The BRAG and GM2003 Models for Glass Dissolution
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0985-NN02-07
The BRAG and GM2003 Models for Glass Dissolution Marc Aertsens Waste and Disposal dept., SCK CEN, Boeretang 200, Mol, B-2400, Belgium ABSTRACT The GM2003 model extends the r(t) glass dissolution model with water diffusion through the diffusion layer. Boron and alkali diffusion through the diffusion layer is described by introducing a retention factor Kd,i between boron/alkali and water in the diffusion layer. Introducing a boron/alkali diffusion coefficient, the BRAG model describes boron/alkali diffusion in the diffusion layer as well. It is shown that both models are consistent with each other and an expression is derived for the boron/alkali diffusion coefficient (BRAG) as a function of both parameters of GM2003: the retention factor Kd,i and the water diffusion coefficient DH2O in the diffusion layer. From dissolution data only, it is possible to fit the value for the boron/alkali diffusion coefficient in the diffusion layer but due to correlations the individual values of both parameters Kd,i and DH2O of GM2003 cannot be determined. From theoretical considerations follows that the Kd,i value for boron/alkali should be slightly larger than 0.1 kg/liter. A user friendly code for the BRAG model allows automatic fits of glass dissolution data in water. INTRODUCTION Estimating the durability of vitrified high-level waste is needed for evaluating the safety of a nuclear waste repository. Therefore, during the last thirty years many glass dissolution experiments have been performed. The interpretation of these experiments occurred by several models. The Glamor project [1], supported by the European Commission, intended to obtain a common interpretation of a high number of selected experiments of glass dissolution in pure water, performed in different institutes located in Europe. Therefore, two models were selected: the r(t) model [2] and the GM model [3]. In a first step, these models were analyzed and discussed in detail. A recent description summarizing these models, their similarities and their differences is given in [4]. However, this review describes a previous version of GM [3], referred to as GM2001. A description of the GM version used in Glamor, called GM2003, was not available then and is presented in this paper. As mentioned in [4], the GM model is essentially the combination of the r(t) model, which assumes that glass dissolution is controlled by silica dissolution, and the Boksay model [5, 4], which takes into account the diffusion of boron and alkali in the diffusion layer, which is the layer of altered glass next to the pristine glass. Since the r(t) model is part of the GM model, we briefly (see [4] for more details) summarize its basics. The r(t) model does not consider the diffusion layer, but only the gel layer. The gel is the altered glass between the original glass/solution interface and the solution side of the diffusion layer. In the r(t) model, silica is supposed to dissolve according to a linear rate law at the gel/diffusion layer interface. Dissolved silica is assumed to diffu
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