| Nuclear energy plays a very important role in the world electricity energy. Most of the present commercial nuclear reactors are light water reactors(LWRs), which merely reach a uranium utilization of 0.6%. The lower resource utilization become a seriously limiting factor for LWR nuclear power. The fast neutron reactor(FNR) seems an approach to solve resource problem, however, the FNR needs fuel reprocessing involving the extraction and separation of actinide isotopes such as plutonium, and this would increase the risk of nuclear proliferation. A breed and burn(B&B) mode can design to breed and burn the fertile fuel in situ without chemical separation in the fuel cycle. Candle(Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy produced) is one of the B&B type reactors.Generally, the candle reactor has two fuel regions: one loads the starter fuel such as enriched uranium, and the other loads the feed fuel such as depleted uranium or thorium. Once the candle reactor is successfully ignited, the distribution of the neutron flux and power density would move along the core axis direction slowly without changing the spatial shape. The candle reactor usually has a giant volume and very deep burnup(~ 40% FIMA), and the nuclides density are very different between the beginning of life and the end of life. Using a Monte-Carlo method to model such a reactor would cost too much computational efforts.A simplified 0D/1D core model can save much computational time, and its neutron balance analysis can provide reasonable estimate results. Neutron balance method was applied to analyze the large B&B cores recently. The neutron balance counts the total neutron production and absorption in a fuel, which is useful for scoping analysis.This thesis will consider the(n,2n)&(n,3n) reaction rates in the neutron balance calculation. Uranium- and thorium-based fuel core are analyzed with and without(n,2n)&(n,3n) consideration for comparison.The Monte-Carlo computational time of 1D approach is only 10% of that with a full candle core model because of the simplified geometry. Then we assess the validity of neutron balance analysis with a simplified 1-D model for determination of the maximum BU attainable in a CANDLE mode. Afterwards, we optimize the configurations of starter fuel and feed fuel for a sodium-cooled core to sustain the CANDLE mode with the validated 1-D model.The core geometry and fuel enrichment can dramatically influence the reactivity swing of the transition state. We assess the numbers of neutrons required in feed fuel, and of the excess neutrons provided by the starter fuel with the neutron balance method. Thus the optimized configuration of starter fuel can be obtained efficiently.The neutron spectrum and parasitic absorption characteristics are different for various fuel types(metal, oxide, oxycarbide), coolant types(sodium, lead bismuth alloy, helium, FLiBe molten salt), and structural material types(HT-9 steel, SiC), which can change the neutron economy of the core. Furthermore, the core radii and fuel volume fractions may also lead to different equilibrium states of a CANDLE reactor. This thesis considers all the factors mentioned above to estimate the optimized configuration of Uranium- and thorium-based fuel core. It is expected to provide a good reference for CANDLE cores design in the future. |
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