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.