Solution composition and particle size effects on the dissolution and solubility of a ThO2 microstructural analogue for UO2 matrix of nuclear fuel

The objective of this study was to investigate the dissolution rate of ThO2 which was synthesised to approximate, as closely as possible, the microstructure of UO2 in a nuclear fuel matrix. The optimal sintering temperature for ThO2 pellets was found to be 1750 ℃, which produced pellets with a microstructure similar to UO2 nuclear fuel pellets, with randomly oriented grains ranging in size from 10 to 30 μm. Dissolution was conducted using ThO2 particles of different size fractions (80 to 160 μm and 2 to 4 mm) in the presence and absence of carbonate, in solutions with pH from 2 to 8 and at 80 ℃. Dissolution rates were calculated from Th released from the solid phase to solution. Particles of ThO2 were also leached with 1 M HNO3 at 80 ℃ in order to investigate the morphological changes at the particle surfaces. The concentration of Th was found to be ≥ 10–9 mol/L at pH ≤ 4, lower than the theoretical solubility of crystalline ThO2. At higher pH values, from 4 to 8, the measured concentrations (10−10 to 10–12 mol/L) were between the theoretical solubility of ThO2 and Th(OH)4. Grain boundaries were shown to exert an influence on the dissolution of ThO2 particles. Using high resolution aqueous solution analysis, these data presented here extend the current understanding of Th solubility in solutio

Bottleable neutral analogues of [B2H5]- as versatile and strongly binding eta2 donor ligands

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B-B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σB-B diborane ligands. Single-crystal X-ray analyses of these complexes show that the X-Cu moiety (X = Cl, OTf, C6F5) lies opposite the bridging hydrogen of the diborane and is near perpendicular to the B-B bond, interacting almost equally with both boron atoms and causing a B-B bond elongation. DFT studies show that σ donation from and π backdonation to the pseudo-π-like B-B bond account for their formation. Astoundingly, these copper σB-B-complexes are inert to ligand exchange with pyridine under either heating or photoirradiation

Thermodynamics and mechanism of protonated cysteine decomposition: a guided ion beam and computational study

pre-printA quantitative molecular description of the decomposition of protonated cysteine, H+Cys, is provided by studying the kinetic energy dependence of threshold collision-induced dissociation (CID) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Primary dissociation channels are deamidation (yielding both NH3 loss and NH4 + formation) and (H2O + CO) loss reactions, followed by an additional six subsequent decompositions. Analysis of the kinetic energy-dependent CID cross sections provides the 0 K barriers for six different reactions after accounting for unimolecular decay rates, internal energy of reactant ions, multiple ion-molecule collisions, and competition among the decay channels. To identify the mechanisms associated with these reactions, quantum chemical calculations performed at the B3LYP/6-311+G(d,p) level were used to locate the transition states (TSs) and intermediates for these processes. Single point energies of the reactants, products, and key optimized TSs and intermediates are calculated at B3LYP, B3P86, and MP2(full) levels using a 6-311+G(2d,2p) basis set. The computational characterization of the elementary steps of these reactions including the structures of the final products is validated by quantitative agreement with the experimental energetics. In agreement with previous work, deamidation is facilitated by anchimeric assistance of the thio group, which also leads to an interesting rearrangement of the intact amino acid identified computationally

The formation of stoichiometric uranium brannerite (UTi2O6) glass-ceramic composites from the component oxides in a one-pot synthesis

Brannerite glass-ceramic composites have been suggested as suitable wasteform materials for high-actinide content wastes, but the formation of glass-ceramic composites containing stoichiometric uranium brannerite (UTi2O6) has not been well-studied. Uranium brannerite glass-ceramic composites were synthesised at by a one-pot cold-press and sinter route from the component oxides. As a comparison, two further samples were produced using an alkoxide-nitrate route. A range of compositions with varying molar ratios of uranium and titanium oxides (from 1:2 to 1:3.20) were synthesised, with a range of different heat treatments (1200 °C for 12–48 h, and 1250 °C for 12 h). All compositions were analysed by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray near-edge spectroscopy, and found to contain UTi2O6 as the majority crystalline phase forming within a glass matrix of nominal stoichiometry Na2AlBSi6O16. In compositions with UO2:TiO2 ratios of 1:2 and 1:2.28, particles of UO2 were observed in the glass matrix, likely due to dissolution of TiO2 in the glass phase; this was prevented by the addition of excess TiO2. This work demonstrates the suitability of this system to produce highly durable wasteforms with excellent actinide waste loading, even with a simple one-pot process. Some grains of brannerite consist of a UO2 particle encapsulated in a shell of UTi2O6, suggesting that brannerite crystallises around particles of UO2 until either the UO2 is fully depleted, or the kinetic barrier becomes too large for further diffusion to occur. We propose that the formation of brannerite within glass-ceramic composites at lower temperatures than that for pure ceramic brannerite is caused by an increase in the rate of diffusion of the reactants within the glass

s-Block amidoboranes: syntheses, structures, reactivity and applications

Metal amidoborane compounds of the alkali- and alkaline earth metals have in recent years found applications in diverse disciplines, notably as hydrogen storage materials, as reagents for the reduction of organic functional groups and as catalysts and intermediates in dehydrocoupling reactions. These functions are connected by the organometallic chemistry of the MNR2BH3 group. This review focusses on central aspects of the s-block amidoborane compounds – their syntheses, structures and reactivity. Well-defined amidoborane complexes of group 2 metals are now available by a variety of solution-phase routes, which has allowed a more detailed analysis of this functional group, which was previously largely confined to solid-state materials chemistry. Structures obtained from X-ray crystallography have begun to provide increased understanding of the fundamental steps of key processes, including amine–borane dehydrocoupling and hydrogen release from primary and secondary amidoboranes. We review structural parameters and reactivity to rationalise the effects of the metal, nitrogen substituents and supporting ligands on catalytic performance and dehydrogenative decomposition routes. Mechanistic features of key processes involving amidoborane compounds as starting materials or intermediates are discussed, alongside emerging applications such as the use of group 1 metal amidoboranes in synthesis. Finally, the future prospects of this vibrant branch of main group chemistry are evaluated

The extremophilic pharmacy:drug discovery at the limits of life

[No abstract

The extremophilic pharmacy:drug discovery at the limits of life

[No abstract

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

The state of the Martian climate

60°N was +2.0°C, relative to the 1981–2010 average value (Fig. 5.1). This marks a new high for the record. The average annual surface air temperature (SAT) anomaly for 2016 for land stations north of starting in 1900, and is a significant increase over the previous highest value of +1.2°C, which was observed in 2007, 2011, and 2015. Average global annual temperatures also showed record values in 2015 and 2016. Currently, the Arctic is warming at more than twice the rate of lower latitudes

Insights into the fabrication and structure of plutonium pyrochlores

Rare earth zirconates, such as Nd2Zr2O7, crystallise with the pyrochlore structure and are a group of materials which have been suggested as potential nuclear waste forms for actinide immobilisation. In this work, a new hydroxide co-precipitation route is presented to investigate the incorporation of Pu into Nd2Zr2O7. The plutonium content was varied between 5 and 10 mol% and the structural uptake and Pu oxidation state were probed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption spectroscopy (XAS). The experimental findings were complemented by DFT ab initio calculations. For all the incorporation mechanisms studied PuO2 was used as the reference reactant state to allow for a direct comparison between the possible Pu uptake scenarios. Analysis of the experimental data suggests that Pu(IV) cations substitute for Nd(III) cations leading to structural distortion of the pyrochlore A-sites. The computed solution energies and bond-distances corroborate the experimental findings and indicate that the excess charge is balanced via the introduction of oxygen at formerly vacant sites