Monitoring synchrotron X-ray-induced radiolysis effects on metal (Fe, W) ions in high-temperature aqueous fluids

Nature and coordination geometry of geologically relevant aqueous Uranium(VI) complexes up to 400 ºC: A review and new data

The structure of the uranyl aqua ion (UO₂²⁺) and a number of its inorganic complexes (specifically, UO₂Cl⁺, UO₂Cl₂⁰, UO₂SO₄⁰, [Formula: see text] , [Formula: see text] and UO₂OH₄^2-) have been characterised using X-Ray absorption spectroscopy/extended X-Ray absorption fine structure (XAS/EXAFS) at temperatures ranging from 25 to 326 ºC. Results of ab initio molecular dynamics (MD) calculations are also reported for uranyl in chloride and sulfate-bearing fluids from 25 to 400 ºC and 600 bar to 20 kilobar (kb). These results are reported alongside a comprehensive review of prior structural characterisation work with particular focus given to EXAFS works to provide a consistent and up-to-date view of the structure of these complexes under conditions relevant to U mobility in ore-forming systems and around high-grade nuclear waste repositories. Regarding reported EXAFS results, average equatorial coordination was found to decrease in uranyl and its sulfate and chloride complexes as temperature rose - the extent of this decrease differed between species and solution compositions but typically resulted in an equatorial coordination number of ∼3-4 at temperatures above 200 ºC. The [Formula: see text] complex was observed at temperatures from 25 to 247 ºC and exhibited no major structural changes over this temperature range. UO₂OH₄^2- exhibited only minor structural changes over a temperature range from 88 to 326 ºC and was suggested to manifest fivefold coordination with four hydroxyl molecules and one water molecule around its equator. Average coordination values derived from fits of the reported EXAFS data were compared to average coordination values calculated using the experimentally derived thermodynamic data for chloride complexes reported by Dargent et al. (2013) and Migdisov et al. (2018b), and for sulfate complexes reported by Alcorn et al. (2019) and Kalintsev et al. (2019). Sulfate EXAFS data were well described by available thermodynamic data, and chloride EXAFS data were described well by the thermodynamic data of Migdisov et al. (2018b), but not by the data of Dargent et al. (2013). The ab initio molecular dynamics calculations confirmed the trends in equatorial coordination observed with EXAFS and were also able to provide an insight into the effect of pressure in equatorial water coordination - for a given temperature, higher pressures appear to lead to a greater number of equatorially bound waters counteracting the temperature effect.

Design of a containment apparatus for synchrotron XAS measurements of radioactive fluid samples under high temperatures and pressures

The simple working principles and versatility of the hydrothermal diamond-anvil cell (HDAC) make it highly useful for synchrotron x-ray studies of aqueous and fluid samples at high pressure-temperature (P-T) conditions. However, safety concerns need to be overcome in order to use the HDAC for synchrotron studies of aqueous radioactive samples at high temperatures and pressures. For accomplishment of such hydrothermal experiments of radioactive materials at synchrotron beamlines, the samples are required to be enclosed in a containment system employing three independent layers of airtight sealing at some synchrotron facilities while enabling access to the sample using several experimental probes, including incoming and outgoing x-rays. In this article, we report the design and implementation of a complete radiological safety enclosure system for an HDAC specialized for high P-T x-ray absorption spectroscopy (XAS) measurements of aqueous solutions containing the actinides at synchrotron beamlines. The enclosure system was successfully tested for XAS experiments using the HDAC with aqueous samples containing depleted uranium at temperatures ranging from 25 to 500 °C and pressures ranging from vapor pressure to 350 MPa.

X-ray-induced reduction of Au ions in an aqueous solution in the presence of support materials and in situ time-resolved XANES measurements

Synchrotron X-ray-induced reduction of Au ions in an aqueous solution with or without support materials is reported. To clarify the process of radiation-induced reduction of metal ions in aqueous solutions in the presence of carbon particles as support materials, in situ time-resolved XANES measurements of Au ions were performed under synchrotron X-ray irradiation. XANES spectra were obtained only when hydrophobic carbon particles were added to the precursor solution containing Au ions. Changes in the shape of the XANES spectra indicated a rapid reduction from ionic to metallic Au in the precursor solution owing to synchrotron X-ray irradiation. In addition, the effects of the wettability of the carbon particles on the deposited Au metallic spots were examined. The deposited Au metallic spots were different depending on the relationship of surface charges between metal precursors and support materials. Moreover, a Au film was obtained as a by-product only when hydrophilic carbon particles were added to the precursor solution containing the Au ions.

X-ray irradiation induced reduction and nanoclustering of lead in borosilicate glass

Under the influence of X-rays an unexpected formation of pure Pb nanoparticles was observed in PbS-doped borosilicate glass.