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We investigate water infiltration in porous matrices made of bitumen and salts with different solubilities. Dispersion, inside bitumen, of either SrSO4or MgSO4at 40% mass fraction was achieved, as a way to model materials used in the context of nuclear waste conditioning. Pulsed field gradient nuclear magnetic resonance (PFG-NMR) and environmental scanning electron microscopy (ESEM) measurements allow us to characterize the evolution of the porous structure as a consequence of water infiltration (due to a leaching phenomenon) for durations up to 1.5 year. PFG-NMR enables performing 1D-imaging of water at different times to monitor its slow seeping inside the material. Profiles observed at the maximum time interval (1.5 year) demonstrate that different leaching behaviors exist depending on whether the salts dispersed inside bitumen are of the soluble or insoluble type. NMR relaxation and diffusion measurements were also taken. Remarkably, when performed in combination with 1D-imaging, such measurements yield information on the surface-to-volume ratio of the water-filled porous network, at different times, as a function of depth. In the case of the matrix-containing insoluble salts, relaxation measurements lead to discrimination between two different water populations differing by their T2or T1values. These values are described in the framework of surface-driven relaxation. A two-step model of leaching is proposed that globally accounts for the different observations (NMR 1D-imaging, relaxation/diffusion, as well as ESEM). The present work gives insight into the leaching behavior of porous bitumen-salt matrices and could be used as an input for modeling their evolution on longer timescales.

Powders and divided solid materials are widely represented as finished or intermediary products in industries as widely varied as foodstuffs, cosmetics, construction, pharmaceuticals, electronic transmission, and energy. Their optimal use requires a mastery of the transformation process based on knowledge of the different phenomena concerned (sintering, chemical reactivity, purity, etc.). Their modelling and understanding need a prior acquisition of sets of data and characteristics which are more or less challenging to obtain. The goal of this study is to present the use of different physico-chemical characterization techniques adapted to uranium-containing powders analyzed either in a raw state or after a specific preparation (ionic polishing). The new developments touched on concern dimensional characterization techniques for grains and pores by image analysis, chemical surface characterization and powder chemical reactivity characterization. The examples discussed are from fabrication process materials used in the nuclear fuel cycle.

The in situ observation of the first stage of sintering of cerium dioxide microspheres was performed using an environmental scanning electron microscope at high temperature (HT-ESEM). The associated morphological modifications were described quantitatively for systems constituted by two single crystal grains, on the one hand, and by two polycrystalline particles, on the other hand. Particularly, the in situ HT-ESEM observations, and subsequent image analysis with homemade image process software, led to assess the evolution of several parameters of interest during isothermal heat treatments, such as neck size, particles radii, dihedral angles between the spheres, and distance between the grains centers. It was then possible to evaluate the activation energies associated with the neck formation for both systems studied, then to identify the different mechanisms involved. The diffusion process operating during the first stage of sintering was also pointed out. Furthermore, the comparison of the results obtained from polycrystalline particles and single crystals, and their confrontation with data coming from numerical computation, led to assess the influence of polycrystallinity on the sintering kinetics. For all the conditions tested, sintering degree was found to be enhanced for polycrystalline particles, mainly because of the contribution of the mechanical rearrangement of crystallites during the neck’s elaboration and of the existence of diffusion paths within the particles. On this basis, polycrystallinity should be considered during numerical computations in order to provide predictive models for the first step of sintering.

Powders and divided solid materials are widely represented as finished or intermediary products in industries as widely varied as foodstuffs, cosmetics, construction, pharmaceuticals, electronic transmission, and energy. Their optimal use requires a mastery of the transformation process based on knowledge of the different phenomena concerned (sintering, chemical reactivity, purity, etc.). Their modelling and understanding need a prior acquisition of sets of data and characteristics which are more or less challenging to obtain. The goal of this study is to present the use of different physico-chemical characterization techniques adapted to uranium-containing powders analyzed either in a raw state or after a specific preparation (ionic polishing). The new developments touched on concern dimensional characterization techniques for grains and pores by image analysis, chemical surface characterization and powder chemical reactivity characterization. The examples discussed are from fabrication process materials used in the nuclear fuel cycle.

Les verres et vitrocéramiques constituent une solution technologique efficace pour réaliser des scellements ou des joints devant fonctionner à haute température. On peut citer en exemple la réalisation de joints d’étanchéité pour pile à combustible à oxyde solide (SOFC solid oxide fuel cell) dont la température de fonctionnement est comprise entre 700 et 900 °C. Toutefois, la longévité de ces joints est limitée par différents facteurs, en particulier par la formation de fissures consécutives aux cycles thermiques. Nous illustrons ici comment le concept d’autocicatrisation peut être mis en œuvre dans le cadre des joints de verre SOFC.

The high temperature oxidation behaviour of cast superalloys strengthened by carbides has been studied as a function of their grain fineness and their dendritic orientation near the surface. These microstructural features have both a significant influence on the oxidation kinetics. The effect depends on the nature of the base metal of the superalloy, on the overall chemical composition or on the types of carbides.

The high temperature oxidation behaviour of cast superalloys strengthened by carbides has been studied as a function of their grain fineness and their dendritic orientation near the surface. These microstructural features have both a significant influence on the oxidation kinetics. The effect depends on the nature of the base metal of the superalloy, on the overall chemical composition or on the types of carbides.

Pure and single phase thorium−uranium(IV) phosphate-diphosphate β-TUPD (orthorhombic form) dense pellets were prepared by two wet chemical routes from low-temperature precursors. The first method was based on the “direct evaporation” of a mixture containing tetravalent actinides and phosphoric acid while the second one involved the initial precipitation of the thorium−uranium(IV) phosphate−hydrogenphosphate hydrate (TUPHPH). Both methods led to single phase sintered samples following a two-step procedure using a uniaxial pressing at room temperature and then a heat treatment at high temperature. The dilatometric study and the determination of the linear shrinkage of the pellets showed that the best densification of the samples prepared via the “precipitation” method was obtained after heating the raw pellet at 1250 °C for 5 h, for initial uranium amount lower than 44.5 wt %(U). For samples prepared via “direct evaporation”, the heat treatment must be prolonged for at least 15 h at this temperature. Moreover, the complete characterization of the samples (EPMA, and so forth) confirmed that the homogeneity of the samples was significantly improved when using TUPHPH as a precursor, probably due to the better reactivity of the initial powder (higher specific surface area, smaller grain size, and so forth). Several leaching tests were performed in various acidic media and in natural waters on β-TUPD sintered samples. All the corresponding normalized dissolution rates remained low (10^-6 to 10^-5 g/(m² day)) even in aggressive media which confirms the high chemical durability of β-TUPD, thus enhancing the potential use of this ceramic in the field of the efficient immobilization of tetravalent actinides.

Because of its low solubility in water and high thermal stability, thorium phosphate diphosphate (TPD), Th4(PO4)4P2O7, seems to be a very good matrix for radionuclide immobilization. During the leaching studies of this compound the formation of a new phase, different from the TPD was observed. In order to identify this phase, two other thorium phosphates: thorium phosphate–hydrogenphosphate (TPHP), Th2(PO4)2HPO4·H2O, and thorium hydroxide phosphate (THOP), Th(OH)PO4, were synthesized in hydrothermal conditions then characterized. It appeared that the neoformed phase was identical to the TPHP.

Thorium phosphate−diphosphate, Th4(PO4)4P2O7, is obtained, at high temperature (800−1250 °C), for a given ratio r = PO4/Th equal to ³/2 from a thorium compound and a phosphating reactant whatever the chemical conditions. This compound, whose solubility is very low, can be loaded in situ and used as a host matrix for radwaste storage. One of the methods for its synthesis involves the precipitation of a precursor, thorium phosphate−hydrogenphosphate. This precursor is used as an adsorber for low concentrations of radionuclides and, after heating in air in the range 800−1200 °C, it is transformed into doped Th4(PO4)4P2O7. The optimum conditions for the precipitation process have been determined. Starting from 0.5−2 M solutions of thorium nitrate or chloride and (NH4)2HPO4 at pH = 9−9.5 at the ratio r = ³/2, a compound of general formula Th2(PO4)2HPO4·nH2O (n = 3−7) is synthesized. Its physicochemical properties have also been studied.

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