Potentially disruptive hydrologic features, events and processes at the Yucca Mountain Site, Nevada

Yucca Mountain, Nevada, has been selected by the United States to be evaluated as a potential site for the development of a geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste. If the site is determined to be suitable for repository development and construction is authorized, the repository at the Yucca Mountain site is planned to be constructed in unsaturated tuff at a depth of about 250 meters below land surface and at a distance of about 250 meters above the water table. The intent of locating a repository in a thick unsaturated-zone geohydrologic setting, such as occurs at Yucca Mountain under the arid to semi-arid climatic conditions that currently prevail in the region, is to provide a natural setting for the repository system in which little ground water will be available to contact emplaced waste or to transport radioactive material from the repository to the biosphere. In principle, an unsaturated-zone repository will be vulnerable to water entry from both above and below. Consequently, a major effort within the site-characterization program at the Yucca Mountain site is concerned with identifying and evaluating those features, events, and processes, such as increased net infiltration or water-table rise, whose presence or future occurrence could introduce water into a potential repository at the site in quantities sufficient to compromise the waste-isolation capability of the repository system