Microscale Isotopic Variation in Uranium Fuel Pellets with Implications for Nuclear Forensics.

Until recently, the analysis and identification of nuclear fuel pellets in the context of a nuclear forensics investigation have been mainly focused on macroscopic characteristics, such as fuel pellet dimensions, uranium enrichment, and other reactor-specific features. Here, we report microscale isotopic heterogeneity observed in different fuel pellet fragments that were characterized in situ by nanoscale secondary ion mass spectrometry (NanoSIMS). The materials analyzed include fuel fragments obtained as part of the Collaborative Materials Exercise (CMX-4) organized by the Nuclear Forensics International Technical Working Group (ITWG), as well as a fuel pellet fragment from a commercial power reactor. Although the commercial fuel pellet showed a homogeneous 235U/238U ratio across the sample (within analytical error), NanoSIMS imaging of the CMX-4 fuel pellet fragments showed distinct microscale variations in the uranium isotopic composition. The average 235U enrichments were 2.2 and 2.9% for the two samples; however, the measured 235U/238U ratios varied between 0.0081 and 0.035 (0.79-3.3 atom % 235U) and between 0.0090 and 0.045 (0.89-4.3 atom % 235U). The measurement of 236U in one of the CMX-4 samples suggested the use of at least three uranium oxide powders of different isotopic compositions ("source terms") in the production of the pellets. These variations were not detected using the conventional bulk, macroscopic techniques applied to these materials. Our study highlights the importance of characterizing samples on the microscale for heterogeneities that would otherwise be overlooked and demonstrates the potential use of NanoSIMS in guiding further nuclear forensic analysis.

Application of modern autoradiography to nuclear forensic analysis.

Modern autoradiography techniques based on phosphorimaging technology using image plates (IPs) and digital scanning can identify heterogeneities in activity distributions and reveal material properties, serving to inform subsequent analyses. Here, we have adopted these advantages for applications in nuclear forensics, the technical analysis of radioactive or nuclear materials found outside of legal control to provide data related to provenance, production history, and trafficking route for the materials. IP autoradiography is a relatively simple, non-destructive method for sample characterization that records an image reflecting the relative intensity of alpha and beta emissions from a two-dimensional surface. Such data are complementary to information gathered from radiochemical characterization via bulk counting techniques, and can guide the application of other spatially resolved techniques such as scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). IP autoradiography can image large 2-dimenstional areas (up to 20×40cm), with relatively low detection limits for actinides and other radioactive nuclides, and sensitivity to a wide dynamic range (105) of activity density in a single image. Distributions of radioactivity in nuclear materials can be generated with a spatial resolution of approximately 50µm using IP autoradiography and digital scanning. While the finest grain silver halide films still provide the best possible resolution (down to ∼10µm), IP autoradiography has distinct practical advantages such as shorter exposure times, no chemical post-processing, reusability, rapid plate scanning, and automated image digitization. Sample preparation requirements are minimal, and the analytical method does not consume or alter the sample. These advantages make IP autoradiography ideal for routine screening of nuclear materials, and for the identification of areas of interest for subsequent micro-characterization methods. In this paper we present a summary of our setup, as modified for nuclear forensic sample analysis and related research, and provide examples of data from select samples from the nuclear fuel cycle and historical nuclear test debris.

Nuclear Forensics: Scientific Analysis Supporting Law Enforcement and Nuclear Security Investigations.

Nuclear forensic science, or "nuclear forensic", aims to answer questions about nuclear material found outside of regulatory control. In this Feature, we provide a general overview of nuclear forensics, selecting examples of key "nuclear forensic signatures" which have allowed investigators to determine the identity of unknown nuclear material in real investigations.

Nuclear forensic analysis of an unknown uranium ore concentrate sample seized in a criminal investigation in Australia.

Early in 2009, a state policing agency raided a clandestine drug laboratory in a suburb of a major city in Australia. During the search of the laboratory, a small glass jar labelled "Gamma Source" and containing a green powder was discovered. The powder was radioactive. This paper documents the detailed nuclear forensic analysis undertaken to characterise and identify the material and determine its provenance. Isotopic and impurity content, phase composition, microstructure and other characteristics were measured on the seized sample, and the results were compared with similar material obtained from the suspected source (ore and ore concentrate material). While an extensive range of parameters were measured, the key 'nuclear forensic signatures' used to identify the material were the U isotopic composition, Pb and Sr isotope ratios, and the rare earth element pattern. These measurements, in combination with statistical analysis of the elemental and isotopic content of the material against a database of uranium ore concentrates sourced from mines located worldwide, led to the conclusion that the seized material (a uranium ore concentrate of natural isotopic abundance) most likely originated from Mary Kathleen, a former Australian uranium mine.

Discrimination of source reactor type by multivariate statistical analysis of uranium and plutonium isotopic concentrations in unknown irradiated nuclear fuel material.

The problem of identifying the provenance of unknown nuclear material in the environment by multivariate statistical analysis of its uranium and/or plutonium isotopic composition is considered. Such material can be introduced into the environment as a result of nuclear accidents, inadvertent processing losses, illegal dumping of waste, or deliberate trafficking in nuclear materials. Various combinations of reactor type and fuel composition were analyzed using Principal Components Analysis (PCA) and Partial Least Squares Discriminant Analysis (PLSDA) of the concentrations of nine U and Pu isotopes in fuel as a function of burnup. Real-world variation in the concentrations of (234)U and (236)U in the fresh (unirradiated) fuel was incorporated. The U and Pu were also analyzed separately, with results that suggest that, even after reprocessing or environmental fractionation, Pu isotopes can be used to determine both the source reactor type and the initial fuel composition with good discrimination.

Imaging and 3D elemental characterization of intact bacterial spores by high-resolution secondary ion mass spectrometry.

We present a quantitative, imaging technique based on nanometer-scale secondary ion mass spectrometry for mapping the 3D elemental distribution present in an individual micrometer-sized Bacillus spore. We use depth profile analysis to access the 3D compositional information of an intact spore without the additional sample preparation steps (fixation, embedding, and sectioning) typically used to access substructural information in biological samples. The method is designed to ensure sample integrity for forensic characterization of Bacillus spores. The minimal sample preparation/alteration required in this methodology helps to preserve sample integrity. Furthermore, the technique affords elemental distribution information at the individual spore level with nanometer-scale spatial resolution and high (microg/g) analytical sensitivity. We use the technique to map the 3D elemental distribution present within Bacillus thuringiensis israelensis spores.