Compositional dependence of crystallization and chemical durability in nepheline (Na₂O•Al₂O₃•2SiO₂) based glasses

Vitrification of sodium and alumina-rich high-level radioactive waste (HLW) into borosilicate glasses faces the problem of nepheline (NaAlSiO4) crystallization during canister-centerline cooling (CCC), which is potentially detrimental to the durability and long-term stability of the final waste form. Some components within the nuclear waste – such as CaO, B2O3, Li2O, Fe2O3, etc. – have been shown to have a profound influence on the propensity of nepheline formation, but the compositionally complex nature of HLW waste makes it difficult to ascertain the mechanisms behind crystallization in the HLW melt during cooling. Hence, this research aims to elucidate the compositional dependence on the structure, crystallization kinetics and chemical durability of simplified HLW glasses designed in the crystallization phase field of nepheline (NaAlSiO4), with an emphasis on understanding the effect of oxides namely CaO, B2O3, Li2O, and Fe2O3. Accordingly, glasses designed in the CaO-Na2O-Al2O3-SiO2, Na2O-Al2O3-B2O3-SiO2, Li2O-Na2O-Al2O3-B2O3-SiO2, and Na2O-Fe2O3-Al2O3-B2O3-SiO2 systems have been the subject of this research. Crystallization studies on glasses in the Na2O–CaO–Al2O3–SiO2 system indicate that the sequence of polymorphic phase transformations in these glass-ceramics is dictated by the compositional chemistry of parent glasses and local environments of different species in the glass structure, for example, sodium environment in glasses becomes highly ordered with decreasing Na2O/CaO ratio, thus favoring the formation of hexagonal nepheline, while cubic polymorph is the stable phase in SiO2–poor glass-ceramics with (Na2O+CaO)/Al2O3 > 1. In the Na2O–Al2O3–B2O3–SiO2 system, crystallization studies indicate that boron suppresses crystallization by staying in the glassy phase and not entering the nepheline crystal. It is found that nepheline crystallization is more strongly suppressed when B2O3 is substituted against Al2O3 than when substituted against SiO2. With increasing B2O3, there is a decrease in the liquidus temperature of the melts along with an increase in viscosity at the liquidus temperature. The increase in viscosity at the liquidus is likely to be the main reason behind suppression in the extent of crystallization in these glasses. Furthermore, the compositional dependence on crystallization and chemical durability is determined in Li2O–Na2O–Al2O3–B2O3–SiO2 glasses by performing Canister Centerline Cooling (CCC) treatments and Product Consistency Tests (PCT). It is found that a direct correlation exists between the extent of nepheline formation and the increase in dissolution of B, Na and Li elements in an aqueous environment. The change in the thermal history of glasses due to different cooling rates is found to have a profound impact on dissolution. Lastly, heat treatments conducted have been conducted as a function of heating atmosphere on glasses in the Na2O–Fe2O3–Al2O3–B2O3–SiO2 system. It is found that while iron coordination in glasses and glass-ceramics changes as a function of glass chemistry, the heating atmosphere during crystallization exhibits a minimal effect on iron redox. The change in the heating atmosphere does not affect the phase assemblage but does affect the microstructural evolution. For future work, it is recommended that more complex compositions be explored in the 7-component Li2O-Na2O-CaO-Fe2O3-Al2O3-B2O3-SiO2 system to understand the combined effect of the species studied in this thesis on crystallization and chemical durability of model HLW glasses.Ph.D.Includes bibliographical reference