84 research outputs found
Vitrification of wastes: from unwanted to controlled crystallization, a review
In this review, we provide a perspective on the science and technology of vitrification of waste. First, we provide a background on the general classes of wastes for which vitrification is currently used for immobilization or is proposed, including nuclear and industrial hazardous wastes. Next, we summarize the issues surrounding solubility of waste ions and resulting uncontrolled crystallization or phase separation. Some newer waste form designs propose a controlled crystallization, resulting in a glass-ceramic. A summary of glass systems and glass-ceramic systems is given, with the focus on immobilizing waste components at high waste loading. Throughout, design and processing considerations are given, and the difference between uncontrolled undesirable and controlled desirable crystallization is offered
Vitrification of wastes: from unwanted to controlled crystallization, a review
In this review, we provide a perspective on the science and technology of vitrification of waste. First, we provide a background on the general classes of wastes for which vitrification is currently used for immobilization or is proposed, including nuclear and industrial hazardous wastes. Next, we summarize the issues surrounding solubility of waste ions and resulting uncontrolled crystallization or phase separation. Some newer waste form designs propose a controlled crystallization, resulting in a glass-ceramic. A summary of glass systems and glass-ceramic systems is given, with the focus on immobilizing waste components at high waste loading. Throughout, design and processing considerations are given, and the difference between uncontrolled undesirable and controlled desirable crystallization is offered
Impacts of composition and beta irradiation on phase separation in multiphase amorphous calcium borosilicates
Borosilicate glasses for nuclear waste applications are limited in waste loading by the precipitation of water-soluble molybdates. In order to increase storage efficiency, new compositions are sought out that trap molybdenum in a water-durable CaMoO4 crystalline phase. Factors affecting CaMoO4 combination and glass-in-glass phase separation in calcium borosilicate systems as a function of changing [MoO3] and [B2O3] are examined in this study in order to understand how competition for charge balancers affects phase separation. It further examines the influence of radiation damage on structural modifications using 0.77 to 1.34 GGy of 2.5 MeV electron radiation that replicates inelastic collisions predicted to occur over long-term storage. The resulting microstructure of separated phases and the defect structure were analyzed using electron microscopy, XRD, Raman and EPR spectroscopy prior to and post irradiation. Synthesized calcium borosilicates are observed to form an unusual heterogeneous microstructure composed of three embedded amorphous phases with a solubility limit ~ 2.5 mol% MoO3. Increasing [B2O3] increased the areas of immiscibility and order of (MoO4)2 â anions, while increasing [MoO3] increased both the phase separation and crystallization temperature resulting in phases closer to metastable equilibrium, and initiated clustered crystallization for [MoO3] > 2.5 mol%. ÎČ-irradiation was found to have favorable properties in amorphous systems by creating structural disorder and defect assisted ion migration that thus prevented crystallization. It also increased reticulation in the borosilicate network through 6-membered boroxyl ring and Si ring cleavage to form smaller rings and isolated units. This occurred alongside an increased reduction of Mo6 + with dose that can be correlated to molybdenum solubility. In compositions with existing CaMoO4 crystallites, radiation caused a scattering effect, though the crystal content remained unchanged. Therefore ÎČ-irradiation can preferentially prevent crystallization in calcium borosilicates for [MoO3] < 2.5 mol%, but has a smaller impact on systems with existing CaMoO4 crystallites
Recommended from our members
Characterization of immiscibility in calcium borosilicates used for the immobilization of Mo 6+ under Auâirradiation
Funder: FfWGFunder: Cambridge Philosophical SocietyAbstract: The aim of this paper was to assess factors affecting primary and secondary phase separation in simplified calcium borosilicate glasses studied for nuclear waste applications. Several glasses with varying [MoO3] and [B2O3] were synthesized and exposed to Auâirradiation to examine compositional effects on glass structure and domain size of separated phases induced by accumulated radiation damage resulting from αâdecay over a ~1000 year timeframe. The produced glasses fell within the immiscibility dome of CaOâSiO2âB2O3 and showed a unique microstructure of embedded immiscibility with three identifiable amorphous phases according to electron microscopy, Raman spectroscopy, and diffraction. These glasses were then bombarded with 7 MeV Au3+ ions to a dose of 3 Ă 1014 ions/cm2 creating an estimated ~1 dpa of damage. Several changes to the morphology, spatial distribution, and size of secondary phases were observed, indicative of significant structural reorganization and changes to the chemical composition of each phase. A general mechanism of coalescence to form larger particles was observed for [MoO3] < 2.5 mol%, whereas segregation to form smaller more evenly distributed particles was seen for [B2O3] †15 mol% and [MoO3] â„ 2.5 mol%. These microscopic changes were concurrent to surfaceâbulk diffusion of Ca and/or Mo ions, where the direction of diffusion was dependent on [B2O3] with a barrier identified at ~20 mol%, as well as crossâphase diffusion of said ions. These modifications occurred in part through the formation of distorted ring structures within the borosilicate network, which enabled the increased dissolution of isolated (MoO4)2â units. Auâirradiation was therefore able to increase the solubility of molybdenum and alter the structure and composition of secondary phases with the extent of modification varying with [MoO3] and [B2O3]/[SiO2], though glasses notably remained heterogeneous. The collective results suggest that radiation and composition can both be used as design tools to modulate the domain size and distribution of separated phases in heterogeneous glasses
Recommended from our members
Mechanism of powellite crystallite expansion within nano-phase separated amorphous matrices under Au-irradiation.
This is the final version of the article. It first appeared from Royal Society of Chemistry via https://doi.org/10.1039/D0CP02447CA fundamental approach was taken to understand the implications of increased nuclear waste loading in the search for new materials for long-term radioisotope encapsulation. This study focused on the formation and radiation tolerance of glass ceramics with selectively induced CaMoO4 as a form to trap the problematic fission product molybdenum. Several samples were synthesised with up to 10 mol% MoO3 within a soda lime borosilicate matrix, exhibiting phase separation on the nano scale according to thermal analysis, which detected two glass transition temperatures. It is predicted that these two phases are a result of spinodal decomposition with Si-O-Ca-O-Si and Si-O-Ca-O-B units, with the latter phase acting as a carrier for MoO3. The solubility limit of molybdenum within this matrix was 1 mol%, after which crystallisation of CaMoO4 occurred, with crystallite size (CS) increasing and cell parameters decreasing as a function of [MoO3]. These materials were then subjected to irradiation with 7 MeV Au3+ ions to replicate the nuclear interactions resulting from α-decay. A dose of 3 Ă 1014 ions per cm2 was achieved, resulting in 1 dpa of damage within a depth of âŒ1.5 ÎŒm, according to TRIM calculations. Glasses and glass ceramics were then analysed using BSE imaging, XRD refinement, and Raman spectroscopy to monitor changes induced by accumulated damage. Irradiation was not observed to cause any significant changes to the residual amorphous network, nor did it cause amorphisation of CaMoO4 based on the relative changes to particle size and density. Furthermore, the substitution of Ca2+ to form water-soluble Na2/NaGd-MoO4 assemblages did not occur, indicating that CaMoO4 is resilient to chemical modification following ion interactions. Au-irradiation did however cause CaMoO4 lattice parameter expansion, concurrent to growth in CS. This is predicted to be a dual parameter mechanism of alteration based on thermal expansion from electronic coupling, and the accumulation of defects arising from atomic displacements.This work was funded by the University of Cambridge, Department of Earth Sciences and EPSRC (Grant No. EP/K007882/1) for an IDS. Additional financial support provided by FfWG and the Cambridge Philosophical Society
Rheology of a sodiumâmolybdenum borosilicate melt undergoing phase separation
During glass production, phase separation can result in the formation of suspended liquid droplets, which can cause changes in the system rheology. In nuclear waste vitrification context, some new glassy matrices may present this phase separation matter, but the mechanisms controlling the viscosity changes have not yet been determined. Here, we measure the viscosity of a sodiumâborosilicate melt containing dissolved MoO3 at different temperatures and subject to different applied shear strain rates. We observe that (i) the viscosity increases sharply as the temperature decreases and (ii) at any constant temperature below 1000°C, the system presents nonâNewtonian response. Using transmission electron microscope observations coupled with viscosity calculations, we interpret the cause of the observed changes as the result of phase separation. We show that the viscosity increase on cooling is in excess of the predicted temperature dependence for a homogeneous melt of the starting composition. The increase is due to the formation of a second phase and is controlled by chemical and structural modifications of the matrix during the loss of the elements that form the droplets. This work provides insights into the rheology of a system composed of two composition sets due to a miscibility gap
Tests of achromatic phase shifters performed on the SYNAPSE test bench: a progress report
The achromatic phase shifter (APS) is a component of the Bracewell nulling
interferometer studied in preparation for future space missions (viz.
Darwin/TPF-I) focusing on spectroscopic study of Earth-like exo-planets.
Several possible designs of such an optical subsystem exist. Four approaches
were selected for further study. Thales Alenia Space developed a dielectric
prism APS. A focus crossing APS prototype was developed by the OCA, Nice,
France. A field reversal APS prototype was prepared by the MPIA in Heidelberg,
Germany. Centre Spatial de Li\`ege develops a concept based on Fresnel's
rhombs. This paper presents a progress report on the current work aiming at
evaluating these prototypes on the SYNAPSE test bench at the Institut
d'Astrophysique Spatiale in Orsay, France
Mechanism of powellite crystallite expansion within nano-phase separated amorphous matrices under Au-irradiation
A fundamental approach was taken to understand the implications of increased nuclear waste loading inthe search for new materials for long-term radioisotope encapsulation. This study focused on theformation and radiation tolerance of glass ceramics with selectively induced CaMoO4as a form to trapthe problematic fission product molybdenum. Several samples were synthesised with up to 10 mol%MoO3within a soda lime borosilicate matrix, exhibiting phase separation on the nano scale according tothermal analysis, which detected two glass transition temperatures. It is predicted that these two phasesare a result of spinodal decomposition with SiâOâCaâOâSi and SiâOâCaâOâB units, with the latterphase acting as a carrier for MoO3. The solubility limit of molybdenum within this matrix was 1 mol%,after which crystallisation of CaMoO4occurred, with crystallite size (CS) increasing and cell parametersdecreasing as a function of [MoO3]. These materials were then subjected to irradiation with 7 MeV Au3+ions to replicate the nuclear interactions resulting froma-decay. A dose of 3ïżœ1014ions per cm2wasachieved, resulting in 1 dpa of damage within a depth ofB1.5mm, according to TRIM calculations.Glasses and glass ceramics were then analysed using BSE imaging, XRD refinement, and Raman spectro-scopy to monitor changes induced by accumulated damage. Irradiation was not observed to cause anysignificant changes to the residual amorphous network, nor did it cause amorphisation of CaMoO4based on the relative changes to particle size and density. Furthermore, the substitution of Ca2+toform water-soluble Na2/NaGdâMoO4assemblages did not occur, indicating that CaMoO4is resilientto chemical modification following ion interactions. Au-irradiation did however cause CaMoO4latticeparameter expansion, concurrent to growth in CS. This is predicted to be a dual parameter mechanismof alteration based on thermal expansion from electronic coupling, and the accumulation of defectsarising from atomic displacements
Characterisation of immiscibility in calcium borosilicates used for the immobilisation of Mo 6+ under Auâirradiation
The aim of this paper was to assess factors affecting primary and secondary phase separation in simplified calcium borosilicate glasses studied for nuclear waste applications. Several glasses with varying [MoO3] and [B2O3] were synthesised and exposed to Auâirradiation to examine compositional effects on the glass structure and domain size of separated phases induced by accumulated radiation damage resulting from αâdecay over a ~1000 year timeframe. The produced glasses fell within the immiscibility dome of CaOâSiO2âB2O3 and showed a unique microstructure of embedded immiscibility with three identifiable amorphous phases according to electron microscopy, Raman spectroscopy and diffraction. These glasses were then bombarded with 7 MeV Au3+ ions to a dose of 3Ă1014 ions/cm2 creating an estimated ~1 dpa of damage. Several changes to the morphology, spatial distribution and size of secondary phases were observed, indicative of significant structural reorganisation and changes to the chemical composition of each phase. A general mechanism of coalescence to form larger particles was observed for [MoO3] < 2.5mol%, while segregation to form smaller more evenly distributed particles was seen for [B2O3] †15mol% and [MoO3] â„ 2.5mol%. These microscopic changes were concurrent to surfaceâbulk diffusion of Ca and/or Mo ions, where the direction of diffusion was dependent on [B2O3] with a barrier identified at ~20mol%, as well as cross phase diffusion of said ions. These modifications occurred in part through the formation of distorted ring structures within the borosilicate network, which enabled the increased dissolution of isolated (MoO4)2â units. Auâirradiation was therefore able to increase the solubility of molybdenum and alter the structure and composition of secondary phases with the extent of modification varying with [MoO3] and [B2O3]/[SiO2], though glasses notably remained heterogeneous. The collective results suggest that radiation and composition can both be used as design tools to modulate the domain size and distribution of separated phases in heterogeneous glasses
Swift heavy ion-irradiated multi-phase calcium borosilicates: implications to molybdenum incorporation, microstructure, and network topology
Abstract: A series of calcium borosilicate glasses with varying [B2O3], [MoO3], and [CaO] were prepared and subjected to 92 MeV Xe ions used to simulate the damage from long-term α-decay in nuclear waste glasses. Modifications to the solubility of molybdenum, the microstructure of separated phases, and the SiâOâB network topology were investigated following five irradiation experiments that achieved doses between 5 Ă 1012 and 1.8 Ă 1014 Xe ions/cm2 in order to test the hypotheses of whether irradiation would induce, propagate, or anneal phase separation. Using electron microscopy, EDS analysis, Raman spectroscopy, and XRD, irradiation was observed to increase the integration of MoO42â by increasing the structural disorder within and between heterogeneous amorphous phases. This occurred through Si/B-O-Si/B bond breakage and reformation of boroxyl and 3/4-membered SiO4 rings. De-mixing of the SiâOâB network concurrently enabled cross directional Ca and Mo diffusion along defect created pathways, which were prevalent along the interface between phases. The initiation and extent of these changes was dependent primarily on the [SiO2]/[B2O3] ratio, with [MoO3] having a secondary effect on influencing the defect population with increasing dose. Microstructurally, these changes to bonding caused a reduction in heterogeneities between amorphous phases by reducing the size and increasing the spatial distribution of immiscible droplets. This general increase in structural disorder prevented crystallization in most cases, but where precipitation was initiated by radiation, it was re-amorphized with increasing dose. These outcomes suggest that internal radiation can alter phase separation tie lines, and can therefore be used as a tool to design certain structural environments for long-term encapsulation of radioisotopes
- âŠ