Hydrothermal conversion of uranyl oxalate into UO 2+ x unravelled by in situ XANES, opening a new avenue for nuclear fuel fabrication.

We evaluated the potential of time-resolved laser-induced fluorescence spectroscopy (TRLFS) combined with chemometric methods for fast identification of U(VI)-bearing minerals in a mining context. We analyzed a sample set which was representative of several environmental conditions. The set consisted of 80 uranium-bearing samples related to mining operations, including natural minerals, minerals with uranium sorbed on the surface, and synthetic phases prepared and characterized specifically for this study. The TRLF spectra were processed using the Ward algorithm and the K-nearest neighbors (KNN) method to reveal similarities between samples and to rapidly identify the uranium-bearing phase and the associated mineralogical family. The predictive models were validated on an independent dataset, and then applied to test samples mostly taken from U mill tailings. Identification results were found to be in accordance with the available characterization data from X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX). This work shows that TRLFS can be an effective decision-making tool for environmental investigations or geological prospection, considering the large diversity of uranium-bearing mineral phases and their low concentration in environmental samples.

Uranium-cerium oxide solid solutions, U1-xCexO2+δ·nH2O, were prepared through hydrothermal conversion of mixed U(IV)-Ce(III) oxalate precursors, cerium being used as a surrogate for plutonium. Whatever the starting pH, the fluorite-type structure of AnO2 was obtained after heating at 250 °C for 24 h. The initial pH of the reaction media appeared to affect significantly the oxide morphology: for pH ≤ 2, the powder was found to be composed of microspheres, whereas for more alkaline pH values, agglomerates of nanocrystallites were found. Furthermore, a study of the hydrothermal treatment duration (T = 250 °C, pH = 8, t = 1-48 h) showed that fluorite-type mixed dioxides started to form after only 1 h, and then became single phase after 3 h. SEM and TEM/EDS analyses revealed that the cationic distribution narrowed with time to finally form highly homogeneous mixed oxides. Such a preparation route was then applied to various cerium incorporation rates and it was found that the formation of U1-xCexO2+δ·nH2O mixed oxides was possible for 0.1 ≤ x ≤ 0.75. In all the systems investigated, the speciation of uranium and cerium was questioned in both the solid and liquid phases. Thermodynamic calculations and evaluation of the O/M ratio in the final oxides led us to understand the complex redox behaviour of uranium and cerium in solution during hydrothermal processes and to propose a conversion mechanism.

Ammonia borane NH3BH3 (AB), a material for solid-state hydrogen storage, can be nanosized by confinement into the porosity of a scaffold like mesoporous silica, carbon cryogel, graphene oxide, ZIF-8 as a metal organic framework, poly (methyl acrylate), boron nitride and manganese oxide. In doing so, nanosized AB is destabilized and shows better dehydrogenation properties than bulk AB in terms of temperature, activation energy, enthalpy and kinetics. Such improvements are due to the confinement-driven nanosizing effect, but not only. A catalytic effect may also have a contribution and, in some cases, it even overpasses the nanosizing effect. These effects are explained in detail herein. The present review aims at reporting the outcomes of the AB confinement strategy to help understand the advantages and to identify the limitations which are still not adequately defined. Based on this analysis, the challenges ahead are listed and discussed, and it appears that there are new opportunities to explore. Though nanosized AB is not mature enough for implementation, it has the potential to be developed further. Avenues worth exploring are given.