Effect of the nature of alkali and alkaline-earth oxides on the structure and crystallization of an aluminoborosilicate glass developed to immobilize highly concentrated nuclear waste solutions

A complex rare-earth rich aluminoborosilicate glass has been proved to be a good candidate for the immobilization of new high level radioactive wastes. A simplified seven-oxides composition of this glass was selected for this study. In this system, sodium and calcium cations were supposed in other works to simulate respectively all the other alkali (R+=Li+, Rb+, Cs+) and alkaline-earth (R'2+=Sr2+, Ba2+) cations present in the complex glass composition. Moreover, neodymium or lanthanum are used here to simulate all the rare-earths and actinides occurring in waste solutions. In order to study the impact of the nature of R+ and R'2+ cations on both glass structure and melt crystallization tendency during cooling, two glass series were prepared by replacing either Na+ or Ca2+ cations in the simplified glass by respectively (Li+, K+, Rb+, Cs+) or (Mg2+, Sr2+, Ba2+) cations. From these substitutions, it was established that alkali ions are preferentially involved in the charge compensation of (AlO4)- entities in the glass network comparatively to alkaline-earth ions. The glass compositions containing calcium give way to the crystallization of an apatite silicate phase bearing calcium and rare-earth ions. The melt crystallization tendency during cooling strongly varies with the nature of the alkaline-earth.Comment: nombre de pages:

Effect of changing the rare earth cation type on the structure and crystallization behavior of an aluminoborosilicate glass

4 pagesAn aluminoborosilicate glass, containing high amount of rare earth (RE) accordingly to the following composition 50.68 SiO2 - 4.25 Al2O3 - 8.50 B2O3 - 12.19 Na2O - 4.84 CaO - 3.19 ZrO2 - 16.35 RE2O3 (wt.%), is currently under study for the immobilization of nuclear waste solutions. In this work, we wanted to investigate the effect of changing the RE cation type on the glass structure and on its crystallization behavior. For this purpose, a glass series was elaborated in which the nature of the RE is varying from lanthanum to lutetium. In this glass series, only little effect was observed on the glass structure. On the contrary, a strong impact was put in evidence on the crystallization behavior through different heat treatments. A slow cooling of the melt at 1°C/min, revealed significant crystallization of apatite Ca2RE8(SiO4)6O2 in sample containing rare earths with ionic radii close to that of calcium. Another heat treatment consisting of successive nucleation and growth stages, performed to force the crystallization in the bulk and reduce any surface crystallization effect, put in evidence the existence of a strongly heterogeneous second rare earth rich silicate phase for samples containing RE with low ionic radius (from Y to Lu)

Impact des conditions de traitements thermiques sur la cristallisation dans un verre nucléaire simplifié

9 pagesL'incorporation du néodyme et du molybdène dans les verres nucléaires est une problématique liée aux risques de cristallisation de certaines phases riches en terres rares (TR) : l'apatite Ca2TR8 (SiO4)6O2 et en molybdène : les molybdates de calcium CaMoO4 et de sodium Na2MoO4. En raison de la capacité de certaines de ces phases à modifier le comportement à long terme de la matrice de confinement, il est impératif de bien comprendre ces phénomènes de cristallisation afin de les maîtriser. Ce travail présente une étude réalisée par DRX et MEB-FEG sur la cristallisation des phases apatite Ca2Nd8(SiO4)6O2 et powellite CaMoO4 en fonction de la température et de la composition dans deux verres aluminoborosilicatés ayant subi un traitement thermique de nucléation et de croissance. Nous avons pu mettre en évidence les domaines de cristallisation des deux phases ainsi que leur microstructure en fonction de la température de croissance. De plus, il est apparu que le néodyme a tendance à inhiber la cristallisation de la powellite

Effet de la nature des ions alcalins et alcalino-terreux sur la structure d'un verre riche en terre

Dans le cadre d'une étude structurale d'un verre de confinement de déchets nucléaires de type aluminoborosilicate et riche en terres rares, l'influence de la nature des ions alcalins ou alcalino-terreux est analysée. Pour cela deux séries de verres ont été élaborées dans lesquelles l'ion Na+ (respectivement l'ion Ca2+) présent dans la composition de référence, est totalement substitué par un autre ion alcalin Li+, K+, Rb+ ou Cs+ (respectivement un autre ion alcalino-terreux Mg2+, Sr2+ ou Ba2+). Ces verres, analysés par spectroscopie d'absorption optique, Raman et RMN 27Al et 11B, ont permis de montrer le fort impact de la nature de l'ion modificateur aussi bien sur la structure du réseau vitreux ( variation du rapport BO3/BO4 et variations locales du degré de polymérisation) que de l'environnement local de la terre rare (diminution du degré de covalence de la liaison Nd-O avec l'augmentation de la force de champ de l'ion modificateur)

Structural study of a rare earth-rich aluminoborosilicate glass containing various alkali and alkaline-earth modifier cations

4 pagesA rare-earth rich aluminoborosilicate glass of composition (given in wt.%): 50.68 SiO2 - 4.25 Al2O3 - 8.50 B2O3 - 12.19 M2O - 4.84 M'O - 3.19 ZrO2 - 16.35 Nd2O3 (where M and M' are respectively an alkali and alkaline earth cation) is currently under study as potential nuclear waste form. In this work, we were interested in the structure of this glass in relation with the modifier cation type. Two different glass series were elaborated by changing separately the nature of the alkaline (M=Li, Na, K, Rb, Cs) and the alkaline-earth (M'=Mg, Ca, Sr, Ba) ions and different structural studies were intended to elucidate the local environment of the rare-earth and the network arrangement. Only slight effect was put in evidence on the covalency degree and the length of Nd-O linkage with a change of M or M', by optical spectroscopy and EXAFS measurements. Raman and MAS NMR (29Si, 27Al, 11B) spectroscopies showed a variation of the polymerization degree of the network with the size of the modifier cation. Finally, the most important feature of this glass composition is related to the AlO4- charge compensation which was proved to be uniquely assured by alkali cations

ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on the network structure

International audienceThe influence of increasing ZrO 2 content on the structural features of a rare earths (RE = Nd, La) bearing soda-lime aluminoborosilicate glass was investigated through a multi-spectroscopic approach (Raman, Zr-EXAFS, 29 Si, 11 B, 27 Al and 23 Na MAS NMR). Particular attention was paid to the modifications occurring in the glassy network and on the distribution of Na + and Ca 2+ ions. Zr 4+ ions were shown to be located in (ZrO 6) 2-sites, connected to the silicate network, and preferentially charge compensated by Na + ions. A favorable competition of Zr 4+ ions against RE 3+ ions and (BO 4)-entities for charge compensators was observed, but no effect was detected on the environment of (AlO 4)-(Daniel Caurant) 2 entities. This competition resulted in a modification of the RE 3+ ions environment with the ZrO 2 content that may affect their solubility in the glassy network

ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on rare earths environment and crystallization

International audienceThe effect of adding increasing ZrO 2 content on the environment of Nd 3+ ions in a glass belonging to the SiO 2-B 2 O 3-Al 2 O 3-Na 2 O-CaO system was investigated both by optical absorption spectroscopy and Nd-EXAFS (L III-edge). In agreement with the evolution of the structure of the glassy network and more particularly of the distribution of Na + and Ca 2+ ions with ZrO 2 addition put in evidence in a previous study, it is shown here that the average Nd-O distance continuously increases whereas the bond covalency decreases with zirconium content. This result can be explained both by the decrease of the amount of non-bridging oxygen atoms (NBOs) and by the increase of the proportion of Ca 2+ ions acting as charge compensators in the neighborhood of neodymium polyhedra in the (Daniel Caurant) 2 depolymerized regions of the glass structure. This evolution is due to a competition in favor of zirconium between Zr and Nd for Na + cations charge compensators. The local structural evolution around neodymium is probably responsible for the evolution of the crystallization tendency-increase of Nd-rich apatite (Ca 2 Nd 8 (SiO 4) 6 O 2) crystallization in the bulk-observed in this work both during melt cooling and glass heating by increasing ZrO 2 content. It is proposed that the nucleating effect of ZrO 2 on apatite crystallization put in evidence here is mainly due to the changes that are indirectly induced by zirconium in the neighborhood of Nd 3+ ions (destabilization) rather than to the formation of Zr-rich crystals that would then act as nucleating phase for apatite

The role of alkalis on the incorporation of iodine in simple borosilicate glasses

International audienceThe purpose of this study was to identify incorporation mechanisms and saturation levels of iodine in borosilicate glasses that are simplified versions of compositions used for nuclear waste containment. Several series of glasses with different alkali contents (22 or 35 mol% Na$_2$O, or 22 mol% of a mixture of Na$_2$O + Li$_2$O, K$_2$O or Cs$_2$O) were loaded with iodine (from 1 000 to 10 000 ppm at.) at 1100 °C in a specially-designed closed system. When the incorporation limit of iodine was reached, alkali iodide crystals were observed (e.g. NaI, KI…). Crystal-free pools of glass were studied by Electron Probe Micro-Analysis (EPMA) to measure the incorporation limit of iodine, which ranges from 892 ppm at. for a cesium–bearing glass to 7260 ppm at. for the glass with 35 mol% Na$_2$O. The composition of the crystals was analyzed by Inductively Coupled Plasma spectroscopy (ICP) and it was systematically found that crystals are enriched in the heavy alkali compared to the bulk composition. Correlations exist between the saturation level and the composition of the crystals. It is concluded that high alkali content in general and high concentration of Na$_2$O in particular is favorable for iodine incorporation