Publications

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Aqueous dissolution of silicate materials exhibits complex temporal evolution and rich pattern formations. Mechanistic understanding of this process is critical for the development of a predictive model for a long-term performance assessment of silicate glass as a waste form for high-level radioactive waste disposal. Here we provide a summary of a recently developed nonlinear dynamic model for silicate material degradation in an aqueous environment. This model is based on a simple self-organizational mechanism: dissolution of silica framework of a material is catalyzed by cations released from material degradation, which in turn accelerate the release of cations. This model provides a systematical prediction of the key features observed in silicate glass dissolution, including the occurrence of a sharp corrosion front, oscillatory dissolution, multiple stages of the alteration process, wavy dissolution fronts, growth rings, incoherent bandings of alteration products, and corrosion pitting. This work provides a new perspective for understanding silicate material degradation and evaluating the long-term performance of these materials as a waste form for radioactive waste disposal.

Montmorillonite with an empirical formula of Na0.2Ca0.1Al2Si4O10(OH)2(H2O)10 is a di-octahedral smectite. Montmorillonite-rich bentonite is a primary buffer candidate for high level nuclear waste (HLW) and used nuclear fuel to be disposed in mild environments. In such environments, temperatures are expected to be ≤ 90oC, the solutions are of low ionic strengths, and pH is close to neutral. Under the conditions outside the above parameters, the performance of montmorillonite-rich bentonite is deteriorated because of collapse of swelling particles as a result of illitization, and dissolution of the swelling clay minerals followed by precipitation of non-swelling minerals. It has been well known that tri-octahedral smectites such as saponite, with an ideal formula of Mg3(Si, Al)4O10(OH)2•4H2O for an Mg-end member (saponite-15A), are less susceptible to alteration under harsh conditions. Recently, Mg-bearing saponite has been favorably considered as a preferable engineered buffer material for the Swedish very deep holes (VDH) disposal concept in crystalline rock formations. In the VDH, HLW is disposed in deep holes at depth between 2,000 m and 4,000 m. At such deployment depths, the temperatures are expected to be between 100oC and 150oC, and the groundwater is of high ionic strength. The harsh chemical conditions of high pH are also introduced by the repository designs in which concretes and cements are used as plugs and buffers. In addition, harsh chemical conditions introduced by high ionic strength solutions are also present in repository designs in salt formations and sedimentary basins. For instance, the two brines associated with the salt formations for the Waste Isolation Pilot Plant (WIPP) in USA have ionic strengths of 5.82 mol•kg-1 (ERDA-6) and 8.26 mol•kg-1 (GWB). In the Asse site proposed for a geological repository in salt formations in Germany, the Q-brine has an ionic strength of ~13 mol•kg-1. In this work, we present our investigations regarding the stability of saponite under hydrothermal conditions in harsh environments.

Numerical modeling of flow and transport through fractured crystalline rock was conducted to identify major factors that affect migration of radionuclides from a high-level nuclear waste repository. The study was based on data collected at the Mizunami Underground Research Laboratory (URL) in Japan. Distributions of fracture parameters were used to generate a selected number of DFN realizations. For each realization the DFN was upscaled to a continuum mesh to provide permeability and porosity fields. The upscaled permeability and porosity fields were then used to study flow and transport through the fractured rock in a site-scale domain. For the present study the focus is on the effect of domain size and on upscaling of DFN to a continuum system. Simulation results and analysis on various upscaling and boundary condition assumptions are presented.

Understanding fluid flow and transport in shale is of great importance to the development of unconventional hydrocarbon reservoirs and nuclear waste repositories. Tracer techniques have proven to be a useful tool for gaining such understanding. Shale is characterized by the presence of nanometer‐sized pores and the resulting extremely low permeability. Chemical species confined in nanopores could behave drastically differently from those in a bulk system and the interaction of these species with pore surfaces is much enhanced due to a high surface/fluid volume ratio, both of which could potentially affect tracer migration and chromatographic differentiation in shale. Nanoconfinement manifests the discrete nature of fluid molecules in transport, therefore enhancing mass‐dependent isotope fractionations. All these effects combined lead to a distinct set of tracer signatures that may not be observed in a conventional hydrocarbon reservoir or highly permeable groundwater aquifer system. These signatures can be used to delineate flow regimes, trace fluid sources, and quantify the rate and extent of a physical/chemical process. Such signatures can be used for the evaluation of cap rock structural integrity, the postclosure monitoring of a geologic repository, or the detection of a possible contamination in a water aquifer by a shale oil/gas extraction.

Understanding the viscosity and friction of a fluid under nanoconfinement is the key to nanofluidics research. Existing work on nanochannel flow enhancement has been focused on simple systems with only one to two fluids considered such as water flow in carbon nanotubes, and large slip lengths have been found to be the main factor for the massive flow enhancement. In this study, we use molecular dynamics simulations to study the fluid flow of a ternary mixture of octane-carbon dioxide-water confined within two muscovite and kerogen surfaces. The results indicate that, in a muscovite slit, supercritical CO2 (scCO2) and H2O both enhance the flow of octane due to (i) a decrease in the friction of octane with the muscovite wall because of the formation of thin layers of H2O and scCO2 near the surfaces; and (ii) a reduction in the viscosity of octane in nanoconfinement. Water reduces octane viscosity by weakening the interaction of octane with the muscovite surface, while scCO2 reduces octane viscosity by weakening both octane-octane and octane-surface interactions. In a kerogen slit, water does not play any significant role in changing the friction or viscosity of octane. In contrast, scCO2 reduces both the friction and the viscosity of octane, and the enhancement of octane flow is mainly caused by the reduction of viscosity. Our results highlight the importance of multicomponent interactions in nanoscale fluid transport. The results presented here also have a direct implication in enhanced oil recovery in unconventional reservoirs.

Our results show that a pseudo-boehmite precursor material can be chemically modified with divalent cationic species, for example, Nickel, to create an effective getter for anionic species. The viability of this novel class of materials is established by a variety of characterization methods, including surface area measurements, scanning electron microscopy, elemental analysis, and sorption capacity measurements. We will present the results of sorption capacity and surface area measurements that show the high sorption capacity of this novel class of getter materials. Our study shows that the divalent cation modification can increase the sorption capacity by as much as a factor of two.

Various versions of deep borehole nuclear waste disposal have been proposed in the past in which effective sealing of a borehole after waste emplacement is generally required. In a high temperature disposal mode, the sealing function will be fulfilled by melting the ambient granitic rock with waste decay heat or an external heating source, creating a melt that will encapsulate waste containers or plug a portion of the borehole above a stack of the containers. However, there are certain drawbacks associated with natural materials, such as high melting temperatures, inefficient consolidation, slow crystallization kinetics, the resulting sealing materials generally being porous with low mechanical strength, insufficient adhesion to waste container surface, and lack of flexibility for engineering controls. In this study, we showed that natural granitic materials can be purposefully engineered through chemical modifications to enhance the sealing capability of the materials for deep borehole disposal. The present work systematically explores the effect of chemical modification and crystallinity (amorphous vs. crystalline) on the melting and crystallization processes of a granitic rock system. The approach can be applied to modify granites excavated from different geological sites. Several engineered granitic materials have been explored which possess significantly lower processing and densification temperatures than natural granites. Those new materials consolidate more efficiently by viscous flow and accelerated recrystallization without compromising their mechanical integrity and properties.

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A fluid flow in a nanochannel highly depends on the wettability of the channel surface to the fluid. The permeability of the nanochannel is usually very low, largely due to the adhesion of fluid at the solid interfaces. Using molecular dynamics (MD) simulations, we demonstrate that the flow of water in a nanochannel with rough hydrophilic surfaces can be significantly enhanced by the presence of a thin layer of supercritical carbon dioxide (scCO2) at the water-solid interfaces. The thin scCO2 layer acts like an atomistic lubricant that transforms a hydrophilic interface into a super-hydrophobic one and triggers a transition from a stick- to- a slip boundary condition for a nanoscale flow. This work provides an atomistic insight into multicomponent interactions in nanochannels and illustrates that such interactions can be manipulated, if needed, to increase the throughput and energy efficiency of nanofluidic systems.

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Understanding of aqueous dissolution of silicate glasses and minerals is of great importance to both Earth science and materials science. Silicate dissolution exhibits complex temporal evolution and spatial pattern formations. Recently, we showed how observed complexity could emerge from a simple self-organizational mechanism: dissolution of the silica framework in a material could be catalyzed by the cations released from the reaction itself. This mechanism enables us to systematically predict many key features of a silicate dissolution process including the occurrence of a sharp corrosion front (vs. a leached surface layer), oscillatory dissolution and multiple stages of the alteration process (e.g., an alteration rate resumption at a late stage of glass dissolution). Here, through a linear stability analysis, we show that this same mechanism can also lead to morphological instability of an alteration front, which, in combination with oscillatory dissolution, can potentially lead to a whole suite of patterning phenomena, as observed on archaeological glass samples, including wavy dissolution fronts, growth rings, incoherent bandings of alteration products, and corrosion pitting. Here, the result thus further demonstrates the importance of the proposed self-accelerating mechanism in silicate material degradation.

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