Modeling the durability of silica additives proposed to protect nuclear waste glass packages in a geological repository

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 Rhone 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 M-add(2) (where M-add is the added silica mass) up to a critical value M-crit at which it increases considerably (from 10(3) or 10(4) to 10(8) years) due to saturation of the adsorption sites. Excess silica consumption is then limited by the typically very low (3 x 10(-3) 1.m(-2)y(-1)) 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 M-add much less than M-crit, but much lower if M-add = M-crit or M-add > M-crit 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.