Role and properties of the gel formed during nuclear glass alteration: importance of gel formation conditions

• Published in: Journal of nuclear materials 2001-09, Vol.298 (1), p.1-10
• Main Authors: Gin, S., Ribet, I., Couillard, M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Building materials. Ceramics. Glasses; Chemical industry and chemicals; Exact sciences and technology; Glasses; Pollution; Radioactive wastes; Structure, analysis, properties; Wastes
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/S0022-3115(01)00573-6

A French SON 68 nuclear glass sample was experimentally altered to assess the mechanisms limiting the glass alteration kinetics, especially during the transition phase between the initial rate r 0 and the final rate under silicon saturation conditions. A glass specimen was altered at the initial rate for one week to form a silicon-depleted non-protective gel; the specimen was then leached under static conditions at a glass-surface-to-solution-volume ( S/ V) ratio of 500 m −1 and the alteration kinetics were compared with those of a pristine glass specimen altered under the same conditions. Unexpectedly, after static leaching the previously leached glass was 2.7 times as altered as the pristine specimen, and the steady-state silicon concentration was twice as high for the previously leached specimen. STEM characterization of the alteration films showed that the initial non-protective gel constituted a silicon pump with respect to the glass, and that the glass alteration kinetics were limited only when a fraction of the gel became saturated with silicon, and exhibited protective properties. This work also shows that silicon recondensation was uniform at micrometer scale: the silicon hydrolyzed at the reaction interface then diffused before recondensing over a length comparable to the gel thickness. In addition to these findings, this investigation suggests a reinterpretation of the effect of the S/ V ratio on the glass alteration kinetics and on the steady-state dissolved silicon concentration.