Vitrification of solid technological waste is currently under investigation. For this type of waste made up of metals, minerals, and organic matters, formulation studies were carried out in the NCAS ...(Na2O–CaO–Al2O3–SiO2) system in order to define a vitrifying additive to treat the entire waste deposit, while maximizing the waste loading. Main challenge related to this type of waste comes from the presence of alumina in very large quantities in the glass/glass–ceramics melt, enhancing the risk of melt solidification due to a fast and massive crystallization. Melt lock‐up can potentially occur at the operating temperature envisaged for the process (1400°C) and is prohibitive because it would lead to a premature stoppage of the process. The results obtained from casting tests, rheological experiments, and thermodynamic modeling enabled to provide an accurate estimation of the risk of melt lock‐up for NCAS compositions. It was highlighted that the composition had a major influence on the temperature at which massive crystallization might occur. From all the results obtained, the maximum Al2O3 content that could be incorporated in the final material was determined to be close to 50 wt%. The composition of a vitrifying additive was also statistically designed to treat the technological waste of interest.
Part of the Research and Development program concerning high level nuclear waste (HLW) glasses aims to assess new glass formulations able to incorporate a high waste content with enhanced properties ...in terms of thermal stability, chemical durability, and process ability. This study focuses on peraluminous glasses of the SiO2 – Al2O3 – B2O3 – Na2O – Li2O – CaO – La2O3 system, defined by an excess of aluminum ions Al3+ in comparison with modifier elements such as Na+, Li+ or Ca2+. To understand the effect of composition on physical properties of glasses (viscosity, density, Tg), a Design Of Experiments (DOE) approach was applied to investigate the peraluminous glass domain. The influence of each oxide was quantified to build predictive models for each property. Lanthanum and lithium oxides appear to be the most influential factors on peraluminous glass properties.
•A Design of Experiment approach to link composition and glass properties.•Adding alkali decreases glass transition temperature.•Adding La2O3 strongly decreases glass melt viscosity.•Adding La2O3 increases density.
This study describes the role of nickel oxide (NiO) in peraluminous glass. Peraluminous glass is defined by an excess of aluminium ions (Al3+) compared to modifier elements such as Na+, Li+ or Ca2+ ...ions. The study investigates the effect NiO incorporation on peraluminous glass homogeneity and process ability. Quenched NiO-containing glass showed an incorporation limit greater than 12 wt% NiO (13.8 mol%), while slowly cooled glass showed a limit of 5 wt% NiO (5.8 mol%). Nickel oxide is also found to have a fludifying effect on peraluminous glasses. When the nickel oxide content is above its incorporation limit, the resulting crystals were found to have a significant effect on glass rheological properties. Finally, hypotheses are proposed as to the structural role of Ni in peraluminous glass based on the experimental results obtained.
•The effect of NiO incorporation on peraluminous glass homogeneity and process ability is investigated•Quenched glass showed an incorporation limit greater than 12 wt% NiO (13.8 mol%)•Slowly cooled glass showed an incorporation limit of 5 wt% NiO (5.8 mol%)•NiO is found to have a fludifying effect•Ni-containing crystals were found to have a significant effect on the glass rheological properties•Hypotheses on the structural role of Ni in peraluminous glass are proposed
Considering the interest of developing new glass matrices able to immobilize higher concentration of high level nuclear wastes than currently used nuclear borosilicate compositions, glasses ...containing high rare earth contents are of particular interest. This study focuses on a peraluminous aluminoborosilicate system SiO2–B2O3–Al2O3–Na2O–CaO–Nd2O3 defined by a peralkaline/peraluminous ratio RP=(Na2O+CaO)/(Na2O+CaO+Al2O3)<0.5. Samples with various contents of Nd2O3 from 0 to 10mol% were studied using DSC, XRD, SEM, TEM, STEM and EMPA methods. The glasses present a high thermal stability even after a slow cooling treatment from the melt. Only a slight mullite crystallization is detected for low Nd2O3 content (≤2.3mol%) and crystallization of a neodymium borosilicate crystalline phase combined to a phase separation occurred at high Nd2O3 content (≥8mol%). The solubility of neodymium in the presence of aluminum is demonstrated, with higher neodymium incorporation amounts than in peralkaline glasses.
•Peraluminous glasses present a very large domain of homogeneity.•Enhancement of neodymium solubility compared to peralkaline glasses•Boron, aluminum and neodymium play synergic roles for network stabilization.•Above neodymium solubility limit, crystallization and/or phase separation occur.
Chromium is a multivalent element with very low solubility in silica-based glasses. It is incorporated into glass via the solubilisation of Cr2O3 as Cr3+ through an acido-basic reaction, and is in ...redox equilibrium with Cr6+ and Cr2+ oxidation states depending on the surrounding atmosphere. The least soluble form of chromium is Cr3+, which is also the most common form of chromium in glass elaborated in air. This study investigated the behavior of chromium in peraluminous glasses, which are potential matrices for nuclear waste containment. Chromium is often present in nuclear waste solutions as a corrosion product coming from the waste reprocessing steps. Consequently, it is important to understand the effect of chromium addition to peraluminous glasses, both in terms of the microstructure and the processing ability of the glass melt. The study investigated the effect of chromium added as a Cr2O3 chromium (+III) oxide on the homogeneity, the viscosity, and the glass transition temperature (Tg) of peraluminous glasses. The chromium redox state and its effect on the incorporation limit of chromium was determined. Peraluminous glasses have an incorporation limit of between 0.50 and 0.55 wt% Cr2O3 in air, and between 1.25 and 1.50 wt% Cr2O3 in a reducing environment. Chromium (+III) oxide was found to have a limited effect on the viscosity of peraluminous glasses up to 2 wt% of Cr2O3, despite the presence of crystals above 0.5 wt%. The addition of chromium oxide had no significant effect on the Tg of the glass, except after a slow cooling of the sample containing the highest content of 2.0 wt% Cr2O3. Two forms of chromium were identified under UV-visible spectroscopy for the glasses elaborated in air: Cr3+ (6-fold) and Cr6+ (4-fold).
This study investigates the effect of iron oxide Fe2O3 on the homogeneity, viscosity, and glass transition temperature (Tg) of peraluminous glasses. As-quenched glass samples elaborated at 1350 °C ...show an incorporation limit around 17.5 wt% of Fe2O3 (10.4 mol%), while slowly cooled glass samples show a limit of 7.5 wt% of Fe2O3 (4.2 mol%). Iron in the peraluminous glasses investigated was detected under three configurations: 6-fold coordinated 6Fe3+ in majority, 4-fold coordinated 4Fe3+ and 6-fold coordinated 6Fe2+. It was observed that Fe2O3 addition has a strong fluidifying effect on peraluminous glasses and tends to slightly decrease the glass transition temperature Tg, depending on its thermal history.
The evolution under alpha-decay radiation of a 241Am doped aluminoborosilicate glass-ceramic was investigated in hot cells DHA-ATALANTE facility in CEA. The cumulative La2O3 - Am2O3 solubility limit ...was voluntarily exceeded, leading to the formation of apatite-like silicate crystals. The crystals, with a hexagonal-shaped morphology, that is characteristic of apatite crystals, have a composition and a cell parameter close to those expected for an apatite phase of stoichiometric composition Ca2(La,Am)8(SiO4)6O2. Structural and microstructural evolutions under alpha self-irradiation were followed for 8 years by regularly analyzing the crystals and the residual glassy matrix. The evolution of the X-Ray Diffraction (XRD) patterns is the result of a progressive radiation-induced amorphization in apatite crystals. The fully amorphous state is reached at an alpha-decay dose of around 3 × 1018 α/g. Raman analyses suggest a modification of the connectivity of the SiO4 tetrahedra of apatite crystals, with a transition from isolated SiO4 units in the crystalline state to connected SiO4 units in the metamict state. The crystalline-to-amorphous transformation is accompanied by an increase in macroscopic volume (swelling), highlighted by RAMAN imaging. This macroscopic dimensional change is also associated to a decohesion of the crystals from the glassy matrix, observed by Scanning Electron Microscopy (SEM). However, optical and SEM images of the glass-ceramic surface do not reveal any significant cracks in the residual glass under alpha self-irradiation, thus showing a quite good stability of this glass-ceramic material.
Abstract Vitrification of solid technological waste is currently under investigation. For this type of waste made up of metals, minerals, and organic matters, formulation studies were carried out in ...the NCAS (Na 2 O–CaO–Al 2 O 3 –SiO 2 ) system in order to define a vitrifying additive to treat the entire waste deposit, while maximizing the waste loading. Main challenge related to this type of waste comes from the presence of alumina in very large quantities in the glass/glass–ceramics melt, enhancing the risk of melt solidification due to a fast and massive crystallization. Melt lock‐up can potentially occur at the operating temperature envisaged for the process (1400°C) and is prohibitive because it would lead to a premature stoppage of the process. The results obtained from casting tests, rheological experiments, and thermodynamic modeling enabled to provide an accurate estimation of the risk of melt lock‐up for NCAS compositions. It was highlighted that the composition had a major influence on the temperature at which massive crystallization might occur. From all the results obtained, the maximum Al 2 O 3 content that could be incorporated in the final material was determined to be close to 50 wt%. The composition of a vitrifying additive was also statistically designed to treat the technological waste of interest.
Lanthanum and neodymium incorporation in simplified high level waste glasses was investigated for SiO2-B2O3-Na2O-Al2O3-CaO compositions quenched from 1200 C, for various La/(La + Nd) (atomic) and ...increasing rare-earth oxides contents. In this system and beyond the solubility limit, rare-earth (RE) elements are reported to form apatite phases with the general formula Ca2RE8(SiO4)6O2. Speciation of these trivalent RE3+ cations in both amorphous network and crystal phases was determined from XRD, SEM, optical absorption at 10 K, Raman spectroscopy, and EPMA. It appeared that RE2O3 solubility was higher for La-rich formulations than for Nd-rich ones, and that an increase in the RE oxide content reduced the connectivity of the network building units through formation of non-bridging oxygens at the expense of the oxygen bridges. This depolymerisation of the glass network did not affect the neodymium environment which consisted of silicate tetrahedra. The composition of the apatite crystals was affected by the La/(La + Nd) of the parent glass and deviation from the ideal composition (Ca2RE8(SiO4)6O2) occurred in the neodymium end of the system. It thus appeared that both RE2O3 solubility and crystal composition strongly depend on the type and crystal chemistry of the RE elements.
The incorporation and partitioning of americium and trivalent lanthanides were investigated in aluminoborosilicate glass-ceramics with apatite-like silicate crystals of general formula ...Ca2(Ln,Am)8(SiO4)6O2.
A microstructural and structural study of two glass-ceramics containing Am2O3 - La2O3 or Nd2O3 - La2O3, respectively, was carried out by XRD, SEM/EDS and EMPA so as to assess a comparison of Lanthanides and Actinides partitioning into apatite crystals and residual glass. Moreover, Raman analyses of residual glasses were performed to compare the role of Am and Nd on the glassy structure.
Results put forward that shape, composition, Ln/Am stoichiometry and cell parameters of apatite crystals of both (Am-La) and (Nd-La) glass-ceramics are very close. This paper thus shows similar results for (Am-La) and (Nd-La) glass-ceramics in terms of apatite/glass partitioning and in terms of structural role on glassy structure. It can be therefore put forward that Nd3+ and Am3+ behaviors are close, either in the glass or in the crystalline structure.
Display omitted
•Actinides and Lanthanides behaviors are compared in glass and in a apatite ceramic phase.•Composition, shape and cell parameters of Ln/Am apatite crystals are very close.•Microstructures of Ln/Am glass-ceramics are very close.•Structures of Ln/Am glassy phases are very close.
PDF
