In this thesis advanced fuel cycles for the incineration, i.e. deep–burn, of weapons–grade
plutonium, reactor–grade plutonium from pressurised light water reactors and reactor–grade
plutonium + the associated Minor Actinides in the 400 MWth Pebble Bed Modular Reactor
Demonstration Power Plant was simulated with the VSOP 99/05 diffusion code. These
results were also compared to the standard 9 g/fuel sphere U/Pu 9.6% enriched uranium
fuel cycle. The addition of the Minor Actinides to the reactor–grade plutonium caused an
unacceptable decrease in the burn–up and thus an unacceptable increase in the heavy metal
(HM) content in the spent fuel, which is intended for direct disposal in a deep geological
repository, without chemical reprocessing. All the Pu fuel cycles failed the adopted safety
limits in that either the maximum fuel temperature of 1130°C, during normal operation, or the
maximum power of 4.5 kW/sphere was exceeded. All the Pu cycles also produced positive
Uniform Temperature Reactivity Coefficients, i.e. the coefficient where the temperature of the
fuel and the graphite moderator in the fuel spheres are varied together. these positive
temperature coefficients were experienced at low temperatures, typically below 700°C. This
was due to the influence of the thermal fission resonance of 241Pu. The safety performance of
the weapons–grade plutonium was the worst. The safety performance of the reactor–grade
plutonium also deteriorated when the heavy metal loading was reduced from 3 g/sphere to 2
g or 1 g.
In view of these safety problems, these Pu fuel cycles were judged to be not licensable in the
PBMR DPP–400 reactor. Therefore a redesign of the fuel cycle for reactor–grade plutonium,
the power conversion system and the reactor geometry was proposed in order to solve these
problems. The main elements of these proposals are:
v
1. The use of 3 g reactor–grade plutonium fuel spheres should be the point of departure.
232Th will then be added in order to restore negative Uniform Temperature Reactivity
Coefficients.
2. The introduction of neutron poisons into the reflectors, in order to suppress the power
density peaks and thus the temperature peaks.
3. In order to counter the reduction in burn–up by this introduction of neutron poisons, a
thinning of the central reflector was proposed. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/8069 |
| Date | January 1900 |
| Creators | Serfontein, Dawid Eduard. |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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