Frédérick Carrel

Detection of nuclear material by photon activation inside cargo containers

Photons with energies above 6 MeV can be used to detect small amounts of nuclear material inside large cargo containers. The method consists in using an intense beam of high-energy photons (bremsstrahlung radiation) in order to induce reactions of photofission on actinides. The measurement of delayed neutrons and delayed gammas emitted by fission products brings specific information on localization and quantification of the nuclear material. A simultaneous measurement of both of these delayed signals can overcome some important limitations due to matrix effects like heavy shielding and/or the presence of light elements as hydrogen. We have a long experience in the field of nuclear waste package characterization by photon interrogation and we have demonstrated that presently the detection limit can be less than one gram of actinide per ton of package. Recently we tried to extend our knowledge to assess the performance of this method for the detection of special nuclear materials in sea and air freights. This paper presents our first results based on experimental measurements carried out in the SAPHIR facility, which houses a linear electron accelerator with the energy range from 15 MeV to 30 MeV. Our experiments were also modeled using the full scale Monte Carlo techniques. In addition, and in a more general frame, due to the lack of consistent data on photonuclear reactions, we have been working on the development of a new photonuclear activation file (PAF), which includes cross sections for more than 600 isotopes including photofission fragment distributions and delayed neutron tables for actinides. Therefore, this work includes also some experimental results obtained at the ELSA electron accelerator, which is more adapted for precise basic nuclear data measurements.

GAMPIX: A new gamma imaging system for radiological safety and Homeland Security Purposes

Localization of radioactive sources is a major issue for many applications in the nuclear industry. Gamma imaging is a powerful non-destructive technique, enabling to locate radioactive hot spots contained in a given area and well-adapted to this topic. Industrial gamma cameras have been commercialized by various companies for several years but present some limitations in terms of sensitivity, weight and ease of use in comparison with the requirements of end-users.

Detection of actinides with an electron accelerator by active photoneutron interrogation measurements

The solution for management of a nuclear waste package is chosen according to its radiological characteristics. One of the most important of these features is the -activity which is due to actinides ( U, U, Pu, etc.) If non-destructive passive methods are not sufficient to quantify the latter, non-destructive active methods based on the fission process represent a solution of interest. First, these methods consist in irradiating a package in order to induce fission reactions on the actinides, and then, to detect the prompt and delayed particles which are emitted following these reactions. Our aim is to conduct neutron interrogation measurements on nuclear waste packages using an electron accelerator as a photoneutron generator. One of the main interests of this approach is that the intensity of the neutron flux can be one or two orders of magnitude higher than the one delivered by a deuterium-tritium generator. With the objective of improving nuclear waste characterization, the development of this method could enable the integration of three complementary techniques on a single measurement cell (active neutron interrogation, active photon interrogation, and high-energy imaging). In this paper, simulation and experimental results are presented. A simulation study using MCNPX has been conducted in order to determine the characteristics of the photoneutron flux emitted by the electron accelerator of the SAPHIR facility owned by CEA LIST. Energy spectra, angular distribution and intensity of the photoneutron flux have been obtained. A photoneutron interrogation measurement cell based on this accelerator has been built and assessed by carrying out measurements on uranium samples. Delayed gamma-ray spectra have been acquired and enabled to confirm the experimental feasibility of our method.

Determination of actinide isotopic composition: Performances of the IGA code on plutonium spectra according to the experimental setup

The IGA code (a French acronym standing for actinides gamma isotopy) is a tool developed by the CEA LIST to determine the isotopic composition of plutonium and uranium, based on the automatic analysis of the γ/X spectrum emitted by a nuclear sample. Its main feature is its generic approach of the problem. As a consequence, the IGA code is a very flexible tool, because no particular experimental setup is imposed to the user for the acquisitions, in terms of energy range, gain, channel number or detector resolution. However, these experimental conditions can have an impact on the quality of the results. So, a study has been carried out to evaluate the effect of different experimental parameters on the IGA performances for plutonium spectra and quantify these performances. The study has been led according to three different approaches: analysis of IGA results on a whole spectrum database coming from French laboratories, analysis on artificially modified spectra, and finally analysis of results on new real spectra acquired in the laboratory on certified samples.

New Experimental Results on the Cumulative Yields From Thermal Fission of

The yields of fission products are one of the main characteristics of the fission process. In the field of nuclear waste package characterization, using Photon Activation Analysis (PAA), these yields are needed in order to optimize a technique enabling the identification of actinides (235U, 238U, 239Pu), based on the detection of delayed gamma-rays. As the lack of data in the field of photofission is strongly penalizing for the tuning of this technique, we designed several measurement campaigns in order to determine the yields of various photofission products. The experiments were based on the detection of delayed gamma-rays and delayed neutrons emitted during the same measurement. The feasibility of this technique was first verified in the context of active neutron interrogation, by comparing experimental results for the thermal fission of 235U and 239Pu with reference values provided by several recent databases (ENDFB 6.8, JEFF 3.1). The method was then applied to active photon interrogation, in order to obtain the yields of nuclides formed by the photofission of 235U and 238U. This paper presents the experimental results obtained with these measurements.

^{235} {\rm U}

CEA is developing a method to prevent illegal traffic of special nuclear material (SNM) at borders, especially sea ports where millions of containers are transiting every day. This nondestructive method is based on the interrogation of the container by high-energy photon and the subsequent detection of particles emitted after the fission. This technique requires the knowledge of nuclear parameters that have been rarely measured for fission induced by photons. Therefore, we present fundamental measurements of these parameters for uranium-238 and uranium-235. Numerical simulations to assess the SNM detection performances are also reported.

and

The tracking of radiation contamination and distribution has become a high priority in the nuclear cycle industry in order to respect the ALARA principle which is a main challenge during decontamination and dismantling activities. To support this need, AREVA/CANBERRA and CEA LIST have been actively carrying out research and development on a gamma-radiation imager. In this paper we will present the new generation of gamma camera, called GAMPIX. This system is based on the Timepix chip, hybridized with a CdTe substrate. A coded mask could be used in order to increase the sensitivity of the camera. Moreover, due to the USB connection with a standard computer, this gamma camera is immediately operational and user-friendly. The final system is a very compact gamma camera (global weight is less than 1 kg without any shielding) which could be used as a handheld device for radioprotection purposes. In this article, we present the main characteristics of this new generation of gamma camera and we expose experimental results obtained during in situ measurements. Even though we present preliminary results the final product is under industrialization phase to address various applications specifications.

^{239}{\rm Pu}

Being able to characterize and classify nuclear waste packages is crucial for an appropriate mode of storage. The alpha-activity due to the presence of actinides inside the packages can be determined using non-destructive active (NDA) methods. CEA-LIST is developing one of those, based on fission induced by photons that allows us to quantify the global mass of actinides in large concrete packages and localize it by the means of a tomography using delayed particles emitted after the fission process.

and From Photofission of

War against CBRN-E threats needs to continuously develop sensors with improved detection efficiency. More particularly, this topic concerns the NR controls for homeland security. A first analysis requires indeed a fast gamma spectrometry so as to detect potential special nuclear materials (SNM). To this aim, plastic scintillators could represent the best alternative for the production of large-scale, low-cost radiation portal monitors to be deployed on boarders, tolls, etc. Although they are known to be highly sensitive to gamma rays, due to their poor resolution, information relative to the nature of the SNM is tricky. Thus, only the Compton edge is obtained after interaction, and no information of the photoelectric peak is observed. This project concerns new developments on a possible pseudogamma spectrometry performed with plastic scintillators. This project is articulated on a combination of two developments: - The design of new materials most suitable for recovering the photoelectric peak after gamma interaction with the scintillator. This work concerns mainly plastic scintillators loading with heavy elements, such as lead or bismuth. - The analysis of the resulting signal with smart algorithms. This work is thus a pluridisciplinary work performed at CEA LIST and embeds 4 main disciplines: MCNPX simulations (simulated spectra), chemistry of materials (preparation of various plastic scintillators with different properties), instrumentation (lab experiments) and smart algorithms. Really impressive results were obtained with the unfolding of simulated spectra at various energies (from 241Am to 60Co) and an innovative approach was proposed to counter-balance the quenching effect of luminescence by heavy elements in plastic scintillators.

^{235} {\rm U}

Radiological characterization of nuclear waste packages is an industrial issue in order to select the best mode of storage. The characterization becomes crucial particularly for waste packages produced at the beginning of the French nuclear industry. For the latter, available information is often incomplete and some key parameters are sometimes missing (content of the package, alpha-activity, fissile mass...) In this case, the use of non-destructive methods, both passive and active, is an appropriate solution to characterize nuclear waste packages and to obtain all the information of interest. In this article, we present the results of a complete characterization carried out on the TE 1060 block, which is a nuclear waste package produced during the 1960s in Saclay. This characterization is part of the DEMSAC (Dismantling of Saclays facilities) project (ICPE part). It has been carried out in the SAPHIR facility, located in Saclay and housing a linear electron accelerator. This work enables to show the great interest of active methods (photon activation analysis and high-energy imaging) as soon as passive techniques encounter severe limitations.