Passive neutron coincidence counting with plastic scintillators for the characterization of radioactive waste drums

B. Simony (1, 2) , B. Pérot (1, 2) , C. Carasco (1, 2) , F. Jallu (1, 2) , N. Saurel (3) , S. Colas (3) , P. Girones (2) , J. Collot (4)

1 LMN - Laboratoire de Mesures Nucléaires
2 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)
3 DAM/VALDUC - VALDUC
4 LPSC - Laboratoire de Physique Subatomique et de Cosmologie

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 3He proportional counters, which have become too costly because of 3He 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 240Pu spontaneous fissions represent more than 85 percent of the total signal when using crosstalk rejection algorithms, the 15 percent remaining coincidences being due to parasitic coincidences caused by ( αn) 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 percent between a point-like fission source and the homogeneous distribution in the drum.

Mots clés: nuclear waste drums; Radioactivity; Plutonium; Neutrons; Ionization; Gamma rays; Crosstalk; ROOT; plastic scintillators; 3He proportional counters; Homogeneous distribution; passive neutron coincidence counting (PNCC); Plastic scintillator; Neutron coincidence counting; Radioactive wastes; Radioactive waste packages; Scintillation counters; Quantitative assessments; MCNPX-PoliMi

Domaines: Physique [physics]; Physique Nucléaire Expérimentale [nucl-ex]

B. Simony, B. Pérot, C. Carasco, F. Jallu, N. Saurel, et al.. Passive neutron coincidence counting with plastic scintillators for the characterization of radioactive waste drums. 2016 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room-Temperature Semiconductor Detector Workshop (NSS/MIC/RTSD), Oct 2016, Strasbourg, France. pp.1-4, ⟨10.1109/TNS.2017.2747119⟩. ⟨cea-01992389⟩