Optimization of neutron collars for Pu-assay—Design capabilities and adequacy of nuclear data

Abstract

On-line monitoring of Pu-rich streams in a reprocessing plant is essential for development of real-time accounting systems. This can be carried out using neutron collars, which are neutron detectors embedded in a moderating medium, wrapped around the pipeline. For an efficient system, the thickness of the moderator and the position of the detectors have to be optimized. Before carrying out optimization studies, one needs to validate the calculational method and the neutron cross section set used for these studies. For this purpose, an experiment was conducted with HDPE as a moderating medium and BF 3 as a neutron detector, with a known Pu concentration. We have carried out 1- and 2-D discrete ordinate transport calculations and Monte-Carlo simulations with two standard neutron cross section data. The studies show that the numerical codes are quite sufficient. The thermalization process is not simulated properly, due to the cross section sets being inadequate. There is a need to have a good cross section set with a large number of groups in both the high-energy and thermal regions. Anisotropic scattering matrices are also necessary, since the moderators are hydrogenous media.