during accidents, the fuel elements with their inherent safety features may be challenged.
A pebble bed reactor has an online fuelling concept, where fuel is circulated through the core.
The fuel is loaded at the top of the core and through gravity, moves down to the bottom where it
is unloaded to either be discarded or to be re–circulated. This is determined by the burnup
measuring system. By circulating the fuel spheres more than once through the reactor a flattened
axial power profile with lower power peaking and therefore lower maximum fuel temperatures
can be achieved. This is an attractive approach to increase the core performance by lowering the
important fuel operating parameters. However, the circulation has an economic impact, as it
increases the design requirements on the burnup measuring system (faster measuring times and
increased circulation). By adopting a multi–pass recycling scheme of the pebble fuel elements it is
shown that the axial power peaking can be reduced
The primary objective for this study is the investigation of the influences on the core design with
regards to the number of fuel passes. The general behaviour of the two concepts, multi–pass
refuelling and a once–through circulation, are to be evaluated with regards to flux and power and
the maximum fuel temperature profiles. The relative effects of the HTR–Modul with its
cylindrical core design and the PBMR 400 MW with its annular core design are also compared to
verify the differences and trends as well as the influences of the control rods on core behaviour.
This is important as it has a direct impact on the safety of the plant (that the fuel temperatures
need to remain under 1600 °C in normal and accident conditions). The work is required at an
early stage of reactor design since it influences design decisions needed on the fuel handling system design and defuel chute decay time, and has a direct impact on the fuel burnup–level
qualification.
The analysis showed that in most cases the increase in number of fuel passes not only flattens the
power profile, but improves the overall results. The improvement in results decreases
exponentially and from ten passes the advantage of having more passes becomes insignificant.
The effect of the flattened power profile is more visible on the PBMR 400 MW than on the
HTR–Modul. The 15–pass HTR–Modul design is at its limit with regards to the measuring time of
a single burnup measuring system. However, by having less passes through the core, e.g. tenpasses,
more time will be available for burnup measurement. The PBMR 400 MW has three
defuel chutes allowing longer decay time which improves measurement accuracy, and, as a result
could benefit from more than six passes without increasing the fuel handling system costs.
The secondary objective of performing a sensitivity analysis on the control rod insertion
positions and the effect of higher fuel enrichment has also been achieved. Control rod efficiency
is improved when increasing the excess reactivity by means of control rod insertion. However,
this is done at lower discharge burnup and shut down margins. Higher enrichment causes an
increase in power peaking and more fuel–passes will be required to maintain the peaking and
temperature margins than before. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/4729 |
| Date | January 2010 |
| Creators | Geringer, Josina Wilhelmina |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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