Fitting Element Profiles for Predicting Glass Dissolution Rates in Synthetic Interstitial Clay Water
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FITTING ELEMENT PROFILES FOR PREDICTING GLASS DISSOLUTION RATES IN SYNTHETIC INTERSTITIAL CLAY WATER Marc Aertsens, Karel Lemmens, Pierre Van Iseghem SCK●CEN, Boeretang 200, B-2400 Mol, Belgium ABSTRACT An analytical model is developed to derive diffusion coefficients from the element profiles of the most mobile glass elements. Congruent dissolution is considered as a transient phenomenon and neglected. In the model, altered glass consists of two sublayers: a gel layer at the solution side, and a diffusion layer at the pristine glass side. In both layers, mobile glass elements diffuse towards the solution, while hydrogen/water diffuses towards the pristine glass. Fitting the element profiles of boron, sodium and lithium leads to diffusion coefficients in the diffusion layer, which at 90°C are typically of the order of 10-22 m2/s to 10-21 m2/s. At 40°C, these diffusion coefficients are typically a factor ten lower. The diffusion coefficients in the gel are by less than a factor two hundred larger than those in the diffusion layer. The diffusion coefficients derived from the profiles in this way are used to predict the dissolution behavior. These predictions, for which a constant value (representing congruent dissolution) is added to the predicted diffusive release, agree reasonably well with the measured dissolution data. Those results support the idea that diffusion determines the long term release of glass matrix elements in synthetic interstitial clay water. Flux conservation at the interface between the gel and the diffusion layer is not consistent with a constant (as a function of time) width of the diffusion layer. Some experimental evidence supports that this width increases with time. INTRODUCTION A possibility for managing high-level nuclear waste (HLW) is to encapsulate it in a glass matrix and dispose this conditioned waste in deep geological layers, e.g. clay. Due to interaction with clay, the glass surface slowly alters, leading to the release of glass elements (including radionuclides) in the environment. For a safety analysis, it is important to estimate the rate of this release process. Therefore, dissolution experiments were performed with two candidate HLW glasses (SM539 and SON68) at two temperatures (40°C and 90°C) in a synthetic interstitial clay (SIC) water solution at a surface to volume ratio SA/V = 100 m-1. The full data of these tests will be published in a report. Possibly, the long term dissolution behavior of this system is diffusion controlled. Due to scatter, it is hard to show this relying only on dissolution data. Therefore, element profiles (SIMS) were taken from the glass samples as well. The purpose is to derive diffusion coefficients from the profiles. These diffusion coefficients can be used to predict the leached quantities, which can be compared with dissolution data. Therefore, we have developed the model Fit_sims. The basic equations of this model are presented in the next section. After solving the model, we briefly discuss it. Then Fit_sims is used to fit element p
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