Passive Neutron Coincidence Counting With Plastic Scintillators for the Characterization of Radioactive Waste Drums

This paper reports a feasibility study, performed by numerical simulation with MCNPX-PoliMi v2.0, for plutonium quantitative assessment in radioactive waste packages by passive neutron coincidence counting with plastic scintillators. Owing to their low cost and good detection efficiency for fast neutrons, plastic scintillators could indeed constitute a good alternative to
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He proportional counters, which have become too costly because of
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He global shortage. However, their high-sensitivity to gamma rays and crosstalk are well-known drawbacks that need to be carefully studied. A measurement system for 118 L drums filled either with metallic or organic technological wastes have been modeled with MCNPX-PoliMi, and output data have been processed with ROOT. A 5-cm-thick lead shield is used in front of the detectors to attenuate plutonium and americium gamma radiation. In the studied cases, triple coincidences due to
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Pu spontaneous fissions represent more than 85% of the total signal when using crosstalk rejection algorithms, the 15% remaining coincidences being due to parasitic coincidences caused by (α, n) reactions or Am and Pu gamma rays. Although crosstalk rejection significantly reduces counting statistics, a few thousand triple coincidences are still recorded for 1 g of Pu homogeneously distributed in both metallic and organic matrices in a 25 min acquisition. For higher masses of Pu, a linear evolution of the number of coincidences with the mass is observed up to about 10 g. In addition, a study of Pu localization effects has shown that the triple coincidence difference is smaller than 20% between a point-like fission source and the homogeneous distribution in the drum.