I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[28515]
Open Access
Abstract: Mn-doped UO2 is under consideration for use as an accident tolerant nuclear fuel. We detail the synthesis of Mn-doped UO2 prepared via a wet co-precipitation method, which was refined to improve the yield of incorporated Mn. To verify the Mn-doped UO2 defect chemistry, X-ray absorption spectroscopy at the Mn K-edge was performed, in addition to X-ray diffraction, Raman spectroscopy and high-energy resolved fluorescence detection X-ray absorption near edge spectroscopy at the U M4-edge. It was established that Mn2+ directly substitutes for U4+ in the UO2 lattice, accompanied by oxygen vacancy (Ov) charge compensation. In contrast to other divalent-element doped UO2 materials, compelling evidence for U5+ in a charge compensating role was not found. This work furthers understanding of the structure and crystal chemistry of Mn-doped UO2, which could show potential advantages as a novel efficient advanced nuclear fuel.
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Jan 2024
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B18-Core EXAFS
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Diamond Proposal Number(s):
[1724]
Open Access
Abstract: ThTi2O6 derived compounds with the brannerite structure were designed, synthesised, and characterised with the aim of stabilising incorporation of U5+ or U6+, at dilute concentration. Appropriate charge compensation was targeted by co-substitution of Gd3+, Ca2+, Al3+, or Cr3+, on the Th or Ti site. U L3 edge X-ray Absorption Near Edge Spectroscopy (XANES) and High Energy Resolution Fluorescence Detected U M4 edge XANES evidenced U5+ as the major oxidation state in all compounds, with a minor fraction of U6+ (2–13%). The balance of X-ray and Raman spectroscopy data support uranate, rather than uranyl, as the dominant U6+ speciation in the reported brannerites. It is considered that the U6+ concentration was limited by unfavourable electrostatic repulsion arising from substitution in the octahedral Th or Ti sites, which share two or three edges, respectively, with neighbouring polyhedra in the brannerite structure.
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Aug 2023
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B18-Core EXAFS
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Lewis R.
Blackburn
,
Luke T.
Townsend
,
Malin C.
Dixon Wilkins
,
Toshiaki
Ina
,
Merve
Kuman
,
Shi-Kuan
Sun
,
Amber R.
Mason
,
Laura J.
Gardner
,
Martin C.
Stennett
,
Claire L.
Corkhill
,
Neil C.
Hyatt
Diamond Proposal Number(s):
[17243]
Open Access
Abstract: Indium (In) is a neutron absorbing additive that could feasibly be used to mitigate criticality in ceramic wasteforms containing Pu in the immobilised form, for which zirconolite (nominally CaZrTi2O7) is a candidate host phase. Herein, the solid solutions Ca1-xZr1-xIn2xTi2O7 (0.10 ≤ x ≤ 1.00; air synthesis) and Ca1-xUxZrTi2-2xIn2xO7 (x = 0.05, 0.10; air and argon synthesis) were investigated by conventional solid state sintering at a temperature of 1350 °C maintained for 20 h, with a view to characterise In3+ substitution behaviour in the zirconolite phase across the Ca2+, Zr4+ and Ti4+ sites. When targeting Ca1-xZr1-xIn2xTi2O7, single phase zirconolite-2M was formed at In concentrations of 0.10 ≤ x ≤ 0.20; beyond x ≥ 0.20, a number of secondary In-containing phases were stabilised. Zirconolite-2M remained a constituent of the phase assemblage up to a concentration of x = 0.80, albeit at relatively low concentration beyond x ≥ 0.40. It was not possible to synthesise the In2Ti2O7 end member compound using a solid state route. Analysis of the In K-edge XANES spectra in the single phase zirconolite-2M compounds confirmed that the In inventory was speciated as trivalent In3+, consistent with targeted oxidation state. However, fitting of the EXAFS region using the zirconolite-2M structural model was consistent with In3+ cations accommodated within the Ti4+ site, contrary to the targeted substitution scheme. When deploying U as a surrogate for immobilised Pu in the Ca1-xUxZrTi2-2xIn2xO7 solid solution, it was demonstrated that, for both x = 0.05 and 0.10, In3+ was successfully able to stabilise zirconolite-2M when U was distributed predominantly as both U4+ and average U5+, when synthesised under argon and air, respectively, determined by U L3-edge XANES analysis.
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Jun 2023
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17782, 24074]
Open Access
Abstract: Crystal chemical design principles were applied to synthesise novel U4+ dominant and titanium excess betafite phases Ca1.15(5)U0.56(4)Zr0.17(2)Ti2.19(2)O7 and Ca1.10(4)U0.68(4)Zr0.15(3)Ti2.12(2)O7, in high yield (85–95 wt%), and ceramic density reaching 99% of theoretical. Substitution of Ti on the A-site of the pyrochlore structure, in excess of full B-site occupancy, enabled the radius ratio (rA/rB = 1.69) to be tuned into the pyrochlore stability field, approximately 1.48 ≲ rA/rB ≲ 1.78, in contrast to the archetype composition CaUTi2O7 (rA/rB = 1.75). U L3-edge XANES and U 4f7/2 and U 4f5/2 XPS data evidenced U4+ as the dominant speciation, consistent with the determined chemical compositions. The new betafite phases, and further analysis reported herein, point to a wider family of actinide betafite pyrochlores that could be stabilised by application of the underlying crystal chemical principle applied here.
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Jun 2023
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17782]
Open Access
Abstract: Portland cement-based grouts used for radioactive waste immobilisation contain a Ca- and Si-rich binder phase, known as calcium–silicate–hydrate (C–S–H). Depending on the blend of cement used, the Ca/Si ratio can vary considerably. A range of C–S–H minerals with Ca/Si ratios from 0.6 to 1.6 were synthesised and contacted with aqueous U(VI) at 0.5 mM and 10 mM concentrations. Solid-state 29Si MAS-NMR spectroscopy was applied to probe the Si coordination environment in U(VI)-contacted C–S–H minerals and, in conjunction with U LIII-edge X-ray absorption spectroscopy analysis, inferences of the fate of U(VI) in these systems were made. At moderate or high Ca/Si ratios, uranophane-type uranyl silicates or Ca-uranates dominated, while at the lowest Ca/Si ratios, the formation of a Ca-bearing uranyl silicate mineral, similar to haiweeite (Ca[(UO2)2Si5O12(OH)2]·3H2O) or Ca-bearing weeksite (Ca2(UO2)2Si6O15·10H2O) was identified. This study highlights the influence of Ca/Si ratio on uranyl sequestration, of interest in the development of post-closure safety models for U-bearing radioactive waste disposal.
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Feb 2023
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[20872, 24074, 28515]
Open Access
Abstract: Advanced Cr-doped UO2 fuels are essential for driving safe and efficient generation of nuclear energy. Although widely deployed, little is known about their fundamental chemistry, which is a critical gap for development of new fuel materials and radioactive waste management strategies. Utilising an original approach, we directly evidence the chemistry of Cr(3+)2O3–doped U(4+)O2. Advanced high-flux, high-spectral purity X-ray absorption spectroscopy (XAS), corroborated by diffraction, Raman spectroscopy and high energy resolved fluorescence detection-XAS, is used to establish that Cr2+ directly substitutes for U4+, accompanied by U5+ and oxygen vacancy charge compensation. Extension of the analysis to heat-treated simulant nuclear fuel reveals a mixed Cr2+/3+ oxidation state, with Cr in more than one physical form, explaining the substantial discrepancies that exist in the literature. Successful demonstration of this analytical advance, and the scientific underpinning it provides, opens opportunities for an expansion in the range of dopants utilised in advanced UO2 fuels.
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Dec 2022
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[20189]
Open Access
Abstract: A primary target towards the clean-up operation of the Fukushima disaster is the retrieval of Molten Core-Concrete Interaction (MCCI) products, presently residing on the basement of the damaged nuclear reactor Units 1–3. MCCI is a fusion of materials, composed of both nuclear fuel cladding and neighbouring structural components. Determining the currently unknown, physical and mechanical properties of MCCI is essential for successful and timely retrieval. In this paper, we aim to experimentally quantify the mechanical properties of a material fabricated to resemble MCCI. A small-scale representative specimen was mechanically tested using Hertzian indentation stepwise loading. Synchrotron X-ray computed tomography was conducted at several loading stages to reveal the sample microstructure and mechanical degradation. The acquired tomograms were analysed by digital volume correlation to measure full-field displacements and strains developed within the sample volume. Young’s modulus and Poisson ratio were determined via this combined methodology.
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Jul 2022
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17782]
Open Access
Abstract: Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and hydrotalcite (Mg6Al2(OH)16CO3·4H2O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing UVI from radioactive waste streams. U LIII-edge X-ray absorption spectroscopy (XAS) was used to probe the UVI coordination environment in contact with these minerals. For the first time, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these UVI-contacted minerals and make inferences on the UVI coordination, in conjunction with the X-ray spectroscopy analyses. The U LIII-edge XAS analysis of the UVI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with 27Al NMR analysis, indicated that a disordered Ca- or Al-bearing UVI secondary phase also formed. For the UVI-contacted hydrotalcite phases, the XAS and 27Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.
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Jan 2022
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17243]
Open Access
Abstract: Within the context of the UK's radioactive waste vitrification programme, which utilises a lithium-sodium borosilicate glass modified with CaO and ZnO to immobilise high level nuclear waste, an investigation was undertaken to evaluate the effects on the structure and chemical durability of altering the CaO to ZnO ratio. Using a 6-component simplified alkali aluminoborosilicate glass, replacement of CaO by ZnO, even in moderate amounts, had a marked effect on the glass structure. Zn K-edge EXAFS identified that Zn existed within two distinct environments, both containing Zn in tetrahedral coordination. At high CaO content, Zn was coordinated in a “hardystonite-like” (Ca2ZnSi2O7) environment, while higher ZnO content induced destabilization and nano-scale phase separation occurred, forming discrete tri-clusters of Si/Al–O–Zn in a “willemite-like” (Zn2SiO4) environment. The presence of these environments was corroborated by thermal analysis and 29Si MAS NMR data. Despite this phase separation, glasses with higher ZnO content were found to exhibit the lowest normalized dissolution rates under dilute conditions, as determined using the Single-Pass Flow-Through methodology. Chemical structure analysis indicates that such behaviour is a result of enhanced polymerization of the glass network in the presence of Zn, and a reduced propensity for Si–O–Zn bond hydrolysis in water, resolving a long-running literature debate on the role of Zn on the dissolution of glass in the forward rate. Evidence is presented that indicates the phase-separated regions dissolve at somewhat different rates. These results enhance understanding of CaO[thin space (1/6-em)]:[thin space (1/6-em)]ZnO-containing glass behaviour, important to assess the long-term safety of radioactive waste management and disposal strategies.
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Jan 2022
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B18-Core EXAFS
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Diamond Proposal Number(s):
[24074]
Abstract: The synthesis, characterization, and crystal structure of a novel (dominant) uranium(V) brannerite of composition U1.09(6)Ti1.29(3)Al0.71(3)O6 is reported, as determined from Rietveld analysis of the high-resolution neutron powder diffraction data. Examination of the UTi2–xAlxO6 system demonstrated the formation of brannerite-structured compounds with varying Al3+ and U5+ contents, from U0.93(6)Ti1.64(3)Al0.36(3)O6 to U0.89(6)Ti1.00(3)Al1.00(3)O6. Substitution of Al3+ for Ti4+, with U5+ charge compensation, resulted in near-linear changes in the b and c unit cell parameters and the overall unit cell volume, as expected from ionic radii considerations. The presence of U5+ as the dominant oxidation state in near-single-phase brannerite compositions was evidenced by complementary laboratory U L3-edge and high-energy-resolution fluorescence-detected U M4-edge X-ray absorption near-edge spectroscopy. No brannerite phase was found for compositions with Al3+/Ti4+ > 1, which would require a U6+ contribution for charge compensation. These data expand the crystal chemistry of uranium brannerites to the stabilization of dominant uranium(V) brannerites by the substitution of trivalent cations, such as Al3+, on the Ti4+ site.
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Nov 2021
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