Decomposition of neutron noise in a reactor into higher-order mode components and investigation of the space and frequency dependence

Abstract

It is a well-known fact that the frequency characteristics of neutron noise induced by the fluctuation of nuclear cross sections have space-dependence that departs from the point kinetics behavior. In this paper, the neutron noise distribution in a two-dimensional BWR core model, which is calculated by solving a two-energy group neutron noise diffusion equation, is decomposed into higher-order mode components using α-mode eigenfunctions. The amplitude and the phase shift of the higher-order mode components have a minor dependence on the frequency, compared to the fundamental mode. Near the neutron noise source, the higher-order mode components account for a major portion of the neutron noise, thereby causing a minor dependence of the neutron noise on the frequency. Near the nodes of the higher-order modes, the fundamental mode is dominant, and the neutron noise exhibits almost the point kinetics behavior. The space dependence and the frequency characteristics of the neutron noise are elucidated by examining the higher order components that are decomposed from the neutron noise distribution.