Space-Time Kinetics and Time-Eigenfunctions.

[摘要] For pulsed-neutron experiments performed in a subcritical reactor, the reactivityobtained from the area-ratio method is sensitive to detector positions. The spatiale®ects in the area-ratio method are induced by both the prompt-neutron harmonicsand the delayed-neutron harmonics in the reactor. The traditional kinetics distortionfactor is only limited to correcting the spatial e®ects induced by the prompt-neutronfundamental mode. The modi¯ed area-ratio method corrects the spatial e®ects byperforming time-dependent numerical simulations of the pulsed-neutron experiments,which are usually time-consuming and practically di±cult. Bell;;s spatial correctionfactor can compensate for the spatial e®ects, but it cannot provide physical expla-nations for the spatial dependence manifest in the experiments.In this thesis, we performed a high-order modal analysis for the area-ratio methodand obtained the spatial correction factors fp and fd to account for spatial e®ectsinduced by the prompt-neutron harmonics and the delayed-neutron harmonics, re-spectively. Numerical simulations indicate that the spatial e®ects are mainly inducedby the prompt-neutron harmonics, and the delayed spatial correction factor fd is closeto 1.0. The high-order prompt-neutron harmonics induce signi¯cant spatial e®ectsand cannot be neglected in calculating the spatial correction factors. Our modalanalysis also shows that the prompt spatial correction factor fp is simply the ratio ofthe detector responses corresponding to the the fundamental k-mode and the promptneutron °ux integrated over the pulse period , if a proper normalization is applied.Thus, it is convenient to calculate fp, and we are able to provide physically intuitiveexplanations on the spatial dependence in the area-ratio method: overestimation ofthe subcriticality in regions close to the external neutron source, and underestimationaway from the source but within the fuel region.In this thesis, the fundamental and high-order harmonics are calculated by IRAM,with the BICGSTAB method applied to solve the corresponding ¯xed source prob-lem. We also demonstrate that IRAM is a more e±cient method than the poweriteration method for solving eigenvalue problems with large dominance ratios.

[发布机构] University of Michigan

[效力级别] Nuclear Engineering and Radiological Sciences

[关键词] Spatail Dependence in the Area-ratio Method;Nuclear Engineering and Radiological Sciences;Engineering;Nuclear Engineering & Radiological Sciences