B18-Core EXAFS
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Diamond Proposal Number(s):
[13559]
Open Access
Abstract: Spent nuclear fuel contains both uranium (U) and high yield fission products, including strontium-90 (90Sr), a key radioactive contaminant at nuclear facilities. Both U and 90Sr will be present where spent nuclear fuel has been processed, including in storage ponds and tanks. However, the interactions between Sr and U phases under ambient conditions are not well understood. Over a pH range of 4–14, we investigate Sr sorption behavior in contact with two nuclear fuel cycle relevant U(IV) phases: nano-uraninite (UO2) and U(IV)–silicate nanoparticles. Nano-UO2 is a product of the anaerobic corrosion of metallic uranium fuel, and UO2 is also the predominant form of U in ceramic fuels. U(IV)–silicates form stable colloids under the neutral to alkaline pH conditions highly relevant to nuclear fuel storage ponds and geodisposal scenarios. In sorption experiments, Sr had the highest affinity for UO2, although significant Sr sorption also occurred to U(IV)–silicate phases at pH ≥ 6. Extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy, and desorption data for the UO2 system suggested that Sr interacted with UO2 via a near surface, highly coordinated complex at pH ≥ 10. EXAFS measurements for the U(IV)–silicate samples showed outer-sphere Sr sorption dominated at acidic and near-neutral pH with intrinsic Sr-silicates forming at pH ≥ 12. These complex interactions of Sr with important U(IV) phases highlight a largely unrecognized control on 90Sr mobility in environments of relevance to spent nuclear fuel management and storage.
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Feb 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[13559, 17376, 17243]
Abstract: Uranium (U) is a radionuclide of key environmental interest due its abundance by mass within radioactive waste and presence in contaminated land scenarios. Ubiquitously present iron (oxyhydr)oxide mineral phases, such as (nano)magnetite, have been identified as candidates for immobilisation of U via incorporation into the mineral structure. Studies of how biogeochemical processes, such as sulfidation from the presence of sulfate-reducing bacteria, may affect iron (oxyhydr)oxides and impact radionuclide mobility are important in order to underpin geological disposal of radioactive waste and manage radioactively contaminated land. Here, this study utilised a highly controlled abiotic method for sulfidation of U(V) incorporated into nanomagnetite to determine the fate and speciation of U. Upon sulfidation, transient release of U into solution occurred (∼8.6 % total U) for up to 3 days, despite the highly reducing conditions. As the system evolved, lepidocrocite was observed to form over a period of days to weeks. After 10 months, XAS and geochemical data showed all U was partitioned to the solid phase, as both nanoparticulate uraninite (U(IV)O2) and a percentage of retained U(V). Further EXAFS analysis showed incorporation of the residual U(V) fraction into an iron (oxyhydr)oxide mineral phase, likely nanomagnetite or lepidocrocite. Overall, these results provide new insights into the stability of U(V) incorporated iron (oxyhydr)oxides during sulfidation, confirming the longer term retention of U in the solid phase under complex, environmentally relevant conditions.
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Feb 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17243]
Open Access
Abstract: U(IV) mobility can be significantly enhanced by colloids in both engineered and natural environments. This is particularly relevant in decommissioning and clean-up of nuclear facilities, such as legacy fuel ponds and silos at the Sellafield site, UK, and in long-term radioactive waste geodisposal. In this study, the product of metallic uranium (U) corrosion under anaerobic, alkaline conditions was characterised, and the interaction of this product with silicate solutions was investigated. The U metal corrosion product consisted of crystalline UO2 nanoparticles (5–10 nm) that aggregated to form clusters larger than 20 nm. Sequential ultrafiltration indicated that a small fraction of the U metal corrosion product was colloidal. When the uranium corrosion product was reacted with silicate solutions under anaerobic conditions, ultrafiltration indicated a stable colloidal uranium fraction was formed. Extended X-ray absorption fine structure (EXAFS) spectroscopy and high resolution TEM confirmed that the majority of U was still present as UO2 after several months of exposure to silicate solutions, but an amorphous silica coating was present on the UO2 surface. This silica coating is believed to be responsible for formation of the UO2 colloid fraction. Atomic-resolution scanning TEM (STEM) indicated some migration of U into the silica-coating of the UO2 particles as non-crystalline U(IV)-silicate, suggesting alteration of UO2 at the UO2-silica interface had occurred. This alteration at the UO2-silica interface is a potential pathway to the formation of U-silicates (e.g. coffinite, USiO4).
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Aug 2019
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[9044, 9598]
Open Access
Abstract: A detailed understanding of the mechanisms and effects of radiation damage in phyllosilicate minerals is a necessary component of the evaluation of the safety case for a deep geological disposal facility (GDF) for radioactive waste. Structural and chemical changes induced by α-particle damage will affect these mineral’s performance as a reactive barrier material (both in the near and far-field) over timescales relevant to GDF integrity. In this study, two examples of chlorite group minerals have been irradiated at α-particle doses comparable to those predicted to be experienced by the clay buffer material surrounding high level radioactive waste canisters. Crystallographic aberrations induced by the focused 4He2+ ion beam are revealed via high-resolution, microfocus X-ray diffraction mapping. Interlayer collapse by up to 0.5 Å is prevalent across both macro- and micro-crystalline samples, with the macro-crystalline specimen displaying a breakdown of the phyllosilicate structure into loosely-connected, multi-oriented crystallites displaying variable lattice parameters. The damaged lattice parameters suggest a localised breakdown and collapse of the OH- rich, ‘brucite-like’ interlayer. Microfocus Fe K-edge X-ray absorption spectroscopy illustrates this defect accumulation, manifest as a severe damping of the X-ray absorption edge. Subtle Fe2+/Fe3+ speciation changes are apparent across the damaged structures. A trend towards Fe reduction is evident at depth in the damaged structures at certain doses (8.76 x 1015 alpha particles/ cm2). Interestingly, this reductive trend does not increase with radiation dose, indeed at the maximum dose (1.26 x 1016 alpha particles/ cm2) administered in this study, there is evidence for a slight increase in Fe binding energy, suggesting the development of a depth-dependant redox gradient concurrent with light ion damage. At the doses examined here, these damaged structures are likely highly reactive, as sorption capacity will, to an extent, be largely enhanced by lattice disruption and an increase in available ‘edge’ sites.
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Feb 2019
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B18-Core EXAFS
I15-Extreme Conditions
I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[12704, 13559, 15276, 15966]
Abstract: Uranium is typically the most abundant radionuclide by mass in radioactive wastes and is a significant component of effluent streams at nuclear facilities. Actinide (IV) (An(IV)) colloids formed via various pathways, including corrosion of spent nuclear fuel, have the potential to greatly enhance the mobility of poorly soluble An(IV) forms, including uranium. This is particularly important in conditions relevant to decommissioning of nuclear facilities and the geological disposal of radioactive waste. Previous studies have suggested that silicate could stabilise U(IV) colloids. Here the formation, composition and structure of U(IV)-silicate colloids under the alkaline conditions relevant to spent nuclear fuel storage and disposal were investigated using a range of state of the art techniques. The colloids are formed across a range of pH conditions (9-10.5) and silicate concentrations (2-4 mM) and have a primary particle size 1-10 nm, also forming suspended aggregates < 220 nm. X-ray absorption spectroscopy, ultrafiltration and scanning transmission electron microscopy confirm the particles are U(IV)-silicates. Additional evidence from X-ray diffraction and pair distribution function data suggests the primary particles are composed of a UO2-rich core and a U-silicate shell. U(IV)-silicate colloids formation correlates with the formation of U(OH)3(H3SiO4)32- complexes in solution indicating they are likely particle precursors. Finally, these colloids form under a range of condition relevant to nuclear fuel storage and geological disposal of radioactive waste and represent a potential pathway for U mobility in these systems.
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Jul 2018
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I10-Beamline for Advanced Dichroism - scattering
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Diamond Proposal Number(s):
[9565]
Open Access
Abstract: The ability for magnetite to act as a recyclable electron donor and acceptor for Fe-metabolizing bacteria has recently been shown. However, it remains poorly understood whether microbe-mineral interfacial electron transfer processes are limited by the redox capacity of the magnetite surface or that of whole particles. Here we examine this issue for the phototrophic Fe(II)-oxidizing bacteria Rhodopseudomonas palustris TIE-1 and the Fe(III)-reducing bacteria Geobacter sulfurreducens, comparing magnetite nanoparticles (d ≈ 12 nm) against microparticles (d ≈ 100–200 nm). By integrating surface-sensitive and bulk-sensitive measurement techniques we observed a particle surface that was enriched in Fe(II) with respect to a more oxidized core. This enables microbial Fe(II) oxidation to occur relatively easily at the surface of the mineral suggesting that the electron transfer is dependent upon particle size. However, microbial Fe(III) reduction proceeds via conduction of electrons into the particle interior, i.e. it can be considered as more of a bulk electron transfer process that is independent of particle size. The finding has potential implications on the ability of magnetite to be used for long range electron transport in soils and sediments.
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Aug 2016
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[9044, 11703, 12064]
Open Access
Abstract: The development of complementary imaging techniques at beamline I18 at Diamond
Light Source (Didcot, UK) to investigate the microstructure of inorganic materials is
described. In particular, the use of X-ray micro-imaging techniques to understand the
effect of alpha radiation on phyllosilicates, and the nature of individual catalytic par-
ticles are reported. Micro X-ray diffraction (
m
XRD) studies of the former materials
have shown structural changes that will affect their adsorption properties, while the
chemistry of the catalyst particles has been investigated using micro X-ray fluorescence,
m
XRD and
m
X-ray absorption near-edge structure mapping. The distribution of a Mo-
promoted Pt nitrobenzene hydrogenation catalyst has shown that some of the Pt pene-
trated to the core of the particle and has the same chemistry as the bulk of the Pt located
on the outside of the particle. The phase distribution in an as-prepared Re-Ti-promoted
Co Fischer-Tropsch catalyst is reported.
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Jun 2016
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B18-Core EXAFS
I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[9044, 9598, 9647]
Open Access
Abstract: A critical radiation damage assessment of the materials that will be present in a Geological
Disposal Facility (GDF) for radioactive waste is a priority for building a safety case. Detailed analysis
of the effects of high-energy
a
-particle damage in phyllosilicates such as mica is a necessity, as these
are model structures for both the clay-based backfill material and the highly sorbent components of a
crystalline host rock. The
a
-radiation stability of biotite mica [general formula:
K(Mg,Fe)3(Al,Si3O10)(F,OH)2
] has been investigated using the 5 MV tandem pelletron at the University of Manchester’s
Dalton Cumbrian Facility (DCF) and both the microfocus spectroscopy (I18) and core X
-ray absorption
spectroscopy (B18) beamlines at Diamond Light Source (U.K.). Microfocus X-ray diffraction
mapping has demonstrated extensive structural aberrations in the mica resulting from controlled exposure
to the focused
4He2+ ion (a-particle) beam. Delivered doses were comparable to
a-particle fluences
expected in the highly active, near-field of a GDF. At doses up to 6.77 displacements per atom (dpa)
in the region of highest particle fluence, biotite mica displays a heterogeneous structural response to
irradiation on a micrometer scale, with sequential dilation and contraction of regions of the structure
perpendicular to the sheets, as well as a general overall contraction of the phyllosilicate layer spacing.
At the peak of ion fluence, the structure collapses under a high point defect density and amorphous
areas are pervasive among altered domains of the original lattice. Such structural alterations are likely
to affect the material’s capacity to sorb and retain escaped radionuclides over long timescales; increased
edge site availability may favor increased sorption while interlayer uptake will likely be reduced due
to collapse. Radiation-induced reduction of structural iron at the region of highest structural damage
across an
a-particle’s track has been demonstrated by Fe
K-edge X-ray absorption near edge spectroscopy (XANES) and local structural disorder has been confirmed by analysis of both potassium
K-edge
XANES and Fe
K-edge extended X-ray absorption fine structure analysis. An infrared absorption study
of deformations in the OH–
stretching region, along with electron probe microanalysis complements
the synchrotron data presented here
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Apr 2016
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[6208, 9044]
Open Access
Abstract: A detailed understanding of the response of mineral phases to the radiation fields experienced in a geological disposal facility (GDF) is currently poorly constrained. Prolongued ion irradiation has the potential to affect both the physical integrity and oxidation state of materials and therefore may alter a structure's ability to react with radionuclides. Radiohalos (spheres of radiation damage in minerals surrounding radioactive (α-emitting) inclusions) provide useful analogues for studying long term α-particle damage accumulation. In this study, silicate minerals adjacent to Th- and U-rich monazite and zircon were probed for redox changes and long/short range disorder using microfocus X-ray absorption spectroscopy (XAS) and high resolution X-ray diffraction (XRD) at Beamline I18, Diamond Light Source. Fe3+→ Fe2+ reduction has been demonstrated in an amphibole sample containing structural OH− groups – a trend not observed in anhydrous phases such as garnet. Coincident with the findings of Pattrick et al. (2013), the radiolytic breakdown of OH− groups is postulated to liberate Fe3+ reducing electrons. Across all samples, high point defect densities and minor lattice aberrations are apparent adjacent to the radioactive inclusion, demonstrated by micro-XRD.
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Dec 2015
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[585]
Abstract: Combined microfocus XAS and XRD analysis of alpha-particle radiation damage haloes around thorium-containing monazite in Fe-rich biotite reveals changes in both short- and long-range order. The total alpha-particles flux derived from the Th and U in the monazite over 1.8 Ga was 0.022 alpha particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short-range order disruption revealed by Fe K-edge EXAFS is manifest by a high variability in Fe-Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K-edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe3+ components of the biotite by interaction with the He-4(2)2+, the result of low and thermal energy electrons produced by the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionization interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing an increase in the trioctahedral layer distances and contraction within the trioctahedral layers. The large amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.
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Aug 2013
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