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Title: Pore Connectivity, Episodic Flow, and Unsaturated Diffusion in Fractured Tuff

Abstract

We use an integrated approach consisting of experiments and complementary pore-scale network modeling to investigate the occurrence of sparsely connected pore spaces in rock matrices at Yucca Mountain, Nevada, and its implication to matrix diffusion. Imbibition results indicate that pore spaces in devitrified tuff are not well-connected, and that this lack of connectivity is further compounded by episodic flow in fractured devitrified tuff with low matrix permeability. A rigorous methodology for investigating chemical transport in fractured rock under episodic conditions, employing a suite of both sorbing and non-sorbing tracers (including radionuclides U-235, Np-237, and Pu-242), has been developed and implemented. In addition, gas diffusion and synchrotron microtomography techniques have been under development to examine the scaling issues of diffusion and pore connectivity. Preliminary results from experiments and modeling work are presented in this paper, in order to reexamine our understanding of matrix diffusion and to evaluate the impact on diffusive radionuclide retardation of episodic fracture flow and low pore connectivity.

Authors:
; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
893980
Report Number(s):
UCRL-PROC-218551
TRN: US0700038
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Conference
Resource Relation:
Conference: Presented at: 2006 International High Level Radioactive Waste Management Conference, Las Vegas, NV, United States, Apr 30 - May 04, 2006
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; 43 PARTICLE ACCELERATORS; 58 GEOSCIENCES; 54 ENVIRONMENTAL SCIENCES; DIFFUSION; FRACTURES; MATRICES; PERMEABILITY; RADIOACTIVE WASTE MANAGEMENT; RADIOISOTOPES; SIMULATION; SYNCHROTRONS; TRANSPORT; TUFF; YUCCA MOUNTAIN

Citation Formats

Hu, Q, Ewing, R P, Tomutsa, L, and Singleton, M J. Pore Connectivity, Episodic Flow, and Unsaturated Diffusion in Fractured Tuff. United States: N. p., 2006. Web.
Hu, Q, Ewing, R P, Tomutsa, L, & Singleton, M J. Pore Connectivity, Episodic Flow, and Unsaturated Diffusion in Fractured Tuff. United States.
Hu, Q, Ewing, R P, Tomutsa, L, and Singleton, M J. 2006. "Pore Connectivity, Episodic Flow, and Unsaturated Diffusion in Fractured Tuff". United States. doi:. https://www.osti.gov/servlets/purl/893980.
@article{osti_893980,
title = {Pore Connectivity, Episodic Flow, and Unsaturated Diffusion in Fractured Tuff},
author = {Hu, Q and Ewing, R P and Tomutsa, L and Singleton, M J},
abstractNote = {We use an integrated approach consisting of experiments and complementary pore-scale network modeling to investigate the occurrence of sparsely connected pore spaces in rock matrices at Yucca Mountain, Nevada, and its implication to matrix diffusion. Imbibition results indicate that pore spaces in devitrified tuff are not well-connected, and that this lack of connectivity is further compounded by episodic flow in fractured devitrified tuff with low matrix permeability. A rigorous methodology for investigating chemical transport in fractured rock under episodic conditions, employing a suite of both sorbing and non-sorbing tracers (including radionuclides U-235, Np-237, and Pu-242), has been developed and implemented. In addition, gas diffusion and synchrotron microtomography techniques have been under development to examine the scaling issues of diffusion and pore connectivity. Preliminary results from experiments and modeling work are presented in this paper, in order to reexamine our understanding of matrix diffusion and to evaluate the impact on diffusive radionuclide retardation of episodic fracture flow and low pore connectivity.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2006,
month = 1
}

Conference:
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  • We use an integrated approach consisting of experiments and complementary pore-scale network modeling to investigate the occurrence of sparsely connected pore spaces in rock matrices at Yucca Mountain, Nevada, and their implications for matrix diffusion. Imbibition results indicate that pore spaces in devitrified tuff are not well-connected, and that this lack of connectivity is further compounded by episodic flow in fractured devitrified tuff with low matrix permeability. A rigorous methodology for investigating chemical transport in fractured rock under episodic conditions, employing a suite of both sorbing and non-sorbing tracers (including radionuclides U-235, Np-237, and Pu-242), has been developed and implemented.more » In addition, gas diffusion and synchrotron microtomography techniques have been under development to examine the scaling issues of diffusion and pore connectivity. Preliminary results from experiments and modeling work are presented in this paper, confirming the need to reexamine our understanding of matrix diffusion and to evaluate the impact on diffusive radionuclide retardation of episodic fracture flow and low pore connectivity.« less
  • 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 conditionsmore » 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 {micro}m). The dry core was packed into a flow reactor, flushed with CO{sub 2}, 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 {sup 3}H, ReO{sub 4}{sup -} (a chemical analog for {sup 99}TcO{sub 4}{sup -}), I{sup -} (for {sup 129}I{sup -}), Sr and Cs (for {sup 90}Sr and {sup 137}Cs), plus the radionuclides {sup 235}U, {sup 237}Np, and {sup 241}Pu. 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 {sup 3}H and ReO{sub 4}{sup -} 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 {sup 3}H, 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.« less
  • Over 270 single-hole (Guzman et al., 1996) and 44 cross-hole pneumatic injection tests (Illman et al., 1998; Illman, 1999) have been conducted at the Apache Leap Research Site (ALRS) near Superior, Arizona. They have shown that the pneumatic pressure behavior of fractured tuff at the site is amenable to analysis by methods which treat the rock as a continuum on scales ranging from meters to tens of meters, and that this continuum is representative primarily of interconnected fractures. Both the single-hole and cross-hole test results are free of skin effect. Single-hole tests have yielded estimates of air permeability at variousmore » locations throughout the tested rock volume, on a nominal support scale of about 1 m. The corresponding log permeability data exhibit spatial behavior characteristic of a random fractal and yield a kriged estimate of how these 1-m scale log permeabilities vary in three-dimensional space (Chen et al., 2000). Cross-hole tests have been analyzed by means of a three-dimensional inverse model (Vesselinov et al., 2000) in two ways: (a) by interpreting pressure records from individual borehole monitoring intervals, one at a time, while treating the rock as if it was spatially uniform; and (b) by using the inverse model to interpret pressure records from multiple tests and borehole monitoring intervals simultaneously, while treating the rock as a random fractal characterized by a power variogram. The first approach has yielded equivalent air permeabilities and air-filled porosities for a rock volume characterized by a length-scale of several tens of meters. Comparable results have been obtained by means of type-curves (Illman and Neuman, 2001). The second approach amounts to three-dimensional pneumatic tomography, or stochastic imaging, of the rock. It has yielded a high-resolution geostatistical estimate of how air permeability and air-filled porosity, defined over grid blocks having a length-scale of 1 m, vary throughout the modeled rock volume. These tomographic images are comparable to those obtained by the kriging of 1-m scale log permeability data from single-hole tests. The results reveal a highly pronounced scale effect in permeability and porosity at the ALRS. We analyze the scaling of permeability at the site on the basis of a recent theory, which is consistent with our representation of the rock as a random fractal.« less