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

V Bottau,R De Stefano,G Faussier,L Tondut,C Eleon,B Perot,P.-G Allinei,C Carasco

doi:10.1109/tns.2019.2959079

Abstract

In the framework of the radioactive waste drum characterization using neutron coincidence counting, the Nuclear Measurement Laboratory of French Alternative Energies and Atomic Energy Commission (CEA) Cadarache is studying plastic scintillators as an alternative to ideal but costly 3He gas proportional counters. Plastic scintillators are at least five times cheaper for the same detection efficiency, and in addition, they detect fast neutrons three orders of magnitude more quickly than 3He detectors. However, they are sensitive to gamma rays with no pulse shape discrimination abilities for large detection volumes, which implies the necessity to identify precisely gamma background sources that may affect the useful signal. This article presents a detailed analysis of the gamma-ray spectrum of a radioactive waste drum containing glove box filters contaminated by plutonium dioxide. Classical gamma emissions following alpha and beta disintegrations are identified, and also those accompanying inelastic scattering (n, $\text{n}^{\prime }$ ) or radiative capture (n, $\gamma$ ) reactions in the whole waste drum, and ( $\alpha $ , n) or ( $\alpha $ , p) reactions in the filtration media, which can lead to neutron-gamma coincidences parasitizing useful coincidences from plutonium spontaneous fissions.

Highlights

I N THE context of passive neutron measurements for nuclear material characterization, conventional coincidence collars based on 3He gas counters permit temporal discrimination of spontaneous fission neutrons and parasitic neutrons from (α, n) reactions. 3He proportional counters are the gold standard because they show a very high capture cross section for thermal neutrons, leading to high detection efficiency when surrounding detectors with polyethylene moderator, while being practically insensitive to gamma rays
It was in quasi-contact with a 120-L radioactive waste drum containing glove box filters contaminated with plutonium dioxide (PuO2)
The detailed gamma-ray spectrometry analysis of a 120-L waste drum filled with plutonium bearing glove box filters allowed us to identify, besides natural background gamma and X-rays (40K, U, and Th chains) and well-known gamma and X-rays due to plutonium and americium radioactive decays, a number of less common gamma rays

Summary

INTRODUCTION

I N THE context of passive neutron measurements for nuclear material characterization, conventional coincidence collars based on 3He gas counters permit temporal discrimination of spontaneous fission neutrons and parasitic neutrons from (α, n) reactions. 3He proportional counters are the gold standard because they show a very high capture cross section for thermal neutrons, leading to high detection efficiency when surrounding detectors with polyethylene moderator, while being practically insensitive to gamma rays. 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, avoiding thermalizing materials and offering a nanosecond response time. Pulse shape discrimination (PSD) liquid and plastic organic scintillators are commonly used to combat gamma sensitivity [5]–[11], but they would not constitute a cost-effective alternative to 3He counters for large radioactive waste package characterization. The difference between neutron and gamma-ray pulse tails is not preserved in large PSD scintillators due to multiple light scattering [12], [13].

EXPERIMENTAL SETUP

SPECTRUM ANALYSIS

Findings

CONCLUSION