International Nuclear Safeguards Research Articles

Dead time correction of Feynman sampled rates suitable for neutron coincidence counting

Neutron coincidence counting (NCC) based on shift-register logic (SRL) is extensively used by international nuclear safeguards inspectorates to verify special nuclear material inventories, particularly Pu. An alternative autocorrelation pulse counting analysis technique is founded on Feynman sampling and makes use of the well-known Feynman-Y statistic. Although the latter approach has been widely adopted by the reactor noise and sub-criticality communities it has not been used by the nonproliferation community. Both techniques may be interpreted using the same theoretical model and so should extract the same fundamental information about the detected neutron pulse train, namely the first and second reduced factorial multiplets that are determined by the joint properties of the measurement item and the surrounding external neutron detector. In the nuclear safeguards domain, a semi-empirical dead time treatment is conventionally applied to the trigger (singles) and doubles rates and, as already noted, the Feynman-Y method is not typically applied. In this article, we revisit the dead time treatment of Robba et al. (1983) for the Feynman sampling method. We show so that a dead time corrected multiplet analysis, familiar to safeguards practitioners, can be extracted. This is a first and necessary step to enabling a comparison of the relative performance of these two autocorrelation techniques on a like basis for deeply subcritical measurement items, which is the domain of concern to international nuclear safeguards inspectorates. From the analytical derivations it emerges that the Robba formalism, with minor corrections, and the commonly used NCC shift-register logic dead time correction formalism yield numerically equivalent corrections for practical purposes over the operational range of safeguards interest.

International safeguards for the final disposal of spent nuclear fuel – why, what and how

Abstract. The objectives of international safeguards are the timely detection of diversion of significant quantities of nuclear material from peaceful nuclear activities to the manufacture of nuclear weapons (or for other purposes), and deterrence of such diversion by the risk of early detection for states with comprehensive safeguards agreements with the International Atomic Energy Agency (IAEA). Following these objectives, several studies have focused on the developments of concepts and methods for safeguarding final disposal facilities as well as on identifying the most feasible technologies that could potentially be deployed for verifying final disposal programmes (IAEA, 1998, 2010, 2018). These activities were coordinated through Member State Safeguards Support Programmes, including the joint tasks on the development of “Safeguards for Geological Repositories” (SAGOR, 1994–2004) and on the “Application of Safeguards to Geological Repositories” (ASTOR, 2005–2017). SAGOR performed a diversion path analysis for spent fuel disposal facilities, determined safeguards technical objectives and identified potential safeguards measures to meet those objectives. ASTOR supported the IAEA in assessing how safeguards measures could be effectively implemented and provided recommendations with respect to developing such measures. Specific verification technologies were developed under other Member State Support Programme tasks. A summary report on the progress and status of safeguards for spent fuel encapsulation plants and geological repositories was completed by ASTOR in 2017. ASTOR also identified areas and actions that need to be accomplished to support safeguards implementation in final disposal facilities, such as (1) establish performance requirements for the design of safeguards technologies relevant to geological disposal of spent fuel, (2) determine specific information needs of states and operators regarding safeguards implementation for geological disposal of spent fuel and develop appropriate guidance, (3) determine specific information needs of IAEA inspectors and analysts and develop a guidance document that provides recommendations for implementing safeguards for a geological repository system under the state-level concept and (4) develop and test appropriate safeguards equipment (IAEA, 2017; Moran et al., 2018). While several measures and technologies related to verifying the geological disposal of spent fuel have been used by the IAEA at other facilities or are in development or testing, other technologies still need to be developed and tested. In addition, ASTOR identified the need for approaches to how information about disposed spent fuel and high-level nuclear waste should be managed, handled, organized, archived, read, interpreted and secured for the long term (for centuries after repository closure and beyond), including an international standard for states and facility operators on information management, data-retention methods and timescales for preserving safeguards data for geological repositories. The presentation will introduce the objectives of international nuclear material safeguards for the final disposal of spent nuclear fuel, highlight the current status of developments and discussions in terms of approaches and technologies for safeguarding geological repositories, and give an outlook on implementing safeguards for final disposal in Germany.

Studying visual search without an eye tracker: an assessment of artificial foveation

Eye tracking is a useful tool for studying human cognition, both in the laboratory and in real-world applications. However, there are cases in which eye tracking is not possible, such as in high-security environments where recording devices cannot be introduced. After facing this challenge in our own work, we sought to test the effectiveness of using artificial foveation as an alternative to eye tracking for studying visual search performance. Two groups of participants completed the same list comparison task, which was a computer-based task designed to mimic an inventory verification process that is commonly performed by international nuclear safeguards inspectors. We manipulated the way in which the items on the inventory list were ordered and color coded. For the eye tracking group, an eye tracker was used to assess the order in which participants viewed the items and the number of fixations per trial in each list condition. For the artificial foveation group, the items were covered with a blurry mask except when participants moused over them. We tracked the order in which participants viewed the items by moving their mouse and the number of items viewed per trial in each list condition. We observed the same overall pattern of performance for the various list display conditions, regardless of the method. However, participants were much slower to complete the task when using artificial foveation and had more variability in their accuracy. Our results indicate that the artificial foveation method can reveal the same pattern of differences across conditions as eye tracking, but it can also impact participants’ task performance.

Partial defect detection using the DCVD and a segmented Region-Of-Interest

The Digital Cherenkov Viewing Device (DCVD) is a safeguards instrument available to international nuclear safeguards inspectors. It is frequently used to verify fuel on the gross defect level, and approved for partial defect verification, i.e. to assess that parts of a fuel assembly have not been diverted. The current limit for partial defect verification with the DCVD is on the 50% level. In the verification process, an analysis methodology is used where the inspector places a Region-Of-Interest (ROI) around the fuel assembly and assesses the total Cherenkov light intensity within this region. The intensity is then compared to a predicted value, and deviations from the predicted value are used to flag fuel assemblies for further investigations. In this work, we investigate a slightly different analysis approach, where the ROI is split into two or three segments to more accurately capture changes in light intensity in different regions of the captured image. The purpose is to increase the sensitivity of the DCVD to partial defects below the 50% level. Based on simulations of a Pressurised Water Reactor 17x17 fuel assembly, we conclude that a partial defect on the 30% level decreases the Cherenkov light intensity by at least 15% using one single ROI, by at least 20% using a ROI with two segments, and by at least 22% using a ROI with three segments. The analysis approach using two or three ROI segments instead of one thus appears to be more sensitive to partial defects, and can enable more accurate detection of partial defects on the 50% level as well as partial defect detection below the 50% level. Validation of the approach using a limited set of measurement data of intact fuel assemblies supports that detection of light intensity reductions by 20% and 22% is possible, while ensuring that the false positive rate is kept sufficiently low. However, an optimization of ROI segment splits as well as a more extended validation of the approach is required before the method can be considered reliable and applicable to all fuel assemblies that the DCVD can verify today.

Simultaneous reconstruction of emission and attenuation in passive gamma emission tomography of spent nuclear fuel

In the context of international nuclear safeguards, the International Atomic Energy Agency (IAEA) has recently approved passive gamma emission tomography (PGET) as a method for inspecting spent nuclear fuel assemblies (SFAs). The PGET instrument is essentially a single photon emission computed tomography (SPECT) system that allows the reconstruction of axial cross-sections of the emission map of an SFA. The fuel material heavily self-attenuates its gamma-ray emissions, so that correctly accounting for the attenuation is a critical factor in producing accurate images. Due to the nature of the inspections, it is desirable to use as little a priori information as possible about the fuel, including the attenuation map, in the reconstruction process. Current reconstruction methods either do not correct for attenuation, assume a uniform attenuation throughout the fuel assembly, or assume an attenuation map based on an initial filtered back-projection reconstruction. We propose a method to simultaneously reconstruct the emission and attenuation maps by formulating the reconstruction as a constrained minimization problem with a least squares data fidelity term and regularization terms. Using simulated data, we show that our approach produces clear reconstructions which allow for a highly reliable classification of spent, missing, and fresh fuel rods.

Eero Arnold Hakkila

Eero Arnold Hakkila, 87, died on Nov. 10, 2018, in Tonto Verde, Arizona. “Arnie became a nuclear chemist after growing up on his family’s Connecticut chicken farm. He had a long, distinguished career at Los Alamos National Laboratory (LANL); one of his career high points was serving as a member of the United Nations International Atomic Energy Agency’s nuclear weapons inspection team in Iraq following the first Gulf War. Arnie was an avid rock collector, bird watcher, skier, golfer, fisherman, and outdoorsman. He married the love of his life, Margaret, in 1988. After retiring from LANL, he and Margaret settled in Tonto Verde, Arizona, where they golfed and spent time relaxing with friends and family.”—Jon Hakkila, son Most recent title: Group leader, International Nuclear Safeguards Division, Los Alamos National Laboratory Education: BS, secondary education, Teachers College of Connecticut, 1953; PhD, chemistry, Ohio State University, 1957 Survivors: Wife, Margaret; sons, Gregg,

Verification of Spent Nuclear Fuel in Sealed Dry Storage Casks via Measurements of Cosmic-Ray Muon Scattering

Most of the plutonium in the world resides inside spent nuclear reactor fuel rods. This high-level radioactive waste is commonly held in long-term storage within large, heavily shielded casks. Currently, international nuclear safeguards inspectors have no stand-alone method of verifying the amount of reactor fuel stored within a sealed cask. Here we demonstrate experimentally that measurements of the scattering angles of cosmic ray muons which pass through a storage cask can be used to determine if spent fuel assemblies are missing without opening the cask. This application of technology and methods commonly used in high-energy particle physics provides a potential solution to this long-standing problem in international nuclear safeguards.

Minor isotope safeguards techniques (MIST): Analysis and visualization of gas centrifuge enrichment plant process data using the MSTAR model

This paper presents a development related to the use of minor isotope safeguards techniques (MIST) and the MSTAR cascade model as it relates to the application of international nuclear safeguards at gas centrifuge enrichment plants (GCEPs). The product of this paper is a derivation of the universal and dimensionless MSTAR cascade model. The new model can be used to calculate the minor uranium isotope concentrations in GCEP product and tails streams or to analyze, visualize, and interpret GCEP process data as part of MIST. Applications of the new model include the detection of undeclared feed and withdrawal streams at GCEPs when used in conjunction with UF6 sampling and/or other isotopic measurement techniques.

Extension of the Dytlewski-style dead time correction formalism for neutron multiplicity counting to any order

Neutron multiplicity counting using shift-register calculus is an established technique in the science of international nuclear safeguards for the identification, verification, and assay of special nuclear materials. Typically passive counting is used for Pu and mixed Pu-U items and active methods are used for U materials. Three measured counting rates, singles, doubles and triples are measured and, in combination with a simple analytical point-model, are used to calculate characteristics of the measurement item in terms of known detector and nuclear parameters. However, the measurement problem usually involves more than three quantities of interest, but even in cases where the next higher order count rate, quads, is statistically viable, it is not quantitatively applied because corrections for dead time losses are currently not available in the predominant analysis paradigm. In this work we overcome this limitation by extending the commonly used dead time correction method, developed by Dytlewski, to quads. We also give results for pents, which may be of interest for certain special investigations. Extension to still higher orders, may be accomplished by inspection based on the sequence presented. We discuss the foundations of the Dytlewski method, give limiting cases, and highlight the opportunities and implications that these new results expose. In particular there exist a number of ways in which the new results may be combined with other approaches to extract the correlated rates, and this leads to various practical implementations.

Timely Verification at Large-Scale Gas Centrifuge Enrichment Plants

ABSTRACTThis article examines challenges in international nuclear safeguards pertaining to the timely detection of highly enriched uranium production at large-scale gas centrifuge enrichment plants. To establish where present gas centrifuge enrichment plant safeguards measures and approaches could be strengthened, we have created a discrete time model for simulating hypothetical misuse scenarios, both through transient phases and at steady-state. We find that timely detection of misuse at modern large-scale facilities presents a challenge for international safeguards. A toolbox of unattended measurement systems, along with remote monitoring, however, could be used to improve detection timeliness, enabling the initiation of follow-up activities, potentially on a rapid time scale. These measures, which would need very low false alarm rates, should be implemented in a graded approach, depending on the characteristics of each enrichment plant and an analysis of plausible acquisition paths for the State in which it is situated. Some of these technologies could provide significant benefit to plant operators.

On the equivalence of the RTI and SVM approaches to time correlated analysis

Recently two papers on how to perform passive neutron auto-correlation analysis on time gated histograms formed from pulse train data, generically called time correlation analysis (TCA), have appeared in this journal Dubi et al. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (673) (2012) 111; Croft et al. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (691) (2012) 152. For those of us working in international nuclear safeguards these treatments are of particular interest because passive neutron multiplicity counting is a widely deployed technique for the quantification of plutonium. The purpose of this letter is to show that the skewness-variance-mean (SVM) approach developed in Dubi et al. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (673) (2012) 111) is equivalent in terms of assay capability to the random trigger interval (RTI) analysis laid out in Croft et al. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (691) (2012) 152. Mathematically we could also use other numerical ways to extract the time correlated information from the histogram data including, for example, what we might call the mean, mean square, and mean cube approach. The important feature however, from the perspective of real world applications, is that the correlated information extracted is the same, and subsequently gets interpreted in the same way based on the same underlying physics model.

Boron-10 Based Neutron Coincidence Counter for Safeguards

The shortage of 3He has triggered the search for effective alternative neutron detection technologies for national security applications, including international nuclear safeguards. Any alternative neutron detection technology must satisfy two basic criteria: it must meet a neutron detection efficiency requirement, and it must be insensitive to gamma-ray interference at a prescribed level while still meeting the neutron detection requirement. For nuclear safeguards, a system must perform measurements in the field with a prescribed precision in a specified time. This paper describes an effort to design, model and test an alternatives-based neutron coincidence counter for nuclear safeguards applications. The technology chosen for use in an alternatives-based uranium neutron coincidence collar was boron-lined proportional counters. Extensive modeling was performed of various system configurations and comparisons were made to measurements on a commercial prototype boron-10 based uranium neutron coincidence collar.

The optimum choice of gate width for neutron coincidence counting

In the measurement field of international nuclear safeguards, passive neutron coincidence counting is used to quantify the spontaneous fission rate of certain special nuclear materials. The shift register autocorrelation analysis method is the most commonly used approach. However, the Feynman-Y technique, which is more commonly applied in reactor noise analysis, provides an alternative means to extract the correlation information from a pulse train. In this work we consider how to select the optimum gate width for each of these two time-correlation analysis techniques. The optimum is considered to be that which gives the lowest fractional precision on the net doublets rate. Our theoretical approach is approximate but is instructional in terms of revealing the key functional dependence. We show that in both cases the same performance figure of merit applies so that common design criteria apply to the neutron detector head. Our prediction is that near optimal results, suitable for most practical applications, can be obtained from both techniques using a common gate width setting. The estimated precision is also comparable in the two cases. The theoretical expressions are tested experimentally using 252Cf spontaneous fission sources measured in two thermal well counters representative of the type in common use by international inspectorates. Fast accidental sampling was the favored method of acquiring the Feynman-Y data. Our experimental study confirmed the basic functional dependences predicted although experimental results when available are preferred. With an appropriate gate setting Feynman-Y analysis provides an alternative to shift register analysis for safeguards applications which is opening up new avenues of data collection and data reduction to explore.

Study of a theoretical model for the measured gate moments resulting from correlated detection events and an extending dead time

Neutron emissions from fissioning nuclear material are temporally correlated. The detection of these correlated neutrons is frequently used to quantify plutonium (Pu) and other fissile materials for international nuclear safeguards and related activities. However, detector dead time affects the observed rates of correlated neutrons in a non-trivial manner, and must be accounted for to obtain accurate results. A major simplification made in the most widely used dead time corrections is that the neutron detections are occurring randomly in time. A few previous attempts at providing a dead time model for correlated neutrons have been limited in scope, have made simplifying assumptions early in the derivation, and have, in general, not been implemented in the broader safeguards community. This paper provides an exact dead time model for correlated neutron detections in a single channel system assuming an updating dead time, and therefore a paralyzable system. This dead time model includes the assumption that a single exponential, with one characteristic decay constant, can describe the system neutron die-away profile. An exact model for the effects of dead time on measured gate moments is derived which is extendable to an arbitrary order of neutron correlation. This dead time model predicts the measured gate moments based on the dead time and underlying detection rates, including the effects from detection rates with an arbitrarily high order of correlation. The effects of dead time on the apparent singles, doubles, triples and quadruples rates using either event triggered, random or mixed gate structure is also derived. Either the equations for the measured gate moments or the apparent multiplicity rates can be numerically inverted to find the dead time corrected multiplicity rates. Although the model has been explicitly solved for rates up to and including quadruples, it is directly extendable to any order of correlation. This model is presented from the perspective of neutron counting. However, the model is mathematically applicable to any series of events which are temporally correlated through an exponential probability distribution and which are subject to a dead time-like filter.

Comparison of active and passive environmental sampling for safeguards applications

Environmental sampling (ES) of surfaces using cotton swipes at nuclear facilities is one of the standard sampling protocols used by International Atomic Energy Agency (IAEA) inspectors to verify treaty compliance. Described herein is a series of studies undertaken to (1) survey a variety of low latent uranium content sampling media for efficacy of collecting nuclear materials and (2) compare active collection of particulates to swipe sampling. Findings from the evaluation of 40 natural and synthetic woven swipes will be presented. Results from a joint Oak Ridge National Laboratory (ORNL) and Savannah River National Laboratory (SRNL) project wherein a variety of passive deposition surfaces and active collections were examined for efficacy of sampling uranium processing effluents will also be detailed. For the swipe material evaluation portion of this work, uranium (U) particulate collection efficiency was evaluated on woven materials as-received and coated with complexants/solvents. The relative merits of each sampling matrix were reviewed on the basis of: (1) latent U background, (2) ease of separating the swipe material from the nuclear components, (3) presence of isobaric interferences from the swipe, and (4) efficiency in collecting uranium from contaminated surfaces. The second series of experiments evaluated the collection characteristics of active aerosol collection versus swipe sampling. ORNL and SRNL are collaborating on the development of the next generation of ES equipment for air grab and constant samples that could become an important addition to the international nuclear safeguards inspector’s toolkit. Described herein are findings from the evaluation of collection efficiency of swipe sampling and active collection using an aerosol contaminant extractor (ACE) developed by SRNL.

Experimental evaluation of a boron-lined parallel plate proportional counter for use in nuclear safeguards coincidence counting

Boron-lined proportional technologies are increasingly being considered as a viable option for the near-term replacement of 3He-based technologies for use in international nuclear safeguards neutron detection and coincidence counting applications. In order to determine the applicability and feasibility of any replacement technology for international safeguards, it must be evaluated against performance parameters specific to nuclear safeguards applications. In this paper, we present an experimental evaluation of a boron-lined parallel plate proportional counter developed by Precision Data Technology, Inc. (PDT). The counter performance was evaluated using a high-rate 252Cf spontaneous fission neutron source and a set of 137Cs gamma-ray sources with a dose rate of 450mR/h at the detector face. The performance data were subsequently compared with an equivalent 3He-based system defined using the Monte Carlo N-particle eXtended (MCNPX) radiation transport code.

Sample preparation method for analysis of swipe samples by inductively coupled plasma mass spectrometry

International nuclear safeguards have been applied for over 30 years to control nuclear activities. Following the recon of undeclared nuclear activities in Iraq in 1991, strengthening of the safeguards system became necessary (1995). From 1996 the detection of undeclared nuclear activities using sensitive and precise analysis of swipe samples became one of the most important parts of IAEA inspections. The aim of this study was the development of a fast and easy sample preparation and analysis method for the bulk uranium and plutonium analysis of swipe samples. For the sample preparation low power microwave-assisted digestion followed by extraction chromatography using TRU® resin was applied. The concentration of analytes of interest and their isotopic composition were determined by inductively coupled plasma sector field mass spectrometry (ICP-SFMS). The analytical performance of the method is in good agreement with the requirements (accuracy, precision, repeatability) of the International Atomic Energy Agency–Network for Analytical Laboratories (IAEA–NWAL) and can be applied for routine analysis. The low detection limits achieved enable the determination of the isotope ratios and isotope concentrations of U and Pu (234U, 235U, 236U, 238U and 239Pu, 240Pu, 241Pu) present in ultratrace concentration levels in swipe samples. The method was tested with real swipe samples collected at Hungarian radioactive source producing facilities.

Use of data from environmental sampling for IAEA safeguards. Case study: uranium with near-natural 235U abundance

Environmental sampling (ES) is one of the measures applied in international nuclear safeguards. The detection capability of safeguards ES relies on a combination of highly sensitive analytical techniques and resourceful data evaluation. The evaluation process is dynamic, employing a variety of tools, information and analytical results. While the presence of uranium or plutonium may be a significant finding in itself, high quality isotopic measurements are essential to associate the material with a specific nuclear activity. This is illustrated in cases where the uranium detected appears to be “natural” or “near-natural”, but in fact can be identified with various nuclear processes.

Protecting Nuclear Safeguards Monitoring Data from Tampering

Effective treaty monitoring requires that nuclear monitoring data be safe from tampering. Tamper-indicating seals and standard data encryption/integrity techniques, however, do not provide sufficient security, especially against electronic and physical attacks in the context of international nuclear safeguards. This article presents an alternative approach for assuring the integrity of monitoring data called the “One-Time Pad of Digits Substitutions” (OPODS). This cipher is a combination of the unbreakable one-time keypad, and the traditional substitution cipher. OPODS provides unbreakable security prior to an adversary trespassing inside nuclear monitoring hardware (even if the trespassing goes undetected), and good security after.

14 - pixel , multiplexed array of gamma-ray microcalorimeters with 47eV energy resolution at 103keV

The authors present a prototype for a high-energy-resolution, high-count-rate, gamma-ray spectrometer intended for nuclear forensics and international nuclear safeguards. The prototype spectrometer is an array of 14 transition-edge-sensor microcalorimeters with an average energy resolution of 47eV (full width at half maximum) at 103keV. The resolution of the best pixel is 25eV. A cryogenic, time-division multiplexer reads out the array. Several important topics related to microcalorimeter arrays are discussed, including cross-talk, the uniformity of detector bias conditions, fabrication of the arrays, and the multiplexed readout. The measurements and calculations demonstrate that a kilopixel array of high-resolution microcalorimeters is feasible.