Johnna B. Marlow’s research while affiliated with Los Alamos National Laboratory and other places

In 2018, a measurement campaign took place with participants from Los Alamos National Laboratory (LANL), the Nuclear Research Centre-Negev (NRCN) and Soreq Nuclear Research Center (SNRC) at the Israeli Research Reactor-1 (IRR-1) in which 14 of the reactor’s used fuel assemblies (FAs) with varied amount of depletion were measured with the nondestructive assay instrument Advanced Experimental Fuel Counter (AEFC). Designed for safeguards purposes, the AEFC measures both neutrons emitted from the FA (passive neutrons) and fission neutrons induced by an external neutron source (in this experiment, ²⁵²Cf). Signals recorded with the AEFC include total neutron count rates (Singles), time-correlated neutron count rates (Doubles), and total gamma-ray count rates. The ²³⁵U content of the FAs was previously assessed by two independent methods: (1) measurement of the transparency of the FA to low-energy gamma rays from an activated rhenium source (rhenium gamma transmission, or the RGT method) and (2) calculation of the ∼30-year burnup history of the core using detailed three-dimensional Monte-Carlo core depletion calculations. The results from the FAs that had been measured via the RGT method were used to construct the calibration curves, which translate the AEFC count rates to ²³⁵U mass. Then, the calibration was evaluated using AEFC measurements of six additional FAs that were not measured via the RGT method. From the results, it was determined the Doubles calibration curve was more reliable than that of the Singles and follows a simple second-order polynomial fit for the whole range of residual ²³⁵U mass content, albeit with larger statistical uncertainty. Detailed uncertainties quantification was conducted for both the AEFC Singles and Doubles. This includes the analysis of statistical uncertainties, calibration uncertainty, and random uncertainties due to the sensitivity of the AEFC to several sources of uncertainty, namely the FA position, FA orientation, interrogation source position, and ambient pool temperature. An overall total uncertainty of 6 g of ²³⁵U is estimated for the Singles and Doubles, which is mainly due to calibration uncertainty (for the Singles) and statistical uncertainty (for the Doubles), and which constitutes 3%–6% of the ²³⁵U total mass in the FAs, depending on their level of depletion.

Accurate and timely characterization of physical properties pertinent to plutonium bearing materials is important for fulfilling nuclear nonproliferation and safeguards goals. Physical properties include the fissile mass, leakage multiplication, and the α-ratio, defined as the ratio of (α,n) neutrons to spontaneous fission neutrons. Traditionally, these properties can be inferred by relating the measured neutron multiplicity count rates to the well-established point kinetics moments equations; the current state-of-the-art utilizes 3He-based detection systems. Organic scintillators have been used extensively to study and measure characteristic signatures in the neutron angular and energy distributions. Previous work has proposed techniques that independently leverage the energy and angle sensitivity of organic scintillators to estimate the α-ratio of plutonium bearing material; however, it is expected that the energy and angular distributions are correlated to one another due to the underlying physics of fission and (α,n) neutron emissions. This work presents experimental results that characterize neutron-neutron angular distribution and subsequently the neutron-neutron energy-angle correlations for plutonium samples of similar mass and multiplication, but varying α-ratio due to the type of low-Z impurity. Full neutron-neutron angular distributions are presented using a low-energy detection threshold of 0.10, 0.15, and 0.20 MeVee (0.73, 0.96, 1.16 MeV neutron-equivalent energy). Neutron anisotropy was quantified by taking the ratio of neutron-neutron coincidences at 180°to those at 90°, where a value of unity indicates a purely isotropic source. The results show that the observed neutron-neutron correlations transition away from fission-induced signal to the cross-talk signal associated with single (α,n) neutrons with increasing α-ratio. Energy-angle correlations are characterized by calculating the neutron anisotropy at various detection thresholds and show positive correlation between the observed anisotropy and the energy of the neutrons.

The uranium neutron coincidence collar uses thermal neutron interrogation to verify the ²³⁵U 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 ²³⁵U mass and burnable poison content in LEU nuclear fuel simultaneously in a timely manner, without requiring additional hardware.

Thermal neutron counters utilized and developed for deployment as non-destructive assay (NDA) instruments in the field of nuclear safeguards traditionally rely on 3He-based proportional counting systems. 3He-based proportional counters have provided core NDA detection capabilities for several decades and have proven to be extremely reliable with range of features highly desirable for nuclear facility deployment. Facing the current depletion of 3He gas supply and the continuing uncertainty of options for future resupply, a search for detection technologies that could provide feasible short-term alternative to 3He gas was initiated worldwide. As part of this effort, Los Alamos National Laboratory (LANL) designed and built a 3He-free full scale thermal neutron coincidence counter based on boron-lined proportional technology. The boron-lined technology was selected in a comprehensive inter-comparison exercise based on its favorable performance against safeguards specific parameters. This paper provides an overview of the design and initial performance evaluation of the prototype High Level Neutron counter—Boron (HLNB). The initial results suggest that current HLNB design is capable to provide ~80% performance of a selected reference 3He-based coincidence counter (High Level Neutron Coincidence Counter, HLNCC). Similar samples are expected to be measurable in both systems, however, slightly longer measurement times may be anticipated for large samples in HLNB. The initial evaluation helped to identify potential for further performance improvements via additional tailoring of boron-layer thickness.

Uranium cylinder assay plays an important role in the nuclear material accounting at gas centrifuge enrichment plants. The Passive Neutron Enrichment Meter (PNEM) was designed to determine uranium mass and enrichment in 30B and 48Y cylinders using total neutron and coincidence counting in the passive mode. 30B and 48Y cylinders are used to hold bulk UF6 feed, product, and tails at enrichment plants. In this paper, we report the results of a Monte-Carlo-based feasibility study for an active uranium cylinder assay system based on the PNEM design. There are many advantages of the active technique such as a shortened count time and a more direct measure of 235U content. The active system is based on a modified PNEM design and uses a 252Cf source as the correlated, active interrogation source. We show through comparison with a random AmLi source of equal strength how the use of a correlated driver significantly boosts the active signal and reduces the statistical uncertainty. We also discuss ways in which an active uranium cylinder assay system can be optimized to minimize background from 238U fast-neutron induced fission and direct counts from the interrogation source.

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 450 mR/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.

The Passive Neutron Enrichment Meter (PNEM) is a nondestructive assay (NDA) system being developed at Los Alamos National Laboratory (LANL). It was designed to determine ²³⁵U mass and enrichment of uranium hexafluoride (UFâ) in product, feed, and tails cylinders (i.e., 30B and 48Y cylinders). These cylinders are found in the nuclear fuel cycle at uranium conversion, enrichment, and fuel fabrication facilities. The PNEM is a ³He-based neutron detection system that consists of two briefcase-sized detector pods. A photograph of the system during characterization at LANL is shown in Fig. 1. Several signatures are currently being studied to determine the most effective measurement and data reduction technique for unfolding ²³⁵U mass and enrichment. The system collects total neutron and coincidence data for both bare and cadmium-covered detector pods. The measurement concept grew out of the success of the Uranium Cylinder Assay System (UCAS), which is an operator system at Rokkasho Enrichment Plant (REP) that uses total neutron counting to determine ²³⁵U mass in UFâ cylinders. The PNEM system was designed with higher efficiency than the UCAS in order to add coincidence counting functionality for the enrichment determination. A photograph of the UCAS with a 48Y cylinder at REP is shown in Fig. 2, and the calibration measurement data for 30B product and 48Y feed and tails cylinders is shown in Fig. 3. The data was collected in a low-background environment, meaning there is very little scatter in the data. The PNEM measurement concept was first presented at the 2010 Institute of Nuclear Materials Management (INMM) Annual Meeting. The physics design and uncertainty analysis were presented at the 2010 International Atomic Energy Agency (IAEA) Safeguards Symposium, and the mechanical and electrical designs and characterization measurements were published in the ESARDA Bulletin in 2011.

One pressing research and development challenge currently facing the nuclear safeguards community is finding an alternative to 3He gas for neutron detection. The high demand for 3He gas for several scientific and global security applications has exceeded the gas supply. 1 This has resulted in the depletion of 3He stockpiles and consequent shortfall in the availability of 3He for conventional neutron detection. As part of finding a viable 3He replacement neutron detection technology for safeguards use, performance requirements specific to safeguards applications must be defined. This article discusses the main detector performance parameters that form nuclear safeguards 3He replacement requirements and provides recommended values for these parameters based on experience with traditional safeguards neutron coincidence counting applications. A 3He replacement detector test program has been established at Los Alamos National Laboratory (LANL) 2 for the evaluation of 3He alternative neutron detection technologies against safeguards-specific performance parameters. Here, the unique features of the LANL test program are also highlighted.

To address the urgent need to find a viable 3He replacement technology, Los Alamos National Laboratory has developed an integrated test program for performance evaluation of 3He replacement technologies with focus on safeguards specific parameters. In addition, a dedicated experimental setup was built to support the experimental activities. Current activity focuses on evaluation of 10B lined proportional counters. This paper presents an overview of the test program as well as the first results from the GE/Reuter-Stokes 10B lined system.