A New Method On Improving The Conventional Resonance Computational Method And Studies On The Effects Of Fuel Rod Spatial Self-shielding

By utilizing resonance and transport-depletion related modules of SCALE5.1 code package, this thesis focuses on the study of resonance calculation. Most of the conventional resonance calculation methods are based on the equivalence theory and are insufficient to consider the spatial self-shielding effects and resonance interference effects due to the adoption of many theoretical simplifications. This thesis begins with the quantitative demonstration of the errors of the conventional method by benchmarking the obtained effective resonance cross sections against the reference ones which are obtained by solving the one-dimensional continuous energy neutron slowing-down equation with the help of the CENTRM module. It then proposes a remedy to deal with the defects of the conventional method by the introduction of a correction factor to the effective resonance cross section. It is demonstrated that the proposed correction factor changes smoothly and monotonously with fuel enrichment,Pu weight percent, fuel temperature and the ratio of the number of fuel nuclides to moderator nuclides, and thus makes it possible to pre-tabulate the correction factor when preparing the multigroup cross section library. While most of the advanced resonance calculation methods currently under investigation greatly increase the complexity and also the computational costs, the proposed resonance calculation method not only corrects the important theoretical defects of the conventional method, but also keeps its merits of simplicity and easy-implementing, which enable it to own a good potential for the future engineering application.Moreover, the thesis also investigates the fuel rods spatial self-shielding effect. Numerical results reveal that for usual fuel rods without burnable absorber, due to the strong resonance self-shielding effect, there is a noticeable change of neutron spectrum along the radial direction of fuel rod. It results in the spatial dependency of the effective resonance cross section for resonant nuclides like uranium-238. However, the current single region resonance model is not able to represent this spatial effect, and it can not represent the spatial dependency of depletion rate of heavy nuclides either. Numerical results also demonstrate that despite of the above-mentioned theoretical deficiency of the current method, due to the error cancellation effect, the eventual error of the overall reactivity is not significant. While for the integral gadolinium burnable absorber fuel rod, due to its extremely large thermal absorption cross sections, its spatial self-shielding effects for thermal neutrons are much more significant than the ones for resonance neutrons. Therefore, for this type of fuel rod, although the aforementioned theoretical defect of the current method still exists, its effects are much less than the ones for the usual fuel rods. These results and conclusions are significant to quantitatively learn the errors of the current resonance computational method and to the development of a new generation core analysis method.Project supported by the National Natural Science Foundation of China (Grant No. 10975098 ).