[en] We have developed two integrated thermocouple (TC) crucible systems that allow precise measurement of sample temperature when using a furnace associated with an environmental scanning electron microscope (ESEM). Sample temperatures measured with these systems are precise (±5C) and reliable. The TC crucible systems allow working with solids and liquids (silicate melts or ionic liquids), independent of the gas composition and pressure. These sample holder designs will allow end users to perform experiments at high temperature in the ESEM chamber with high precision control of the sample temperature. (authors)

[en] Protons dynamics in borosilicate glasses of various compositions (SiO₂/B₂O₃/NaO/CaO/X, with X = Al₂O₃ or ZrO₂) at various stages of alteration have been characterized at a picosecond scale using quasielastic neutron scattering (QENS). The results obtained were compared to the one from porous silica-based material (MCM41) having pore wall surfaces with Si-OH, Al-OH, or Zr- OH terminal groups and pore sizes around 2.3 nm. The composition and the nano-porosity of the alteration layers were characterized using solution analysis, small angle X-ray scattering and transmission electron microscopy. The strength hydrogen bonds of confined water were studied using thermogravimetric analysis and differential thermal analysis. The results showed that the free water bonding and the mobility of protons depend on the altered glass composition and that the residence time of water obtained from QENS analysis is apparently correlated to the alteration rate of the glass. Moreover, whereas the composition of the alteration layer can partially explain this result, the presence of ions in the leachate filling the gel nano-porosity seems to be the main factor affecting the water/protons mobility. The latter result is really important since the ion solvation and the ion adsorption on the surface can strongly impact the hydrolysis rate of the pore wall of the gel and can also modify the kinetics of dissolved elements recondensation into the nano-pore. (authors)

[en] The preparation of a synthetic uranothorite with desired formula Th_0.5U_0.5SiO₄ was performed under hydrothermal conditions from a mixture of tetravalent thorium and uranium in hydrochloric solution with sodium metasilicate. The XRD Rietveld analysis revealed that the system obtained was composed by two crystallized phases. The first one corresponded to uranium-depleted Th_0.57U_0.43SiO₄ with a = 7.0571(1) angstrom and c = 6.2998(1) angstrom that confirmed the formation of a solid solution between thorite and coffinite end-members. On the other hand, U-enriched Th_0.21U_0.79O₂ dioxide was also pointed out while the formation of amorphous SiO₂ was stated from elementary analyses and mu-Raman spectroscopy. This latter also led to confirm the formulation of the silicate by rejecting unambiguously the presence of structural hydroxyl groups or water molecules. (authors)

[en] An easy way of preparation, based on the precipitation of U"4"+ or UO_2"2"+ cations by urea in the presence of PEG at T = 90-120 C, was set up to prepare shape-controlled spherical uranium oxides. The parametric study of heating time and temperature allowed us to tailor the size of the meso-crystals obtained, which varied from 50 to 250 nm. For U(IV)-based samples, advanced characterization by the combination of SEM and HR-TEM observations and PXRD and SAXS measurements revealed a hierarchical organization of the powder with three different levels. The first one corresponded to small crystallites of about 3 nm, which grouped into spherical agglomerates of 15-20 nm and then re-aggregated to produce the bigger spheres observed (up to 200 nm in diameter). On the other hand, U(VI)-bearing spherical aggregates were found to be more likely in a metastable form, evolving towards the precipitation of crystalline meta-schoepite. In the last step, the samples prepared at low temperature were fired between 700 and 1000 C under various atmospheres in order to tailor the final O/M ratio in the resulting oxides. In spite of the important chemical modifications associated, the precursors were generally found to present pseudomorphic conversion towards the final high temperature oxides and still exhibited a spherical form, provided the conditions of calcination were properly selected. (authors)

[en] In the literature, a previous work has validated the application of the generalized JMAK equation to model the crystal dissolution kinetics in silicate melts for T > T-liq. Because this dissolution can happen at temperatures below T-liq, the purpose of this work was to test this model for T < T-liq. To do so, the dissolution kinetics of rare earth (RE) silicate in a borosilicate melt were studied by following (with image analysis) the evolution of the crystal fraction as a function of time and temperature. The application of the model enables the mechanism limiting the dissolution to be known, i.e. the diffusion and the activation energy of the dissolution (475 kJ/mol). These conclusions are the same as those resulting from the application for T > T-liq. This shows the continuity of the dissolution around T-liq which is, in addition, confirmed by the chemical profile study performed by microprobe analysis. Moreover, it validates the application of the JMAK model for crystal dissolution whatever the temperature. Furthermore, the study of the crystal morphologies during dissolution indicates that a mechanism called crystal attack/splitting occurs for T > T-liq and leads to a change in value of the Avrami exponent. (authors)

[en] The microstructural and mineralogical changes associated with heating calcined mixtures of Al(NO₃)₃⋅9H₂O–NaNO₃ have been studied. This system is a simplified analogue of high-level radioactive waste calcine, one of the raw materials used in the vitrification process employed for waste management. The decomposition (dehydration and denitration) and formation of secondary crystalline phases have been studied by differential thermal and gravimetric analysis (DTA and TGA), and heat-treated products characterized by X-ray diffraction, Raman spectroscopy, Nuclear Magnetic Resonance (NMR) and Transmission Electron Microscopy (TEM). It is found that pure Al(NO₃)₃⋅9H₂O transforms to amorphous Al₂O₃ at a temperature of ∼180 °C, well below that of the calcination process (500 °C). This amorphous Al₂O₃ is highly porous with a high specific surface area, but may in turn convert to denser γ-Al₂O₃ and α-Al₂O₃ with increasing temperature. On the other hand, pure NaNO₃ remains stable up to ∼880 °C, despite a solid–liquid transition at ∼320 °C. For Al(NO₃)₃⋅9H₂O–NaNO₃ mixtures, the products of calcination at 500 °C are found to consist of very fine porous material containing Na, Al and O, in addition to a variable proportion of well-defined crystals consisting of Na, and O. Heating these mixtures to temperatures of up to 1000 °C shows that for the case 80% Al(NO₃)₃⋅9H₂O −20% NaNO₃ (weight%) a variety of crystalline sodium aluminates is formed (NaAlO₂, NaAl₁₁O₁₇, NaAl₆O_9.5), while for the 50–50 mixture, only NaAlO₂ is found. In large amounts, addition of alumina thus leads to the formation of crystalline phases rich in Al₂O₃ that are responsible for hardening the calcine as the temperature rises. The kinetics of nitrogen loss from NaNO₃ are also found to be influenced by the relative proportion of Al(NO₃)₃⋅9H₂O in the calcine, larger amounts of Al leading to denitration at lower temperature. These results constitute the necessary background for understanding chemical reactions between the calcined waste and the glass precursor

[en] The Phenix nuclear power plant has been a French Sodium Fast Reactor (SFR) prototype producing electrical power between 1973 and 2010. The power was monitored using ex-core neutron measurements. This kind of measurement instantly estimates the power but needs to be often calibrated with the heat balance thermodynamic measurement. Large safety and security margins have then been set not to derive above the nominal operating point. It is important for future SFR to reduce this margin and working closer to the nominal operating point. This work deals with the use of delayed gamma to measure the power. The main activation product contained in the primary sodium coolant is the ²⁴Na which is not convenient for neutron flux measurement due to its long decay period. The experimental study done at the Phenix reactor shows that the use of ²⁰F as power tagging agent gives a fast and accurate power measurement closed to the thermal balance measurement thanks to its high energy photon emission (1.634 MeV) and its short decay period (11 s). (authors)