A massive interest has been generated lately by the improvement of solid-state magic-angle spinning (MAS) NMR methods for the study of a broad range of paramagnetic organic and inorganic materials. The open-shell cations at the origin of this paramagnetism can be metals, transition metals, or rare-earth elements. Actinide-bearing compounds and their 5f unpaired electrons remain elusive in this intensive research area due to their well-known high radiotoxicity. A dedicated effort enabling the handling of these highly radioactive materials now allows their analysis using high-resolution MAS NMR (>55 kHz). Here, the study of the local structure of a series of actinide dioxides, namely, ThO2, UO2, NpO2, PuO2, and AmO2, using solid-state 17O MAS NMR is reported. An important increase of the spectral resolution is found due to the removal of the dipolar broadening proving the efficiency of this technique for structural analysis. The NMR parameters in these systems with numerous and unpaired 5f electrons were interpreted using an empirical approach. Single-ion model calculations were performed for the first time to determine the z component of electron spin on each of the actinide atoms, which is proportional to the shifts. A similar variation thereof was observed only for the heavier actinides of this study.

Production of actinide oxide powder via dry thermal decomposition of corresponding oxalates is currently carried out on the industrial scale at temperatures exceeding 500°C. Although it is simple, this method presents some disadvantages such as high decomposition temperature with a direct effect on the surface area, pre-organised morphology of the nanoparticles affecting the sintering behaviour, etc. We have recently proposed the decomposition of An IV -oxalates under hot compressed water conditions as a straightforward way to produce reactive actinide oxide nanocrystals. This method could be easily applied at low temperatures (95-250°C) in order to generate highly crystalline nano-AnO 2 . We present here the formation conditions of AnO 2 (An = Th, U, Np, and Pu) and some associated solid solutions, their stability, and grain growth during thermal treatment. The involvement of water molecules in the mechanism of the oxalate decomposition under the hot compressed water conditions has been demonstrated by an isotopic exchange reaction during the thermal treatment of the hydrated oxalate in H 2 ij 17 O] through MAS-NMR and Raman techniques. CrystEngComm, 2018, 20, 4614-4622 This journal is |

Safe management of high level nuclear waste is a worldwide significant issue for which vitrification has been selected by many countries. There exists a crucial need for improving our understanding of the ageing of the glass under irradiation. While external irradiation by ions provides a rapid simulation of damage induced by alpha decays, short lived actinide doping is more representative of the reality. Here, we report radiological NMR experiments to compare the damage in International Simplified Glass (ISG) when irradiated by these two methods. In the 0.1 mole percent 244Cm doped glass, accumulation of high alpha decay only shows small modifications of the local structure, in sharp contrast to heavy ion irradiation. These results reveal the ability of the alpha particle to partially repair the damage generated by the heavy recoil nuclei highlighting the radiation resistance of nuclear glass and the difficulty to accurately simulate its behaviour by single ion beam irradiations.

Transparent nanostructured glasses show interesting properties for optical fibers or laser beam applications. Binary calcium silicate glasses are known to undergo phase separation in silica-rich compositions. However, adding up to a few mole percent of alumina seems to inhibit the phase separation. By adjusting the amount of alumina added to demixing calcium silicate glasses, we managed to obtain transparent, but nanostructured, glasses with either silica-rich or calcium aluminosilicate-rich nanodomains of a controlled size down to 5 nm for compositions beyond the limits of the immiscibility domain. Therefore, the limits of the immiscibility domain of the SiO 2 ÀAl 2 O 3 ÀCaO ternary diagram must be extended to consider the presence of nanosized domains. An atomic-scale analysis of these glasses was performed using 29 Si and 27 Al nuclear magnetic resonance (NMR) experiments, showing that, as the size of the domains decreased, repolymerization of the silicon network and formation of aluminum-rich clusters were observed.

The valence state of uranium has been confirmed for the three sodium uranates NaU(V)O3/[Rn](5f(1)), Na4U(VI)O5/[Rn](5f(0)), and Na2U(VI)2O7/[Rn](5f(0)), using X-ray absorption near-edge structure (XANES) spectroscopy. Solid-state (23)Na magic angle spinning nuclear magnetic resonance (MAS NMR) measurements have been performed for the first time, yielding chemical shifts at -29.1 (NaUO3), 15.1 (Na4UO5), and -14.1 and -19 ppm (Na1 8-fold coordinated and Na2 7-fold coordinated in Na2U2O7), respectively. The [Rn]5f(1) electronic structure of uranium in NaUO3 causes a paramagnetic shift in comparison to Na4UO5 and Na2U2O7, where the electronic structure is [Rn]5f(0). A (23)Na multi quantum magic angle spinning (MQMAS) study on Na2U2O7 has confirmed a monoclinic rather than rhombohedral structure with evidence for two distinct Na sites. DFT calculations of the NMR parameters on the nonmagnetic compounds Na4UO5 and Na2U2O7 have permitted the differentiation between the two Na sites of the Na2U2O7 structure. The linear thermal expansion coefficients of all three compounds have been determined using high-temperature X-ray diffraction: αa = 22.7 × 10(-6) K(-1), αb = 12.9 × 10(-6) K(-1), αc = 16.2 × 10(-6) K(-1), and αvol = 52.8 × 10(-6) K(-1) for NaUO3 in the range 298-1273 K; αa = 37.1 × 10(-6) K(-1), αc = 6.2 × 10(-6) K(-1), and αvol = 81.8 × 10(-6) K(-1) for Na4UO5 in the range 298-1073 K; αa = 6.7 × 10(-6) K(-1), αb = 14.4 × 10(-6) K(-1), αc = 26.8 × 10(-6) K(-1), αβ = -7.8 × 10(-6) K(-1), and αvol = -217.6 × 10(-6) K(-1) for Na2U2O7 in the range 298-573 K. The α to β phase transition reported for the last compound above about 600 K was not observed in the present studies, either by high-temperature X-ray diffraction or by differential scanning calorimetry.

Uranium−neptunium mixed dioxides are considered as fuels and targets for the transmutation of the minor actinides in fast neutron reactors. Hereafter, a local and atomic scale structural analysis was performed on a series of U 1−x Np x O 2 (x = 0.01; 0.05; 0.20; 0.50; 0.75; 0.85) synthesized by the sol−gel external gelation method, for which longer range structural analysis indicates that the process yields solid solutions. The oxidation state of IV for uranium and neptunium cations was confirmed using U L III and Np L III edge X-ray absorption near edge structure (XANES). The atomic scale structure was probed with 17 O magic angle spinning nuclear magnetic resonance (MAS NMR) for the anion. Structural distortions due to the substitution of U by the smaller Np cation were detected by 17 O MAS NMR.

For nuclear fuel related applications, the oxygen stoichiometry of mixed oxides UMO is an essential property as it affects fuel properties and may endanger the safe operation of nuclear reactors. A careful review of the open literature indicates that this parameter is difficult to assess properly and that the nature of the defects, i.e., oxygen vacancies or U, in aliovalent cation-doped UO is still subject to controversy. To confirm the formation of U, we have investigated the room-temperature stable ULaO phase using several experimental methods (e.g., XRD, XANES, and NMR) confirmed by theoretical calculations. This paper presents the experimental proof of U and its effect we identified in both electronic and local structure. We observe that U is formed in quasi-equimolar proportion as La in ULaO (y = 0.06, 0.11, and 0.22) solid solutions. The fluorite structure is maintained despite the cationic substitution, but the local structure is affected as variations of the interatomic distances are found. Therefore, we provide here the definitive proof that the substitution of U with La is not accommodated by the creation of O vacancies as has often been assumed. The UO fluorite structure compensates the incorporation of an aliovalent cation by the formation of U in quasi-equimolar proportions.

A concept to integrate a commercial high-resolution, magic angle spinning nuclear magnetic resonance (MAS-NMR) probe capable of very rapid rotation rates (70 kHz) in a hermetically sealed enclosure for the study of highly radiotoxic materials has been developed and successfully demonstrated. The concept centres on a conventional wide bore (89 mm) solid-state NMR magnet operating with industry standard 54 mm diameter probes designed for narrow bore magnets. Rotor insertion and probe tuning take place within a hermetically enclosed glovebox, which extends into the bore of the magnet, in the space between the probe and the magnet shim system. Oxygen-17 MAS-NMR measurements demonstrate the possibility of obtaining high quality spectra from small sample masses (∼10 mg) of highly radiotoxic material and the need for high spinning speeds to improve the spectral resolution when working with actinides. The large paramagnetic susceptibility arising from actinide paramagnetism in (Th 1−x U x )O 2 solid solutions gives rise to extensive spinning sidebands and poor resolution at 15 kHz, which is dramatically improved at 55 kHz. The first 17 O MAS-NMR measurements on NpO 2+x samples spinning at 55 kHz are also reported. The glovebox approach developed here for radiotoxic materials can be easily adapted to work with other hazardous or even air sensitive materials.