Crystallization behavior of iron‐ and boron‐containing nepheline (Na₂O·Al₂O₃·2SiO₂) based model high‐level nuclear waste glasses

Abstract

This study focuses on understanding the relationship between iron redox, composition, and heat‐treatment atmosphere in nepheline‐based model high‐level nuclear waste glasses. Glasses in the Na2O–Al2O3–B2O3–Fe2O3–SiO2 system with varying Al2O3/Fe2O3 and Na2O/Fe2O3 ratios have been synthesized by melt‐quench technique and studied for their crystallization behavior in different heating atmospheres—air, inert (N2), and reducing (96%N2–4%H2). The compositional dependence of iron redox chemistry in glasses and the impact of heating environment and crystallization on iron coordination in glass‐ceramics have been investigated by Mössbauer spectroscopy and vibrating sample magnetometry. While iron coordination in glasses and glass‐ceramics changed as a function of glass chemistry, the heating atmosphere during crystallization exhibited minimal effect on iron redox. The change in heating atmosphere did not affect the phase assemblage but did affect the microstructural evolution. While glass‐ceramics produced as a result of heat treatment in air and N2 atmospheres developed a golden/brown colored iron‐rich layer on their surface, those produced in a reducing atmosphere did not exhibit any such phenomenon. Furthermore, while this iron‐rich layer was observed in glass‐ceramics with varying Al2O3/Fe2O3 ratio, it was absent from glass‐ceramics with varying Na2O/Fe2O3 ratio. An explanation of these results has been provided on the basis of kinetics of diffusion of oxygen and network modifiers in the glasses under different thermodynamic conditions. The plausible implications of the formation of iron‐rich layer on the surface of glass‐ceramics on the chemical durability of high‐level nuclear waste glasses have been discussed.

A data‐driven approach for predicting nepheline crystallization in high‐level waste glasses

“…13,24,25 However, in the literature, contradicting reports are also present. Deshkar et al 26 examined the impact of varying CaO for Na 2 O and SiO 2 in very simplified glass systems. They did not observe that CaO inhibits nepheline formation when replacing Na 2 O, but instead report that increasing the CaO/SiO 2 ratio induces the formation of cubic carnegieite, a mineral with the same nominal composition as nepheline but a different structure.…”

Interfacial structures of spinel crystals with borosilicate nuclear waste glasses from molecular dynamics simulations

“…During the processing of nuclear waste glasses, due to complex chemistry of the waste materials, crystalline phases can form and some poses serious issues in water glass processing. For example, the formation of nepheline crystals in waste glasses is detrimental to the chemical durability due to consuming glass‐forming oxides such as SiO 2 and Al 2 O 3 during crystal formation . Iron nickel (NiFe 2 O 4 ) spinel crystal is another type of crystal that can form during the vitrification process.…”

Understanding the structure, thermal, and optical properties in Al2O3‐incorporated La2O3‐TiO2‐Nb2O5 glasses

“…However, high connectivity does not mean good ability against crystallization. For example, although stoichiometric nepheline glass (NaAlSiO 4 ) has very high connectivity and polymerization, it is easy to crystallize 27,28 . After the heat treatment under T p for this series of glass samples, it was observed that the Nb‐Al‐5 glass formed four crystalline phases, which did not occur with the less‐stable other samples.…”

“…has very high connectivity and polymerization, it is easy to crystallize. 27,28 After the heat treatment under T p for this series of glass samples, it was observed that the Nb-Al-5 glass formed four crystalline phases, which did not occur with the less-stable other samples. The observed increase in the GFA with Al 2 O 3 incorporation can be interpreted by the confusion principle together with the increase of various chemical compounds.…”