Son68 Glass Dissolution Kinetics at High Reaction Progress: Mechanisms Accounting for The Residual Alteration Rate
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SON68 GLASS DISSOLUTION KINETICS AT HIGH REACTION PROGRESS: MECHANISMS ACCOUNTING FOR THE RESIDUAL ALTERATION RATE P. Frugier*, S. Gin*, J.E. Lartigue*, E. Deloule** *Commissariat à l’Énergie Atomique (CEA) Valrhô DTCD/SECM/LCLT BP 17171, 30207 Bagnols-sur-Cèze Cedex, France **CRPG 15, rue Notre Dame des Pauvres, BP 20, 54501 VANDOEUVRE LES NANCY Cedex ABSTRACT In a highly confined medium corresponding to geological repository conditions, the alteration rate of the French SON68 inactive nuclear reference glass drops by about four orders of magnitude below the initial rate. However, extended experiments lasting months or years provide evidence of a virtually constant or slowly decreasing residual alteration rate. Although very low, this rate could account for most of the altered glass thickness after 10 000 years. Experiments at high temperatures and especially high glass-surface-area-to-solution-volume ratios were performed to reveal and quantify the predominant mechanisms underlying the residual rate. The authors describe the characterization of the solution chemistry, the crystallized secondary phases, and the amorphous gel observed after alteration of the French SON68 inactive reference glass, and discuss their implications in terms of long-term behavior modeling. A slow diffusion mechanism is identified in the solid, involving alkalis in particular but also boron. This mechanism results in higher concentrations in solution that affect the system chemistry, not only by slightly modifying the pH and element speciation in solution (e.g. silicon), but also by inducing the precipitation of new crystallized secondary phases that can consume glass constituent elements in the same way as simple solution renewal. Diffusion and the precipitation of secondary phases are two mechanisms to be considered to account for the residual rate. INTRODUCTION It has been established for nearly two decades that in a medium with little or no solution renewal the dissolution rate of SON68 borosilicate glass, the inactive reference for French nuclear glass, diminishes by several orders of magnitude from the initial rate r0. More recently, a residual alteration rate was observed and measured based on the concentrations of boron and alkali in solution. The value was less than 40 nm/year at 90°C and pH=9.5 [1]. The existence of a residual alteration rate is today an established fact that has been observed by various authors in a large number of glass compositions [1-4]. There is currently no consensus on the description of hydrolysis and condensation of the glass network elements between the “hydrated glass” [4] and the passivating amorphous gel [1], or on the nature of the elements that should be taken into account in formulating a rate law. Nevertheless, the participants in the European GLAMOR contract [5, 6] are agreed on the need to supplement the hydrolysiscondensation reactions with transport reactions in the glass or amorphous phase. They also concur on the need to identify the predominant mechanisms under residual rate conditions, de
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