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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Introducing a risk aggregation rationale for mapping risks to aquifers from point- and diffuse-sources–proof-of-concept using contamination data from industrial lagoons

Highlights: • Aquifer risk mapping is introduced for contaminants acting as point/diffuse-sources. • Innovations are framed by risk cells based on Source-Pathway-Receptor-Source (SPRC). • Point-source (Risk Cell 1) and diffuse-source (Risk Cell 2) use sets of algorithms. • Algorithms include MODPATH, DRASTIC, GWPI and are integrated on a modular basis. • Risk Cells 1/2 are aggregated giving fit-for-purpose results as proof-of-concept. - Abstract: Proof-of-concept for a methodology is presented on mapping risks to aquifers impacted from point- and diffuse-sources, where mapping or indexing refers to relative but not absolute values. The methodology is generic but tested by investigating impacts of a risk exposure from industrial wastewater lagoons. The methodology is innovative for using the qualitative Source-Pathways-Receptors-Consequences (SPRC) framework to aggregate risks from both point-sources and diffuse-sources through breaking down a study area into SPRC risk cells. In this paper, two risk cells are required as: (i) Cell 1 is directly impacted from a point-source; and (ii) Cell 2 is impacted by diffuse-sources to slowly contaminate the aquifer by infiltration over a large area. Indexing both types of risk cells generically comprise three tiered processes: (i) binary indexing establishes if a grid cell is at a potential risk or not; (ii) graded indexing measures the strength of the risk pathways from source to receptors; and (iii) local indexing measures intrinsic potentials at the grid cell to propagate the risk. These three processes apply to both point- and diffuse-sources but with different mathematical formulations. The proof-of-concept for the methodology of risk aggregation using the SPRC framework is supported by results of a study area, in which a set of performance metrics are used by comparing with the measurements. The results are found to be fit-for-purpose for serving as proactive management tools and to provide a deeper insight into potential impacts of adverse effects.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Iodine effective diffusion coefficients through volcanic rock: Influence of iodine speciation and rock geochemistry

Accurate prediction of the subsurface transport of iodine species is important for the assessment of long-term nuclear waste repository performance, as well as monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty, given that radioiodine decays into radioxenon. However, the transport of iodine through intact geologic media is not well understood, compromising our ability to assess risk associated with radioiodine migration. The current study's goal is to quantify the matrix diffusion of iodine species through saturated volcanic rock, with particular attention paid to the redox environment and potential speciation changes. Here, diffusion experiments were run for iodide through lithophysae-rich lava, lithophysae-poor lava, and welded tuff, whereas iodate diffusion was studied through welded tuff. Iodine transport was compared with a conservative tracer, HDO, and effective diffusion coefficients were calculated. Likely due to a combination of size and anion exclusion effects, iodine species diffused more slowly than the conservative tracer through all rock types tested. Furthermore, oxidation of iodide to iodate was observed in the lithophysae-poor lava, affecting transport. Results provide much needed data for subsurface transport models that predict radioiodine migration from underground sources, and indicate the pressing need for geochemical and redox interactions to be incorporated into these models.

58 GEOSCIENCES↗
DOE OSTI2017

Oxygen diffusion in Gd-doped mixed oxides

Molecular dynamics simulations have been performed to investigate oxygen transport in (U x Pu x-1 ) 0.95 Gd 0.05 O 1.975 , (U x Th x-1 ) 0.95 Gd 0.05 O 1.975 and (Pu x Th x-1 ) 0.95 Gd 0.05 O 1.975 between 1000 and 3200 K. Oxygen diffusivity and corresponding activation energies are examined and compared to values for the undoped (U x Pu x-1 )O 2 , (U x Th x-1 )O 2 and (Pu x Th x-1 )O 2 systems where compositions between end members display enhanced diffusivity. Below the superionic transition oxygen diffusivity for the Gd doped systems is orders of magnitude greater compared to their undoped counterparts. But, enhanced diffusivity for doped mixed actinide cation compositions is not observed compared to doped end members. Furthermore, changes in activation energy suggest changes in diffusion regime, which correspond to the creation of thermally activated oxygen defects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Ab-initio informed cluster dynamics simulation of self- and Xe diffusivity in uranium mononitride under irradiation

Uranium mononitride (UN) is one of the ceramic nuclear fuel alternatives to oxide fuels considered for light water reactor and advanced reactor designs, as it presents significant advantages such as high uranium density (better economics) and high thermal conductivity and melting point (increased safety). Self- and fission gas diffusivities need to be better understood, given that they influence key fuel performance phenomena like swelling and fission gas release. Recently, radiation enhanced diffusivity was investigated in UN by means of cluster dynamics simulations relying on empirical potential-based parameterizations, the reliability of which highly depends on the interatomic potential accuracy. Here, in this work, we refine this approach by determining, using ab-initio calculations, the properties of defect clusters containing vacancies, self-interstitials and Xe impurities. We also consider larger clusters than previous studies. The obtained dataset (formation enthalpies, entropies, and migration barriers) is used to parameterize a cluster dynamics model of mobile clusters, and to calculate the defect cluster concentrations under irradiation. This gives us access to the radiation enhanced self- and fission gas diffusivities. Although the resulting diffusivities are close to the values reported in the literature, we find important qualitative differences in the diffusion mechanisms. Capturing the correct mechanisms is crucial to properly describe the chemistry and fission rate dependence of the model.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Investigation of species-mass diffusion in binary-species boundary layers at high pressure using direct numerical simulations

Here, direct numerical simulations of single-species and binary-species temporal boundary layers at high pressure are performed with special attention to species-mass diffusion. The working fluids are nitrogen or a mixture of nitrogen and methane. Mean profiles and turbulent fluctuations of mass fraction show that their qualitative characteristics are different from those of streamwise velocity and temperature, due to the different boundary conditions. In a wall-parallel plane near the wall, the streamwise velocity and temperature have streaky patterns and the fields are similar. However, the mass fraction field at the same location is different from the streamwise velocity and temperature fields indicating that species-mass diffusion is not similar to the momentum and thermal diffusion. In contrast, at the centre and near the edge of the boundary layer, the mass fraction and temperature fields have almost the same pattern, indicating that the similarity between thermal and species-mass diffusion holds away from the wall. The lack of similarity near the wall is traced to the Soret effect that induces a temperature-gradient-dependent species-mass flux. As a result, a new phenomenon has been identified for a non-isothermal binary-species system – uphill diffusion, which in its classical isothermal definition can only occur for three or more species. A quadrant analysis for the turbulent mass flux reveals that near the wall the Soret effect enhances the negative contributions of the quadrants. Due to the enhancement of the negative contributions, small species-concentration fluid tends to be trapped near the wall.

42 ENGINEERING↗
DOE OSTI2025

Bio-Oil Impact on Water Diffusion and Durability of Bitumen: Influence of Aging and Salinity

Bio-oils derived from inexpensive biomass offer environmentally friendly options for modifying bitumen for improved durability or rejuvenating aged material, although their impact on bitumen moisture resistance can be mixed. A better understanding of water diffusion into bitumen and its interactions with bio-oil compounds would aid the development of effective bio-oils. In this study, the effect of water exposure on bio-oil-modified bitumen blends was examined by de-wetting contact angle measurements and differential FTIR spectroscopy. The bio-oils improved the anti-stripping behavior of the bitumen but also increased water absorption which could weaken cohesive strength. Short-term thermal aging increased water diffusion for all bitumen blends including the control, probably due to the presence of more oxidized compounds. Negative peaks of alkanes and polar groups in the differential FTIR data suggest the co-diffusion of surfactant molecules towards the bitumen-water interface. Basic pH increased de-wetting of some bitumen blends from silica, possibly by attacking the silica surface itself. The presence of salt in solution altered the bitumen surface composition through the formation of salt complexes with bitumen or bio-oil compounds. In particular, the formation of calcium-carboxylate complexes appeared to greatly improve anti-stripping effects of bio-oils. Water diffusion into most of the bitumen blends was insensitive to pH or salt concentration except for a few notable outliers. Identifying the compounds responsible for increasing or decreasing water diffusion in these outlier cases would be valuable developing future bio-oil formations that avoid or promote those compounds.

Hung, Albert M.↗
DOE OSTI2016

Determination of krypton diffusion coefficients in uranium dioxide using atomic scale calculations

We present a study of the diffusion of krypton in UO 2 using atomic scale calculations combined with diffusion models adapted to the system studied. The migration barriers of the elementary mechanisms for interstitial or vacancy assisted migration are calculated in the DFT + U framework using the nudged elastic band method. The attempt frequencies are obtained from the phonon modes of the defect at the initial and saddle points using empirical potential methods. The diffusion coefficients of Kr in UO 2 are then calculated by combining this data with diffusion models accounting for the concentration of vacancies and the interaction of vacancies with Kr atoms. We determined the preferred mechanism for Kr migration and the corresponding diffusion coefficient as a function of the oxygen chemical potential μ O or nonstoichiometry. For very hypostoichiometric (or U-rich) conditions, the most favorable mechanism is interstitial migration. For hypostoichiometric UO 2 , migration is assisted by the bound Schottky defect and the charged uranium vacancy, V U 4– . Around stoichiometry, migration assisted by the charged uranium–oxygen divacancy (V UO 2– ) and V U 4– is the favored mechanism. Finally, for hyperstoichiometric or O-rich conditions, the migration assisted by two V U 4– dominates. Kr migration is enhanced at higher μ O , and in this regime, the activation energy will be between 4.09 and 0.73 eV depending on nonstoichiometry. The experimental values available are in the latter interval. Since it is very probable that these values were obtained for at least slightly hyperstoichiometric samples, our activation energies are consistent with the experimental data, even if further experiments with precisely controlled stoichiometry are needed to confirm these results. Finally, the mechanisms and trends with nonstoichiometry established for Kr are similar to those found in previous studies of Xe.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Influence of Hydrogen Bonding on the Surface Diffusion of Molecular Glasses: Comparison of Three Triazines

Surface grating decay measurements have been performed on three closely related molecular glasses to study the effect of intermolecular hydrogen bonds on surface diffusion. The three molecules are derivatives of bis(3,5-dimethyl-phenylamino)-1,3,5-triazine and differ only in the functional group R at the 2-position, with R being C 2 H 5 , OCH 3 , and NHCH 3 , and referred to as “Et”, “OMe”, and “NHMe”, respectively. Of the three molecules, NHMe forms more extensive intermolecular hydrogen bonds than Et and OMe and was found to have slower surface diffusion. For Et and OMe, surface diffusion is so fast that it replaces viscous flow as the mechanism of surface grating decay as temperature is lowered. In contrast, no such transition was observed for NHMe under the same conditions, indicating significantly slower surface diffusion. This result is consistent with the previous finding that extensive intermolecular hydrogen bonds slow down surface diffusion in molecular glasses and is attributed to the persistence of hydrogen bonds even in the surface environment. Here, this result is also consistent with the lower stability of the vapor-deposited glass of NHMe relative to those of Et and OMe and supports the view that surface mobility controls the stability of vapor-deposited glasses.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Swelling and Diffusion during Methanol Sorption into Hydrated Nafion

Diffusion within polymer electrolyte membranes is often coincident with time-dependent processes such as swelling and polymer relaxation, which are factors that limit their ability to block molecular crossover during use. The solution-diffusion model of membrane permeation, which is the accepted theory for dense polymers, applies only to steady-state processes and does not address dynamic internal structural changes that can accompany permeation. To begin discovery of how such changes can be coupled to the permeation process, we have constructed here a stochastic multiscale reaction-diffusion model that examines time-dependent methanol uptake into and swelling of hydrated Nafion. Several potential mechanisms of diffusion and polymer response are tested. The simulation predictions are compared to real-time Fourier transform infrared attenuated total reflectance spectroscopy (FTIR-ATR) absorbance reported in the literature [ Hallinan , D. T. , Jr. ; Elabd , Y. A. J. Phys. Chem. B 2007 , 111 , 13221 - 13230 ]. Of the proposed polymer response mechanisms, only one, a reaction-limited, local response to increasing methanol concentration that takes the entire experimental time frame of 600 s, produces simulated FTIR-ATR data consistent with experiment. The simulations show that water diffusion out of the membrane is minimal during methanol sorption and that changes in the measured infrared absorbances are due primarily to the increase in methanol concentration accompanied by dilution of water during swelling. Swelling involves densification of the polymer structure even as there is an overall volume expansion of the film. Potential connections between the polymer densification and molecular-level structural changes of Nafion in methanol are discussed. These results indicate that the interaction between methanol and Nafion serves to increase Nafion's capacity to accommodate large volumes of methanol-water solutions, facilitating increased permeation across the membrane relative to pure water.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Effects of Nonequilibrium Atomic Structure on Ionic Diffusivity in LLZO: A Classical and Machine Learning Molecular Dynamics Study

To improve the performance of electrochemical devices, it is essential to understand the effects of nonequilibrium motifs in solids, such as grain boundaries, amorphous phases, and highly strained regions, on atomic-scale transport and stability. Molecular dynamics simulations are used to explore the combined effect of far-from-equilibrium atomic structures and the choice of interatomic potential on ionic diffusivity predictions for Li 7 La 3 Zr 2 O 12 (LLZO), a promising solid electrolyte for all-solid-state batteries. Amorphization and high strain are considered using both classical Buckingham interatomic potentials and machine learning force fields. Here we find that both crystalline expansion and amorphization tend to slow diffusion, although the different physical encodings in the two potentials impact the properties in different ways. We trace these variations to a combination of structural and transport factors, the contributions of which are deconvoluted computationally. Graph-based analysis reveals that the variations for amorphous LLZO arise from the connectivity of diffusion pathways within the predicted structures, which generally correlates with diffusivity and is notably higher for structures generated by the machine learning force fields. Our study provides additional insight into the relationship between atomic structure and diffusivity in LLZO, while also highlighting the need for care in choosing and validating potentials to simulate far from equilibrium structures.

25 ENERGY STORAGE↗
DOE OSTI2017

Vacancies and Vacancy-Mediated Self Diffusion in Cr 2 O 3 : A First-Principles Study

Charged and neutral vacancies and vacancy mediated self diffusion in alpha-Cr2O3 were investigated using first principles density functional theory (DFT) and periodic supercell formalism. The vacancy formation energies of charged defects were calculated using the electrostatic finite-size corrections to account for electrostatic interactions between supercells and the corrections for the bandgap underestimation in DFT. Calculations predict that neutral oxygen (O) vacancies are predominant in chromium (Cr)-rich conditions and Cr vacancies with -2 charge state are the dominant defects in O-rich conditions. The charge transition levels of both O and Cr vacancies are deep within the bandgap indicating the stability of these defects. Transport calculations indicate that vacancy mediated diffusion along the basal plane has lower energy barriers for both O and Cr ions. The most favorable vacancy mediated self diffusion processes correspond to the diffusion of Cr ion in 3+ charge state and O ion in 2- state, respectively. Our calculations reveal that Cr triple defects comprised of Cr in octahedral interstitial sites with two adjacent Cr vacancies along the c-axis have a lower formation energy compared to that of charged Cr vacancies. The formation of such triple defects facilitate Cr self diffusion along the c-axis.

Medasani, Bharat (ORCID:0000000220734162)↗
DOE OSTI2018

Revisiting Anisotropic Diffusion of Carbon Dioxide in the Metal–Organic Framework Zn 2 (dobpdc)

The diffusion of gases confined in nanoporous materials underpins membrane and adsorption-based gas separations, yet relatively few measurements of diffusion coefficients in the promising class of materials, metal–organic frameworks (MOFs), have been reported to date. Recently we reported self-diffusion coefficients for 13 CO 2 in the MOF Zn 2 (dobpdc) (dobpdc 4– = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) which has one-dimensional channels with a diameter of approximately 2 nm [Forse, A. C.; J. Am. Chem. Soc. 2018, 140, 1663-1673]. By analyzing the evolution of the residual 13 C chemical shift anisotropy line shape at different gradient strengths, we obtained self-diffusion coefficients both along (D ∥ ) and between (D ⊥ ) the one-dimensional MOF channels. The observation of nonzero D ⊥ was unexpected based on the single crystal X-ray diffraction structure and flexible lattice molecular dynamics simulations, and we proposed that structural defects may be responsible for self-diffusion between the MOF channels. In this paper we revisit this analysis and show that homogeneous line broadening must be taken into account to obtain accurate values for D ⊥ . In the presence of homogeneous line broadening, intensity at a particular NMR frequency represents signal from crystals with a range of orientations relative to the applied magnetic field and magnetic gradient field. To quantify these effects, we perform spectral simulations that take into account homogeneous broadening and allow improved D ⊥ values to be obtained. Finally, our new analysis best supports nonzero D ⊥ at all studied dosing pressures and shows that our previous analysis overestimated D ⊥ .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

First-principles investigation of native interstitial diffusion in Cr2O3

First principles density functional theory (DFT) investigation of native interstitials and the associated self-diffusion mechanisms in a-Cr2O3 reveals that interstitials are more mobile than vacancies of corresponding species. Cr interstitials occupy the unoccupied Cr sublattice sites that are octahedrally coordinated by 6 O atoms, and O interstitials form a dumbbell configuration orientated along the [221] direction (diagonal) of the corundum lattice. Calculations predict that neutral O interstitials are predominant in O-rich conditions and Cr interstitials in +2 and +1 charge states are the dominant interstitial defects in Cr-rich conditions. Similar to that of the vacancies, the charge transition levels of both O and Cr interstitials are located deep within the bandgap. Transport calculations reveal a rich variety of interstitial diffusion mechanisms that are species, charge, and orientation dependent. Cr interstitials diffuse preferably along the diagonal of corundum lattice in a two step process via an intermediate defect complex comprising a Cr interstitial and an adjacent Cr Frenkel defect in the neighboring Cr bilayer. This mechanism is similar to that of the vacancy mediated Cr diffusion along the c-axis with intermediate Cr vacancy and Cr Frenkel defect combination. In contrast, O interstitials diffuse via bond switching mechanism. O interstitials in -1 and -2 charge states have very high mobility compared to neutral O interstitials.

Medasani, Bharat K.↗
DOE OSTI2018

Diffusion Mechanisms of Radiolytic Species in Irradiated Al (Oxy-)Hydroxides

Rare-event simulations (climbing image nudge elastic band and metadynamics) were performed using density functional theory (DFT) and a hybrid exchange-correlation functional to elucidate and quantify the diffusion mechanisms of radiation-induced species (O - , H 0 , and H 2 ) in boehmite (γ-AlOOH) and gibbsite (γ-Al(OH) 3 ). The two Al (oxy-)hydroxide phases are known to have much different radiolytic activities, but the underlying mechanisms remain unknown. The DFT calculations revealed that O - diffusion occurred via proton-coupled hole transfer with high energy barriers in both phases. In contrast, energy barriers for H 0 transfers were generally lower in boehmite than in gibbsite, suggesting a more facile diffusion in boehmite of H radicals to the surface, where H 2 formation can take place. Another key difference was the ability of H 0 and H 2 to diffuse across the structural layers in gibbsite but not in boehmite. Therefore, while the formation of O and H radicals was energetically favored in gibbsite compared to boehmite, the DFT calculations indicated that the mechanisms of diffusion are responsible for the higher H 2 yields measured for boehmite compared to gibbsite

Shen, Zhizhang↗
DOE OSTI2019

Sorption and Transport of Vapors in ZIF-11: Adsorption, Diffusion, and Linker Flexibility

The adsorption and diffusion of alcohol and hydrocarbon vapors in zeolitic imidazolate frameworks (ZIF)-11 are reported as a function of loading at 308 K. The hydrophobicity of the benzimidazole linkers provides moderate ideal adsorbed solution theory selectivities for adsorptive alcohol/water separations until high relative humidity conditions. Sorbates well beyond the crystallographic pore aperture are admitted into the framework at decreasing diffusivities with increasing molecular size. The diffusion of methane is also observed as a function of temperature, revealing a nonconstant activation energy of diffusion, which is linked to the temperature-dependent flexibility of the ligands comprising the six- and eight-membered aperture rings, as revealed through solid-state NMR. Compared to ZIF-8, ZIF-11 surprisingly demonstrates higher diffusivities and lower activation energies despite the smaller crystallographic aperture size. This phenomenon is explained via molecular dynamics simulations that highlight the greater flexibility of the benzimidazole linker in ZIF-11 relative to the 2-methylimidazolate linker in ZIF-8 and specifies the importance of structural flexibility in identifying diffusionally-selective materials.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Diffusion of NiH on Ni(111) and of Ni with Adsorbed H on Ni(111) in the Context of Ni Coarsening in Solid Oxide Cells

In the Ni-based hydrogen electrode of a solid oxide cell (SOC), Ni coarsening is an important degradation mechanism and could be enhanced by NiH diffusing on the Ni particle surfaces. Here, in this work, the average lifetime and diffusion distance of NiH on Ni(111) are computed using density-functional theory and kinetic Monte Carlo methods. It is found that NiH is extremely short-lived and, thus, cannot promote coarsening in the SOC. Also, the diffusion of Ni on Ni(111) is shown to be at most slightly accelerated by H along NiH dissociation paths involving the movement of Ni with a nearby H. Based on this result, coarsening is not likely to be dramatically accelerated by the diffusion of Ni with H on Ni(111). However, support is provided for the experimental procedure of measuring the product of surface coverage and single-species diffusivity of Ni on Ni(111) in a hydrogen-rich atmosphere.

Ni coarsening↗
DOE OSTI2020

A Particle-Based Conditional Sampling Scheme for the Simulation of Transport in Fractured Rock With Diffusion Into Stagnant Water and Rock Matrix

In situ experiments and field-scale characterization studies have pointed out that, in fractured crystalline media, groundwater flow is highly channelized. This implies that, at the scale of a single fracture, only part of the fracture surface area is in contact with flowing water, while the rest of in-plane water is essentially stagnant and can be accessed by solutes via molecular diffusion. Despite their importance for contaminant retention, to date, there are no numerical or analytical approaches that could be used to assess the implication of stagnant water zones on solute transport in realistic large-scale Discrete Fracture Network-based models. In this study, we present an efficient and flexible algorithm for the simulation of transport in fractured rock with diffusion into stagnant water and rock matrix. The algorithm is a generalization of a previously developed numerical framework for time domain particle tracking in sparsely fractured rock. The key of the generalization is that total time in fracture (τ f ) is first evaluated using a Monte Carlo sampling and then a second sampling is performed conditioned on τ f . The algorithm has been successfully validated against existing independent solutions and the implication of diffusion into stagnant water and secondary diffusion into the matrix has been assessed for a realistic modeling scenario. The results show that, due to diffusion into stagnant water, contaminants are more strongly retarded. This increased retention is more significant for sorbing species, as a larger number of sorption sites is accessible. A high sensitivity to the flowing channel/stagnant water zone geometry has also been observed.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Super-diffusion of excited carriers in semiconductors

The ultrafast spatial and temporal dynamics of excited carriers are important to understanding the response of materials to laser pulses. Here we use scanning ultrafast electron microscopy to image the dynamics of electrons and holes in silicon after excitation with a short laser pulse. We find that the carriers exhibit a diffusive dynamics at times shorter than 200 ps, with a transient diffusivity up to 1,000 times higher than the room temperature value, D 0 ≈30 cm 2 s –1 . The diffusivity then decreases rapidly, reaching a value of D 0 roughly 500 ps after the excitation pulse. We attribute the transient super-diffusive behaviour to the rapid expansion of the excited carrier gas, which equilibrates with the environment in 100–150 ps. Numerical solution of the diffusion equation, as well as ab initio calculations, support our interpretation. Lastly, our findings provide new insight into the ultrafast spatial dynamics of excited carriers in materials.

36 MATERIALS SCIENCE↗