HR: 17:15h
AN: H14C-05    [Abstracts]
TI: Coupling Seepage and Radionuclide Transport in and Around Emplacement Drifts at Yucca Mountain
AU: * Zhang, G
EM: gxzhang@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, United States
AU: Spycher, N
EM: NSpycher@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, United States
AU: Sonnenthal, E
EM: ELSonnenthal@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, United States
AU: Steefel, C
EM: CISteefel@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, United States
AB: The proposed nuclear waste repository of the United States is located at Yucca Mountain, Nevada. Waste packages will be placed in deep (~350 m) underground drifts in volcanic tuff. Seepage may potentially occur at the repository drifts when the drifts get rewetted after a dryout period. The potential seepage water will be quickly evaporated or boiled to near dryness as long as it falls on the top of the hot waste package leading to formation of brine, precipitation of salts and volatilization of gases. These processes may potentially impact the long-term safety of waste packages in the drift. The objectives of this study are to: (1) develop a quantitative model of coupled thermal, hydrological, and chemical (THC) processes potentially leading to brine formation, salt precipitation and gas volatilization on top of waste packages and/or a drip shield and (2) dynamically integrate such a model into the larger-scale models of processes within and around waste emplacement drifts, as well as into the smaller-scale waste-package corrosion models. Process models were implemented into an existing reactive transport numerical simulator, TOUGHREACT, to allow modeling of (1) evaporative concentration to very high ionic strength (up to 40 molal), (2) boiling point elevation due to dissolved salts, (3) boiling/evaporation to dryness, and (4) salt deliquescence. An integrated near-field and in-drift THC simulation was run using a vertical 2-D grid extending from near the ground surface to the groundwater table, and covering a width equal to half the design drift spacing of 81 m. The integrated model was then used to simulate a discrete dripping event within the drift. The model considered the release of radionuclides into seepage water as this water contacts the waste package and flows through the invert. The precipitation of uranophane and Np-uranophane was also considered. These minerals form in the invert from the neutralization of mildly acidic seepage water by clay minerals. The main findings from this modeling effort are as follows: (1) the near-field and in-drift brine chemical evolution is dominated by the precipitation of NaCl, CaSO₄, and CaCO₃; (2) the generation of acid gases at high evaporative concentration yields P_HCl ~10^-7 bar at boiling temperatures, with pH staying > 5 in condensation areas; (3) the clay minerals in the invert neutralize the pH of seepage water, although this result is sensitive to assumptions regarding the kinetics of reactions with clays; (4) the drift invert may act as a pH buffer that promotes the precipitation of uranophane and impedes further downward migration of radionuclides at elevated concentrations, however, the pH buffer effect is subject to the content and composition of the clay minerals in the invert. The model captures some of the processes involved in salt formation and radionuclide transport, and can be further applied to capturing the details of radionuclide transport between the waste form and the rock through the invert.
DE: 1000 GEOCHEMISTRY
DE: 1800 HYDROLOGY
SC: Hydrology [H]
MN: 2007 Fall Meeting