Analysis of the Structure of Heavy Ion Irradiated LaFeO 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.

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

ThTiO derived compounds with the brannerite structure were designed, synthesised, and characterised with the aim of stabilising incorporation of U or U, at dilute concentration. Appropriate charge compensation was targeted by co-substitution of Gd, Ca, Al, or Cr, on the Th or Ti site. U L edge X-ray Absorption Near Edge Spectroscopy (XANES) and High Energy Resolution Fluorescence Detected U M edge XANES evidenced U as the major oxidation state in all compounds, with a minor fraction of U (2-13%).

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

Crystal chemical design principles were applied to synthesise novel U dominant and titanium excess betafite phases CaUZrTiO and CaUZrTiO, 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/r = 1.69) to be tuned into the pyrochlore stability field, approximately 1.

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 CaZrTiO) is a candidate host phase. Herein, the solid solutions CaZrInTiO (0.10 ≤ x ≤ 1.

Oxidative dissolution of Cr-doped UO nuclear fuel.

Alternative UO nuclear fuels, incorporating Cr as a dopant, are currently in use in light-water reactors. Dissolution experiments using Cr-doped UO, performed as a function of Cr content in a simplified groundwater solution and under oxic conditions, established that the addition of Cr to the UO matrix systematically reduced the normalised dissolution rate of U at 25 and 40 °C. This effect was most notable under dilute solution conditions, and is the result of galvanic coupling between Cr and U, resulting from the presence of Cr in the UO matrix, as corroborated by activation energy determination.

Spectroscopic identification of Ca-bearing uranyl silicates formed in C-S-H systems.

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.

Cr solid solution in UO evidenced by advanced spectroscopy.

Advanced Cr-doped UO 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 CrO-doped UO.

Phase Evolution in the CaZrTiO-DyTiO 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 Dy incorporation within the self-charge balancing zirconolite CaZrDyTiO solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.

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.

Spectroscopic evaluation of U-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 (CaAl(SO)(OH)·26HO) and hydrotalcite (MgAl(OH)CO·4HO) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing U from radioactive waste streams.

Synthesis, Characterization, and Crystal Structure of Dominant Uranium(V) Brannerites in the UTiAlO System.

The synthesis, characterization, and crystal structure of a novel (dominant) uranium(V) brannerite of composition UTiAlO is reported, as determined from Rietveld analysis of the high-resolution neutron powder diffraction data. Examination of the UTiAlO system demonstrated the formation of brannerite-structured compounds with varying Al and U contents, from UTiAlO to UTiAlO. Substitution of Al for Ti, with U charge compensation, resulted in near-linear changes in the and unit cell parameters and the overall unit cell volume, as expected from ionic radii considerations.

Chemical state mapping of simulant Chernobyl lava-like fuel containing material using micro-focused synchrotron X-ray spectroscopy.

Uranium speciation and redox behaviour is of critical importance in the nuclear fuel cycle. X-ray absorption near-edge spectroscopy (XANES) is commonly used to probe the oxidation state and speciation of uranium, and other elements, at the macroscopic and microscopic scale, within nuclear materials. Two-dimensional (2D) speciation maps, derived from microfocus X-ray fluorescence and XANES data, provide essential information on the spatial variation and gradients of the oxidation state of redox active elements such as uranium.

Thermal treatment of Cs-exchanged chabazite by hot isostatic pressing to support decommissioning of Fukushima Daiichi Nuclear Power Plant.

Ion exchange materials are used widely for the removal of radionuclides from contaminated water at nuclear licensed sites, during normal operating procedures, decommissioning and in accident clean-up, such as the ongoing recovery operation at the Fukushima Daiichi nuclear power plant. Framework silicate inorganic ion exchange materials, such as chabazite ((NaKCaMg)[AlSiO]•9.7HO), have shown particular selectivity towards Cs uptake, but their safe storage poses a number challenges requiring conditioning into passively safe waste packages of minimal volume.

Low-Temperature Nitridation of FeO by Reaction with NaNH.

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 NH gas. One such method is the reaction between metal oxides and NaNH 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).

Characterization of and Structural Insight into Struvite-K, MgKPO·6HO, an Analogue of Struvite.

Struvite-K (MgKPO·6HO) is a magnesium potassium phosphate mineral with naturally cementitious properties, which is finding increasing usage as an inorganic cement for niche applications including nuclear waste management and rapid road repair. Struvite-K is also of interest in sustainable phosphate recovery from wastewater and, as such, a detailed knowledge of the crystal chemistry and high-temperature behavior is required to support further laboratory investigations and industrial applications. In this study, the local chemical environments of synthetic struvite-K were investigated using high-field solid-state Mg and K MAS NMR techniques, alongside P MAS NMR and thermal analysis.

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.

Immobilizing Pertechnetate in Ettringite via Sulfate Substitution.

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, this work explores how rapid formation of ettringite [CaAl(SO)(OH)·26(HO)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO.

Development, characterization and dissolution behavior of calcium-aluminoborate glass wasteforms to immobilize rare-earth oxides.

Calcium-aluminoborate (CAB) glasses were developed to sequester new waste compositions made of several rare-earth oxides generated from the pyrochemical reprocessing of spent nuclear fuel. Several important wasteform properties such as waste loading, processability and chemical durability were evaluated. The maximum waste loading of the CAB compositions was determined to be ~56.

Compositional Dependence of Solubility/Retention of Molybdenum Oxides in Aluminoborosilicate-Based Model Nuclear Waste Glasses.

Molybdenum oxides are an integral component of the high-level waste streams being generated from the nuclear reactors in several countries. Although borosilicate glass has been chosen as the baseline waste form by most of the countries to immobilize these waste streams, molybdate oxyanions (MoO) exhibit very low solubility (∼1 mol %) in these glass matrices. In the past three to four decades, several studies describing the compositional and structural dependence of molybdate anions in borosilicate and aluminoborosilicate glasses have been reported in the literature, providing a basis for our understanding of fundamental science that governs the solubility and retention of these species in the nuclear waste glasses.

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.

Geochemical association of Pu and Am in selected host-phases of contaminated soils from the UK and their susceptibility to chemical and microbiological leaching.

Understanding the biogeochemical behaviour and potential mobility of actinides in soils and groundwater is vital for developing remediation and management strategies for radionuclide-contaminated land. Pu is known to have a high Kd in soils and sediments, however remobilization of low concentrations of Pu remains a concern. Here, some of the physicochemical properties of Pu and the co-contaminant, Am, are investigated in contaminated soils from Aldermaston, Berkshire, UK, and the Esk Estuary, Cumbria, UK, to determine their potential mobility.

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.

Real-time gamma imaging of technetium transport through natural and engineered porous materials for radioactive waste disposal.

We present a novel methodology for determining the transport of technetium-99m, a γ-emitting metastable isomer of (99)Tc, through quartz sand and porous media relevant to the disposal of nuclear waste in a geological disposal facility (GDF). Quartz sand is utilized as a model medium, and the applicability of the methodology to determine radionuclide transport in engineered backfill cement is explored using the UK GDF candidate backfill cement, Nirex Reference Vault Backfill (NRVB), in a model system. Two-dimensional distributions in (99m)Tc activity were collected at millimeter-resolution using decay-corrected gamma camera images.

Arsenic bioremediation by biogenic iron oxides and sulfides.

Microcosms containing sediment from an aquifer in Cambodia with naturally elevated levels of arsenic in the associated groundwater were used to evaluate the effectiveness of microbially mediated production of iron minerals for in situ As remediation. The microcosms were first incubated without amendments for 28 days, and the release of As and other geogenic chemicals from the sediments into the aqueous phase was monitored. Nitrate or a mixture of sulfate and lactate was then added to stimulate biological Fe(II) oxidation or sulfate reduction, respectively.