Earth Sciences Division Berkeley Lab
Nuclear Waste Program
Annual Report 2000–2001
FRACTURES AND UZ PROCESSES AT YUCCA MOUNTAIN Jennifer J. Hinds and Gudmundur S. Bodvarsson Contact: Jennifer J. Hinds, 510/486-7107, jhinds@lbl.gov
RESEARCH OBJECTIVES The objective of this research is to evaluate the importance of fractures at different scales on a variety of processes in the unsaturated zone at Yucca Mountain. These processes are ambient liquid flow, radionuclide transport, and coupled thermal-hydrological processes. Additionally, we evaluate the importance of including all fractures or a subset of mapped fractures in the analysis of these processes.
APPROACH Fractures are integral structural features at Yucca Mountain, providing highly permeable pathways for air and liquid to flow through unsaturated rocks. They are also potential pathways for radionuclide migration. Though millions of fractures exist in the unsaturated zone, the portion of these fractures relevant to assessing the performance of a potential repository depends upon the process being evaluated. Ambient Flow Processes—Water flows mainly through fractures in the low-porosity welded tuffs occurring in the unsaturated zone at Yucca Mountain. Yet only a fraction of the widely dispersed and connected fractures is believed to actively conduct water under ambient conditions. Also, within an active fracture, only a limited area of the rock matrix is in contact with percolating water (as a result of flow fingering). Thus, much of the fracture system does not participate in liquid flow, and the transfer of water between fractures and the rock matrix is limited. Radionuclide Transport Processes—Transport patterns are expected to mimic flow patterns under ambient conditions. Consequently, only fractures contributing to flow are important. However, the existence of small fractures and microfractures connected to larger fractures that transmit water may be important for transport, because they penetrate matrix blocks and can enhance matrix diffusion, thereby improving repository performance. Coupled Thermal-Hydrological Processes—If a repository is constructed at Yucca Mountain, the heat generated from radioactive decay may cause boiling and vaporization of pore water in the rock surrounding the waste packages. Much of the vapor generated will migrate into fractures and be driven away from the repository drifts, creating a dryout zone. When the vapor encounters cooler rock away from the drifts, it will condense, and saturation will increase locally in all fractures.
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Part of the condensate may then imbibe into the matrix, while some of the condensate will remain in the fractures, become mobile, and drain around the drift opening.
ACCOMPLISHMENTS Tens of thousands of fractures have been mapped and described as part of the site characterization of Yucca Mountain. The majority of the mapped fracture data come from fractures with trace lengths greater than 1 m; however, small fractures (< 1 m in length) are far more numerous. Some small-scale fracturemapping studies have been conducted in the ECRB Cross Drift, and additional studies have been proposed.
SIGNIFICANCE OF FINDINGS Numerical analyses should consider all fracture data in light of the processes to be modeled. For mountain-scale ambientflow-modeling applications, only fractures that actively contribute to large-scale flow are believed to be significant. Since the specific fractures transmitting water are unknown, we recommend using mapped data for coarse fractures (≥ 1 m in length) to estimate fracture-network characteristics and suggest adopting a fracture-matrix interaction variable to match observed field data. For mountain-scale transport modeling, small fractures and microfractures connected to larger active fractures may be important for diffusion and should be evaluated. Coupled thermal-hydrological models should reflect information about fractures at all scales within the condensation zone, and the full fracture-matrix interface area should be used.
RELATED PUBLICATION Hinds, J.J., and G.S. Bodvarsson, Role of fractures in processes affecting potential repository performance, Proceedings of the Ninth International High-Level Radioactive Waste Management Conference, Las Vegas, Nevada, April 29–May 3, 2001.
ACKNOWLEDGMENTS This work was supported by the Director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy, through Memorandum Purchase Order EA9013MC5X between Bechtel SAIC Company, LLC, and Berkeley Lab. The support is provided to Berkeley Lab through U.S. Department of Energy Contract No. DE-AC03-76SF00098.
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