Solid-state NMR examination of alteration layers on nuclear waste glasses

Solid-state nuclear magnetic resonance (NMR) is a powerful tool for probing the role and significance of alteration layers in determining the kinetics for the corrosion of nuclear waste glass. NMR methods are used to probe the chemical structure of the alteration layers to elucidate information about their chemical complexity, leading to increased insight into the mechanism of altered layer formation. Two glass compositions were examined in this study: a glass preliminarily designed for nuclear waste immobilization (called AFCI) and a simplified version of this AFCI glass (which we call SA1R). Powdered glasses with controlled and known particle sizes were corroded in ASTM type I water at 90 C for periods of one and five months with a glass surface-area to solution-volume ratio of 100,000 m^{- 1}. ¹H-²⁹Si cross-polarization Carr-Purcell-Meiboom-Gill (CP-CPMG) magic angle spinning (MAS) NMR, ¹H-²⁷Al CP-MAS NMR, ¹H- ¹¹B CP-MAS NMR, and ¹H-²³Na CP-MAS NMR experiments provided isolated structural information about the alteration layers, which differ in structure from that of the pristine glass. Both glasses studied here develop alteration layers composed primarily of ^[IV]Si species. Aluminum is also retained in the alteration layers, perhaps facilitated by the observed increase in coordination from ^[IV]Al to ^[VI]Al, which correlates with a loss of charge balancing cations. The mechanism of increasing coordination appears to occur through an unstable ^[V]Al intermediate. ¹H-¹¹B CP-MAS NMR observations indicated a retention of boron in the hydrated glass layers, which has not been characterized by previous work. For the AFCI glass, secondary phase formation begins during the corrosion times considered here, and these new phases are detected within the alteration layers. We identify new phases (termed as precursor phases) as crystalline sodium metasilicates. An important finding is that simple glass compositions, while providing general trends about the formation of alteration layers, do not account for all of the various reaction products that occur in the corrosion of more complex nuclear waste glass compositions.