Search

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 sodium‐borosilicate 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 non‐Newtonian 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. [ABSTRACT FROM AUTHOR]

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 Au‐irradiation to examine compositional effects on glass structure and domain size of separated phases induced by accumulated radiation damage resulting from α‐decay over a ~1000 year timeframe. The produced glasses fell within the immiscibility dome of CaO−SiO2−B2O3 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 × 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] ≥ 2.5 mol%. These microscopic changes were concurrent to surface‐bulk 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 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)2− units. Au‐irradiation 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. [ABSTRACT FROM AUTHOR]

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 Si–O–B network topology were investigated following five irradiation experiments that achieved doses between 5 × 1012 and 1.8 × 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 MoO42− 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 Si–O–B 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. [ABSTRACT FROM AUTHOR]

Radioactive waste vitrification has been carried out industrially in several countries for nearly 40 years. Research into the formulation and long term behavior of high and intermediate level waste glasses, mainly borosilicate compositions, is still continuing in order to (i) safely condition new types of wastes and (ii) design and demonstrate the safety of the disposal of these long-lived waste forms in a deep geological repository. This article presents a summary of current knowledge on the formulation, irradiation resistance and the chemical durability of these conditioning materials, with a special focus on the fate of radionuclides during glass processing and aging. It is shown that, apart from the situation for certain elements with very low incorporation rate in glass matrices, vitrification in borosilicate glass can enable waste loadings of up to ~20 wt% while maintaining the glass homogeneity for geological time scales and guaranteeing a high stability level in spite of irradiation and water contact. [ABSTRACT FROM AUTHOR]

Phase separation in sodium borosilicate glasses containing molybdenum oxide has been studied in a compositional range of interest for nuclear glasses. Two series of quaternary compositions (63.5SiO 2 -19.6Na 2 O-16.9B 2 O 3) 1-x (MoO 3) x with 0 ≤ x ≤ 3 mol.% MoO 3 have been synthesized and characterized, by a range of complementary experimental techniques both in situ in temperature, by viscometry and hot stage SEM, and ex-situ, by HRTEM, PXRD, EPMA and dissolution calorimetry. The phase separation temperatures between silicate and molybdenum oxide rich liquids were determined for each composition. Molybdenum oxide solubility limits were established for quenched and annealed glasses.For molybdenum oxide contents exceeding these limits, crystalline sodium mono-molybdate precipitates in the glasses as shown by XRD results and the fractions of precipitates were indirectly quantified by MoO 3 mole balance and enthalpy balance relying on solution calorimetry experiments. The experimental results were compared with thermodynamic calculations performed with Thermo-Calc software and a CALPHAD database of the SiO 2 -Na 2 O-MoO 3 ternary system. The ternary model allows the calculation of the nature and quantity of the precipitated phases in reasonable agreement with the experiments, even if, in the current state of development of the thermodynamic database, the absence of boron restricts the relevance of the calculations. Thus, for the construction of a predictive thermodynamic calculation tool, the addition of boron to the database is the most promising prospect. [ABSTRACT FROM AUTHOR]

The effects of phase separation by nucleation and growth as well as those of crystallization on the structure and the chemical durability of sodium borosilicate glasses enriched in molybdenum oxide were investigated. Increasing concentrations of MoO 3 (0 to 10% molar) were added to glasses which were homogeneous (MoO 3 ≤ 1.4% molar) or with crystallizations and separated phases (MoO 3 > 1.4% molar). Their structure was determined by 11 B, 23 Na, 29 Si and 95 Mo MAS NMR, and 23 Na MQMAS NMR, coupled with DFT calculations of NMR parameters. Their microstructure and morphology were characterized by X-ray diffraction and by electronic microscopy. These complementary approaches pointed out a decrease in the number of non-bridging oxygens (NBOs) within the vitreous matrix, first initiated by an increase in polymerization for glasses with a low molybdenum oxide content (< 2 % molar), and combined with the decrease in the number of tetrahedral boron for higher contents. Correlations between the chemical durability, the structure and the microstructure of the glasses were then obtained at low and high dissolution reaction progress. The modifications to the chemical composition and to the structure of the surrounding glass induced by increasing MoO 3 strongly influenced the matrix durability. Moreover, the nature and the morphology of the separated phases influence the dissolution kinetics by modifying the solution chemistry. [ABSTRACT FROM AUTHOR]

Abstract In order to increase the waste loading efficiency in nuclear waste glasses, alternate glass ceramic (GC) materials are sought that trap problematic molybdenum in a water-durable CaMoO 4 phase within a borosilicate glass matrix. In order to test the radiation resistance of these candidate wasteforms, accelerated external radiation can be employed to replicate long-term damage. In this study, several glasses and GCs were synthesized with up to 10 mol% MoO 3 and subjected to 92 MeV Xe ions with fluences ranging between 5 × 1012 to 1.8 × 1014 ions/cm2. The main mechanisms of modification following irradiation involve: (i) thermal and defect-assisted diffusion, (ii) relaxation from the ion's added energy, (iii) localized damage recovery from overlapping ion tracks, and (iv) the accumulation of point defects or the formation of voids that created significant strain and led to longer-range modifications. Most significantly, a saturation in alteration could be detected for fluences greater than 4 × 1013 ions/cm2, which represents an average structure that is representative of the maximum damage state from these competing mechanisms. The results from this study can therefore be used for long-term structural projections in the development of more complex GCs for nuclear waste applications. Graphical abstract Image 1 Highlights • Radiation response of CaMoO 4 bearing glasses examined using XRD, SEM, and Raman. • Minor amorphization of crystallites detected, but bulk of particles show resistance. • Modifications to crystallinity and amorphous network saturate for doses > 4 × 1013 ions/cm2. • Saturation caused by competing processes of damage creation and thermal recovery. [ABSTRACT FROM AUTHOR]

Due to their low solubility, some elements, such as lanthanides, can lead to crystalline phases in borosilicate glass during cooling. The impact of rare earth (lanthanum and neodymium) silicate crystallization on glass structural configuration as well as on glass chemical durability was investigated in soda-lime borosilicate glasses. Partially crystallized glasses containing a single Ca 2 RE 8 (SiO 4 ) 6 O 2 (RE = La, Nd) phase (ranging from 1 to 36 wt.%) were synthesized by heat treating homogeneous glasses. Structural characterizations of vitreous and crystalline parts were carried out by 29 Si, 11 B, 23 Na and 17 O MAS and MQMAS NMR. The crystallization leads to a reorganization of the cation distribution around the rare earth elements. By keeping a part of the lanthanum and the calcium, crystallization causes a decrease in the number of NBOs in the vitreous part. A part of the sodium, initially near the lanthanum, becomes available to form Si–O–Na bonds and to convert trihedral boron into tetrahedral boron. Through the study of the alteration of different materials – homogeneous, crystallized and surrounding glasses, rare earth silicate ceramics, and separated mixtures of glass and crystals – a precise description could be made of the influence of the presence of crystals has on chemical durability. For the various alteration regimes, it was thus shown that the alteration kinetics increase proportionally to the crystal content, and that apatite-type crystals do not lead to a composition gradient on the glass–crystal interface. Although the crystals formed were more durable than the vitreous matrix, the chemical durability of partially crystallized glasses depends on the composition and structure of the surrounding glass. [ABSTRACT FROM AUTHOR]

The influence of the temperature and quenching rate on the structure of a borosilicate glass was studied by high-resolution solid-state 11B, 23Na, 29Si nuclear magnetic resonance (NMR) and high-temperature Raman spectroscopy. Data were obtained for glass in the solid state after annealing and quenching at cooling rates covering four orders of magnitude as well as in the liquid state from Raman experiments and from calorimetry and Theological data. Nuclear magnetic resonance measurements were used to calibrate the Raman spectra in order to quantify the change in boron coordination with temperature. This result can then be used to determine the fictive temperature of the glass directly from the boron coordination. The fictive temperature, heat capacity, and configurational entropy are extracted from calorimetry and viscosity measurements. Changes in the boron coordination account for only 25% of the configurational heat capacity of the liquid. The structural parameters capable of accounting for the remaining quantity are discussed on the basis of structural data, both local (inhomogeneity of the sodium distribution) and medium-range (from NMR parameter distribution). It has thus been shown that, although the B-O-B angular distributions of the boroxol rings (and probably the Si-O-Si distributions) are not affected by temperature, a structural disorder is identified through the angular distributions of the bonds linking borate and silicate groups. [ABSTRACT FROM AUTHOR]

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 CaMoO 4 crystalline phase. Factors affecting CaMoO 4 combination and glass-in-glass phase separation in calcium borosilicate systems as a function of changing [MoO 3 ] and [B 2 O 3 ] 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% MoO 3. Increasing [B 2 O 3 ] increased the areas of immiscibility and order of (MoO 4 ) 2 − anions, while increasing [MoO 3 ] increased both the phase separation and crystallization temperature resulting in phases closer to metastable equilibrium, and initiated clustered crystallization for [MoO 3 ] > 2.5 mol%. β-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 Mo 6 + with dose that can be correlated to molybdenum solubility. In compositions with existing CaMoO 4 crystallites, radiation caused a scattering effect, though the crystal content remained unchanged. Therefore β-irradiation can preferentially prevent crystallization in calcium borosilicates for [MoO 3 ] < 2.5 mol%, but has a smaller impact on systems with existing CaMoO 4 crystallites. [ABSTRACT FROM AUTHOR]

Ruthenium, a fission product arising from the reprocessing of spent uranium oxide (UOX) fuel, crystallizes in the form of acicular RuO2 particles in high-level waste containment glass matrices. These particles are responsible for significant modifications in the physicochensical behavior of the glass in the liquid state, and their formation mechanisms are a subject of investigation. The chemical reactions responsible for the crystallization of RuO1 particles with acicular or polyhedral shape in simplified radioactive waste containment glass are described. In situ high-temperature environmental scanning electron microscopy (ESEM) is used to follow changes in morphology and composition of the ruthenium compounds funned by reactions at high temperature between a simplified RuO2NaNO5 precursor and a sodium borosilicate glass (Si02-B203-Na20). The key parameter in the formation of acicular or polyhedral RuO1 crystals is the chemistry of the ruthenium compound under oxidized conditions (RuIV, RuV). The precipitation of needle-shaped RuO2 crystals in the melt might be associated with the formation of an intermediate Ru compound (Na3Ru O4) before dissolution in the melt, allowing Ru concentration gradients. The formation of polyhedral crystals is the result of the direct incorporation of RuO2 crystals in the melt followed by an Ostwald ripening mechanism. [ABSTRACT FROM AUTHOR]

• Alkalis and rhenium volatilize congruently at about 700 °C from a borosilicate glass melt. • Cesium and rhenium volatilization during vitrification implies the formation of a solid aerosol. • Demixed molybdic phases formed in glass melt have a similar composition as aerosol particles during vitrification. • An ELPI+ cascade impactor was used to measure particles from 0.006 µm to 9.84 µm in the vitrification process. A side effect of high level waste vitrification at 1100 °C is the volatilization of radioactive cesium (Cs), recaptured further on in the process as a solid aerosol by a dust scrubber. Feedback from industry suggests that Cs combines with technetium (Tc) – simulated here by rhenium (Re), chosen for its similar behavior – to form volatile species. This work characterized Cs volatilization and aerosol formation. The first experiment involved elaborating and analyzing a simplified glass enriched with Cs and Re , which reinforce the volatilization phenomena. The results showed that a demixed liquid phase rich in Cs, Re , Mo, and Na formed in the liquid melt, which is reminiscent of a molybdic phase evoked in the literature. Moreover, analyzing the condensed gasses led to the conclusion that the alkalis and Re volatilize congruently at around 700 °C. In the second experiment, a series of measurements of the gas phase characterized aerosols throughout the process. The size distribution results showed that the particles formed in the furnace were mostly submicronic, and that the aerosols in the calciner and the dust scrubber included nanoparticles and micronic particles. SEM-EDS characterization revealed the composition of a spherical particle from the furnace made of two nested phases: one rich in Cs- Re -O, and the other rich in Na-Mo-O. The analyses performed showed that Cs and Re could be found in every particle sampled throughout the process, and that the micronic particles might originate from the aggregation of submicronic particles because of composition similarities. Thus, the observations led to a better understanding of the behavior of the elements of interest in both the melt and the gas phase. [ABSTRACT FROM AUTHOR]