Basic Study on Regional Oscillation of Nuclear Reactor by Using Coupled Core from a Neutronics

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One can observe a regional oscillation of power in a large sized nuclear reactor. To reveal the mechanism of this phenomenon, a basic study was conducted by using coupled cores constructed in the Kyoto University Critical Assembly (KUCA) of which spatial coupling coefficient was close to that of existing Boiling Water Reactor (BWR).The core separation distance and the fuel pitch in the core were systematically varied to measure the eigenvalue separation (ES) of the spatial first-harmonic mode which is closely related to the reactor stability. The Feynman-alpha, the rod drop and the flux tilt methods were applied to measure the ES values. They were agreed with each other within experimental errors. It was found that the flux tilt method based on the measurement of neutron flux distribution by an optical fiber scintillation detector is powerful to obtain an accurate ES value. Whereas the Feynman-alpha method seems to be difficult to measure ES in strongly coupled core where the first-har … More monic mode is much smaller than the fundamental mode. It was observed that ES becomes larger with decreasing the core separation distance or the fuel pitch.The neutronics calculations were executed to analyze the experimental results on the basis of either diffusion or transport theory. The result of transport calculation agreed better with the experimental result than that of diffusion calculation, since the transport effect was large in the present analysis. For the calculation of the first-mode, although the mesh effect was smaller, the three-dimensional, the energy group collapsing and the transport effects were larger than those for the fundamental-mode. This fact indicates that the larger correction would be necessary in the first-mode calculation compared with the fundamental-mode calculation.Moreover, traials were conducted to apply higher order perturbation theory to calculate the flux tilt and to measure the transmission time of the change in neutron flux from one core to the other core.However, the fruit has not been obtained yet. Less