Report on laboratory scale thermally-coupled processes experiments

Yucca Mountain Site Characterization Project (YMP) is studying Yucca Mountain, Nevada as a potential repository for high-level nuclear wastes. The studies include predictions of the quantity and composition of water in the repository near-field environment that will affect the release rate of radioactive nuclides from waste packages, and the transport of the nuclides through the rock mass adjacent to these packages. The radioactive decay heat from the high- level nuclear waste may increase the temperature in the rock mass to the extent that coupled thermal-mechanical-hydrological-chemical (TMHC) processes may exist in the originally -partially-saturated Topopah Spring tuff-the host rock for the potential repository in Yucca Mountain. Modeling the coupled TMHC processes is necessary to predict the quantity and quality of water in the near-field environment for the entire life span of a repository (tens of thousands of years). In situ thermal tests are required to build up the confidence level of the coupled TMHC models. The purposes of conducting the laboratory studies of the coupled TMHC processes are to enhance our understanding of those processes, and to assist the interpretation of the field test results. Laboratory experiments deal with controlled experimental and boundary conditions, smaller sample sizes, and simpler geometrical configurations (e.g., regular shape and single fracture). These characteristics make the laboratory results suitable for understanding the processes. This in turn will make incorporation of these processes in model calculations more manageable. However, it should be noted that small sample size and simple geometrical configuration make the results of the laboratory tests unsuitable for direct use in predicting behaviors of in situ rock mass. The laboratory tests included in this reporting period are summarized below, along with projection of future work. This report fulfills the level 4 Milestone ID: SPL7A5M4

Fracture/matrix flow experiments results

The impact of vapor diffusion and its potential enhancement are of concern with respect to the performance of the potential nuclear waste repository at Yucca Mountain. Under non-isothermal conditions, such as those prevailing in the near-field environment, gas-phase diffusion of water vapor (a condensable component) may be enhanced as compared to isothermal conditions. Two main phenomena are responsible for this enhancement (Philip and DeVries 1957, p. 226). Normally, diffusive transport of water vapor is obstructed by the presence of liquid islands in the pore throats, and diffusion is reduced at higher saturations. However, under a thermal gradient, a vapor-pressure gradient develops in the gas phase, causing water to evaporate from one side of the liquid island and to diffuse in the gas phase to a liquid island of lower temperature, where it condenses (Figure 1). Water flows through the liquid island as a result of differences in meniscus curvature between the two sides. This difference is caused by the temperature gradient between the liquid-vapor interfaces on the two ends of the liquid island. The evaporation-condensation process repeats itself on the other side of the liquid island; the result is an enhanced diffusive flux through the medium

Effects of X-ray dose on rhizosphere studies using X-ray computed tomography

X-ray Computed Tomography (CT) is a non-destructive imaging technique originally designed for diagnostic medicine, which was adopted for rhizosphere and soil science applications in the early 1980s. X-ray CT enables researchers to simultaneously visualise and quantify the heterogeneous soil matrix of mineral grains, organic matter, air-filled pores and water-filled pores. Additionally, X-ray CT allows visualisation of plant roots in situ without the need for traditional invasive methods such as root washing. However, one routinely unreported aspect of X-ray CT is the potential effect of X-ray dose on the soil-borne microorganisms and plants in rhizosphere investigations. Here we aimed to i) highlight the need for more consistent reporting of X-ray CT parameters for dose to sample, ii) to provide an overview of previously reported impacts of X-rays on soil microorganisms and plant roots and iii) present new data investigating the response of plant roots and microbial communities to X-ray exposure. Fewer than 5% of the 126 publications included in the literature review contained sufficient information to calculate dose and only 2.4% of the publications explicitly state an estimate of dose received by each sample. We conducted a study involving rice roots growing in soil, observing no significant difference between the numbers of root tips, root volume and total root length in scanned versus unscanned samples. In parallel, a soil microbe experiment scanning samples over a total of 24 weeks observed no significant difference between the scanned and unscanned microbial biomass values. We conclude from the literature review and our own experiments that X-ray CT does not impact plant growth or soil microbial populations when employing a low level of dose (<30 Gy). However, the call for higher throughput X-ray CT means that doses that biological samples receive are likely to increase and thus should be closely monitored

Quantifying fracture geometry with X-ray tomography: Technique of Iterative Local Thresholding (TILT) for 3D image segmentation

This paper presents a new method—the Technique of Iterative Local Thresholding (TILT)—for processing 3D X-ray computed tomography (xCT) images for visualization and quantification of rock fractures. The TILT method includes the following advancements. First, custom masks are generated by a fracture-dilation procedure, which significantly amplifies the fracture signal on the intensity histogram used for local thresholding. Second, TILT is particularly well suited for fracture characterization in granular rocks because the multi-scale Hessian fracture (MHF) filter has been incorporated to distinguish fractures from pores in the rock matrix. Third, TILT wraps the thresholding and fracture isolation steps in an optimized iterative routine for binary segmentation, minimizing human intervention and enabling automated processing of large 3D datasets. As an illustrative example, we applied TILT to 3D xCT images of reacted and unreacted fractured limestone cores. Other segmentation methods were also applied to provide insights regarding variability in image processing. The results show that TILT significantly enhanced separability of grayscale intensities, outperformed the other methods in automation, and was successful in isolating fractures from the porous rock matrix. Because the other methods are more likely to misclassify fracture edges as void and/or have limited capacity in distinguishing fractures from pores, those methods estimated larger fracture volumes (up to 80 %), surface areas (up to 60 %), and roughness (up to a factor of 2). These differences in fracture geometry would lead to significant disparities in hydraulic permeability predictions, as determined by 2D flow simulations

Experimental tests of enhancement of thermal vapor diffusion in Topopah Spring Tuff

Investigation of the behavior of VOCs in ground water across fine and coarse grained geological contacts using a medium-scale physical model

Electro-Osmotic Remediation of Fine-Grained Sediments

The coupled-flow phenomenon, electro-osmosis, whereby water flow results from an applied electrical potential gradient, is being used at Lawrence Livermore National Laboratory to induce water flow through deep (25-40 meters below surface) fine-grained sediments. The scoping work described here lays the groundwork for implementation of this technology to remediate solvent-contaminated clayey zones at the LLNL site. The electro-osmotic conductivity (k{sub e}) measured in-situ between two 37 m deep wells, 3 m apart of 2.3 x 10{sup -9} m{sup 2}/s-V is in good agreement with the value determined from bench-top studies on the core extracted from one of the wells of 0.94 {+-} 0.29 x 10{sup -9} m{sup 2}/s-V. Hydraulic conductivity (k{sub h}) of the same core is measured to be 2.03 {+-} 0.36 x 10{sup -10} m/s. Thus, a voltage gradient of 1 V/cm produces an effective hydraulic conductivity of {approx}1 x 10{sup -7} m/s; an increase in conductivity of nearly three orders of magnitude

Soil water retention measurements using a combined tensiometer-coiled domain reflectometry probe.

The objective of the presented study was to developa single probe that can be used to determine soil water retention curves in both laboratory and field conditions, by incIuding a coiled TDR probe around the porous cup of a standard tensiometer