Effets cumulatifs et compétitifs des éléments chimiques sur l’altération des verres nucléaires

This work takes place in the context of the long-term behavior of nuclear glasses under repository conditions. The main objective is to identify, understand and compare the effects of some chemical elements present in the glass composition and/or in the repository media (Zn, Mg, Ni, Co, Fe, Ca, Gd, Ce, K, Cs, Cr and Ag) on the processes involved in glass alteration by water. The cumulative or competitive nature of the effects of these chemical elements was determined. To reach this goal, a 6 oxides simple glass (ISG) has been altered for more than 500 days in a solution containing one or more of the chemical elements of interest.The results indicated that Zn, Mg, Ni, Co and Fe elements increase glass alteration forming secondary phases with the same structure and stoichiometry (trioctahedral smectites). To form, these silicates consume chemical elements (Si, Al) from the environment and induce a pH decrease until a limiting value of pH. Beyond this pH the precipitation of secondary phases is inhibited and these chemical elements can be integrated into the gel, replacing Ca whose solubility increases at lower pH. As long as they form secondary phases, the effects of these elements are cumulative. Rare earths Gd and Ce also increase glass alteration forming secondary phases but their effects are lower as they contain less silicon. These elements are not integrated in the gel. Chromium increases glass alteration by precipitating with Ca and leading to a less protective gel, depleted in Ca. Silver precipitates as AgCl and has no effect on the alteration of the glass.The chemical elements K, Cs and Ca limit glass alteration by integrating into the gel and slowing down the transport phenomena therein. This integration is competitive: the order of integration (quantity and effectiveness glass alteration limitation) is the following Ca >> Cs > K. Thus, the increase of glass alteration may be proportional to the quantity of elements promoting the precipitation of secondary phases, but the pH decrease limits the process. The effects of the elements that reduce glass alteration by incorporating into the gel, are quantitatively limited by the gel composition and its ability to incorporate them, and qualitatively by the nature of the elements.Cette thèse s’inscrit dans l’étude du comportement à long terme des verres nucléaires en conditions de stockage. Son objectif est de déterminer, de comprendre et de comparer les effets de certains éléments chimiques présents dans la composition du verre et/ou dans le milieu de stockage (Zn, Mg, Ni, Co, Fe, Ca, Gd, Ce, K, Cs, Cr et Ag) sur les différents processus mis en jeux lors de l’altération aqueuse des verres nucléaires. Dans ce cadre, une attention particulière a été portée sur le caractère cumulatif ou compétitif des effets de ces éléments chimiques. Pour cela, un verre simple à 6 oxydes (ISG) a été altéré pendant plus de 500 jours dans une solution contenant un ou plusieurs des éléments chimiques d’intérêt. Les éléments Zn, Mg, Ni, Co et Fe augmentent l’altération du verre en formant des phases secondaires de même structure (smectites tricotaédriques) et de même stœchiométrie (à l’élément près). Leur précipitation consomme des éléments chimiques du milieu (Si, Al) et induit une diminution de pH. Ce processus se maintient jusqu’à l’atteinte d’un pH limite propre à chaque phase secondaire, au-dessous duquel leur précipitation est inhibée. Par la suite, ces éléments peuvent s’intégrer dans le gel d’altération en remplacement du Ca rendu plus soluble par la baisse du pH. Tant qu’ils forment des phases secondaires, les effets de ces éléments sont cumulatifs. Les terres rares Gd et Ce augmentent aussi l’altération du verre en formant des phases secondaires mais leurs effets sont plus faibles car elles sont moins silicatées. Ces éléments, ne s’intègrent pas dans le gel. Le Cr précipite avec le Ca pour former une phase qui appauvrit le gel en Ca, entraine une diminution du pH et augmente l’altération du verre.Les éléments K, Cs et Ca limitent l’altération du verre en s’intégrant dans le gel et en ralentissant les phénomènes de transport en son sein. Cette intégration est compétitive : l’ordre d’intégration (quantité et efficacité sur la limitation de l’altération) est le suivant Ca >> Cs > K. L’élément Ag précipitant sous forme d’AgCl, n’a pas d’effet sur l’altération du verre : cette phase ne modifie ni le milieu, ni le développement de la pellicule d’altération.Ainsi, l’augmentation de l’altération pourrait être proportionnelle à la quantité d’éléments favorisant la précipitation de phases secondaires, mais la diminution de pH qui l’accompagne limite ce processus. Les effets des éléments qui diminuent l’altération du verre en s’incorporant dans le gel, sont limités en quantité par la composition du gel et sa capacité à les recevoir, et en qualité par la nature même des éléments

Modeling of IEAP based actuator using a linear model with localized parameters

International audienceCurrently, ionic electro-active polymers (IEAP) using increases due to their interesting characteristics i.e., large strain response, soft actuation, similarities to biological muscles, . However, IEAP actuators have a complex behavior. This work presents a study on the modeling of an IEAP based trilayer actuator [PEDOT:PSS-PEO/NBR-PEO/PEDOT:PSS-PEO] using the bond-graph methodwhich is a graphical approach based on energetic exchanges. As power is a universal currency in physical systems, it allows taking into account the electrochemo-mechanical phenomena and the interactions between them. The proposed model is able to predict the electro-chemical (current, voltage) and chemomechanical (force, displacement) behaviors of the actuator. Most relevant, withthis model, simulation results are in good keeping with experimental ones. In addition, this model can be used to estimate physical values that could not directly be measured. This work is illustrated with simulation results and experimental validation

Impact of Fe, Mg and Ca elements on glass alteration: Interconnected processes

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Impact of Zn, Mg, Ni and Co elements on glass alteration: Additive effects

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Impact of iron and magnesium on glass alteration: Characterization of the secondary phases and determination of their solubility constants

International audienceIn this study, the effects of iron and magnesium on International Simple Glass (ISG) alteration were studied throughout 511 days of aqueous leaching experiments. The aim was to determine by thorough characterization, the nature of the alteration products that control glass alteration. Iron and magnesium were added separately or together in solution as FeCl2 and MgCl2 salts, with monthly additions to compensate for their consumption. The alteration degree was determined by leachate analyses (ICP-AES) and the alteration products composition, morphology and microstructure were characterized (SEM, TEM-EDX and XRD).The results indicated that magnesium and iron increase glass alteration, forming tri-octahedral smectites with the same (Fe + Mg)/Si ratio. With iron, two kinds of silicates precipitate with the same composition but with a different morphology, whereas with magnesium alone, a single Mg-silicate forms. Moreover, it was found that the glass alteration rate drops when the pH stabilizes at a minimum value of 7.8 for Mg-silicates and 6.2 for Fe-silicates. At this point the secondary silicates stop precipitating. This result was confirmed by geochemical simulation and the solubility product of these silicates was estimated considering the presence or absence of aluminum in their structure.Finally, a two-step process was proposed to explain the location of the secondary phase precipitation: firstly in solution and at the solution/gel interface, and secondly in a highly porous upper zone of the gel

Drivers of Water Transport in Glass: Chemical or Topological Effect of the Glass Network?

International audienceThe relation existing between water transport in glass, topology, chemical elements (network former and charge compensator), and their structural role in glass were investigated through glass topology modeling, glass and water structure analyses, and water diffusion characterization. Two series of aluminosilicate glasses with and without boron and having various ratios of charge compensators CaO/Na 2 O were used. The glass structure was characterized using Raman spectroscopy and nuclear magnetic resonance. Their topologies, i.e., the density of bottlenecks and the interstitial sites, were obtained by molecular dynamics. For glasses without boron, the substitution of Na by Ca leads to the strengthening of the glass network. The results are similar for glasses with boron except if the amount of Na is not sufficient. In this case, a part of Ca is required as a charge compensator for AlO 4 and BO 4 units. This last result is important since it highlights that the presence of boron and the CaO/Na 2 O ratio drives the roles of Ca inside the glass structure. Moreover, glass topology is driven by boron presence and to a lesser extent by CaO/Na 2 O ratio. Water transport characterized by the duration of the predominance of the hydration/interdiffusion processes, the apparent water diffusion coefficient, and the water structure in the hydrated glass were studied and determined using X-ray reflectivity and attenuated total reflectance infrared spectroscopy. The results show that the duration of the predominance of the hydration/interdiffusion processes is driven by the ability of Si-OX bonds to be hydrolyzed as much as the fraction of free water clusters in hydrated glass. Moreover, for aluminosilicate glasses, we show that water transport is mainly driven by glass topology through the role of Ca and its amount in glass. Indeed, Ca strengthens the glass network by decreasing the density of the bottleneck allowing the diffusion of water molecules. When boron is added to the glass, water transport may be mainly driven by the chemical interactions between the water molecules, Ca, and the network former of glass matrix

Alteration of a Mimas MOX fuel subjected to water radiolysis: surface characterization by optical/SEM image analysis and Raman spectroscopy.

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Behavior of Conducting Polymer-Based Micro-Actuators Under a DC Voltage

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Behavior of Conducting Polymer-Based Micro-Actuators Under a DC Voltage

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SiC-TiC nanocomposite for bulk solar absorbers applications: Effect of density and surface roughness on the optical properties

International audienceIn this study, the potential of SiC-TiC nano-composites as solar absorbers has been studied. For solar thermal applications, materials with high solar absorptance and low emittance are ideally sought for (spectral selectivity). A semi-molecular sol-gel synthesis route leading to nanometric homogenous composites was described. The resulting SiC-TiC nanocomposite powder was sintered at different temperatures to produce samples with various relative densities (from 57% to 96%). The samples morphology and composition were characterized by several techniques including Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDX), X-Ray Diffraction (XRD), carbon and oxygen elemental analyses. The link between the surface roughness and the relative density was precised and the effects on the optical properties (0.25–25 µm wavelength range) were studied. Comparisons were made with pure SiC and pure TiC samples with various relative densities.Overall, the sample emittance was found to strongly decrease with the increase in the relative density, leading to a great increase in the spectral selectivity, despite a little decrease in the solar absorptance. The TiC-SiC composite has an intermediate reflectance compared to the pure SiC and the pure TiC samples. With an absorptance of 0.76, an emittance of 0.44 and a selectivity of 1.74, the denser SiC-TiC could be a good candidate for bulk solar applications