The United States is currently planning to bury high-level radioactive wastes from commercial nuclear power plants and from weapons production in a mined repository in tuff deposits under Yucca Mountain, Nevada. Radioactive decay in these wastes will create steep thermal gradients in the rocks surrounding the repository. This heat might cause the pore water in these tuffs to evaporate, migrate as vapor to cooler areas, and condense to form a solution that can aggressively dissolve the surrounding tuff. Under certain circumstances the silica-rich, bicarbonate solutions produced by this process could migrate under the influence of gravity back toward the canisters where the water would again evaporate leaving amorphous silica behind to form a low permeability silica cap above the repository. Over time, a perched water table containing carbonate and colloidal silica-rich brine could develop above the repository. Thermoelastic fracturing of the silica cap might allow this brine to flow into the repository. We developed a vertical thermal gradient experiment that simulates the silica dissolution and transport processes and used it to determine that each Joule of heat can leach and transport as much as 1.85(+ or -0.56)X10 (super -8) g of silica from a crushed sample of the Topopah Springs Member of the Paintbrush Tuff. Based on this value and the thermal models of Moujaes and Lei (1995), we estimate that the heat from each canister of waste might transport as much as 31.7 kg of silica during the first 100 years and as much as 136 kg of silica during the first 1,000 years after emplacement. Assuming a reasonable fracture density and a 30 mX30 m area of influence for each canister, we estimate that the silica cap above the repository could grow to be 2.1 mm thick after 100 yr and 9.1 mm thick after 1,000 yr.