Editorial: Analytical methods in nuclear forensics

As nuclear technology receives broader acceptance, the safeguarding of nuclear materials becomes a concern for the international community. New analytical methodologies that enable the interrogation of intercepted nuclear materials and the ability to derive definitive conclusions about the origin and processing history of these materials is critically important from a safeguarding perspective. Nuclear forensics uses complementary signatures to constrain the origin and history of nuclear materials and to ensure that the materials subjected to safeguards agreement are not diverted from their declared peaceful uses. The precision of analytical methods routinely used for interrogation of nuclear materials is critical for the law enforcement agencies and the international organizations to make timely decisions regarding these materials and implement punitive actions on the bad actors. In sample-limited scenarios, signatures—like microscale morphology and heterogeneity, which can be obtained using non-destructive techniques—become increasingly important to supplement information obtained from destructive analyses. This Research Topic brings research articles from four research groups representing improvements in both destructive and non-destructive analytical methodologies used to interrogate nuclear materials for safeguards purposes.

The study led by the safeguards science and technology group at Los Alamos National Laboratory (LANL) presents uranium chemical state determination and mapping capabilities for a hyperspectral X-ray imaging (HXI) instrument. This work presents the first published spectral mapping results on actinide materials, facilitating the detection of trace elements and chemical state determination at the nanogram scale. The ability of the HXI instrument to support nuclear forensic analysis of small samples is demonstrated. The energy resolution of the HXI translates to lower trace element detection thresholds to support a wide range of applications (e.g., metallurgy, semiconductors, materials science, geology, etc.). The HXI is compatible with low-vacuum operating modes in commercial scanning electron microscopes (SEMs), allowing investigation of liquid-phase studies in biology or chemical catalysis. Though less sensitive than “chemical state measurement via X-ray emission spectroscopy” at a synchrotron, the wider availability of SEMs makes the HXI “a more accessible technique for routine analysis in an industrial setting.” Using Monte-Carlo calculations, the time needed to distinguish uranium compounds from one another is estimated to be around 15 s. These results, presented by Carpenter et al., lay the foundation for further development of a useful nuclear forensics analytical tool.

Authors from the Atomic Weapons Establishment (AWE, UK) and University of Bristol (UK) describe the development of nuclear forensic tools that can be applied to thorium materials. Thorium is considered an indirect use material. Higginson et al. provide a summary of possible chronometric signatures for thorium materials. The nuclear forensic analytical methods applicable to thorium materials are distinct compared to more common uranium and plutonium materials. A laboratory scale recreation of the processing pathway historically used in the UK to process bulk monazite ore ((REE,Th)PO₄ where REE = Ce, La, Nd) to thorium dioxide (ThO₂) product (using a well characterized thorium ore originating from Kerala, India that was used in the initial UK test reactor experiments) is reported. Measurements traced a variety of ‘fingerprint’ material properties and impurities through the processing. It was shown that radio-chronometry, relative rare earth element abundances, impurities and microscopy were useful forensic signatures for thorium.

Authors (Ervin and Blakemore) from the University of Kansas (United States) reports “electroanalytical studies of the interconversion of crown-encapsulated U(VI)/U(V) complexes”, including quantification of heterogeneous electron-transfer. The redox properties of actinide-containing species are known to “influence their reactivity, speciation and interfacial behavior”. “Digital simulations of conventional cyclic voltammetry data with Au or Pt electrodes” were used to extract the parameters governing the electrode behavior, including heterogeneous electron-transfer rate constant (k⁰), transfer coefficient (α) and the diffusion coefficients of U^VIO₂ and U^VO₂. “Rotating ring-disk electrode studies were used to directly interrogate the chemical reversibility of U(VI)-U(V) interconversion”. This investigation demonstrated that hydrodynamic methods are valuable for design of electroanalytical approaches to distinguish actinide species and for study of their redox properties. These material characteristics are important fingerprints from a safeguards and nuclear forensic perspective.

Schreiber et al. describe new capabilities for nuclear forensics using ultra-high-resolution microcalorimeter gamma spectroscopy. The energy resolution achievable with the instrument allows isotope identification and quantification in small quantities of nuclear materials. Microcalorimeter detectors operate at ultra-low temperatures (<0.1 K), allowing “measurement of spectra with full width half maximum (FWHM) of less than 100 eV at 100 keV”. LANL has delivered three instruments–one Spectrometer Optimized for Facility Integrated Applications (SOFIA) and two High Efficiency and Resolution Microcalorimeter Spectrometers (HERMES) – to analytical laboratories routinely handling nuclear materials. Microcalorimetry results are presented for “recently irradiated materials, nuclear materials from throughout the nuclear fuel cycle, and for medical isotope products”. Microcalorimetry can also provide fission product gamma-rays that are otherwise not feasible to observe without elaborate chemical separations. Microcalorimeter technology provides additional signatures to better constrain the origin and intended use of nuclear materials.

In summary we, editors of the Research Topic on Analytical Methods in Nuclear Forensics, are very pleased to have helped with the publication of four excellent papers relevant to the safeguards and nuclear forensic communities.

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Footnotes

Edited and reviewed by: Edgar C. Buck, Pacific Northwest National Laboratory (DOE), United States