Analysis of the Structure of Heavy Ion Irradiated LaFeO3 Using Grazing Angle X-ray Absorption Spectroscopy

Crystalline ceramics are candidate materials for the immobilization of radionuclides, particularly transuranics (such as U, Pu, and Am), arising from the nuclear fuel cycle. Due to the α-decay of transuranics and the associated recoil of the parent nucleus, crystalline materials may eventually be rendered amorphous through changes to the crystal lattice caused by these recoil events. Previous work has shown irradiation of titanate-based ceramics to change the local cation environment significantly, particularly in the case of Ti which was shown to change from 6- to 5-fold coordination. Here, this work expands the Ti-based study to investigate the behavior in Fe-based materials, using LaFeO3 as an example material. Irradiation was simulated by heavy ion implantation of the bulk LaFeO3 ceramic, with the resulting amorphous layer characterized with grazing angle X-ray absorption spectroscopy (GA-XAS). Insights into the Fe speciation changes exhibited by the amorphized surface layer were provided through quantitative analysis, including pre-edge analysis, and modeling of the extended X-ray absorption fine structure (EXAFS), of the GA-XAS data.

A multimodal X-ray spectroscopy investigation of uranium speciation in ThTi2O6 compounds with the brannerite structure

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.

Crystal chemical design, synthesis and characterisation of U(IV)-dominant betafite phases for actinide immobilisation

Crystal chemical design principles were applied to synthesise novel U⁴⁺ dominant and titanium excess betafite phases Ca_1.15(5)U_0.56(4)Zr_0.17(2)Ti_2.19(2)O₇ and Ca_1.10(4)U_0.68(4)Zr_0.15(3)Ti_2.12(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 (r_A/r_B = 1.69) to be tuned into the pyrochlore stability field, approximately 1.48 ≲ r_A/r_B ≲ 1.78, in contrast to the archetype composition CaUTi₂O₇ (r_A/r_B = 1.75). U L₃-edge XANES and U 4f_7/2 and U 4f_5/2 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.

Underpinning the use of indium as a neutron absorbing additive in zirconolite by X-ray absorption spectroscopy

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 Ca_1-xZr_1-xIn_2xTi₂O₇ (0.10 ≤ x ≤ 1.00; air synthesis) and Ca_1-xU_xZrTi_2-2xIn_2xO₇ (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 Ca_1-xZr_1-xIn_2xTi₂O₇, 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 Ca_1-xU_xZrTi_2-2xIn_2xO₇ 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.

Cr2+ solid solution in UO2 evidenced by advanced spectroscopy

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 Cr^2+/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.

Phase Evolution in the CaZrTi2O7-Dy2Ti2O7 System: A Potential Host Phase for Minor Actinide Immobilization

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy³⁺ incorporation within the self-charge balancing zirconolite Ca_1–xZr_1–xDy_2xTi₂O₇ solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δx = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L₃-edge XANES region confirmed that Dy was present uniformly as Dy³⁺, remaining analogous to Am³⁺. Fitting of the EXAFS region was consistent with Dy³⁺ cations distributed across both Ca²⁺ and Zr⁴⁺ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy³⁺ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi₂O₇–REE³⁺Ti₂O₇ binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE³⁺ cations.

Zirconolite (CaZrTi₂O₇) ceramics are candidate wasteform materials for Pu and other minor actinides. Herein, the Ca_1−xZr_1−xDy_2xTi₂O₇ solid solution was fabricated by a conventional mixed oxide synthesis, with Dy³⁺ included as a structural simulant for Am³⁺. A phase transformation from zirconolite-2M to zirconolite-4M was observed at low Dy concentrations (0.20 ≤ x ≤ 0.30) after which cubic pyrochlore was stabilized as the dominant phase. Observations and interpretations were supported by electron diffraction and X-ray absorption spectroscopic methods.

Spectroscopic evaluation of UVI-cement mineral interactions: ettringite and hydrotalcite

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.

HERMES - a GUI-based software tool for pre-processing of X-ray absorption spectroscopy data from laboratory Rowland circle spectrometers

HERMES, a graphical user interface software tool, is presented, for pre-processing X-ray absorption spectroscopy (XAS) data from laboratory Rowland circle spectrometers, to meet the data handling needs of a growing community of practice. HERMES enables laboratory XAS data to be displayed for quality assessment, merging of data sets, polynomial fitting of smoothly varying data, and correction of data to the true energy scale and for dead-time and leakage effects. The software is written in Java 15 programming language, and runs on major computer operating systems, with graphics implementation using the JFreeChart toolkit. HERMES is freely available and distributed under an open source licence.

Synthesis, Characterization, and Crystal Structure of Dominant Uranium(V) Brannerites in the UTi2-xAlxO6 System

The synthesis, characterization, and crystal structure of a novel (dominant) uranium(V) brannerite of composition U_1.09(6)Ti_1.29(3)Al_0.71(3)O₆ is reported, as determined from Rietveld analysis of the high-resolution neutron powder diffraction data. Examination of the UTi_2-xAl_xO₆ system demonstrated the formation of brannerite-structured compounds with varying Al³⁺ and U⁵⁺ contents, from U_0.93(6)Ti_1.64(3)Al_0.36(3)O₆ to U_0.89(6)Ti_1.00(3)Al_1.00(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.

Symmetry and the Role of the Anion Sublattice in Aurivillius Oxyfluoride Bi2TiO4F2

The photocatalytic and dielectric behaviors of Aurivillius oxyfluorides such as Bi₂TiO₄F₂ depend sensitively on their crystal structure and symmetry but these are not fully understood. Our experimental work combined with symmetry analysis demonstrates the factors that influence anion order and how this might be tuned to break inversion symmetry. We explore an experimental approach to explore anion order, which combines Rietveld analysis with strain analysis.

Aurivillius oxyfluoride Bi₂TiO₄F₂ shows a promising photocatalytic behavior, which has been related to its polar structure, but polar properties have been difficult to reproduce in bulk samples. We explore a possible anion order and how this could give polar structures when combined with other distortions. We highlight the influence of strain on anion order and therefore the properties in these systems.

Low-Temperature Nitridation of Fe3O4 by Reaction with NaNH2

Low-temperature soft chemical synthesis routes to transition-metal nitrides are of interest as an alternative to conventional high-temperature ammonolysis reactions involving large volumes of chemotoxic NH₃ gas. One such method is the reaction between metal oxides and NaNH₂ at ca. 200 °C to yield the counterpart nitrides; however, there remains uncertainty regarding the reaction mechanism and product phase assemblage (in particular, noncrystalline components). Here, we extend the chemical tool box and mechanistic understanding of such reactions, demonstrating the nitridation of Fe₃O₄ by reaction with NaNH₂ at 170-190 °C, via a pseudomorphic reaction. The more reduced Fe₃O₄ precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with Fe₂O₃. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and ⁵⁷Fe Mössbauer spectroscopy, comprised 49-59 mol % ε-Fe_2+xN, accompanied by 29-39 mol % FeO_1-xN_x and 8-14 mol % γ″-FeN. The oxynitride phase was apparently noncrystalline in the recovered product but could be crystallized by heating at 180 °C. Although synthesis of transition-metal nitrides is achieved by reaction of the counterpart oxide with NaNH₂, it is evident from this investigation that the product phase assemblage may be complex, which could prove a limitation if the objective is to produce a single-phase product with well-defined electrical, magnetic, or other physical properties for applications. However, the significant yield of the FeO_1-xN_x oxynitride phase identified in this study opens the possibility for the synthesis of metastable oxynitride phases in high yield, by reaction of a metal oxide substrate with NaNH₂, with either careful control of H₂O concentration in the system or postsynthetic hydrolysis and crystallization.

Nuclear forensic signatures and structural analysis of uranyl oxalate, its products of thermal decomposition and Fe impurity dopant

Uranyl oxalate (UO2C2O4·xH2O) may exist at the back-end of the nuclear fuel cycle (NFC) as an intermediate in spent fuel reprocessing. The conditions used in aqueous reprocessing and thermal treatment can affect the physical and chemical properties of the material. Furthermore, trace impurities, such as Fe, may incorporate into the structure of these materials. In nuclear forensics, understanding relationships between processing variables aids in determination of provenance and processing history. In this study, the thermal decomposition of UO2C2O4·3H2O and phase analysis of its thermal products are examined. Their morphologies are discussed with respect to a matrix of solution processing conditions.

Characterisation and disposability assessment of multi-waste stream in-container vitrified products for higher activity radioactive waste

Materials from GeoMelt® In-Container Vitrification (ICV)™ of simulant UK nuclear wastes were characterised to understand the partitioning of elements, including inactive surrogates for radionuclide species of interest, within the heterogeneous products. Aqueous durability analysis was performed to assess the potential disposability of the resulting wasteforms. The vitrification trial aimed to immobilise a variety of simulant legacy waste streams representative of decommissioning operations in the UK, including plutonium contaminated material, Magnox sludges and ion-exchange materials, which were vitrified upon the addition of glass forming additives. Two trials with different wastes were characterised, with the resultant vitreous wasteforms comprising olivine and pyroxene crystalline minerals within glassy matrices. Plutonium surrogate elements were immobilised within the glassy fraction rather than partitioning into crystalline phases. All vitrified products exhibited comparable or improved durability to existing UK high level waste vitrified nuclear wasteforms over a 28 day period.

Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in CsLn 2Ti2NbO10 Dion-Jacobson Phases

The Dion-Jacobson (DJ) family of perovskite-related materials have recently attracted interest due to their polar structures and properties, resulting from hybrid-improper mechanisms for ferroelectricity in n = 2 systems and from proper mechanisms in n = 3 CsBi2Ti2NbO10. We report here a combined experimental and computational study on analogous n = 3 CsLn 2Ti2NbO10 (Ln = La, Nd) materials. Density functional theory calculations reveal the shallow energy landscape in these systems and give an understanding of the competing structural models suggested by neutron and electron diffraction studies. The structural disorder resulting from the shallow energy landscape breaks inversion symmetry at a local level, consistent with the observed second-harmonic generation. This study reveals the potential to tune between proper and hybrid-improper mechanisms by composition in the DJ family. The disorder and shallow energy landscape have implications for designing functional materials with properties reliant on competing low-energy phases such as relaxors and antiferroelectrics.

Correction: Multimodal X-ray microanalysis of a UFeO4 particle: evidence for the environmental stability of ternary U(V) oxides from depleted uranium munitions testing

Correction for 'Multimodal X-ray microanalysis of a UFeO4 particle: evidence for the environmental stability of ternary U(v) oxides from depleted uranium munitions testing' by Daniel E. Crean et al., Environ. Sci.: Processes Impacts, 2020, DOI: 10.1039/d0em00243g.

Nanoscale mechanism of UO2 formation through uranium reduction by magnetite

Uranium (U) is a ubiquitous element in the Earth's crust at ~2 ppm. In anoxic environments, soluble hexavalent uranium (U(VI)) is reduced and immobilized. The underlying reduction mechanism is unknown but likely of critical importance to explain the geochemical behavior of U. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite. Through high-end spectroscopic and microscopic tools, we demonstrate that the reduction proceeds first through surface-associated U(VI) to form pentavalent U, U(V). U(V) persists on the surface of magnetite and is further reduced to tetravalent UO2 as nanocrystals (~1-2 nm) with random orientations inside nanowires. Through nanoparticle re-orientation and coalescence, the nanowires collapse into ordered UO2 nanoclusters. This work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for the molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination.

Multimodal X-ray microanalysis of a UFeO4: evidence for the environmental stability of ternary U(v) oxides from depleted uranium munitions testing

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.

Solubility, speciation and local environment of chlorine in zirconolite glass–ceramics for the immobilisation of plutonium residues

Understanding chlorine speciation and incorporation in glass ceramic wasteforms underpins future treatment of contaminated plutonium residues.

An improved laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for analytical applications in materials chemistry research

X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e., synchrotron x-ray light sources. In this paper, we report the design and performance of an improved XAFS and XES spectrometer based on the general conceptual design of Seidler et al. [Rev. Sci. Instrum. 85, 113906 (2014)]. New developments include reduced mechanical degrees of freedom, much-increased flux, and a wider Bragg angle range to enable extended x-ray absorption fine structure (EXAFS) measurement and analysis for the first time with this type of modern laboratory XAFS configuration. This instrument enables a new class of routine applications that are incompatible with the mission and access model of the synchrotron light sources. To illustrate this, we provide numerous examples of x-ray absorption near edge structure (XANES), EXAFS, and XES results for a variety of problems and energy ranges. Highlights include XAFS and XES measurements of battery electrode materials, EXAFS of Ni with full modeling of results to validate monochromator performance, valence-to-core XES for 3d transition metal compounds, and uranium XANES and XES for different oxidation states. Taken en masse, these results further support the growing perspective that modern laboratory-based XAFS and XES have the potential to develop a new branch of analytical chemistry.

Nonresonant valence-to-core x-ray emission spectroscopy of niobium

The valence-to-core (V2C) portion of x-ray emission spectroscopy (XES) measures the electron states close to the Fermi level. These states are involved in bonding, thus providing a measure of the chemistry of the material. In this article, we show the V2C XES spectra for several niobium compounds. The Kβ″ peak in the V2C XES results from the transition of a ligand 2s electron into the 1s core-hole of the niobium, a transition allowed by hybridization with the niobium 4p. This location in energy of this weak peak shows a strong ligand dependence, thus providing a sensitive probe of the ligand environment about the niobium.

Role of Microstructure and Surface Defects on the Dissolution Kinetics of CeO2, a UO2 Fuel Analogue

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesized CeO2 analogue for UO2 fuel. Dissolution was performed on the following: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90 °C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce(3+) influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long term performance of spent fuel in geological disposal.

Solution Composition Effects on the Dissolution of a CeO2 analogue for UO2 and ThO2 nuclear fuels

This study investigates the dissolution of CeO2, an isostructural analogue for UO2 and ThO2, which was synthesized to closely approximate the microstructure of a spent nuclear fuel matrix. Dissolution of CeO2 particles was performed in simplified solutions representative of saline, near-neutral and alkaline ground waters that may be encountered in geological disposal scenarios, and in acidic medium for comparison. The normalized mass loss of cerium was found to be significantly influenced by the formation of colloidal particles, especially in the near-neutral and alkaline solutions investigated. The normalized dissolution rate, RL(Ce), k (g m-2 d-1), in these two solutions was found to be similar, but significantly lower than in a nitric acid medium. The activation energies based on the normalized release rate of cerium, at 40°C, 70°C and 90°C in each solution, were in the range of 24 ± 3 kJ mol-1 to 27 ± 7 kJ mol-1, indicative of a surface-mediated dissolution mechanism. The mechanism of dissolution was postulated to be similar in each of the solutions investigated, and further work is proposed to investigate the role of carbonate on the CeO2 dissolution mechanism.

Contribution of energetically reactive surface features to the dissolution of CeO2 and ThO2 analogues for spent nuclear fuel microstructures

In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthetic CeO2 and ThO2, spent nuclear fuel analogues that approximate as closely as possible the microstructure characteristics of fuel-grade UO2 but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) was investigated during dissolution. The effects of surface polishing on dissolution rate were also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called "instant release fraction" of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude.

Microanalytical X-ray imaging of depleted uranium speciation in environmentally aged munitions residues

Use of depleted uranium (DU) munitions has resulted in contamination of the near-surface environment with penetrator residues. Uncertainty in the long-term environmental fate of particles produced by impact of DU penetrators with hard targets is a specific concern. In this study DU particles produced in this way and exposed to the surface terrestrial environment for longer than 30 years at a U.K. firing range were characterized using synchrotron X-ray chemical imaging. Two sites were sampled: a surface soil and a disposal area for DU-contaminated wood, and the U speciation was different between the two areas. Surface soil particles showed little extent of alteration, with U speciated as oxides U3O7 and U3O8. Uranium oxidation state and crystalline phase mapping revealed these oxides occur as separate particles, reflecting heterogeneous formation conditions. Particles recovered from the disposal area were substantially weathered, and U(VI) phosphate phases such as meta-ankoleite (K(UO2)(PO4) · 3H2O) were dominant. Chemical imaging revealed domains of contrasting U oxidation state linked to the presence of both U3O7 and meta-ankoleite, indicating growth of a particle alteration layer. This study demonstrates that substantial alteration of DU residues can occur, which directly influences the health and environmental hazards posed by this contamination.

Remediation of soils contaminated with particulate depleted uranium by multi stage chemical extraction

Contamination of soils with depleted uranium (DU) from munitions firing occurs in conflict zones and at test firing sites. This study reports the development of a chemical extraction methodology for remediation of soils contaminated with particulate DU. Uranium phases in soils from two sites at a UK firing range, MOD Eskmeals, were characterised by electron microscopy and sequential extraction. Uranium rich particles with characteristic spherical morphologies were observed in soils, consistent with other instances of DU munitions contamination. Batch extraction efficiencies for aqueous ammonium bicarbonate (42-50% total DU extracted), citric acid (30-42% total DU) and sulphuric acid (13-19% total DU) were evaluated. Characterisation of residues from bicarbonate-treated soils by synchrotron microfocus X-ray diffraction and X-ray absorption spectroscopy revealed partially leached U(IV)-oxide particles and some secondary uranyl-carbonate phases. Based on these data, a multi-stage extraction scheme was developed utilising leaching in ammonium bicarbonate followed by citric acid to dissolve secondary carbonate species. Site specific U extraction was improved to 68-87% total U by the application of this methodology, potentially providing a route to efficient DU decontamination using low cost, environmentally compatible reagents.

The Use of Surrogates in Waste Immobilization Studies: A Case Study of Plutonium

Surrogates are widely used in the research and development of nuclear wasteforms, providing detailed insight into the chemical and physical behaviour of the wasteform whilst avoiding the widespread (restricted and costly) use of radiotoxic elements in the laboratory. However, caution must be exercised when dealing with surrogates since no single element or compound perfectly mimics all aspects of the behaviour of another. In this paper we present a broad discussion of the use of surrogates in waste immobilization, drawing upon and highlighting our research into glass and ceramic wasteforms for the immobilization of bulk PuO2.