Contribution of atom-probe tomography to a better understanding of glass alteration mechanisms: Application to a nuclear glass specimen altered 25 years in a granitic environment

Journal Article · 07 April 2013 · Chemical Geology

DOI: https://doi.org/10.1016/j.chemgeo.2013.04.001 · OSTI ID:1097327

Gin, Stephane ^[1]; Ryan, Joseph V. ^[2]; Schreiber, Daniel K. ^[2]; Neeway, James J. ^[2]; Cabie, M. ^[3]

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1. CEA Marcoule DTCD SECM LCLT, Bagnols-sur-Ceze (France)
2. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
3. Aix-Marseille Universite, CP2M, Marseille (France)

Here, we report and discuss results of atom probe tomography (APT) and energy-filtered transmission electron microscopy (EFTEM) applied to a borosilicate glass sample of nuclear interest altered for nearly 26 years at 90°C in a confined granitic medium in order to better understand the rate-limiting mechanisms under conditions representative of a deep geological repository for vitrified radioactive waste. The APT technique allows the 3D reconstruction of the elemental distribution at the reactive interphase with sub-nanometer precision. Profiles of the B distribution at pristine glass/hydrated glass interface obtained by different techniques are compared to show the challenge of accurate measurements of diffusion profiles at this buried interface on the nanometer length scale. Our results show that 1) Alkali from the glass and hydrogen from the solution exhibit anti-correlated 15 ± 3 nm wide gradients located between the pristine glass and the hydrated glass layer, 2) boron exhibits an unexpectedly sharp profile located just at the outside of the alkali/H interdiffusion layer; this sharp profile is more consistent with a dissolution front than a diffusion-controlled release of boron. The resulting apparent diffusion coefficients derived from the Li and H profiles are D_Li = 1.5 × 10^-22 m².s^-1 and D_H = 6.8 × 10^-23 m².s^-1. These values are around two orders of magnitude lower than those observed at the very beginning of the alteration process, which suggests that interdiffusion is slowed at high reaction progress by local conditions that could be related to the porous structure of the interphase. As a result, the accessibility of water to the pristine glass could be the rate-limiting step in these conditions. More generally, these findings strongly support the importance of interdiffusion coupled with hydrolysis reactions of the silicate network on the long-term dissolution rate, contrary to what has been suggested by recent interfacial dissolution-precipitation models for silicate minerals.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1097327
• Report Number(s): PNNL-SA--92809; 47580; 46403; 42495; AF5805000
• Journal Information: Chemical Geology, Journal Name: Chemical Geology Vol. 349-350; ISSN 0009-2541
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English