HR: 09:30h
AN: H11H-07    [Abstracts]
TI: Radionuclide Transport in Fractured Tuff under Episodic Flow Conditions
AU: * Hu, Q
EM: hu7@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550 United States
AU: Sun, Y
EM: sun4@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550 United States
AU: Ewing, R P
EM: ewing@iastate.edu
AF: Iowa State University, 2101 Agronomy Hall, Ames, IA 50011 United States
AB: The current conceptual model of radionuclide transport in unsaturated fractured rock includes water movement in fractures, with migration of the entrained radionuclides being retarded by diffusion into and sorption within the rock matrix. Water infiltration and radionuclide transport through low-permeability unsaturated fractured rock are episodic and intermittent in nature, at least at local scales. Under episodic flow conditions, the matrix is constantly imbibing or draining, and this fluctuating wetness both drives two-way advective movement of radionuclides, and forces changes in the matrix diffusivity. This work is intended to examine, both experimentally and numerically, how radionuclide transport under episodic flow conditions is affected by the interacting processes of imbibition and drainage, diffusion, and matrix sorption. Using Topopah Spring welded volcanic tuff, collected from the potential repository geologic unit at Yucca Mountain for storing high-level nuclear waste, we prepared a saw-cut fracture core (length 10.2 cm, diameter 4.4 cm, and fracture aperture 100 microns). The dry core was packed into a flow reactor, flushed with CO₂, then saturated via slow pumping (0.01 mL/min) of synthetic groundwater. The fractured core was then flushed with air at 97% relative humidity (to simulate in situ unsaturated fractured rock conditions at Yucca Mountain), then the episodic transport experiment was conducted. Episodic flow involved 4 cycles of tracer solution flow within the fracture, followed by flushing with high humidity air. Each flow episode contained a different suite of non-sorbing and sorbing tracers, which included ³H, ReO₄^- (a chemical analog for ⁹⁹TcO₄^-), I^- (for ¹²⁹I^-), Sr and Cs (for ⁹⁰Sr and ¹³⁷Cs), plus the radionuclides ²³⁵U, ²³⁷Np, and ²⁴¹Pu. These radionuclides span a variety of sorption strengths and represent a large fraction of the radionuclides of concern at the potential Yucca Mountain repository. Meanwhile, the non-sorbing ³H and ReO₄^- serve as diffusivity tracers with different aqueous diffusion coefficients. Liquid effluent from the flow reactor was collected for multi-elemental analyses using ICP-MS, as well as liquid scintillation counting for ³H, to obtain the breakthrough curves of non- or less-retarded tracers. After the flow-tests were complete, the flow reactor was opened and the distribution of strongly retarded tracers within the fractured core characterized by laser ablation coupled with ICP-MS. A numerical model was developed, based on the NUFT (Non-isothermal, Unsaturated-saturated Flow and Transport) computer code, to describe the experimental system, compare with, and interpret experimental results. This work was supported by the United States Department of Energy (DOE), Office of Civilian Radioactive Waste Management (OCRWM), Office of Science and Technology and International (OST&I). This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1838 Infiltration
DE: 1847 Modeling
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: Fall Meeting 2005