11:45 AM - 12:00 PM
[MAG34-05] The forefront of uranium speciation research: speciation of trace uranium in the environment by advanced X-ray
spectroscopy
Keywords:Transition edge sensor, HERFD-XANES, uranium, biotite
[Introduction] Uranium (U) is used as a fuel for nuclear power generation, and its migration behavior in the environment is important to understand due to its usefulness as a resource and its chemical toxicity. Since the migration behavior of U in environment greatly depending on its chemical species, it is inevitable to understand the chemical species of U in environmental samples. X-ray spectroscopy has been used as a tool to investigate the chemical species of U in the environment because of its high sensitivity and elemental selectivity measurement. However, speciation of trace amounts of U in environmental samples is generally difficult due to interference from co-existing elements in the samples. Analytical methods for precisely determining chemical species in the presence of measurement interferences are important tools not only for studying the environmental behavior of actinides from the Fukushima Daiichi Nuclear Power Plant accident, but also for expanding into a wide range of research fields such as geological disposal of radioactive wastes. In this study, we report on the recent progress of U speciation studies in environmental samples using advanced X-ray spectroscopy, such as (1) high-energy-resolution fluorescence detection-X-ray absorption near-edge structure (HERFD-XANES) spectroscopy and (2) the use of a transition edge sensor (TES) as an X-ray detector.
[Experiments]
(1) Fluorescence XANES measurements using a silicon drift detector (SDD) and HERFD-XANES measurements using a X-ray emission spectrometer were performed at BL39XU, SPring-8. UO2 , FeUO4, and UO2(NO3)2(H2O)6 were used as U standard samples. The biotite sample was taken from a boring core of a uranium mine.
(2) Experiments using TES as an X-ray spectrometer and detector were performed at BL37XU, SPring-8. A NIST 610 glass standard was used to compare the energy resolution in the hard X-ray region between SDD and TES. The biotite sample was taken from a boring core of a uranium mine. The biotite thin sample was resin-filled and polished on both sides using lapping films. The thickness of the bioiite sample was about 100 μm. A beam size of microbeam X-rays was ca. 1 μm. The distribution of U and Rb in biotite was investigated by X-ray fluorescence intensities of U Lα line at 13.612 keV and Rb Kα line at 13.395 keV at an incident energy of 17.2 keV.
[Results and Discussion]
(1) U speciation study by HERFD-XANES
First, U reference samples of U(IV)O2, FeU(V)O4, and U(VI)O2(NO3)2(H2O)6 were used to compare the difference of XANES spectra by using SDD detectors and HERFD-XANES spectra by X-ray emission spectrometer. The measurement with a conventional SDD detector showed a single X-ray absorption peak for the compounds, whereas the measurement with an X-ray emission spectrometer showed a peak splitting for FeUO4. Based on the HERFD-XANES spectra, the chemical species of U in biotite in the environmental samples were investigated, and it was found that U species in biotite were mixture of U(IV) and U(VI).
(2) U speciation study by TES-μ-XRF-XANES
The energy resolutions of SDD and TES at around 14 keV were measured by the NIST610 glass standard. With the energy resolution of the SDD detector, it is difficult to separate the fluorescent X-ray of U Lα line from the that of Rb Kα line in the XRF spectrum. Meanwhile, the fluorescent X-rays of U Lα and Rb Kα lines were fully separated by TES. The μ-XRF mapping analysis of a biotite thin sample showed that the Rb intensity tended to be low at the sites where U intensity was high, suggesting that U accumulation in biotite occurred at the sites where interlayer cations are lost due to weathering. Since Lα fluorescent energy of minor actinides such as Np and Pu exist in this energy region around 14 keV, interference-free analysis using TES can be widely applied to the study of trace actinides in the environment.
[Experiments]
(1) Fluorescence XANES measurements using a silicon drift detector (SDD) and HERFD-XANES measurements using a X-ray emission spectrometer were performed at BL39XU, SPring-8. UO2 , FeUO4, and UO2(NO3)2(H2O)6 were used as U standard samples. The biotite sample was taken from a boring core of a uranium mine.
(2) Experiments using TES as an X-ray spectrometer and detector were performed at BL37XU, SPring-8. A NIST 610 glass standard was used to compare the energy resolution in the hard X-ray region between SDD and TES. The biotite sample was taken from a boring core of a uranium mine. The biotite thin sample was resin-filled and polished on both sides using lapping films. The thickness of the bioiite sample was about 100 μm. A beam size of microbeam X-rays was ca. 1 μm. The distribution of U and Rb in biotite was investigated by X-ray fluorescence intensities of U Lα line at 13.612 keV and Rb Kα line at 13.395 keV at an incident energy of 17.2 keV.
[Results and Discussion]
(1) U speciation study by HERFD-XANES
First, U reference samples of U(IV)O2, FeU(V)O4, and U(VI)O2(NO3)2(H2O)6 were used to compare the difference of XANES spectra by using SDD detectors and HERFD-XANES spectra by X-ray emission spectrometer. The measurement with a conventional SDD detector showed a single X-ray absorption peak for the compounds, whereas the measurement with an X-ray emission spectrometer showed a peak splitting for FeUO4. Based on the HERFD-XANES spectra, the chemical species of U in biotite in the environmental samples were investigated, and it was found that U species in biotite were mixture of U(IV) and U(VI).
(2) U speciation study by TES-μ-XRF-XANES
The energy resolutions of SDD and TES at around 14 keV were measured by the NIST610 glass standard. With the energy resolution of the SDD detector, it is difficult to separate the fluorescent X-ray of U Lα line from the that of Rb Kα line in the XRF spectrum. Meanwhile, the fluorescent X-rays of U Lα and Rb Kα lines were fully separated by TES. The μ-XRF mapping analysis of a biotite thin sample showed that the Rb intensity tended to be low at the sites where U intensity was high, suggesting that U accumulation in biotite occurred at the sites where interlayer cations are lost due to weathering. Since Lα fluorescent energy of minor actinides such as Np and Pu exist in this energy region around 14 keV, interference-free analysis using TES can be widely applied to the study of trace actinides in the environment.