Characterization of the cosmic ray induced neutron multiplicity background of a He-3 passive drum counter for plutonium waste verification.

摘要

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 estimatingplutonium mass, even for complex non-homogeneous matrices, such as waste drums. The JRC Drum Monitor is arecently refurbished passive neutron counter intended for safeguards verifications of 55-gallon drums in Europeannuclear fuel cycle facilities. This instrument incorporates 148 3He tubes arranged in a 4? geometry. It is designed tomaximize counting efficiency for conditioned waste drums of low Pu content. Minimization of structuralcomponents in the drum cavity was undertaken to lessen background from cosmic ray-induced spallation neutronsin high Z materials. The detection system is optimized for standard neutron multiplicity counting to yield the massof plutonium through the absolute determination of the spontaneous fission rate of the waste drum. Prior toredeployment in the field a series of measurements were carried out to characterize the cosmic ray inducedneutron background in the multiplicity counter. The measured signal frequency distributions and their factorialmoments were investigated for a variety of experimental setups. The influence of the presence of passive matricesof various densities (mock waste drums), and materials of high atomic number (Z) (such as lead and tungsten) inthe cavity were investigated in detail.