Dissolution of simulated nuclear waste glass at high surface area to solution volume, high pH and 70 °C: comparison of international simple glass and SON68 glass.

Long-term static dissolution experiments, lasting up to ∼1500 days, were conducted on International Simple Glass (ISG) and SON68 glass under hyperalkaline pH, at 70 °C, and at a very high glass surface area to solution volume ratio. The study compared (1) glass dissolution kinetics, (2) secondary phase formation, and (3) the microstructure of the altered glass and secondary phase interface. Boron release indicated rapid initial dissolution followed by a slowdown mainly due to a significant pH drop.

Deconvoluting Cr states in Cr-doped UO nuclear fuels via bulk and single crystal spectroscopic studies.

Cr-doped UO is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U in the fluorite lattice as Cr with formation of additional oxygen vacancies.

Monazite-Type SmPO as Potential Nuclear Waste Form: Insights into Radiation Effects from Ion-Beam Irradiation and Atomistic Simulations.

Single-phase monazite-type ceramics are considered as potential host matrices for the conditioning of separated plutonium and minor actinides. Sm-orthophosphates were synthesised and their behaviour under irradiation was investigated with respect to their long-term performance in the repository environment. Sintered SmPO pellets and thin lamellae were irradiated with 1, 3.

Insight Into Disorder, Stress and Strain of Radiation Damaged Pyrochlores: A Possible Mechanism for the Appearance of Defect Fluorite.

We have examined the irradiation response of a titanate and zirconate pyrochlore-both of which are well studied in the literature individually-in an attempt to define the appearance of defect fluorite in zirconate pyrochlores. To our knowledge this study is unique in that it attempts to discover the mechanism of formation by a comparison of the different systems exposed to the same conditions and then examined a range of techniques that cover a wide length scale. The conditions of approximately 1 displacement per atom He ions were used to simulate long term waste storage conditions as outlined by previous results from Ewing in a large enough sample volume to allow for neutron diffraction, as not attempted previously.

Incorporation of iodine into uranium oxyhydroxide phases.

Herein, we have synthesised a novel uranium oxyhydroxide (UOH) phase, RbK[(UO)O(OH)]·(IO), under hydrothermal conditions which intercalates IO a hybrid salt-inclusion and host-guest mechanism. The mechanism is based on favorable intermolecular bonding between disordered Rb/K and IO ions and hydroxyl and layer void positions respectively. To examine whether the intercalation may occur ubiquitously for UOH phases, the known UOH mineral phases metaschoepite ([(UO)O(OH)]·12HO), compreignacite (K[(UO)O(OH)]·7HO) and also related β-UO(OH) were synthesised and exposed to aqueous I and IO for 1 month statically at RT and 60 °C in air and the solid analysed using laser ablation inductively coupled plasma mass spectroscopy.

Chromium Doped UO-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels.

Cr-doped UO as a modern nuclear fuel type has been demonstrated to increase the in-reactor fuel performance compared to conventional nuclear fuels. Little is known about the long-term stability of spent Cr-doped UO nuclear fuels in a deep geological disposal facility. The investigation of suitable model materials in a step wise bottom-up approach can provide insights into the corrosion behavior of spent Cr-doped nuclear fuels.

Dissolution Kinetics of International Simple Glass and Formation of Secondary Phases at Very High Surface Area to Solution Ratio in Young Cement Water.

Static dissolution experiments were carried out with the reference International Simple Glass under hyperalkaline pH at 70 °C and very high SA/V ratio. Three aspects of glass dissolution behavior were investigated, (1) the rate drop regime and the residual rate (stage II), (2) the formation of secondary phases including thermodynamic aspects, and (3) the microstructure of the interface of altered glass and secondary phases. A very low residual rate of 6 × 10 g/md was determined based on boron release, which was several orders of magnitude lower than the initial rate established between the start of the experiments and the first sampling on day 59.

Extreme condition high temperature and high pressure studies of the K-U-Mo-O system.

Herein the first examples of alkali earth uranyl molybdates synthesised using extreme conditions of high temperature and high pressure (HT/HP) methods, namely K2[UO2(Mo2O7)2], K2[(UO2)2(Mo(vi)4Mo(iv)(OH)2)O16], K3[(UO2)6(OH)2(MoO4)6(MoO3OH)] and K5[(UO2)10MoO5O11OH]·H2O, are described and characterised. K2[UO2(Mo2O7)2] forms a monoclinic 2D layered structure in space group P21/c that consists of interlinking Mo2O7 dimers that link isolated UO22+ moieties forming [UO2(Mo2O7)2]2- layers which are separated by K+ cations. K2[(UO2)2(Mo(vi)4Mo(iv)(OH)2)O16] forms a disordered triclinic 3D framework structure in space group P1[combining macron].

On the change in UO redox reactivity as a function of HO exposure.

To assess the long-term leaching behaviour of UO, the main constituent of spent nuclear fuel, the oxidative dissolution of UO pellets was studied at high HO exposures ranging from 0.33 mol m to 1.36 mol m.

Correlative Atom Probe Tomography and Transmission Electron Microscopy Analysis of Grain Boundaries in Thermally Grown Alumina Scale.

We employed correlative atom probe tomography (APT) and transmission electron microscopy (TEM) to analyze the alumina scale thermally grown on the oxide dispersion-strengthened alloy MA956. Segregation of Ti and Y and associated variation in metal/oxygen stoichiometry at the grain boundaries and triple junctions of alumina were quantified and discussed with respect to the oxidation behavior of the alloy, in particular, to the formation of cation vacancies. Correlative TEM analysis was helpful to avoid building pragmatically well-looking but substantially incorrect APT reconstructions, which can result in erroneous quantification of segregating species, and highlights the need to consider ionic volumes and detection efficiency in the reconstruction routine.

Analysis of the Secondary Phases Formed by Corrosion of U₃Si₂-Al Research Reactor Fuel Elements in the Presence of Chloride Rich Brines.

Corrosion experiments with non-irradiated U₃Si₂-Al research reactor fuel samples were carried out in synthetic MgCl₂-rich brine to identify and quantify the secondary phases because depending on their composition and on their amount, such compounds can act as a sink for the radionuclide release in final repositories. Within the experimental period of 100 days at 90 °C and anoxic conditions the U₃Si₂-Al fuel sample was completely disintegrated. The obtained solids were subdivided into different grain size fractions and non-ambient X-ray diffraction (XRD) was applied for their qualitative and quantitative phase analysis.

Comparison of Uranium(VI) and Thorium(IV) Silicates Synthesized via Mixed Fluxes Techniques.

Two uranium and two thorium silicates were obtained using high temperature mixed fluxes methods. K(UO)SiO crystallizes in the P2/ c space group and contains open-branched sechser (six) single silicate chains, whereas K(UO)SiO crystallizes in the C2/ c space group and is built of unbranched achter (eight) silicate chains. The crystals of K(UO)SiO and K(UO)SiO are related by increasing U/Si molar ratios, and both structures contain the same secondary building units (SBUs), [USi] heptamers.

Effects of Te(IV) Oxo-Anion Incorporation into Thorium Molybdates and Tungstates.

The exploration of phase formation in the Th-Mo/W-Te systems has resulted in four mixed oxo-anion compounds from high-temperature solid-state reactions: ThWTe2O9, Th(WO4)(TeO3), ThMoTe2O9, and Th2(MoO4)(TeO3)3. All four compounds contain edge-sharing thorium polyhedra linked by MoO4/WO6 and different tellurium oxo-groups to form three-dimensional frameworks. In ThWTe2O9, each helical Th based chain is connected by four tungstotellurite clusters resulting in a building fragment which has a cross-section of four-leafed clovers.

Uptake of Ra during the recrystallization of barite: a microscopic and time of flight-secondary ion mass spectrometry study.

A combined macroscopic and microanalytical approach was applied on two distinct barite samples from Ra uptake batch experiments using time of flight-secondary ion mass spectrometry (ToF-SIMS) and detailed scanning electron microscopy (SEM) investigations. The experiments were set up at near to equilibrium conditions to distinguish between two possible scenarios for the uptake of Ra by already existent barite: (1) formation of a Ba1-xRaxSO4 solid solution surface layer on the barite or (2) a complete recrystallization, leading to homogeneous Ba1-xRaxSO4 crystals. It could be clearly shown that Ra uptake in all barite particles analyzed within this study is not limited to the surface but extends to the entire solid.

Novel fundamental building blocks and site dependent isomorphism in the first actinide borophosphates.

Three novel uranyl borophosphates, Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]H2O (AgNBPU-1), Ag(2-x)(NH4)3[(UO2)2{B2P5O(20-x)(OH)x}] (x = 1.26) (AgNBPU-2), and Ag(2-x)(NH4)3[(UO2)2{B2P(5-y)AsyO(20-x)(OH)x}] (x = 1.43, y = 2.