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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. [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]

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]

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]