Alain Mariani

The help of simulation codes in designing waste assay systems using neutron measurement methods: Application to the alpha low level waste assay system PROMETHEE 6

Abstract The design of a specific nuclear assay system for a dedicated application begins with a phase of development, which relies on information from the literature or on knowledge resulting from experience, and on specific experimental verifications. The latter ones may require experimental devices which can be restricting in terms of deadline, cost and safety. One way generally chosen to bypass these difficulties is to use simulation codes to study particular aspects. This paper deals with the potentialities offered by the simulation in the case of a passive–active neutron (PAN) assay system for alpha low level waste characterization; this system has been carried out at the Nuclear Measurements Development Laboratory of the French Atomic Energy Commission. Due to the high number of parameters to be taken into account for its development, this is a particularly sophisticated example. Since the PAN assay system, called PROMETHEE (prompt epithermal and thermal interrogation experiment), must have a detection efficiency of more than 20% and preserve a high level of modularity for various applications, an improved version has been studied using the MCNP4 (Monte Carlo N-Particle) transport code. Parameters such as the dimensions of the assay system, of the cavity and of the detection blocks, and the thicknesses of the nuclear materials of neutronic interest have been optimised. Therefore, the number of necessary experiments was reduced.

Alpha-particle low-level waste control : Improvement of the promethee 6 assay system performances

Abstract The PROMpt, Epithermal and THErmal interrogation Experiment, version 6 (1996) (PROMETHEE 6) assay system for alpha-particle low-level waste characterization, developed for research and development purposes, includes both passive and active neutron measurement methods. Developed at the Commissariat à l’Energie Atomique, Cadarache Centre, in cooperation with COGEMA, its aim is to reach the incinerating alpha-particle waste requirements (<50 Bq[α]/g of crude waste, i.e., ~50 μg of Pu per drum) in 118-l “European” drums (460 mm in diameter and 750 mm high). Good preliminary results were presented: detection limits of ~0.12 mg of effective 239Pu in total active neutron counting and 0.08 mg of effective 239Pu in coincident active neutron counting [empty cavity, measurement time of 15 min, neutron generator emission of 1.6 × 108 s-1 (4π)]. Those results are improved with the use of a higher neutron source emission [GENIE 36 generator, neutron emission of 2.4 × 109 s-1 (4π)] and working on the configuration of the detector units. In the total counting mode, the gain is a factor of ~4 in a cellulose matrix and 3.1 in a polyvinyl chloride matrix. In the coincidence counting mode, these factors are 1.8 and 1.7, respectively. After a very short description of PROMETHEE 6, this paper presents the last and best performances that were obtained with the increased neutron source. Studies on the detection limit variations with the use of borated shields in front of the detection units and around the neutron generator also are dealt with.

PROMETHEE: An Alpha Low Level Waste Assay System Using Passive and Active Neutron Measurement Methods

Abstract The development of a passive-active neutron assay system for alpha low level waste characterization at the French Atomic Energy Commission is discussed. Less than 50 Bq[α] (about 50 μg Pu) per gram of crude waste must be measured in 118-l “European” drums in order to reach the requirements for incinerating wastes. Detection limits of about 0.12 mg of effective 239Pu in total active neutron counting, and 0.08 mg of effective 239Pu coincident active neutron counting, may currently be detected (empty cavity, measurement time of 15 min, neutron generator emission of 1.6 × 108 s-1 [4π]). The most limiting parameters in terms of performances are the matrix of the drum—its composition (H, Cl...), its density, and its heterogeneity degree—and the localization and self-shielding properties of the contaminant.

Applications of the associated particle technique

The associated particle technique (APT) is a “neutron in - gamma out” non destructive characterization method recently brought out of the laboratory thanks to the development of transportable associated particle neutron generators. The possibility to determine a 3D map of the elements present in an inspected volume makes this technique very attractive in complement to X- or gamma-ray imaging. Since the last few years, the Nuclear Measurement Laboratory of CEA Cadarache, France, is studying several applications for homeland security and radioactive waste characterization, in the frame of national and European projects. Last achievements for the inspection of cargo containers or suspect objects abandoned in public area, underwater explosive detection or material identification in radioactive waste will be overviewed.

Gamma-Ray Signatures Improvement of the EURITRACK Tagged Neutron Inspection System Database

The EURopean Illicit TRAfficking Countermeasures Kit (EURITRACK) inspection system uses 14 MeV neutrons produced by the D(T,n α) reaction to detect explosives in cargo containers. Reactions induced by fast neutrons inside the container produce gamma rays, which are detected in coincidence with the associated alpha particle, the detection of which allows the neutron direction to be determined. The neutron path length is obtained from a neutron time-of-flight measurement, thus allowing the origin of the gamma rays inside the container to be determined, while the chemical composition of the target material is correlated with their energy spectrum. Gamma-ray spectra have been collected with the inspection portal equipped with large volume NaI (Tl) detectors, in order to build a database of signatures for various elements (C, O, N, Fe, Pb, Al, Na, Si, Cl, Cu, Zn) with a low energy threshold of 0.6 MeV. The spectra are compared with previous ones, which were acquired with a 1.35 MeV threshold. The new library is currently being tested to unfold the energy spectra of transported goods into elemental contributions. Results are compared with data processed with the old 1.35 MeV threshold database, thus illustrating the improvement for material identification.

Detection of chemical agents with a portable neutron inspection system

The detection of special elements that can be used by terrorists in improvised chemical weapons can be performed by the spectroscopic analysis of neutron-induced gamma rays. Numerical simulations and experiments have been performed with the aim of designing a portable inspection system. Both thermal neutron analysis (TNA) and the associated particle technique (APT) have been studied within French CBNRE (chemical, biological, nuclear, radiological and explosive) R&D program. The results obtained so far show that TNA and APT can be used in complement to reach the largest possible panel of detectable elements.

Materials characterisation with the Associated Particle Technique

Since the last past years, the Nuclear Measurement Laboratory of CEA Cadarache, France, together with Partners from European and National projects, has been studying the application of fast neutron interrogation with the Associated Particle Technique for material identification in different areas of homeland and maritime security, and for the characterisation of the materials constituting radioactive waste. Fast 14 MeV neutrons are produced from the 3H(1H,n)α fusion reaction in a sealed tube neutron generator embedding an alpha detector. The alpha particle is used to tag neutron direction and emission time, thus allowing the electronic selection of neutron-induced gamma spectra in the voxels of interest. Gamma rays emitted by tagged neutron interactions on the present nuclei (C, O, N, Fe, AI, Si, CI, etc.) are recorded with spectroscopic detectors and analysed to determine elemental proportions, thus allowing material identification. Investigations have been conducted for the detection of explosives, illicit drugs and contraband materials in cargo containers, for the inspection of objects lying on the seafloor suspected to contain explosives like mines, born bs, torpedoes, etc., for the recognition of an improvised chemical device, and for material identification in radioactive waste packages. Recently the detection of special nuclear materials is being investigated using time correlation analysis between induced fission particles, instead of gamma-ray spectroscopy. The paper presents an overview of these studies and last results.

Improvement of the calibration database of the EURITRACK tagged neutron inspection system

The EURopean Illicit TRAfficking Countermeasures Kit (EURITRACK) inspection system uses 14 MeV neutrons produced by the D(T,nα) reaction to detect explosives in cargo containers. Reactions induced by fast neutrons inside the container produce gamma rays, which are detected in coincidence with the associated alpha particle, the detection of which allows to determine the neutron direction. The neutron path length is obtained from a neutron time-of-flight measurement, thus allowing to determine the origin of the gamma rays inside the container, while the chemical composition of the target material is correlated with their energy spectrum. Gamma-ray spectra have been collected with the inspection portal equipped with large volume NaI (Tl) detectors, in order to build a database of signatures for various elements (C, O, N, Fe, Pb, Al, Na, Si, Cl, Cu, Zn) with a low energy threshold of 0.6 MeV. The spectra are compared with previous ones, which were acquired with a 1.35 MeV threshold. The new library is currently being tested to unfold the energy spectra of transported goods into elementary contributions. Results are compared with data processed with the old 1.35 MeV threshold database, thus illustrating the improvement for material identification.

Improvement of the matrix effect compensation in active neutron measurement by simulated annealing algorithm (June 2009)

To answer safety authority requirements and to optimise the management of radioactive waste produced in retrieval and decommissioning activities, which contains a large variety of matrix materials, the accuracy of neutron measurement techniques has to be continuously improved. Active neutron measurements such as the Differential Die-Away (DDA) technique involving pulsed neutron generator as the neutron source, are widely applied to determine the fissile content of waste packages. Unfortunately, the main drawback of such techniques is coming from the lack of knowledge of the waste matrix composition. Thus, the matrix effect correction for the DDA measurement is an essential improvement in the field of fissile material content determination. Different solutions have been developed to compensate the effect of the matrix on the neutron measurement interpretation for a long time. In Low-Level radioactive Waste (LLW) packages examination, the most widely used methods are based on neutron flux monitoring using small 3He proportional counters added inside the detection device and associated to the “Matrix Interrogation Source” (MIS) measurement. This technique was originally developed for passive neutron measurement. It needs a specific measurement step which can be operated with the neutron generator or, most of the time, with an external isotopic neutron source such as 252Cf located as closed as possible to the waste drum. This step represents a limiting factor for the examination management and duration. In this context, this paper describes a new approach developed with the goal of increasing the accuracy of the matrix effect correction and reducing the measurement time. This is a major objective in the Non Destructive Assay (NDA) especially to enhance industrial process efficiency of large number of waste packages inspection. It deals with an innovative matrix correction method for radioactive waste embedded in a large variety of matrices regarding the density range (0.07 – 0.9 g.cm−3) as well as the composition (wood representative of hydrogenized matrix, PVC, iron, etc.). The implementation of this method is based on the analysis of the raw signal with an optimisation algorithm called the simulated annealing algorithm. This algorithm needs a reference data base of Multi-Channel Scaling (MCS) spectra, to fit the raw signal. The construction of the MCS library involves a learning phase to define and acquire the DDA signals as representative as possible of the real measurement conditions. This database has been provided by a set of active signals from experimental matrices (mock-up waste drums of 118 litres) recorded in a specific device dedicated to neutron measurement research and development of the Nuclear Measurement Laboratory of CEA-Cadarache, called PROMETHEE 6. This equipment has been designed to reach an empty cavity detection efficiency of 25%. It is equipped with a pulsed (D-T) neutron generator which can reach an average neutron emission rate up to 2.4 109 ns−1 with a pulse duration of 200 µs. This high technology performance allows achieving very low detection limits with the classical DDA measurement of fissile matter located in light waste matrices (close to 30 µg of 239Pu with an active total measurement time of 900 s). The simulated annealing algorithm is applied to make use of the effect of the matrices on the total active signal of DDA measurement. Furthermore, as this algorithm is directly applied to the raw active signal, it is very useful when active background contributions can not be easily estimated and removed. Most of the cases tested during this work which represents the feasibility phase of the method, are within a 4% agreement interval with the expected experimental value. Moreover, one can notice that without any compensation of the matrix effect, the classical DDA prompt neutron signal analysis may induce an underestimation of more than a factor of 200 on the fissile mass determination for the cases tested in this study. The unexpected so good agreement is a very promising result for the method knowing that the compositions of the mock-up drums are quite representative of the most frequently encountered matrices in LLW packages. This work is the first step of a more general thought carried out to increase the relevance of the whole treatment of DDA measurements from innovative electronic tools (specific fast charge amplifiers, list mode data card system…) up to optimised home made algorithms developed for the post-treatment of the measurements recorded by the list mode data card system.