Final Report for Subcontract B541028, Pore-Scale Modeling to Support "Pore Connectivity" Research Work
This report covers modeling aspects of a combined experimental and modeling task in support of the DOE Science and Technology Program (formerly OSTI) within the Office of Civilian Radioactive Waste Management (OCRWM). Research Objectives The research for this project dealt with diffusive retardation: solute moving through a fracture diffuses into and out of the rock matrix. This diffusive exchange retards overall solute movement, and retardation both dilutes waste being released, and allows additional decay. Diffusive retardation involves not only fracture conductivity and matrix diffusion, but also other issues and processes: contaminants may sorb to the rock matrix, fracture flow may be episodic, a given fracture may or may not flow depending on the volume of flow and the fracture's connection to the overall fracture network, the matrix imbibes water during flow episodes and dries between episodes, and so on. The objective of the project was to improve understanding of diffusive retardation of radionuclides due to fracture / matrix interactions. Results from combined experimental/modeling work were to (1) determine whether the current understanding and model representation of matrix diffusion is valid, (2) provide insights into the upscaling of laboratory-scale diffusion experiments, and (3) help in evaluating the impact on diffusive retardation of episodic fracture flow and pore connectivity in Yucca Mountain tuffs. Questions explored included the following: (1) What is the relationship between the diffusion coefficient measured at one scale, to that measured or observed at a different scale? In classical materials this relationship is trivial; in low-connectivity materials it is not. (2) Is the measured diffusivity insensitive to the shape of the sample? Again, in classical materials there should be no sample shape effect. (3) Does sorption affect diffusive exchange in low-connectivity media differently than in classical media? (4) What is the effect of matrix saturation on the effective diffusivity? Is it different for low-connectivity media than for classical media? (5) In addition to changing the matrix saturation (and thereby the diffusion coefficient), the wetting/drying cycles drive water into, and then out of, the matrix. How do these mass flow cycles affect the long-term exchange of solutes between the fracture and the matrix? Can it be treated as a simple increase in effective diffusivity? Is it a local or a global effect? Is the effect different in low-connectivity media? The modeling portion of this project primarily focused on how diffusion varies with pore connectivity, and it also connected the experimental work to theory.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 948963
- Report Number(s):
- LLNL-SR-411009; TRN: US0901879
- Country of Publication:
- United States
- Language:
- English
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