Modeling the Durability of Silica Additives Proposed to Protect Nuclear Waste Glass Packages in a Geological Repository
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ABSTRACT Silica additives have been proposed to protect nuclear waste glass packages in deep geological repositories by presaturating silicon adsorption sites* in the media surrounding the package (engineered barrier, canister and overpack corrosion products). The durability of such additives was estimated using a one-dimensional dissolution-transport model developed at the CEA's Rh6ne Valley Research Center for the French reference glass in typical disposal sites. The silica additive would be placed between the wasteform and the overpack and thus dissolved before the waste glass. Its lifetime is proportional to Madd (where MaJd is the added silica mass) up to a critical value Mjrit at which it increases considerably (from 103 or l04 to 108 years) due to saturation of the adsorption 2 sites. Excess silica consumption is then limited by the typically very low (3 x 10-3 lIm- y-I ) groundwater flow in the geological site. After consumption of the added silica, the evolution of the altered waste glass fraction is comparable to the evolution without added silica if Mjdd Mrit. These findings were confirmed by some experimental evidence, and establish that a few kilograms of added silica for a 400 kg package can considerably enhance the waste glass durability.
INTRODUCTION Metallic corrosion products-notably from alteration of the overpack-and engineered barrier materials have a high capacity for attracting silicon and radionuclides from water, generally described in terms of adsorption capacity* [1,2]. In the case of a waste package surrounded by water, the effect of the high adsorption capacity would be to maintain the glass alteration rate at its initial value (from 0.001 to 0.1 wt% of the package per year, depending on the temperature**) until the adsorption sites are saturated (typically 104 to 106 years [3]). In order to prevent this undesirable effect of the containment barriers, a silica additive consisting of glass frit or silica gel has been proposed for incorporation between the canister and overpack or in the engineered barrier material. The beneficial effects of added silica have been demonstrated experimentally [4] in the presence of smectite 4a. In a repository configuration, the additives would be accessible to water before the glass matrix, ensuring the latter is surrounded by a silica-saturated solution; its presence would also presaturate the silicon adsorption sites in the layer of metallic corrosion products and in the engineered barrier. However, the effectiveness of silica additives depends on their durability, which must be evaluated. The theoretical assessment described here was performed at the CEA's Rh6ne Valley Research Center for the French reference glass in typical repository sites, using a one-dimensional model with allowance for added silica and glass dissolution and for silicon transport in the 2 * The use of "adsorptioncapacity" and "adsorption sites" is discussed in the hypotheses of the model (p. ) 2 2 **Annual alteration rates of a 400 kg wasteform (1.7 tn surface area) based on expe
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