Protective Effect of the Alteration Gel: A Key Mechanism in the Long-Term Behavior of Nuclear Waste Glass
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Protective Effect of the Alteration Gel: A Key Mechanism in the Long-Term Behavior of Nuclear Waste Glass S. Gin Commissariat à l’Énergie Atomique (CEA), Rhône Valley Research Center, DRRV/SCD, BP 171, 30207 Bagnols-sur-Cèze Cedex, France Email: [email protected] ABSTRACT Experimental alteration results are presented in which a French SON 68 (R7T7-type) nuclear containment glass specimen was first altered under static conditions for 600 days at an S/V ratio of 5000 m-1 before transferring it to pure water. The experiment was designed to assess the diffusion barrier properties of the gel formed during the preliminary alteration phase. Contrary to predictions by kinetic models based only on chemical affinity, the renewed alteration of the specimen in pure water was very limited. Measurements at close intervals showed that the maximum alteration rate under these conditions was 7 × 10-3 g·m-2d-1, or about r0/300, whereas a pristine glass coupon in contact with pure water is altered at r0. The renewed alteration behavior is attributed to partial (5–10%) dissolution of the existing gel, resulting in a slight degradation of its protective properties. Saturation conditions, at a different level than in the preliminary phase, were reached within a few days. The saturation of the solution with respect to silicon, generally interpreted as a glass/solution (Grambow) or gel/solution (Bourcier) equilibrium, is shown to depend not only on the alteration conditions, but also on the specimen leaching history, and is thus not an inherent glass property. This experiment confirms the idea that the gel formed under saturation conditions controls the kinetics of SON 68 glass alteration by means of a diffusion barrier effect. It also raises a number of issues concerning the concept of a “residual rate” and the possible relations between the quasi steady-state Si concentrations observed in solution and the protective properties of the gel. 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]. Similar behavior has also been reported for basaltic glass, which is considered to be a suitable natural analog of nuclear waste glass [2,3]. The drop in the alteration rate is generally 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 ([4,5]. The models describing these kinetics were developed from Grambow’s work in the mid-1980s [4] based on considerations arising from transition state theory [6]. On the basis of a single
type of experiment, Chick et al. [7] reached the general conclusion that the alteration films did not constitute effective diffusion barriers. Chemical affinity models have encountered not only theoretical but also experimental obstacles, however [8]. The discrepancies between a
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