The United States plans to bury high level radioactive waste from commercial power reactors and from nuclear weapons manufacturing in Yucca Mountain, Nevada. Yucca Mountain, located about 80 miles northwest of Las Vegas, consists of horizontally bedded tuff deposits. Although the region is very arid, enough water exists in the tuffs to create a vapor dominated geothermal system as the pore water evaporates, circulates and recondenses. This study examines how silica leaching might occur as a result of water-tuff interactions in Yucca Mountain after the emplacement of heat-producing nuclear waste canisters. A vertical thermal gradient experiment (VTGE) was designed and built in order to simulate the water cycling scenario where water in the tuff is vaporized by the heat from the canisters, moves to cooler regions along fractures and condenses as a liquid which flows downward toward the hot canisters. This experiment was used to measure the rate of silica leaching from Yucca Mountain tuff at various heat fluxes. The results show that under the experimental conditions silica is leached from the tuff sample very effectively (about 1.85 x 10 -8 g per Joule of heat transferred). With such a rate, significant amount of amorphous silica (135 kg per canister for the first 1,000 years after emplacement) could be leached, transported and deposited above the repository horizon; the resulting low permeability zone might change the geological and hydrological properties of the host rock. A significant amount of colloidal silica was formed in the solution soon after the water recycling began. Such colloidal silica could adsorb and transport radionuclides released from breached waste canisters more efficiently than that when radionuclides act alone. The results indicate that silica leaching is a potential problem for the current designs of the Yucca Mountain repository. / Master of Science