HR: 0830h
AN: H21D-0837    [PDF]
TI: Yucca Mountain, a Likely Geologic Repository
AU: * Andrews, R
EM: robert_andrews@ymp.gov
AF: BSC, 1180 Town Center Dr., Las Vegas, NV 89144 United States
AB: The Department of Energy is evaluating whether Yucca Mountain, Nevada, is likely to meet applicable radiation protection standards established by the Nuclear Regulatory Commission and the Environmental Protection Agency to become the geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste. A number of engineered and natural barriers, among them the saturated zone (SZ), are expected to act as obstructions to the release of radionuclides from the potential repository. Radionuclides may move away from the potential geologic repository as either dissolved or colloidal constituents in groundwater. Although groundwater is the transport mechanism, the entire SZ is expected to delay the transport of radionuclides to the accessible environment and reduce the concentration of radionuclides before they reach the accessible environment. Along the flow path from the potential repository to the accessible environment, the water table transitions from fractured volcanic tuffs to alluvium. Transport processes in the more permeable volcanic tuffs include advective transport dominated by fracture flow, matrix diffusion, sorption in the matrix, and dispersion. Transport processes in the alluvium include advective transport, sorption, and dispersion. The site-scale conceptual model is a synthesis of what is known about flow and transport processes at the level of detail required for Total System Performance Assessment (TSPA). Its mathematical and the associated numerical approaches are designed to quantify the uncertainty in the permeability of geologic units and to accurately represent the flow and transport processes therein. Uncertainties are explicitly incorporated into the flow and transport abstractions through key parameters and conceptual models. An inverse approach is used to estimate the distribution of rock permeability that resulted in calculated values of hydraulic head that best match measured values. Inverse methods also yield rates of lateral flow across model boundaries compatible with results from the regional-scale flow model. Confidence in the model was built by comparing calculated to observed hydraulic heads, estimated to measured permeabilities, and lateral flow rates calculated by the site-scale model to those from the regional-scale flow model. In addition, it was confirmed that the flow paths leaving the region of the potential repository are consistent with those inferred from gradients of measured head and from water chemistry data. The results of the site-scale SZ flow and transport model analysis comprise breakthrough curves for radionuclides at the interface between the SZ and the biosphere (20~km from the potential repository). The importance of the breakthrough curves cannot be assessed independently of the TSPA because they do not contain information on the length of time between waste emplacement, the failure of waste packages, and the time until radionuclides reach the SZ. Nevertheless, for radionuclides not subject to sorption (e.g., carbon), simulated transport times generally were $<$1,000~yr. For radionuclides subject to minor sorption in the alluvium, simulated transport times were 1,000--2,000~yr. For radionuclides that irreversibly attach to colloids (actinides), simulated transport times in the SZ were somewhat $<$10,000~yr. Delay in the migration of colloids with attached radionuclides in the SZ results from filtration and resuspension processes. For radionuclides subject to moderate to high sorption, simulated transport times were in excess of 10,000~yr.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2003 Fall Meeting