Year
2013
Abstract
Waste streams at certain fuel cycle and research facilities may contain measurable amounts of plutonium. Passive neutron multiplicity counting is a well-established non-destructive assay technique for estimating plutonium mass, even for complex non-homogeneous matrices, such as waste drums. The JRC Drum Monitor is a recently refurbished passive neutron counter intended for safeguards verifications of 55-gallon drums in European nuclear fuel cycle facilities. This instrument incorporates 148 3He tubes arranged in a 4? geometry. It is designed to maximize counting efficiency for conditioned waste drums of low Pu content. Minimization of structural components in the drum cavity was undertaken to lessen background from cosmic ray-induced spallation neutrons in high Z materials. The detection system is optimized for standard neutron multiplicity counting to yield the mass of plutonium through the absolute determination of the spontaneous fission rate of the waste drum. Prior to redeployment in the field a series of measurements were carried out to characterize the cosmic ray induced neutron background in the multiplicity counter. The measured signal frequency distributions and their factorial moments were investigated for a variety of experimental setups. The influence of the presence of passive matrices of various densities (mock waste drums), and materials of high atomic number (Z) (such as lead and tungsten) in the cavity were investigated in detail.