Field tests and model analyses of seepage into drift

Abstract This paper focuses on field test results and model analyses of the first set of five niche seepage tests conducted in the Exploratory Studies Facility at Yucca Mountain. The test results suggest that (1) a niche opening (short drift excavated for this study) acts as a capillary barrier; (2) a seepage threshold exists; and (3) the seepage is a fraction of the liquid released above the ceiling. Before seepage quantification, air injection and liquid release tests at two niche locations were conducted to characterize the fracture flow paths. Nearly two-order-of-magnitude changes in air permeability values were measured before and after niche excavation. The dyed liquid flow paths, together with a localized wet feature potentially associated with an ambient flow path, were mapped during dry excavation operations. After niche excavation, the seepage is quantified by the ratio of the water mass dripped into a niche to the mass released above the opening at selected borehole intervals. For the first set of five tests conducted at Niche 3650 site, the ratios range from 0% (no dripping for two tests) to 27.2%. Changes in flow path distributions and water accumulation near seepage threshold were observed on the niche ceiling. The seepage test results compare reasonably well with model results without parameter adjustments, using capillary barrier boundary condition in the niche and two-dimensional and three-dimensional conceptualizations to represent discrete fracture and fracture network for the flow paths. Model analyses of the niche tests indicate that the seepage is very sensitive to the niche boundary condition and is moderately sensitive to the heterogeneity of the fracture flow paths and to the strengths of matrix imbibition. Strong capillary strength and large storage capacity of the fracture flow paths limit the seepage. High permeability value also enhances diversion and reduces seepage for low liquid release rate.

[1]  Benjamin Ross,et al.  The diversion capacity of capillary barriers , 1990 .

[2]  C. Tsang,et al.  Solute channeling in unsaturated heterogeneous porous media , 1997 .

[3]  J. R. Philip.  Some general results on the seepage exclusion problem , 1990 .

[4]  Unsaturated seepage and subterranean holes: Conspectus, and exclusion problem for circular cylindrical cavities , 1989 .

[5]  C. Tsang,et al.  Geohydrologic Data and Models of Rainier Mesa and Their Implications to Yucca Mountain , 1993 .

[6]  G. D. LeCain.  PNEUMATIC TESTING IN 45-DEGREE-INCLINED BOREHOLES IN ASH-FLOW TUFF NEAR SUPERIOR, ARIZONA , 1995 .

[7]  G. Bodvarsson,et al.  Overview of scientific investigations at Yucca Mountain—the potential repository for high-level nuclear waste , 1999 .

[8]  T. N. Narasimhan,et al.  A PRACTICAL METHOD FOR MODELING FLUID AND HEAT FLOW IN FRACTURED POROUS MEDIA , 1985 .

[9]  T. N. Narasimhan,et al.  Hydrologic Mechanisms Governing Fluid Flow in a Partially Saturated, Fractured, Porous Medium , 1985 .

[10]  Thomas A. Buscheck,et al.  The implications of episodic nonequilibrium fracture-matrix flow on site suitability and total system performance , 1992 .

[11]  S. Webb.  Generalization of Ross' tilted capillary barrier diversion formula for different two‐phase characteristic curves , 1997 .

[12]  W. E. Wilson,et al.  Conceptual hydrologic model of flow in the unsaturated zone, Yucca Mountain, Nevada , 1984 .

[13]  Van Genuchten,et al.  A closed-form equation for predicting the hydraulic conductivity of unsaturated soils , 1980 .

[14]  Chin-Fu Tsang,et al.  Tracer transport in a stochastic continuum model of fractured media , 1996 .

[15]  Curtis M. Oldenburg,et al.  On numerical modeling of capillary barriers , 1993 .

[16]  J. R. Philip.  Asymptotic solutions of the seepage exclusion problem for elliptic‐cylindrical, spheroidal, and strip‐ and disc‐shaped cavities , 1989 .

[17]  Chin-Fu Tsang,et al.  Modeling studies and analysis of seepage into drifts at Yucca mountain , 1999 .