Dependence of the local/global decomposition of neutron noise on heterogeneity

The local and global components of neutron noise are now attributed to pure attenuation and reactivity fluctuations, respectively. In the present work, the authors investigate the relative importance of each component as a function of frequency and/or fuel spacing. For that, a one-dimensional explicitly heterogeneous reactor model in the radial direction is used. One neutron energy group is considered together with a two-group slowing down kernel. The local relaxation length for the fuel spacing in boiling water reactors (BWRs) is found equal to the diffusion length of the homogenized core; with increasing fuel spacing, it decreases asymptotically to the moderator diffusion length. Finally, the results of a previous investigation, in which slowing down was not included, are confirmed; i.e., (a) the total radial relaxation length in the vicinity of the detector depends strongly on the degree of heterogeneity and (b) for actual BWR configurations, the total radial relaxation length is equal to that of the homogenized core, thus it should be rather insensitive to axial position (void fraction) in the core