Three-group three-dimensional analysis of the random neutron field in bare homogeneous reactors and the concepts of local and global neutron noise

Abstract

A three-group theory of neutron noise is developed and it is shown that the neutron-noise field in homogeneous multiplying media can be separated into a component with a long attenuation length (global component) and two local components whose attenuation lengths in water-moderated reactors are very small and of the order of the diffusion length of thermal neutrons in the medium. The theory is subsequently applied to the investigation of the neutron noise induced by axially propagating stochastic fluctuations of the moderator density in a simple 1-D BWR model and the evaluated neutron cross-spectra and phases are compared to the corresponding ones obtained by employing the two-group model with two-group data derived by condensing the three-group data in the reactor spectrum. In this respect, it is shown that the results obtained by using the three- and the two-group models are almost identical.