Infrared (IR) and Raman spectroscopies and differential thermal analysis (DTA) to better understand the fundamental structure, optical, and thermal characteristics of the glasses. After a range of these glasses were synthesized, optimal compositions were formed into glass disks and subjected to gamma irradiation. Glass disks were characterized both before and after irradiation by microscope imaging, measuring the refractive index, density, and UV-VIS-IR transmission spectra. The final total dose the samples were subjected to was ~2.5 MGy. Ternary samples showed a less than 0.4% change in density and refractive index and minimal change in transmission window. The glasses also resisted cracking as seen in microscope images. Overall, many glass compositions were developed that possess operating temperatures above 500 °C, where conventional chalcogenide glasses such as As2S3 and have Tgs from ~200-300 °C, and these glasses have a greater than Tc – Tg values larger than 100 °C and this shows that these glasses have good thermal stability of Tg such that they can be fabricated into optical fibers and as such can be considered candidates for high temperature infrared fiber optics. Initial fiber fabrication efforts showed that selected glasses could be drawn but larger samples would be needed for further development and optimization

The Differential Die-Away (DDA) technique is a highly sensitive non-destructive assay method for characterizing and detecting the presence of fissile material within an item of interest. DDA utilizes a series of pulses from a neutron generator (NG) to actively interrogate an item of interest. The die-away time of the neutron population induced by this active interrogation and the integral of the total differential die-away signal can be used to characterize items such as nuclear waste drums and spent nuclear fuel assemblies. Los Alamos National Laboratory (LANL) conceptualized, designed, and fabricated a DDA instrument that was deployed for field test measurements at the Central Interim Storage Facility for Spent Nuclear Fuel (Clab) in Oskarshamn, Sweden. The instrument performed multiple static measurements at fixed locations and dynamic axial scans of 15 pressurized water reactor (PWR) and 10 boiling water reactor (BWR) spent fuel assemblies, collecting both passive and active measurement data. The static assays of the assemblies measured the differential die-away signal, die-away time, and total passive neutron emission rate to create calibration curves for the evaluation of assembly multiplication, burnup, initial enrichment, effective fissile mass, and total elemental plutonium mass. Each calibration curve was optimized by minimizing the relative root mean square error (RRMSE) of assembly assay results compared to declared assembly parameters. The same quantities were also measured with the axial scans, and the resulting data were applied in two ways: (1) in the creation of calibration curves to improve evaluation of the same safeguards parameters as static assays, and (2) for comparison to simulation. In most cases, across both PWR and BWR assemblies, axial scan data improved the estimation of the above parameters, quantified by decreasing the calibration curve RRMSE. These axial scan results demonstrate the ability of the DDA instrument and analysis method to characterize spent PWR and BWR fuel as well as, or better than, a static assay of the same assembly. Furthermore, the DDA instrument's unique ability to obtain both active and passive data in a single, axial scan of an entire spent fuel assembly represents a more efficient and accurate way of assaying spent fuel for verification purposes. These results represent a significant advancement for characterizing spent nuclear fuel compared to current technologies.

The active uranium neutron coincidence collar provides a means of non-destructively assaying the fissile linear density of Light Water Reactor fresh fuel assemblies containing low enriched uranium. These neutron collars can operate in two modes: a thermal and a fast mode. In fast mode, a neutron collar has an added cadmium (Cd) liner in the sample cavity of the detector to reduce the impact of the burnable poison (thermal neutron absorber) on the detector signal (doubles). The main advantage for operating in fast mode is a detected signal that is less dependent of the burnable neutron poison content and thus less dependent on facility operator declarations. The drawback is that operating in fast mode requires a longer measurement time (similar to hour vs tens of minutes for thermal mode) to achieve the statistically needed precision in the measurements. The trend in the modern reactor fuel assemblies is moving to higher burnup by using higher initial enrichment and, consequently, a higher number of burnable poison rods to compensate the initial neutron reactivity. The increase of the burnable poison loading has motivated the development of a new generation of high efficiency fast neutron collars to allow practical measurements in-field by nuclear inspectors. This paper describes the development and performance evaluation of a new generation of neutron collars, for both boiling water reactor (BWR) and pressurized water reactor (PWR) fuels, jointly developed between the US Department of Energy, through Los Alamos National Laboratory, and the Euratom Safeguards Directorate of the European Commission. We present here calibrations with reference fuel assemblies at Los Alamos National Laboratory as well as the results of in-field measurement campaigns in fuel fabrication plants with modern commercial fuel assemblies. The experimental results show that a typical PWR verification can be made in a total time of 30 min with an uncertainty in the measured mass of 2% at one standard deviation (1 sigma). A BWR verification can be made in 47 min with an uncertainty in the measured mass of 1.9% at 1 sigma, or a total time of 20 min with 1 sigma uncertainty in the measured mass of 2.5%.

Differential Die-Away Self-Interrogation (DDSI) is a method by which the characteristic die-away of neutrons from spontaneous and induced fissions is used to characterize a spent nuclear fuel assembly. A nondestructive assay (NDA) instrument was built at Los Alamos National Laboratory to implement and test the DDSI method. The DDSI instrument contains He-3 detectors which measure thermal neutrons, and the time and location of detection of each neutron is recorded via list-mode data acquisition. The instrument was sent to the Clab interim storage facility in Sweden for measurement and characterization of 50 spent pressurized water reactor (PWR) and boiling water reactor (BWR) fuel assemblies. The result was over 40 h of neutron list-mode data from a wide variety of fuel assemblies with high enough efficiency to perform neutron coincidence counting, i.e. detection of time-correlated neutrons from fission. Analysis algorithms for characterization of the fuel assemblies were tested on the Swedish spent fuel dataset. Using the measured data, multiplication, fissile mass, initial enrichment, burnup, and total plutonium mass were determined in the 50 assemblies with root mean square errors ranging from 1.5% for PWR multiplication to 11.4% for BWR fissile mass. The results in this work demonstrate that the DDSI concept is capable of characterizing spent power reactor fuel with levels of accuracy that are compatible with the requirements and objectives of various applications such as safeguards verification or facility material control and accounting.

This report represents a deliverable in the Safeguards Technology WBS # Project Title, 24.1.3.1, Task 3 (Report on laboratory-based performance testing of the FNPC). A new, simplified method for the verification of the 235U mass in fresh LEU fuel assemblies that is relatively independent of burnable poison content is being developed. LWR fresh fuel is currently verified using the uranium neutron coincidence collar (UNCL), which relies on an AmLi source to induce fission in the 235U. AmLi neutrons are slowed down in polyethylene, so the induced fission is primarily due to thermal neutrons. Burnable poisons, which are added to nuclear fuel assemblies to extend the lifetime of the fuel, absorb neutrons that would otherwise have induced fission in the fuel, thereby reducing the neutron count rate and thus the measured 235U mass. To remove this bias factor a correction is applied based on the operator’s declared burnable poison content. The new technique uses 238U spontaneous fission in LWR fresh fuel rods to self-interrogate the 235U mass. The 238U spontaneous fission neutrons have a hard neutron energy spectrum, which means that the technique is less sensitive to burnable poisons than the AmLi based systems. This project has continued the work done over the past two years to study this new technique using the optimized 3He based fastneutron passive collar (FNPC). Fabrication of the FNPC has been completed and measurements are in progress using the LANL mockup PWR fuel assembly. Also, preliminary evaluations of advanced analysis techniques and unattended mode operation have been performed. In FY18, the 3He tube based detector was compared with a 10B sealed cell based detector for the same set of PWR fuel assemblies in the Rodeo-II program. The results were that the optimized 3He detector had higher efficiency and less weight than the 10B based system where both systems had about the same cost. Thus, the FY19 work continued exclusively with the 3He based detector; although, either system had the potential for future applications. However, the high efficiency of the FPNC (~24%) is critical for precise measurements of the triples rate necessary in the multiplicity advanced analysis technique which can solve for the burnable poison content.

This report describes a new method for the verification of fresh low enrichment uranium (LEU) fuel assemblies that can reduce International Atomic Energy Agency (IAEA) inspection time in the field and be completely independent of operator declared burnable poison content. The passive measurement makes unattended mode operation practical, and thus provides the potential for reducing inspector time in the field for future applications. The new simplified passive neutron measurement uses the 238U spontaneous fission in LEU fresh fuel rods to self-interrogate the 235U mass in the neighboring rods. The measured response increases linearly with the increase in the linear density (LD) of 235U. The relatively high efficiency of the passive neutron detector (24%) provides good statistical precision (~ 2%) for the singles, doubles, and triples counting rates in the thermal-neutron mode. The three observables makes possible multiplicity analysis so that both the 235U LD and the effective burnable poison content can be determined independent of the operator declarations. This closes the potential diversion path of miss-declaration the burnable poison content. This report presents the detector design, the as-built system, the detector parameter measurements, the calibration, and the assay uncertainty estimates. The calibration measurements were performed using the Los Alamos Laboratory mockup PWR fuel assembly. The measurements were made in both the fastneutron mode (with Cd/Gd metal liners) and the thermal-neutron mode (no Cd/Gd liners).

The ability to perform nondestructive assay (NDA) of fresh fuel assemblies with high precision in reasonable measurement times is significant for nuclear nonproliferation and safeguards. The current methodology used by the International Atomic Energy Agency (IAEA) for NDA of fresh fuel assemblies involves a He-3 based Uranium Neutron Collar (UNCL) system, which is an active coincidence counting system designed to measure time-correlated neutron doubles from induced fissions within the assembly. The UNCL system provides an easily implementable process of quantifying the 235U linear density of the fresh fuel assemblies; however, operational limitations of the UNCL include long assay times and adverse sensitivity to poisoned assemblies. This has engendered investigation in alternative detector materials for fast-neutron counting to rectify some of these current limitations in the UNCL. The potential improvements of using fast-neutron organic scintillators (e.g. liquid scintillators, stilbene, PSDcapable plastics, etc.) was compared directly to the original Uranium Neutron Collar (UNCL) in-detail during the initial Advanced Neutron Detection Technology Rodeo. The simulation results showed that these alternative detector materials can provide an improvement in the assay time required to reach a suitable precision due to both their lower sensitivity to the interrogation source neutrons (i.e. AmLi neutrons) and their inherently shorter coincidence gate widths. The IAEA has developed a liquid scintillator-based Fast-Neutron Collar (FNCL) as a potential replacement system for the UNCL.

The uranium neutron coincidence collar uses thermal neutron interrogation to verify the U-235 mass in low-enriched uranium (LEU) fuel assemblies in fuel fabrication facilities. Burnable poisons are commonly added to nuclear fuel to increase the lifetime of the fuel. The high thermal neutron absorption by these poisons reduces the active neutron signal produced by the fuel. Burnable poison correction factors or fast-mode runs with Cd liners can help compensate for this effect, but the correction factors rely on operator declarations of burnable poison content, and fast-mode runs are time-consuming. This paper describes a new analysis method to measure the U-235 mass and burnable poison content in LEU nuclear fuel simultaneously in a timely manner, without requiring additional hardware.

The UNCL has been used for over thirty years for the verification of the 235U content of LWR nuclear fuel.The most recent version of the detector, the UNCL-II, consists of four HDPE blocks, three containing 3He proportional counters and a fourth containing a slot for an AmLi source. [1] The 3He proportional counters have a nominal active length of 33 cm, a diameter of 2.54 cm and a gas pressure of 4 atm. The UNCL-II has cavity dimensions 41.4 x 23.4 x 23.4 cm.

Active neutron coincidence systems are commonly used by international inspectorates to verify a material balance across the various stages of the nuclear fuel cycle. The Uranium Neutron Coincidence Collar (UNCL) is one such instrument; it is used to measure the linear density of U-235 (g U-235/cm of active length in assembly) in fresh light water reactor fuel in nuclear fuel fabrication facilities. The UNCL and other active neutron interrogation detectors have historically relied on americium lithium ((AmLi)-Am-241) sources to induce fission within the sample in question. Californium-252 is under consideration as a possible alternative to the traditional 241AmLi source. This work relied upon a combination of experiments and Monte Carlo simulations to demonstrate the technical basis for the replacement of 241AmLi sources with Cf-252 sources by evaluating the statistical uncertainty in the measurements incurred by each source and assessing the penetrability of neutrons from each source for the UNCL.

The passive neutron collar approach removes the effect of poison rods when using a 1mm Gd liner. This project sets out to solve the following challenges: BWR fuel assemblies have less mass and less neutron multiplication than PWR; and effective removal of cosmic ray spallation neutron bursts needed via QC tests.