Low-temperature lithium diffusion in simulated high-level boroaluminosilicate nuclear waste glasses

• A novel method of studying low T alkali–alkali exchange in glass has been developed.
• Diffusion coefficients aid in bounding ion exchange reaction for HLW glass models.
• The research highlights breaking of water bond in glass corrosion.
• An interdiffusion model is used to successfully fit experimental results.

Ion exchange is recognized as an integral, if underrepresented, mechanism influencing glass corrosion. However, due to the formation of various alteration layers in the presence of water during the corrosion process, it is difficult to conclusively deconvolute the mechanisms of ion exchange from other simultaneously occurring processes. In this work, an inert non-aqueous solution was used as an alkali source material to isolate ion exchange and study the solid-state diffusion of lithium. Specifically, the experiments involved contacting simulated nuclear waste glass coupons, the 6-oxide CJ6 and the 26-oxide SON68, with a non-aqueous solution of 6LiCl dissolved in dimethyl sulfoxide at 90 °C for various time periods. The depth profiles of major elements in the glass coupons were measured using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Lithium interdiffusion coefficients, DLi, were then calculated based on the measured depth profiles. The results indicate that the penetration of 6Li is faster in the simplified CJ6 (D6Li ≈ 4.0–8.0 × 10− 21 m2/s) compared to the more complex SON68 (D6Li ≈ 2.0–4.0 × 10− 21 m2/s). These values are roughly an order of magnitude greater than measured water diffusion in glasses at similar temperatures. Additionally, sodium ions present in the glass were observed to participate in ion exchange reactions with lithium from the solution. Implications of the diffusion coefficients obtained in the absence of alteration layers to the long-term performance of nuclear waste glasses in a geological repository system are also discussed.