Proliferation of nuclear materials and classified information poses grave threats to national security.
This can include nuclear weapons program knowledge as well as the potential for theft and diversion of uranium feedstocks and special nuclear materials (SNMs, i.e., enriched uranium and plutonium). Uranium oxides, principally UO₂, are widely used as fuel in reactors and are the most common form of illicitly trafficked SNM. It is therefore crucial to develop nuclear forensic signatures and methods that can determine points of origin, synthesis processes and storage conditions, and intent to do harm. While baseline nuclear forensic methods (e.g., inductively coupled plasma, ICP, secondary-ion mass spectrometry, SIMS, and scanning electron microscopy, SEM) are effective in many cases, analysis of recovered material is still often incomplete, such as with micro-particles, which present challenges of small sample size, heterogeneity, and the presence of multiple phases. Moreover, if the processing history for the parent material is unknown, or if characteristics are not retrievable from nuclear databases, then baseline methods (phase identification, impurity quantification, and isotope analysis) will be less conclusive. Therefore, there is an ongoing and urgent need for additional signatures and new methods to further constrain forensic analysis. Molecular signatures (i.e., molecular structures of trace impurity metals and dopants, their oxidation states, and host phase structural defects) encode nuanced information about individual chemical processing steps used during synthesis, independent of feedstock origin or final product.
Stoichiometric technological UO₂ corrodes over time to U(VI) oxyhydroxide (e.g., meta-schoepite minerals) at low temperatures or to U₃O₈ at high temperatures. Crucially, the intermediate alteration products—UO₂+x (x = 0–0.25) and more oxidized products—have unique molecular signatures that can be detected using X-ray absorption spectroscopy (XAS). In contrast, molecular signatures cannot be extracted from traditional mass spectroscopy-based forensic techniques, which provide primarily compositional information (including nuclear mass), are slow (requiring weeks of preparation), and are destructive to samples. Moreover, while X-ray diffraction (XRD) can quantify unit cell contraction upon UO2 oxidation in pure bulk materials, it is very difficult to use XRD to detect and quantify unit cell changes in small particles (nano- to micro-scale) embedded in complex materials such as soils, which is an important category of forensic samples. Although uraninite oxidation has been the focus of much research over the years, there remain important gaps in our knowledge of the reaction pathway and process rates under environmental conditions. These gaps are particularly significant in the context of forensic analyses, where alteration pathways and kinetics accuracy are critical.
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