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Influence de la morphologie du gel sur la cinétique d'altération des verres borosilicatés : rôle du calcium et du zirconium

Abstract : This study is related to the question of the long%term behaviour of the high%level activity nuclear waste confinement glass. In contact with water, a hydrated amorphous silica layer, known as the “gel” is formed at the glass surface. This gel results from the departure of the most soluble elements and from the mechanisms of hydrolysis and recondensation of the silicate (siloxane ?) bonds. In certain conditions, the gel can limit the exchanges between the glass and the solution. Our hypothesis to explain this phenomenon is the closure of the gel pores. This thesis actually presents an original insight into silicate glass alteration, usually described by chemical thermodynamic considerations, and highlights the impact of morphologic transformations in the glass alteration layer on the slowing down of the leaching kinetics. Since the composition of the actual nuclear waste glasses is too complex to isolate the influence of one specific element on the global leaching kinetics, it is preferable to use simpler glasses. Consequently, we studied five oxide based glasses SiO2%B2O3%Na2O%CaO%ZrO2. They correspond to the different categories of nuclear glass components, and can be classified according to their reactivity with water as soluble (B, Na), partially soluble (Si, Ca) and insoluble (Zr). Two series of glasses were synthesized by substituting CaO for Na2O on the one hand and ZrO2 for SiO2 on the other hand. In the first place, we looked into the calcium and zirconium specific impacts on the leaching kinetics. The substitution of sodium for calcium was found to improve the reticulation of the vitreous network, inducing a decrease in the final degree of corrosion. Concerning zirconium, we observed that it prevents the dissolution of nearby silicon, which leads to a decrease of the initial dissolution rate. However, the presence of this insoluble element delays the drop of the alteration rate and leads paradoxically to an increase in the alteration degree. In order to explain this unexpected behaviour, the glass alteration layer morphology was investigated by small%angle X%ray scattering. These experiments highlighted the restructuring of porous network during the glass alteration process and showed that the restructuring is limited by the increase of the zirconia content. This suggests that the restructuring of the glass alteration layer is at the origin of the major drop in the alteration rate observed for the low zirconia content glasses. Besides, this hypothesis has been demonstrated with various experimental techniques. Both time%of%flight secondary%ion mass spectroscopy (TOF%SIMS) that provides an evaluation of dye penetration into the gel pores and neutron scattering with index matching by exchange of H2O%D2O mixture showed that the porosity closed during the course of corrosion in the glass without zirconia, but remained open in the high zirconia%content glasses. These experiments, associated to numerical simulations at mesoscopic scale by a Monte Carlo method, clearly establish a relationship between the gel morphology and alteration kinetics for these series of simplified glasses simulating the nuclear glass composition.
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https://pastel.archives-ouvertes.fr/pastel-00004828
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Submitted on : Wednesday, July 21, 2010 - 10:55:59 AM
Last modification on : Thursday, April 25, 2019 - 1:44:57 AM
Long-term archiving on: : Friday, October 22, 2010 - 3:26:18 PM

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  • HAL Id : pastel-00004828, version 1

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Céline Cailleteau. Influence de la morphologie du gel sur la cinétique d'altération des verres borosilicatés : rôle du calcium et du zirconium. Matériaux. Ecole Polytechnique X, 2008. Français. ⟨pastel-00004828⟩

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