Illicit Nuclear Material Research Articles

Finding the Source of Illicit Nuclear Material

Finding the Source of Illicit Nuclear Material

Tuning the index of refraction of a polyvinyl toluene and polystyrene copolymer toward a heterogenous, index‐matched neutron detector

AbstractOne class of neutron detectors for illicit nuclear materials are capture‐gated detectors, which use organic scintillators to slow neutrons while emitting fluorescent light and elements that have high neutron capture cross‐sections to provide a second signal. Homogeneous detectors composed of neutron capturing metallo‐organics within plastic darken due to their chemical instability, while heterogeneous detectors frequently result in non‐transparent material due to a mismatch of the refractive index. These detectors are often polymerized through bulk polymerization, but there is little data available on this process applied to mixtures of polystyrene (PS) and polyvinyl toluene (PVT), two commonly used polymers in plastic scintillators. This work presents bulk polymerization processing toward an index‐matched, heterogeneous capture‐gated neutron detector based on PS and PVT copolymers with a range of refractive indices. Specifically 1:3, 1:1, and 3:1 PS:PVT ratios were manufactured and their refractive indices, measured by refractometry, were compared to a theoretical model based on a mixture of the refractive indices of pure PS and PVT. Finally, a composite of PS/PVT and an Ohara S‐BAL42 glass was developed to confirm the index‐matching capability of the process as a step toward developing a heterogenous, capture‐gated neutron detector with high light transmission efficiencies allowed by index‐matched materials.

High-fidelity photoneutron detection via neutron activation analysis

To interdict illicit radioactive sources and prevent nuclear terror, governments have deployed portal monitor systems at ports of entry. Such portal monitors rely upon the detection of spontaneously-emitted radiation signatures, however, detection scenarios become more challenging for highly enriched uranium, which emits relatively few spontaneous radiation signatures. Photon active interrogation techniques can augment current cargo inspection scenarios and improve detection capabilities for illicit nuclear material. A high-energy photon interrogation source can induce photofission in nuclear material, producing strong radiation signatures for detection. Prompt fission signals are of specific interest because prompt neutrons are greater than 99% of the fission signal. When using detectors capable of spectroscopy, e.g., pulse shape discrimination-capable organic scintillators, the intense interrogating source often will cause pulse pile-up, degrading particle classification. In this work, we demonstrate photoneutron detection via neutron activation of aluminum and iron. The experiments rely upon nuclear reactions that can only be induced by neutrons, and are robust against pile-up, thereby providing high-confidence neutron rates. In this work, we apply neutron activation analysis for the detection of high-energy neutrons produced by high-energy photon irradiation; the depleted uranium sample was irradiated with a 9-MeV bremsstrahlung beam. We find that the irradiated depleted uranium produced an aluminum activation flux of 1.77 × 105 n cm −2 s−1 and an iron-56 activation flux of 1.05 × 105 n cm −2 s−1. The ability to detect radioactivity induced by neutrons created in (γ, n) and photofission reactions demonstrates a new approach for photon active interrogation to identify nuclear materials.

Nonparametric Dense-Object Detection Algorithm for Applications of Cosmic-Ray Muon Tomography

We present an algorithm that utilizes data generated by cosmic-ray muon-scattering tomography for passive nondestructive detection of dense objects. Our clustering-based approach uses a nonparametric statistical test based on a reference case to determine the presence of high-density high-$Z$ material, such as illicit nuclear material hidden inside a shipping canister. The algorithm outputs a single decision value for the absence or presence of illicit material without the need to perform a detailed visual tomographic reconstruction and/or the need to rely on human interpretation, in contrast to many other muon-based imaging techniques. The performance of the algorithm is demonstrated using experimental data obtained with the Cosmic-Ray Inspection and Passive Tomography detector from two setups consisting of a lead flask, a 55-gallon drum filled with sand, and 2 kg of metallic depleted uranium (DU). The results of these experiments illustrate that the high-density lead flask can be automatically distinguished from the background, with an area under the receiver-operating-characteristic curve (AUC) of 0.90 using less than 90 s of data; the lead flask with 2 kg of DU inside can be detected within the 55-gallon drum containing sand in under 5 min with an AUC of 0.91. Our experimental results illustrate the efficacy of this algorithm for the identification of dense objects using reference background measurements. Practical applications of this algorithm are discussed.

Nuclear forensic signatures of studtite and α-UO3 from a matrix of solution processing parameters

At the back-end of the nuclear fuel cycle, spent nuclear fuel (SNF) may be processed in solution to form a precipitate of uranyl peroxide tetrahydrate, also known as studtite ([(UO2)(O2)(H2O)2]•2H2O). The physical and chemical properties of studtite are influenced by the solution processing conditions employed, thus, the processing history of the precipitate may be elucidated from these characteristics. This is useful in the field of nuclear forensics, where the provenance of intercepted illicit nuclear material is of interest. Studtite is often calcined to ≥500 °C to form alpha uranium trioxide (α-UO3), an oxide prevalent in waste storage. In this study, the crystal structure of UO3 is analysed, with a discussion on the α/α’-UO3 C2mm symmetry. The morphological and structural signatures of both studtite and UO3 are investigated, with respect to a matrix of solution processing conditions and the thermal treatment of studtite.

Development of an optical sensor for measuring opacity changes in polyvinyl toluene scintillators

Polyvinyl toluene (PVT) based detectors are used in radiation portal monitors (RPMs) throughout the world to detect the trafficking of illicit nuclear material. PVT scintillators, which are normally optically clear, have been observed to suffer internal fogging throughout their volume due to prolonged exposure to varying environmental conditions. These changes could lead to reduced performance for RPMs that utilize plastic scintillators. In this research, a proof of concept system, consisting of different color light emitting diodes (LEDs) and a single optical sensor (OS), were used to examine the change in light transmission through a PVT scintillator. This Optical Monitoring System (OMS), coupled with an environmentally exposed PVT detector, was tested in an environmental chamber where it was subjected to changes in temperature and humidity ranging from 55 °C at 100% relative humidity to −20 °C at 40% relative humidity. At temperatures below 10 °C, light transmission was reduced by 81% ± 8% for blue LEDs, 84% ± 5% for yellow LEDs, and 49% ± 4% for green LEDs. Similar reductions in detected light were not recorded when the OMS was tested with only air between the LEDs and OS. Therefore, the significant reductions in transmitted light were attributed to changes occurring within the PVT scintillator. These results indicate that a significant reduction in PVT opacity occurs due to wide environmental changes. A device such as the OMS could be used to track these changes and provide users an early indication that a portal monitor is suffering from reduced performance.

Data for training and testing radiation detection algorithms in an urban environment

The detection, identification, and localization of illicit nuclear materials in urban environments is of utmost importance for national security. Most often, the process of performing these operations consists of a team of trained individuals equipped with radiation detection devices that have built-in algorithms to alert the user to the presence nuclear material and, if possible, to identify the type of nuclear material present. To encourage the development of new detection, radioisotope identification, and source localization algorithms, a dataset consisting of realistic Monte Carlo–simulated radiation detection data from a 2 in. × 4 in. × 16 in. NaI(Tl) scintillation detector moving through a simulated urban environment based on Knoxville, Tennessee, was developed and made public in the form of a Topcoder competition. The methodology used to create this dataset has been verified using experimental data collected at the Fort Indiantown Gap National Guard facility. Realistic signals from special nuclear material and industrial and medical sources are included in the data for developing and testing algorithms in a dynamic real-world background.

Monitoring the long term stability of civil buildings through the MRPC telescopes of the EEE Project

Cosmic ray muons are a penetrating component of extensive air showers created in the Earth atmosphere by the interaction of highly energetic primary particles, mostly protons, which continuously bombard our Planet. The secondary cosmic radiation is the result of the complex interplay between the production cross section and the interaction mechanisms with the atmosphere (including the energy loss, multiple scattering and particle decay). Cosmic muons have been considered since several decades as a powerful probe to exploit our environment, from muography of volcanoes to absorption radiography of possible hidden rooms inside large structures, such as Pyramids, to the detection of high-Z illicit nuclear materials inside containers and many other applications of social interest.This paper discusses the possibility to employ the Multigap Resistive Plate Chambers (MRPC) of the Extreme Energy Events (EEE) Project as muon tracking detectors to monitor the long term stability of civil buildings and structures when used in conjunction with additional detectors. For this application the average direction of the cosmic muon tracks passing through the MRPC telescope and an additional detector located some distance apart in the same building may be reconstructed with good precision and any small variation over long time acquisition periods may be monitored. The performance of such setup is discussed and experimental results from first coincidence measurements obtained with a 40 × 60 cm2 scintillator detector operated in the same building with one of the EEE telescopes, at about 15 m vertical distance from it, are presented. Simple Monte Carlo and GEANT simulations were also carried out to evaluate typical acceptance values for the operating conditions employed so far, to extrapolate to other geometrical configurations, and to evaluate multiple scattering effects.

Retrospective characterization of special nuclear material in time and space

A 4.5 kg sphere of α-phase plutonium was subjected to passive imaging using optically stimulated luminescence dosimetry techniques via inverse square modeling under cylindrical symmetry around the dosimeter array. The results showed angular resolution in the localization capability close to 1° due to axial resolution below 1 cm. Radial resolution was much worse having an offset of 16 cm using only point source geometry estimates for the commercial dosimeters. Using MCNP™ to reconstruct the profile demonstrated a substantial improvement in reconstructing the relative response as opposed to assuming simple point source geometry. From this, an inverse solving approach known as DRAM was used to estimate source distribution in addition to location. These results are considered regarding their implications for nuclear nonproliferation to the extent they demonstrate potential to determine whether illicit nuclear material had historically been kept in any specific location or alternatively, whether such materials had not been kept in a location they were claimed to have been stored. Having measured the materials location and knowing the integrated measurement time then allows estimating amount of material via dose (or alternatively, knowing the assay could give storage time estimates).

Maritime Interdiction of North Korean Ships under UN Sanctions

To be effective in shaping state conduct, the liberalism and idealism that informs public international law must contend with geopolitical realities and the role of power in the international system. David D. Caron was unafraid to address this dichotomy. His work bridged epistemic communities and offered concrete approaches to some of the most vexing international problems. Caron’s work on radioactive waste and nuclear weapons at sea, for example, manifests a profound understanding of the threats posed by the proliferation of nuclear weapons and illicit nuclear materials in the oceans, and how international law reduces these ghastly perils. By providing boundaries for state behavior and fashioning a stability of expectations, international law deepens military and environmental security, and thereby reduces geopolitical risks. Inspired by the contributions of Caron and Harry Scheiber in exploring international law as a tool for addressing the threat of nuclear weapons and material at sea, this Article demonstrates how the legal process is the best tool available, albeit an imperfect one, to counter North Korea’s maritime proliferation of nuclear weapons and technology. The nuclear weapons program of the People’s Democratic Republic of Korea (DPRK) emerged within a complex regional political reality. Protected by U.S. extended deterrence, the Republic of Korea (ROK) has prospered for nearly seventy years despite lying directly under the guns of North Korea, which is enabled by China and Russia to serve as a cudgel to oppose American presence in the region. The DPRK detonated its first nuclear device in 1993, challenging the U.S. nuclear security umbrella and opening the specter of nuclear proliferation. If the United States and ROK actively punish North Korea with military force, North Korea likely would lash out in all directions. If they relent and accept the DPRK into the nuclear club, the consequences could prove equally deadly. International law lies within this dilemma as the only credible option for containing the rogue state’s nuclear ambitions. The Charter of the United Nations (UN) and the United Nations Convention on the Law of the Sea (UNCLOS) operate in tandem for this purpose. The Charter may be considered a constitution for the world; UNCLOS has been called the constitution for the oceans. These seminal treaties work together in a powerful way to restrain North Korea’s nuclear program. Specifically, the UN Security Council has invoked its authority in Chapter VII of the Charter to address threats to the peace by harnessing the legal competence of flag state, port state, and coastal state authority reflected in UNCLOS to strangle North Korea’s access to oceanic trade, crippling its economy and undermining its ability to spread nuclear material and weapons. The struggle to develop and enforce international rules to stop North Korea’s nuclear program is a story still unfolding. But decades of progress in international law and state practice have combined with tighter sanctions by the Security Council to dramatically cripple North Korea’s ability to develop and share nuclear weapons and supporting material and technology. The consequences of failure are genuinely terrifying, as even a single nuclear detonation anywhere in the world would prove catastrophic to global economic and political stability.

Comparative Investigation between In Situ Laser Ablation Versus Bulk Sample (Solution Mode) Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Analysis of Trinitite Post-Detonation Materials.

In the event of the interception of illicit nuclear materials or detonation of a nuclear device, timely and accurate deciphering of the chemical and isotopic composition of pertinent samples is pivotal in enhancing both nuclear security and source attribution. This study reports the results from a first time (to our knowledge), detailed comparative investigation conducted of Trinitite post-detonation materials using both solution mode (SM) and laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS) techniques. Trace element abundances determined for bulk Trinitite samples subsequent to digestion and preparation for SM-ICP-MS analysis compare favorably to calculated median concentrations based on LA-ICP-MS analyses for the identical samples. The trace element concentrations obtained by individual LA-ICP-MS analyses indicate a large scatter compared to the corresponding bulk sample SM-ICP-MS results for the same sample; this feature can be attributed to the incorporation into the blast melt of specific, precursor accessory minerals (minerals in small quantities, such as carbonates, sulfates, chlorites, clay, and mafic minerals) present at ground zero. The favorable comparison reported here validates and confirms the use of the LA-ICP-MS technique in obtaining accurate forensic information at high spatial resolution in nuclear materials for source attribution purposes. This investigation also reports device-like (240)Pu/(239)Pu ratios (∼0.022) for Pu-rich regions of the blast melt that are also characterized by higher Ca and U contents, which is consistent with results from previous studies.

Motion correction for passive radiation imaging of small vessels in ship-to-ship inspections

Passive radiation detection remains one of the most acceptable means of ascertaining the presence of illicit nuclear materials. In maritime applications it is most effective against small to moderately sized vessels, where attenuation in the target vessel is of less concern. Unfortunately, imaging methods that can remove source confusion, localize a source, and avoid other systematic detection issues cannot be easily applied in ship-to-ship inspections because relative motion of the vessels blurs the results over many pixels, significantly reducing system sensitivity. This is particularly true for the smaller watercraft, where passive inspections are most valuable. We have developed a combined gamma-ray, stereo visible-light imaging system that addresses this problem. Data from the stereo imager are used to track the relative location and orientation of the target vessel in the field of view of a coded-aperture gamma-ray imager. Using this information, short-exposure gamma-ray images are projected onto the target vessel using simple tomographic back-projection techniques, revealing the location of any sources within the target. The complex autonomous tracking and image reconstruction system runs in real time on a 48-core workstation that deploys with the system.

Uranium yellowcake trafficking incidents in Africa

The perpetuation of an otherwise astute fissile material safeguards model by the current non-proliferation regime raises a number of concerns, not least the efficacy of the approach in reducing clandestine weapon developments and the nuclear terrorism threat. Mindful of potential shifts in illicit nuclear material and weapon acquisition, following an excessive focus on fissile material safeguards and the proliferation of nuclear weapon technology, this article uses proliferation trend and scenario analysis in a bid to identify potential proliferation threats and non-proliferation opportunities. The results of the article's assessment of reported incidents (1992–2013) involving theft, unauthorised possession, and attempts to smuggle or sell highly enriched uranium (HEU), plutonium, low-enriched uranium (LEU), natural uranium and uranium yellowcake reveal a decline in occurrences involving fissile material but an increase in uranium yellowcake incidents, particularly in Africa. Presumptions that yellowcake may have provided the newest threat to clandestine weapon developments and nuclear terrorism wane amid concerns over possible biases in reporting, scepticism over an organised and demand-driven nuclear black market, and the difficulties of non-state actors’ development of crude nuclear weapons off the tedious yellowcake conversion-enrichment-fabrication pathway. To secure Africa's uranium yellowcake from potential proliferators, the article proposes concerted domestic, regional and multilateral non-proliferation efforts.

Performance bounds for mismatched decision schemes with Poisson process observations

This paper develops a framework for analyzing the performance loss in fixed time interval decision algorithms that are based on observations of time-inhomogeneous Poisson processes, when some parameters characterizing the observation process are not known exactly. Key to the development is the formulation of an analytically computable performance metric which can be used in lieu of the true, but intractable, error probabilities. The proposed metric is obtained by identifying analytical upper bounds on the error probabilities in terms of the uncertain parameters. Using these tools, it is shown that performance degrades gracefully as long as the true values of the parameters remain within a neighborhood of the nominal values used in decision making. The results find direct application to problems of detecting illicit nuclear materials in transit.

Prioritized interdiction of nuclear smuggling via tabu search

We describe an optimization model for interdicting smuggling of illicit nuclear material. Modeled on a directed network, the interdictor’s goal is to select radiation detector installation locations, subject to a budget constraint, to minimize a smuggler’s evasion probability. A smuggler, characterized by a random origin–destination pair, traverses the network knowing the detector locations. An additional layer of uncertainty is captured when considering a set of budget scenarios in place of a fixed budget value. An optimal solution to this model yields a priority list of detector installation locations. After the budget is revealed, detectors are installed from highest to lowest priority until the budget is exhausted. This notion captures time-phased securing of a transportation network as resources become available. Modeled as bilevel stochastic mixed-integer programs, both the fixed and random budget models are computationally challenging. Hence, we develop a tabu search heuristic to approximately solve both problem types, and we present computational results to assess the performance of our algorithm.

The Global Nuclear Detection Architecture and the Deterrence of Nuclear Terrorism

The Global Nuclear Detection Architecture is a worldwide system for detecting illicit radiological or nuclear (R/N) material and R/N weapons. The technical and nontechnical detection capabilities of the architecture can contribute to the deterrence of nuclear terrorism by increasing the risks and costs of mounting an R/N attack. Risks include the danger of one or more encounters with elements of the architecture, uncertainties about the locations and other characteristics of detection capabilities, discovery upon encounter with detection capabilities, and attack failure upon discovery. Among the costs are the added manpower, money, materiel, time, and operational difficulties entailed by attempts to evade or defeat detection capabilities. The deterrent effect of the detection architecture—part of a broader effort to counter nuclear terrorism—warrants greater attention and emphasis.

The Supervised Normalized Cut Method for Detecting, Classifying, and Identifying Special Nuclear Materials

The detection of illicit nuclear materials is a major tool in preventing and deterring nuclear terrorism. The detection task is extremely difficult because of physical limitations of nuclear radiation detectors, shielding by intervening cargo materials, and the presence of background noise. We aim at enhancing the capabilities of detectors with algorithmic methods specifically tailored for nuclear data. This paper describes a novel graph-theory-based methodology for this task. This research considers for the first time the utilization of supervised normalized cut (SNC) for data mining and classification of measurements obtained from plastic scintillation detectors that are of particularly low resolution. Specifically, the situation considered here is for when both energy spectra and the time dependence of such data are acquired. We present here a computational study, comparing the supervised normalized cut method with alternative classification methods based on support vector machine (SVM), specialized feature-reducing SVMs (i.e., 1-norm SVM, recursive feature elimination SVM, and Newton linear program SVM), and linear discriminant analysis (LDA). The study evaluates the performance of the suggested method in binary and multiple classification problems of nuclear data. The results demonstrate that the new approach is on par or superior in terms of accuracy and much better in computational complexity to SVM (with or without dimension or feature reduction) and LDA with principal components analysis as preprocessing. For binary and multiple classifications, the SNC method is more accurate, more robust, and is computationally more efficient by a factor of 2–80 than the SVM-based and LDA methods.

Using container inspection history to improve interdiction logistics for illicit nuclear materials

Strengthening the United States' ability to prevent adversaries from smuggling nuclear materials into the country is a vital and ongoing issue. The prospect of additional countries, such as Iran, obtaining the know-how and equipment to produce these special nuclear materials in the near future underscores the need for efficient and effective inspection policies at ports and border crossings. In addition, the reduction of defense and homeland security budgets in recent years has made it increasingly important to accomplish the interdiction mission with fewer funds. Addressing these complications, in this article, we present a novel two-port interdiction model. We propose using prior inspection data as a low-cost way of increasing overall interdiction performance. We provide insights into two primary questions: first, how should a decision maker at a domestic port use detection data from the foreign port to improve the overall detection capability? Second, what are potential limitations to the usefulness of prior inspection data—is it possible that using prior data actually harms decision making at the domestic port? We find that a boundary curve policy (BCP) that takes into account both foreign and domestic inspection data can provide a significant improvement in detection probability. This BCP also proves to be surprisingly robust, even if adversaries are able to infiltrate shipments during transit. © 2013 Wiley Periodicals, Inc. Naval Research Logistics 60: 433-448, 2013

Isotopic Fingerprinting of the World’s First Nuclear Device Using Post-Detonation Materials

In the event of a rogue nuclear attack or interception of illicit nuclear materials, timely forensic investigations are critical for accurate source attribution. Uranium (U) and plutonium (Pu) isotopic ratios of intercepted materials or postdetonation samples are, perhaps, the most valuable evidence in modern nuclear forensics. These ratios simultaneously provide information regarding the material's ''age'' (i.e., time elapsed since last purification), actinide concentrations, and relevant isotopic ratios/enrichment values. Consequently, these isotope signatures are invaluable in determining the origin, processing history, and intended purpose of any nuclear material. Here we show, for the first time, that it is feasible to determine the U and Pu isotopic compositions of historic nuclear devices from their postdetonation materials utilizing in situ U isotopic measurements. The U isotopic compositions of trinitite glass, produced subsequent to the world's first atomic explosion, indicate two sources: the device's tamper, composed of natural U that underwent fission during detonation, and natural U from the geological background. Enrichments in (234,235,236)U reflect the in situ decay of (238,239,240)Pu, the fuel used in the device. Time-integrated U isotopic modeling yields "supergrade" compositions, where (240)Pu/(239)Pu ≈ 0.01-0.03 and (238)Pu/(239)Pu ≈ 0.00011-0.00017, which are consistent with the Pu originating from the Hanford reactor. Spatially resolved U isotopic data of postdetonation debris reveal important details of the device in a relatively short time frame (hours). This capacity serves as an important deterrent to future nuclear threats and/or terrorist activities and is critical for source attribution and international security.

The state of nuclear forensics

Nuclear terrorism has been identified as one of the most serious security threats facing the world today. Many countries, including the United States, have incorporated nuclear forensic analysis as a component of their strategy to prevent nuclear terrorism. Nuclear forensics involves the laboratory analysis of seized illicit nuclear materials or debris from a nuclear detonation to identify the origins of the material or weapon. Over the years, a number of forensic signatures have been developed to improve the confidence with which forensic analysts can draw conclusions. These signatures are validated and new signatures are discovered through research and development programs and in round-robin exercises among nuclear forensic laboratories. The recent Nuclear Smuggling International Technical Working Group Third Round Robin Exercise and an on-going program focused on attribution of uranium ore concentrate provide prime examples of the current state of nuclear forensics. These case studies will be examined and the opportunities for accelerator mass spectrometry to play a role in nuclear forensics will be discussed.