Abstract
An environmentally aged radioactive particle of UFeO4 recovered from soil contaminated with munitions depleted uranium (DU) was characterised by microbeam synchrotron X-ray analysis. Imaging of uranium speciation by spatially resolved X-ray diffraction (μ-XRD) and X-ray absorption spectroscopy (μ-XAS) was used to localise UFeO4 in the particle, which was coincident with a distribution of U(v). The U oxidation state was confirmed using X-ray Absorption Near Edge Structure (μ-XANES) spectroscopy as +4.9 ± 0.15. Le-Bail fitting of the particle powder XRD pattern confirmed the presence of UFeO4 and a minor alteration product identified as chernikovite (H3O)(UO2)(PO4)·3H2O. Refined unit cell parameters for UFeO4 were in good agreement with previously published values. Uranium-oxygen interatomic distances in the first co-ordination sphere were determined by fitting of Extended X-ray Absorption Fine Structure (μ-EXAFS) spectroscopy. The average first shell U-O distance was 2.148 ± 0.012 Å, corresponding to a U valence of +4.96 ± 0.13 using bond valence sum analysis. Using bond distances from the published structure of UFeO4, U and Fe bond valence sums were calculated as +5.00 and +2.83 respectively, supporting the spectroscopic analysis and confirming the presence of a U(v)/Fe(iii) pair. Overall this investigation provides important evidence for the stability of U(v) ternary oxides, in oxic, variably moist surface environment conditions for at least 25 years.
Highlights
Radioactive and hot particles are introduced into the environment by a number of civil and military nuclear events, including nuclear power plant (NPP) accidents, effluent discharges from nuclear fuel reprocessing, nuclear weapons testing and acts of war.[1]
Ternary compounds in the U–Fe–O system are of interest in the interaction of uranium wastes with iron oxides[3] and as a component of corium in severe nuclear power plant accidents.[4,5]
The high X-ray photon ux and small spot size achievable with modern microfocus synchrotron X-ray beamlines allows the use of localised X-ray absorption spectroscopy (XAS) techniques to probe the oxidation state and chemical environment of elements in radioactive and hot particles, which may not be amenable to regular preparation or characterisation methods.[20,21]
Summary
Information on the chemical speciation of U in a particle containing the ternary oxide UFeO4 is established by multi-modal synchrotron X-ray microscopy. The high X-ray photon ux and small spot size achievable with modern microfocus synchrotron X-ray beamlines allows the use of localised X-ray absorption spectroscopy (XAS) techniques to probe the oxidation state and chemical environment of elements in radioactive and hot particles, which may not be amenable to regular preparation or characterisation methods.[20,21] The particle in this study was recovered from soils contaminated as a result of depleted uranium munitions test ring.[22] The use of these techniques offers a direct measure of the uranium oxidation state to demonstrate the presence of U(V) in UFeO4, and provides evidence on the environmental behaviour of this compound. Coupled with re nement of micro X-ray diffraction data, and elemental analysis by microfocus X-ray uorescence spectroscopy (m-XRF), these techniques provide an integrated methodology for detailed chemical characterisation of radioactive and hot particles of a scale commensurate with, or greater, than the X-ray footprint
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