The Yucca Mountain Project

Ambient Testing

Fracture Flow, Fracture Matrix Interaction Test

These tests specifically address the conditions under which, and the rate at which, moisture could seep into niches (short drifts) in the repository region. Test goals are to determine the amount of seepage into drifts under a variety of episodic pulse events and to identify potential fast flow paths. Seepage into drifts controls the conditions in the ESF environment, the amount of water contacting waste, the rate of waste mobilization, and the transport of radionuclides within the drift and ultimately outside the repository region.

Ambient hydrologic conditions of the repository rock were measured in borehole clusters and niches located in the ESF. In addition, water containing dye was released in the boreholes to simulate an episodic pulse of natural water moving through the rock.

The seepage threshold tests were completed at Niche 3650 (3650m from the ESF North Portal). Moisture monitoring at Niche 3655 by the U.S. Geological Survey (USGS) will continue for one more year. The plan is to switch the testing and monitoring activities between these two niches in the near future.

Scientists will use the results to predict whether water percolating downward above a mined opening (waste emplacement drift or tunnel) will drip into the opening because of gravity, or migrate around the opening due to capillary retention (i.e., a drift acting as a barrier). It is important to determine whether a mined opening acts as a capillary barrier because the barrier may help isolate the waste by preventing water from entering the drift and coming into contact with waste packages.

Air-permeability tests in borehole clusters were performed to characterize the rock and to select borehole intervals for subsequent liquid-release tests. These liquid-release tests involved controlled release of water with dye tracers. Niches were then excavated and examined for the presence of dyed water to determine if and how far the dyed water had traveled.

Tests were repeated at various flow rates and injection volumes to further quantify seepage. Numerical modeling was used to design the tests and predict the outcome, and test results were used to further refine the models. After completion of the active testing phase, long-term monitoring of moisture was conducted, using sensors installed in the niches. Moisture measurements were used to calculate water flow and to quantify the interaction between fast flow paths and rock matrix.

For more information, please contact:
Joe Wang
Ph: 510-486-6753
email: jswang@lbl.gov

Investigations Flow and Transport in Fractured Welded Tuffs

Research Summary

Wetting-front movement, flow-field evolution, and drainage of fracture flow paths were evaluated within the Topopah Spring welded tuff at Yucca Mountain, Nevada. Equipment and techniques were developed for in situ quantification of formation intake rates, flow velocities, seepage rates, and volumes of fracture flow paths.  Localized injections of tracer-laced liquid into a low-permeability zone (LPZ) and a high-permeability zone (HPZ) along a borehole were detected in two boreholes below the point of injection.  For the LPZ tests, water did not seep into an excavated slot  that defined the lower boundary of the test bed, and the liquid-intake rate under constant-head conditions was observed to steadily decrease by two orders of magnitude.  In the HPZ, liquid-intake rates under constant-head conditions were significantly higher and did not exhibit a strong systematic decline.  HPZ tests were also conducted under a range of constant-flow conditions.  Slot seepage rates showed intermittent responses and the percentage of injected water recovered in the slot increased as each test progressed. A maximum of 80% of the injected water was recovered during high-rate injection tests. The flow path volumes were found to increase during the course of each HPZ test.  

Analyses of the breakthrough curves show that flow and transport pathways are dynamic, rather than fixed, and related to the liquid release rates.  Fractures act as the main conducting pathways, with minimal fracture-matrix interactions, under high injection rates. Fracture-matrix interactions, as well as matrix flow, can significantly influence flow and transport under low release rates.  Observations of tracer concentrations rebounding in the seepage water following interruption of flow provide evidence of mass exchange between the fast-flowing fractures and other slow- and/or no-flow regions.  The tests also show the applicability of fluorinated benzoate tracers in situations where multiple tracers of similar physical properties are warranted.  

Numerical modeling was a key component of this research effort. Data from this experiment were used to make quantitative comparisons with numerical simulations of liquid-release experiments. The purpose of the modeling was to aid in experimental design, predict experimental results, and study the physical processes accompanying liquid flow through unsaturated fractured welded tuff.  The model used cubic elements arranged in a regular three-dimensional grid to represent a 24 m3 block of fractured tuff.  High-permeability fracture elements located deterministically preserve the connectivity of the fracture network, which is crucial to its ability to conduct fluid.  Because element thickness is much greater than fracture aperture, fracture elements were assigned properties of a fracture continuum rather than of an individual fracture.  The fracture network was constructed using fracture geometry data taken from a fracture map of the walls and ceiling of the alcove adjacent to the field test site.  The network was then refined using the results of air-permeability tests.  Model results suggest that it may not be sufficient to conceptualize the fractured tuff as consisting of high-permeability fractures embedded in a low-permeability matrix.  The need to include a secondary fracture network (with distinct characteristics from the network of larger mapped fractures) is demonstrated by comparison to the liquid flow observed in the field.

Related Publications

Salve, R., J.S.Y. Wang and C. Doughty, 2002. Liquid flow in unsaturated fractured welded tuffs: I. Field Investigations, Journal of Hydrology, 256: 60-79.

Doughty, C., R. Salve, and J.S.Y. Wang, 2002. Liquid flow in unsaturated fractured welded tuffs: II. Numerical Modeling, Journal of Hydrology, 256: 80-105.

Hu, Q., R. Salve, W. Stringfellow, and J.S.Y. Wang, 2001. Field tracer transport tests in unsaturated fractured tuff, Journal of Contaminant Hydrology, 51: 1-12

For more information, please contact:
Rohit Salve
ph: 510.486.6416
email: r_salve@lbl.gov

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