Preliminary bounds on the expected postclosure performance of the Yucca Mountain Repository Site, southern Nevada

Abstract

On the basis of current data and understanding of site characteristics at Yucca Mountain, the likely performance range of a mined repository for spent nuclear fuel can be calculated. Low flux through the unsaturated zone results in groundwater travel times to the water table that probably exceed 10,000 years and may exceed 100,000 years, far longer than required by the Nuclear Regulatory Commission (NRC). The low flux will also limit releases of waste from the waste packages, probably to annual amounts less than one millionth of the mass of the waste inventory remaining 1000 years after repository closure; the corresponding releases of curies would be well within the allowable releases set by the NRC. Geochemical retardation by sorption and diffusion will slow radionuclide movement relative to groundwater flow by factors of hundreds to thousands for many waste species. In combination, these site conditions provide a high degree of confidence that no releases to the accessible environment will occur during the first 10,000 years after repository closure, the time period for which the Environmental Protection Agency (EPA) has set release limits. Carbon 14, technetium 99, iodine 129, and various nuclides of uranium sorb poorly on the tuffs along the flow paths and, together with uranium daughter products, will be the first radionuclides to arrive at the water table. The total radioactivity produced by these and later arriving contaminants will remain far below the allowable releases, even for periods of millions of years, if expected flux conditions prevail. If the flux is currently greater than the values inferred from the measured in situ moisture contents of the volcanic rocks or if it were to increase in the future, fracture flow and attendant short flow times to the water table could occur. Even if rapid fracture flow were to occur, release of wastes to the accessible environment would probably remain low with respect to the EPA's limits because diffusion of radionuclides from the fractures into the rock matrix would ensure slow migration of most of the wastes through the sorbing matrix.