SON68 Glass Dissolution Kinetics at High Reaction Progress: Experimental Evidence of the Residual Rate
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SON68 Glass Dissolution Kinetics at High Reaction Progress: Experimental Evidence of the Residual Rate S. Gin and P. Frugier Commissariat à l’Énergie Atomique – CEA Valrhô DIEC/SESC, BP 17171, 30207 Bagnols-sur-Cèze Cedex, France ABSTRACT SON68 borosilicate glass dissolution involves several parallel mechanisms: interdiffusion, hydrolysis of the silicate network, recondensation of dissolved species leading to the formation of an amorphous gel layer and precipitation of secondary crystalline phases. In a highly confined medium corresponding to geological disposal conditions, the gel formed during SON68 glass alteration quickly becomes protective and the hydrolysis rate drops by about 4 orders of magnitude compared with the initial dissolution rate. This study focuses on the state of reaction progress corresponding to apparent saturation of the solution. At 90°C in static mode, new experimental data show that the decreasing rate due to the protective gel formation is followed by a quasi-constant residual rate. Several hypotheses are discussed. We first demonstrate that silica dissociation related to a slowly rising pH is not a valid explanation. This hypothesis proposed 15 years ago was consistent with a rate controlled by affinity (equilibrium between the glass and solution). The hypothetical precipitation of secondary crystalline phases that consume elements from the protective gel layer is then considered in the light of data obtained with a simplified 8oxide glass, but this mechanism is not sufficient to account for residual rate of the SON68 glass. Other hypotheses based on the gel evolution (dissolution, evolution of the porous texture) are also proposed. The proposed mechanisms imply the persistence of a protective gel with a constant thickness. Finally, this paper shows that the residual rate could be a decisive phenomenon affecting the long term behavior of glass in disposal conditions. INTRODUCTION It has been established for nearly two decades that in a medium with little or no solution renewal the dissolution rate of a nuclear waste glass diminishes by several orders of magnitude from the initial rate r0 [1]. The drop in the alteration rate was initially attributed to the effect of chemical affinity, i.e. the diminishing difference between the chemical potential of the solution (which increases as the solution becomes saturated) and the constant chemical potential of the glass [2]. New data now suggest that the kinetics are limited only by the transport of reactive species through the gel layer [3]. The drop in the SON68 glass alteration rate depends to a considerable extent on the alteration conditions and in particular on the removal of hydrolyzed silica from the surface. Thus, in highly dilute media (high solution flow rate and/or low S/V ratio) or in contact with clay or metallic corrosion products capable of sorbing silica, a low-density and relatively nonprotective gel forms on the glass. Conversely, in a confined medium (low flow rate and/or high S/V ratio), the gel is dense and highly prot
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