Detection of special nuclear material (SNM) being smuggled into the US through ports of entry has been identified as a crucial capability for ensuring the safety and security of the US from radiological threats. Programs such as the NNSA's Second Line of Defense aim to deploy detection systems, both domestically and abroad, in an attempt to interdict the SNM before it reaches its destination. Active interrogation (AI) is a technique that relies on the detection of emitted particles which are produced when SNM is bombarded with a source of high energy photons or neutrons. This work presents a general framework that allows for fast radiation transport modeling of AI scenarios by generating families of response functions which depict neutron, gamma, or electron radiation exiting various regions within the problem, per unit source of radiation entering the region. The solution for a given scenario, typically the detector count rate, is computed by injecting a source term into the first region and applying the appropriate response functions, in sequence, for each subsequent region. For the AI systems modeled in this work, the source is an electron beam in a linear accelerator. Subsequent response functions create and transport bremsstrahlung photons into the SNM, and transport neutrons born in the problem to a detector. The computed solution is comparable to that of a full Monte Carlo simulation, but is assembled in orders of magnitude less time from pre-computed response function libraries. The ability to rapidly compute detector spectra for complicated AI scenarios opens up research and analysis possibilities not previously possible, including conducting parametric studies of scenarios spanning a large portion of the threat space and generating detector spectra used for conditioning and testing of alarm algorithms. / text

Em instalações nucleares o controle do material nuclear é uma das atividades da maior importância. A Comissão Nacional de Energia Nuclear (CNEN) e a Agencia Internacional de Energia Atomica (AIEA) quando de suas inspeções rotineiras tem os dados fornecidos como um fator de segurança. Ter um sistema de controle de material nuclear que permita a qualquer momento reportar a quantidade e a localização dos diversos itens a serem inspecionados é um fator de primordial importância nos dias de hoje. Neste trabalho objetivou-se aprimorar um sistema já existente utilizando para seu desenvolvimento uma plataforma mais amigável através da linguagem de programação VisualBasic (Microsoft Corporation) para facilitar a equipe de operação do Reator IEA-R1 o fornecimento de dados que possibilitem o melhor controle dos materiais nucleares do Reator IEA-R1. Esses dados tem permitido o desenvolvimento de trabalhos a serem apresentados em congressos nacionais ou internacionais bem como em dissertações de mestrado ou teses de doutorado. O programa foi desenvolvido para atender as exigências das normas de salvaguarda da CNEN e da AIEA, mas suas funções podem ser ampliadas conforme as necessidades futuras. Este sistema poderá ser utilizado em outros reatores que por ventura sejam contruidos no pais, pois é bem pratico e sua utilização permite um um controle efetivo sobre o material nuclear da instalação. / In nuclear facilities, the nuclear material control is one of the most important activities. The National Commission of Nuclear Energy (CNEN) and the International Atomic Energy Agency (IAEA), when inspecting routinely, regards the data provided as a major safety factor. Having a control system of nuclear material that allows the amount and location of the various items to be inspected, at any time, is a key factor today. The objective of this work was to enhance the existing system using a more friendly platform of development, through the VisualBasic programming language (Microsoft Corporation), to facilitate the operation team of the reactor IEA-R1 Reactor tasks, providing data that enable a better and prompter control of the IEAR1 nuclear material. These data have allowed the development of papers presented at national and international conferences and the development of master´s dissertations and doctorate theses. The software object of this study was designed to meet the requirements of the CNEN and the IAEA safeguard rules, but its functions may be expanded in accordance with future needs. The program developed can be used in other reactors to be built in the country, since it is very practical and allows an effective control of the nuclear material in the facilities.

Em instalações nucleares o controle do material nuclear é uma das atividades da maior importância. A Comissão Nacional de Energia Nuclear (CNEN) e a Agencia Internacional de Energia Atomica (AIEA) quando de suas inspeções rotineiras tem os dados fornecidos como um fator de segurança. Ter um sistema de controle de material nuclear que permita a qualquer momento reportar a quantidade e a localização dos diversos itens a serem inspecionados é um fator de primordial importância nos dias de hoje. Neste trabalho objetivou-se aprimorar um sistema já existente utilizando para seu desenvolvimento uma plataforma mais amigável através da linguagem de programação VisualBasic (Microsoft Corporation) para facilitar a equipe de operação do Reator IEA-R1 o fornecimento de dados que possibilitem o melhor controle dos materiais nucleares do Reator IEA-R1. Esses dados tem permitido o desenvolvimento de trabalhos a serem apresentados em congressos nacionais ou internacionais bem como em dissertações de mestrado ou teses de doutorado. O programa foi desenvolvido para atender as exigências das normas de salvaguarda da CNEN e da AIEA, mas suas funções podem ser ampliadas conforme as necessidades futuras. Este sistema poderá ser utilizado em outros reatores que por ventura sejam contruidos no pais, pois é bem pratico e sua utilização permite um um controle efetivo sobre o material nuclear da instalação. / In nuclear facilities, the nuclear material control is one of the most important activities. The National Commission of Nuclear Energy (CNEN) and the International Atomic Energy Agency (IAEA), when inspecting routinely, regards the data provided as a major safety factor. Having a control system of nuclear material that allows the amount and location of the various items to be inspected, at any time, is a key factor today. The objective of this work was to enhance the existing system using a more friendly platform of development, through the VisualBasic programming language (Microsoft Corporation), to facilitate the operation team of the reactor IEA-R1 Reactor tasks, providing data that enable a better and prompter control of the IEAR1 nuclear material. These data have allowed the development of papers presented at national and international conferences and the development of master´s dissertations and doctorate theses. The software object of this study was designed to meet the requirements of the CNEN and the IAEA safeguard rules, but its functions may be expanded in accordance with future needs. The program developed can be used in other reactors to be built in the country, since it is very practical and allows an effective control of the nuclear material in the facilities.

High energy neutron and charged particle radiation cause microstructural and mechanical degradation in structural metals and alloys, such as phase segregation, void swelling, embrittlement and creep. Radiation induced damages typically limit nuclear materials to a lifetime of about 40 years. Next generation nuclear reactors require materials that can sustain over 60 - 80 years. Therefore it is of great significance to explore new materials with better radiation resistance, to design metals with favorable microstructures and to investigate their response to radiation. The goals of this thesis are to study the radiation responses of several nanostructured metallic thin film systems, including Ag/Ni multilayers, nanotwinned Ag and nanocrystalline Fe. Such systems obtain high volume fraction of boundaries, which are considered sinks to radiation induced defects. From the viewpoint of nanomechanics, it is of interest to investigate the plastic deformation mechanisms of nanostructured films, which typically show strong size dependence. By controlling the feature size (layer thickness, twin spacing and grain size), it is applicable to picture a deformation mechanism map which also provides prerequisite information for subsequent radiation hardening study. And from the viewpoint of radiation effects, it is of interest to explore the fundamentals of radiation response, to examine the microstructural and mechanical variations of irradiated nanometals and to enrich the design database. More importantly, with the assistance of in situ techniques, it is appealing to examine the defect generation, evolution, annihilation, absorption and interaction with internal interfaces (layer interfaces, twin boundaries and grain boundaries). Moreover, well-designed nanostructures can also verify the speculation that radiation induced defect density and hardening show clear size dependence. The focus of this thesis lies in the radiation response of Ag/Ni multilayers and nanotwinned Ag subjected to charged particles. The radiation effects in irradiated nanograined Fe are also investigated for comparison. Radiation responses in these nanostructured metallic films suggest that immiscible incoherent Ag/Ni multilayers are more resistant to radiation in comparison to their monolithic counterparts. Their mechanical properties and radiation response show strong layer thickness dependence in terms of radiation hardening and defect density. Coherent twin boundaries can interact with stacking fault tetrahedral and remove them effectively. Twin boundaries can actively absorb radiation induced defects and defect clusters resulting in boundary migration. Size dependence is also found in nanograins where fewer defects exhibit in films with smaller grains.

In this thesis,zirconium carbide (ZrC) layers were deposited on graphite substrates using a CVD reactor at temperatures ranging from 1250 °C to 1450 °C in steps of 50 °C. The deposited layers were characterised by XRD, Raman Spectroscopy and SEM.ZrCsamples were also prepared by spark plasma sintering (SPS), at 1700, 1900 and 2100 °C at 50 MPa for 10 minutes. The phase and microstructure after the sintering process were investigated by XRD and SEM. Iridium (Ir) thin films were deposited on these ZrCsamples and annealed in vacuum at temperatures of 600 and 800 °C for 2h. The phase composition, solid-state reactions and surface morphology were investigated by GIXRD and SEM. XRD was used to identify the phases present in the as-deposited and annealed samples. It showed that Ir2Zr was the initial phase formed at 600 °C. At temperature 800 °C IrZr formed. / Thesis (PhD (Physics))--University of Pretoria, 2019. / University of Pretoria / Physics / PhD (Physics) / Unrestricted

Large passive detectors used in screening for special nuclear materials at ports of entry are characterized by poor spectral resolution, making identification of radionuclides a difficult task. Most identification routines, which fit empirical shapes and use derivatives, are impractical in these situations. Here I develop new, physics-based methods to determine the presence of spectral signatures of one or more of a set of isotopes. Gamma-ray counts are modeled as Poisson processes, where the average part is taken to be the model and the difference between the observed gamma-ray counts and the average is considered random noise. In the linear part, the unknown coefficients represent the intensites of the isotopes. Therefore, it is of great interest not to estimate each coefficient, but rather determine if the coefficient is non-zero, corresponding to the presence of the isotope. This thesis provides new selection algorithms, and, since detector data is undoubtedly finite, this unique work emphasizes selection when data is fixed and finite.

A hypothetical smuggling of material suitable for a nuclear weapon is known as a threat scenario. There is a considerable effort by the U.S. government to reduce this threat by placing radiation detectors at key interdiction points around the world. These detectors provide deterrence and defense against smuggling attempts by scanning vehicles, ships, and pedestrians for threat objects. Formulating deployment strategies for these detectors within the global transportation network requires an understanding of the complex interactions between the attributes of a smuggler and the detection systems. These strategies are rooted in the continued development of novel detection systems and alarm algorithms. Radiation transport simulation provides a means for characterizing detection system response to threat scenarios. However, this task is computationally expensive with existing radiation transport codes. Furthermore, the degrees of freedom in smuggler and threat scenario attributes create a large, constantly evolving problem space. Previous research has demonstrated that decomposing the scenario into independently simulated components using Green's functions can simulate photon detector signals with coarse energy resolution. This dissertation presents a general form of this approach, applicable to a wide range of threat scenarios through physics enhancements and numerical treatments for high energy resolution photon transport, neutron transport, and time dependent transport. While each Green's function implicitly captures the full transport phase-space within each component, these new methods ensure that this information is preserved between components. As a result, detector signals produced from full forward transport simulations can be replicated within 20% while requiring multiple orders of magnitude less computation time. This capability is presented as a general threat scenario simulation platform which can efficiently model a large problem space while preserving the full radiation transport phase-space. / text

Classification problems arise in so many different parts of life – from sorting machine parts to diagnosing a disease. Humans make these classifications utilizing vast amounts of data, filtering observations for useful information, and then making a decision based on a subjective level of cost/risk of classifying objects incorrectly. This study investigates the translation of the human decision process into a mathematical problem in the context of a border security problem: How does one find special nuclear material being smuggled inside large cargo crates while balancing the cost of invasively searching suspect containers against the risk of al lowing radioactive material to escape detection? This may be phrased as a classification problem in which one classifies cargo containers into two categories – those containing a smuggled source and those containing only innocuous cargo. This task presents numerous challenges, e.g., the stochastic nature of radiation and the low signal-to-noise ratio caused by background radiation and cargo shielding. In the course of this work, we will break the analysis of this problem into three major sections – the development of an optimal decision rule, the choice of most useful measurements or features, and the sensitivity of developed algorithms to physical variations. This will include an examination of how accounting for the cost/risk of a decision affects the formulation of our classification problem. Ultimately, a support vector machine (SVM) framework with F -score feature selection will be developed to provide nearly optimal classification given a constraint on the reliability of detection provided by our algorithm. In particular, this can decrease the fraction of false positives by an order of magnitude over current methods. The proposed method also takes into account the relationship between measurements, whereas current methods deal with detectors independently of one another.

Due to increasing terrorist threats and illegal proliferation of nuclear material and technology, there is a need for increased research in the area of detection of smuggled fissile material, some of which is designated by the International Atomic Energy Agency as special fissile material. This thesis focuses on a hypothetical scenario in which a terrorist organization has managed to smuggle an amount of special fissile material onto a personal recreational watercraft and sail it into a marina. If the boat could be forced to go through a detector system, then the contents could be interrogated and a determination made of whether any special fissile material was aboard. This thesis examines the hypothesis that active interrogation may be used successfully in the detection of special fissile material in such an environment. It shows that it is feasible to use an active neutron system to detect a significant quantity of special fissile material onboard a small boat via the differential dieaway technique. The MCNP Monte Carlo transport code was used to simulate the use of a pulsed neutron generator to induce fission in the fissile material and then estimate the detector response. The detector modeled was based on elastic scattering-induced recoil protons using pure hydrogen gas. There was a significant difference between the system with and without the presence of fissile material, and the estimated detector response for the system with fissile material present was shown to be sufficiently greater than the response due to background radiation only. Additionally, dose was estimated and found to be small enough that the system would not likely pose a significant radiological health risk to passengers on the boat.

Cette thèse étudie la détection de matière nucléaire avec la technique de la particule associée pour l’inspection de bagages abandonnés ou de conteneurs maritimes dans le domaine de la sécurité. Le principe consiste à mesurer, avec des scintillateurs plastique, les coïncidences entre particules de fissions induites par des neutrons de 14 MeV produits par un générateur basé sur la réaction 2H(3H,n)4He et équipé d’un détecteur alpha à localisation pour déterminer le temps d’émission et la direction du neutron opposé. La détection d’au moins trois particules de fission en coïncidence avec la particule qui permet de discriminer les matières nucléaires des matériaux bénins. Le système d’acquisition et les outils de simulation ont été qualifiés en passif avec des sources radioactives puis en actif avec le générateur et diverses cibles, validant les estimations de performances de systèmesd’inspection de bagages abandonnés ou de conteneurs maritimes réalisées par simulation numérique avec le code MCNP-PoliMi. Il est ainsi possible de détecter en quelques minutes, quelques kg d’uranium au centre d’un container rempli d’une matrice fer mêmesi l’échantillon est masqué par du plomb, à l’aide du signal des neutrons prompts de fission. La détection est plus difficile dans les matrices organiques en raison de la diffusion des neutrons interrogateurs et de fission sur les noyaux d’hydrogène. Par ailleurs, l’utilisation de scintillateurs plastiques à la place des compteurs gazeux à 3He a été évaluée pour caractériser le plutonium dans les colis de déchets radioactifs par mesure passive des coïncidences. La détection des neutrons de fission est beaucoup plus rapide,ce qui permet de minimiser le bruit accidentel dû aux réactions (,n). Les scintillateurs sont cependant plus sensibles aux rayonnements gamma et à la diaphonie entre détecteurs voisins, ce qui nécessite d’exploiter les coïncidences de multiplicité 3 avec un traitement des données spécifique pour limiter la diaphonie. / This thesis investigates the detection of Special Nuclear Materials (SNM) by neutroninterrogation with the Associated Particle Technique (APT). 14 MeV neutrons areproduced from the 3H(2H,n)α fusion reaction in a sealed tube neutron generatorembedding a position-sensitive alpha detector. The alpha detector determines thedirection of the nearly opposite neutron and its time of flight. The detection of at leastthree prompt fission particles in coincidence with the tagged neutron signs the presenceof SNM. The acquisition system and simulation tools have been qualified in passive modewith radioactive sources and active mode with the generator and various targets,validating the simulation of inspection systems with MCNP-PoliMi. Calculations showthat the detection of a few kilograms of shielded SNM with the ATP is possible in ironcargo container, with the prompt fission neutrons signal. Detection is more difficult inorganic matrices due to tagged- and prompt fission neutrons scattering on hydrogennuclei. Furthermore, the use of plastic scintillators instead of 3He counters was studied tocharacterize the plutonium in the radioactive waste by passive coincidences measurement.Measurements at fast time scales of fast-neutrons instead of the long time scales ofthermal-neutrons reduce random coincidences that can occur with high (,n) reactionrate. The scintillators are however sensitive to gamma rays and cross-talk betweenadjacent detectors. Therefore, we used data-analysis algorithms to minimize cross-talkcontribution to measured three-fold coincidences.