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

Abstract

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 3 He proportional counters, which have become too costly because of 3 He global shortage. However, their highsensitivity to gamma rays and cross talk 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 has 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 240 Pu spontaneous fissions represent more than 85% of the total signal when using cross-talk rejection algorithms, the 15% remaining coincidences being due to parasitic coincidences caused by (,n) reactions or Am and Pu gamma rays. Although cross-talk 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.

Sorting fission from parasitic coincidences of neutrons and gamma rays in plastic scintillators using particle times of flight

“…However, the shortage of 3 He gas has greatly increased the cost of these detectors, leading to a worldwide effort to find possible alternatives, especially in the field of homeland security and safeguards [2], [3]. The Nuclear Measurements Laboratory of CEA Cadarache (France) makes studies of lowcost EJ-200 Polyvinyl Toluene (PVT) plastic scintillators as a potential alternative for neutron coincidence collars [4], [5]. Efficient neutron detection with 3 He counters requires neutron thermalization, which lasts several tens of microseconds, whereas plastic proton recoil detectors are directly sensitive to fast neutrons, which need just a few tens of nanoseconds to cover the source-detector distance.…”

High-Resolution Gamma Spectrometry of a Plutonium Bearing Waste Drum With High-Energy Reaction-Induced Gamma Rays

“…However, the global shortage of 3 He gas [1] has greatly increased the cost in the past 15 years, leading to worldwide initiatives to look for cheaper alternatives with equivalent performances [2]. The Nuclear Measurements Laboratory of French Alternative Energies and Atomic Energy Commission (CEA), Nuclear Energy Division (DEN), Cadarache, is currently studying the use of low-cost polyvinyl toluene (PVT) plastic scintillators as a potential alternative [3], [4] that have the advantage of being sensitive to fast neutrons, thus avoiding thermalizing materials and offering a nanosecond response time. Consequently, very short coincidence windows of a few tens of nanoseconds allow greatly reducing accidental coincidences compared to three orders of magnitude slower 3 He detection systems, mainly due to the time needed for neutron thermalization.…”