Diffusion Of Uranium Research Articles

Fuel-Cladding Chemical Interaction of a Prototype Annular U-10Zr Fuel with Fe-12Cr Ferritic/Martensitic Cladding

As an alternative fuel form, annular metallic fuel design eliminated the liquid sodium bond between the fuel and the cladding, providing back-end fuel cycle and other benefits. The fuel-cladding chemical interaction (FCCI) of annular fuel also presents new features. Here, state-of-the-art electron microscopy and spectroscopy techniques were used to study the FCCI of a prototype annular U-10wt%Zr (U-10Zr) fuel with ferritic/martensitic HT-9 cladding irradiated to 3.3% fission per initial heavy atom (FIMA). Compared to sodium-bonded solid fuels, negligible amounts of lanthanides were found in the FCCI layer in the investigated helium bonded annular fuel. Instead, most of the lanthanides were retained in the newly formed UZr2 phase at the fuel center region. The interdiffusion of iron and uranium resulted in tetragonal (U,Zr)6Fe phase (space group I4/mcm) and cubic (U,Mo)(Fe,Cr)2 phase (space group Fd3 m). The (U,Mo)(Fe,Cr)2 phase contains a high density of voids and intergranular uranium monocarbides of NaCl-type crystal structure (space group Fm3 m). At the interface of the interdiffusion zone and inner cladding, a porous lamellar structure composed of alternating Cr-rich layers and U-rich layers was observed. Next to the lamellar region, the unexpected phase transformation from body centered cubic (BCC) ferrite (α-Fe) to tetragonal binary Fe-Cr σ phase (space group P42/mnm) occurred and tetragonal Fe-Cr-U-Si phase (space group I4/mmm) was identified. Due to the diffusion of carbon into the interdiffusion zone, carbon depletion inside the HT-9 led to the disappearance of the martensite lath structure, and intergranular U-rich carbides formed as a result of the diffusion of uranium into the cladding. These detailed new findings reveal the unique features of the FCCI behavior of annular U-Zr fuels, which could be a promising alternative fuel form for high burnup fast reactor applications.

Effect of Leifsonia sp. on retardation of uranium in natural soil and its potential mechanisms

Uranium mining and milling activities for many years resulted in release of uranium into the adjoining soil in varying degrees. Bioremediation approaches (i.e., immobilization via the action of bacteria) resulting in uranium bearing solid is supposed as an economic and clean in-situ approach for the treatment of uranium contaminated sites. This study purposes to determine the immobilization efficiency of uranium in soil by Leifsonia sp. The results demonstrated that cells have a good proliferation ability under the stress of uranium and play a role in retaining uranium in soil. Residual uranium in active Leifsonia-medium group (66%) was higher than that in the controls, which was 31% in the deionised water control, 46% in the Leifsonia group, and 47% in the medium group, respectively. This indicated that Leifsonia sp. facilitates the immobilization efficiency of uranium in soil by converting part of the reducible and oxidizable fraction of uranium into the residual fraction. X-ray photoelectron fitting results showed that tetravalent states uranium existed in the soil samples, which indicated that the hexavalent uranium was converted into tetravalent by cells. This is the first report of effect of Leifsonia sp. on uranium immobilization in soil. The findings implied that Leifsonia sp. could, to some extent, prevent the migration and diffusion of uranium in soil by changing the chemical states into less toxicity and less risky forms.

Kinetic Parameters and Diffusivity of Uranium in FLiNaK and ClLiK

Anodic stripping voltammetry (ASV) and cyclic voltammetry (CV) measurements at 773, 823, and 873 K were made of uranium trifluoride (UF3) in lithium fluoride-sodium fluoride-potassium fluoride eutectic (FLiNaK) and uranium trichloride (UCl3) in lithium chloride-potassium chloride eutectic (ClLiK). ASV data were used to estimate the charge transfer coefficients, exchange current densities, and activation energies of the uranium reactions. Charge transfer coefficients of both salt systems were within the range of 0.17 to 0.38. Exchange current densities in the fluoride and chloride salts were estimated within the range of 0.060 to 0.12 A cm−2. Activation energy of uranium exchange current was 16.0 kJ mol−1 in the fluoride salt and 28.9 kJ mol−1 in the chloride salt. Kinetics of charge transfer were found to be faster in FLiNaK. Analyses of the CV data suggest the electrochemical system was diffusion controlled and irreversible in the chloride and fluoride salt mixtures. Diffusion coefficients of uranium in the range of temperatures were on the order of 10−5 cm2 s−1 in both systems. Greater values of diffusivity in ClLiK are attributed to its lower density compared with FLiNaK. Activation energy of uranium diffusion in the fluoride and chloride salt mixtures were 53.4 and 89.4 kJ mol−1, respectively.

Adsorption of U(VI) on montmorillonite in the presence of ethylenediaminetetraacetic acid

The adsorption, diffusion and migration behaviors of uranium may be highly influenced by organics in natural environments. Ethylenediaminetetraacetic acid (EDTA) is one of the most important small organics in groundwaters, especially at the radioactive contaminated sites. In this work, we studied U(VI) adsorption on montmorillonite in the presence of EDTA with batch method. Our results indicated that U(VI) adsorption was significantly different in the presence and absence of EDTA. The equimolar EDTA inhibited the U(VI) adsorption at a wide pH range while only enhanced the adsorption at pH ˜7.0. The pseudo-second-order model showed the best fit for describing the adsorption kinetics in comparison with three other popular models. The influence of different addition sequences on adsorption of EDTA and U(VI) on montmorillonite was also investigated, and the results indicated that EDTA played the role as a bridge between montmorillonite surface sites and U(VI). The X-ray photoelectron spectroscopy and attenuated total reflectance Fournier-transform infrared spectroscopy also suggested the formation of EDTA-bridge inner-sphere surface complexes on montmorillonite. This work provides the valuable information of U(VI) adsorption on montmorillonite in the presence of EDTA, and helps us understand the diffusion and migration behaviors of uranium in natural environments.

Mussel-inspired anti-biofouling and robust hybrid nanocomposite hydrogel for uranium extraction from seawater

A major challenge of uranium extraction from seawater (UES) is to effectively block the biofouling without destroying the ecological balance, especially prevent the attachment of macroalgae on the surface of the adsorbent. Herein, a robust montmorillonite-polydopamine/polyacrylamide nanocomposite hydrogel is reported by a two-step method, including PDA intercalation MMT and further free radical polymerization with AM monomers. The interpenetrating structure of hydrogel lead to high water permeability with the swelling ratio of 51, which could fully facilitate the internal accessible sites exposure and increase the uranium diffusion. As a result, a high adsorption capacity of 44 mg g−1 was achieved in lab-scale dynamic adsorption. Most importantly, the prepared anti-biofouling hydrogel adsorbents display excellent anti-adhesion ability towards Nitzschia after 8 days contact. The adsorption capacity of uranium can reach 2130 μg g−1 in algae-contained simulated seawater. This hydrogel also exhibited a long service life of acceptable mechanical strength and adsorption capacity after at least 6 adsorption-desorption cycles. This new anti-biofouling nanocomposite hydrogel shows great potential as a new generation adsorbent for UES.

Uranium diffusion and time‐dependent adsorption–desorption in soil: A model and experimental testing of the model

AbstractMost past research on uranium (U) transport and reaction in the environment has been concerned with groundwater contamination and not with uptake by plants or soil biota, both of which operate over much smaller time and distance scales. We developed and tested a model of U diffusion and reaction in soil at scales appropriate for uptake by plant roots, based on a model we developed in an earlier paper. The model allows for the speciation of U with hydroxyl, carbonate and organic ligands in the soil solution, and the nature and kinetics of sorption reactions with the soil solid. The model predictions were compared with experimentally measured concentration–distance profiles of U in soil adjusted to different pHs and CO2 pressures. Excellent agreement between observed and predicted profiles was obtained using model input parameters measured or otherwise estimated independently of the concentration–distance profiles, showing that the model was a correct description of the system and all important processes were allowed for. The importance of the kinetics of U adsorption and desorption for the timescale of diffusion through the soil is highlighted. The results are discussed in terms of the uptake of U by plant root systems, as modelled in the earlier paper.Highlights We developed a model of U diffusion and reaction in soil on scales relevant to uptake by plant roots. We tested the model against measured diffusion profiles and obtained excellent agreement. The kinetics of U adsorption–desorption reactions are important. Reaction kinetics measured in shaken suspensions or flow‐through systems are likely to be misleading.

Carbon diffusion in molten uranium: an ab initio molecular dynamics study

In this work we used ab initio molecular dynamics within the framework of density functional theory and the projector-augmented wave method to study carbon diffusion in liquid uranium at temperatures above 1600 K. The electronic interactions of carbon and uranium were described using the local density approximation (LDA). The self-diffusion of uranium based on this approach is compared with literature computational and experimental results for liquid uranium. The temperature dependence of carbon and uranium diffusion in the melt was evaluated by fitting the resulting diffusion coefficients to an Arrhenius relationship. We found that the LDA calculated activation energy for carbon was nearly twice that of uranium: 0.55 ± 0.03 eV for carbon compared to 0.32 ± 0.04 eV for uranium. Structural analysis of the liquid uranium–carbon system is also discussed.

Lattice constant in nonstoichiometric uranium dioxide from first principles

Nonstoichiometric uranium dioxide experiences a shrinkage of its lattice constant with increasing oxygen content, in both the hypostoichiometric and the hyperstoichiometric regimes. Based on first-principles calculations within the density functional theory $\text{(DFT)}+U$ approximation, we have developed a point defect model that accounts for the volume of relaxation of the most significant intrinsic defects of ${\mathrm{UO}}_{2}$. Our point defect model takes special care of the treatment of the charged defects in the equilibration of the model and in the determination of reliable defect volumes of formation. In the hypostoichiometric regime, the oxygen vacancies are dominant and explain the lattice constant variation with their surprisingly positive volume of relaxation. In the hyperstoichiometric regime, the uranium vacancies are predicted to be the dominating defect,in contradiction with experimental observations. However, disregarding uranium vacancies allows us to recover a good match for the lattice-constant variation as a function of stoichiometry. This can be considered a clue that the uranium vacancies are indeed absent in ${\mathrm{UO}}_{2+x}$, possibly due to the very slow diffusion of uranium.

Uranium migration in spark plasma sintered W/UO2 CERMETS

W/UO2 CERMET samples were sintered in a Spark Plasma Sintering (SPS) furnace at various temperature under vacuum and pressure. High Resolution Transmission Electron Microscopy (HRTEM) with Energy Dispersive Spectroscopy (EDS) was performed on the samples to determine interface structures and uranium diffusion from the UO2 particles into the tungsten matrix. Local Electrode Atom Probe (LEAP) was also performed to determine stoichiometry of the UO2 particles. It was seen that uranium diffused approximately 10–15 nm into the tungsten matrix. This is explained in terms of production of oxygen vacancies and Fick's law of diffusion.

Edge effects on the characteristics of uranium diffusion on graphene and graphene nanoribbons**Project supported by the International S & T Cooperation Program of China (Grant No. 2014DFG60230), the National Natural Science Foundation of China (Grant Nos. 91326105, 21306220, and 21501189), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA02040104).

The first principles density-functional theoretical calculations of U adatom adsorption and diffusion on a planar graphene and quasi-one-dimensional graphene nanoribbons (GNRs) are performed. An energetic preference is found for U adatom diffusing to the hollow sites of both graphene and GNRs surface. A number of U distinctive diffusion paths either perpendicular or parallel to the ribbon growth direction are examined. The edge effects are evidenced by the calculated energy barriers of U adatom diffusion on armchair and zigzag nanoribbons surfaces. The calculation results indicate that the diffusion of U adatom from the inner site toward the edge site is a feasible process, particularly in zigzag GNR. It is viable to control the initial morphology of nuclear carbon material to retard the diffusion and concentration of nuclides.

Examination of the impact of electron–phonon coupling on fission enhanced diffusion in uranium dioxide using classical molecular dynamics

Abstract

Uranium and cesium diffusion in fuel cladding of electrogenerating channel

The results of reactor tests of a carbonitride fuel in a single-crystal cladding from a molybdenum-based alloy can be used in substantiating the operational reliability of fuels in developing a project of a megawatt space nuclear power plant. The results of experimental studies of uranium and cesium penetration into the single-crystal cladding of fuel elements with a carbonitride fuel are interpreted. Those fuel elements passed nuclear power tests in the Ya-82 pilot plant for 8300 h at a temperature of about 1500°C. It is shown that the diffusion coefficients for uranium diffusion into the cladding are virtually coincident with the diffusion coefficients measured earlier for uranium diffusion into polycrystalline molybdenum. It is found that the penetration of uranium into the cladding is likely to occur only in the case of a direct contact between the cladding and fuel. The experimentally observed nonmonotonic uranium-concentration profiles are explained in terms of predominant uranium diffusion along grain boundaries. It is shown that a substantially nonmonotonic behavior observed in our experiment for the uranium-concentration profile may be explained by the presence of a polycrystalline structure of the cladding in the surface region from its inner side. The diffusion coefficient is estimated for the grain-boundary diffusion of uranium. The diffusion coefficients for cesium are estimated on the basis of experimental data obtained in the present study.

Effect of fission Xe on diffusion of oxygen and uranium in UO2: a molecular dynamics study

We have investigated the effect of fission Xe on diffusion of oxygen and uranium in UO2. Survey of trapping and solution of Xe have been presented. Incorporation and solution energies of Xe for various solution sites have been calculated by Mott–Littleton approach. Results suggest that the most stable trapping site for Xe depends on stoichiometry regions. The effect on diffusion of oxygen and uranium introduced by Xe has been estimated by classical molecular dynamics simulations. It is found that Xe atoms in any trapping site raise the values of oxygen diffusion coefficients and depress activation energies. Besides, the effect introduced by Xe decreases with an increase in trapping site size.

Laser ablation U-series analysis of fossil bones and teeth

Over the past decade, we have applied laser ablation ICP mass spectrometry U-series analysis to a large number of bones and teeth. The method is fast and provides high-resolution data of U-series isotopes, which give insights into the complexity of uranium migration into, within and out of bones. In this paper, we present our laser ablation approach in detail, from the experimental set up, to data reduction and uranium diffusion modelling. Laser ablation analysis can now be applied with minimum damage. Complete diffusion data sets can be obtained by laser drilling which leaves a hole of around 200μm in diameter and 1–2mm depth. If the natural U-diffusion follows a simple single-stage process, valuable age information can be obtained. In other cases, highly complex U-migrations have been observed, which make any age assessment impossible. Two examples from Wadjak and Callao illustrate the potential of nearly non-destructive laser ablation U-series analysis of human skeletal tissues. The results demonstrate the physical presence of modern humans in south-east Asia at a time when our species just started to migrate into Europe.

Effects of compression on porous texture of clay powder: Application to uranium diffusion

This paper presents the results of tests conducted on the compaction properties of clay. Swelling clays of Algeria are being examined as components of the buffer material. Knowledge of the porous texture in clay samples is a useful element in the microstructural characterization of such materials. The prepared materials were characterized by X-ray diffraction, Differential Thermal and Thermo Gravimetric Analysis, Laser Granulometry, Scanning Electron Microscopy, mercury porosimetry and nitrogen adsorption/desorption to obtain information about their structure and surface texture. The cationic exchange of clay minerals, in particular smectites, influences the cation retention and diffusion processes. These processes influence the migration of contaminants from waste disposal sites through clay barrier. In the present work we studied the effect of cation exchange and compaction on the textural properties of clay. Pellets of the Na-saturated and bulk clay samples were prepared by compression of the powder samples using uniaxial stress at different pressures in order to obtain different densities. The pressures of 31–127MPa are applied for the production of cylindrical samples with diameter of 20mm and heights of 15–30mm. The effect of compaction of clay on the diffusion behavior of uranium was studied for the safety assessment of the radioactive waste. The diffusion process of uranium in compacted clay has been modeled by second Fick's law taking into account the effect of sorption and considering the non-steady state. The diffusion coefficients and profiles concentration values of uranium were calculated by computational method using numerical program based on Newton–Raphson algorithm.

Calixarene Cleansing Formulation for Uranium Skin Contamination

An oil-in-water cleansing emulsion containing calixarene molecule, an actinide specific chelating agent, was formulated in order to improve the decontamination of uranium from the skin. Commonly commercialized cosmetic ingredients such as surfactants, mineral oil, or viscosifying agents were used in preparing the calixarene emulsion. The formulation was characterized in terms of size and apparent viscosity measurements and then was tested for its ability to limit uranyl ion permeation through excoriated pig-ear skin explants in 24-h penetration studies. Calixarene emulsion effectiveness was compared with two other reference treatments consisting of DTPA and EHBP solutions. Application of calixarene emulsion induced the highest decontamination effect with an 87% decrease in uranium diffusion flux. By contrast, EHBP and DTPA solutions only allowed a 50% and 55% reduction of uranium permeation, respectively, and had the same effect as a simple dilution of the contamination by pure water. Uranium diffusion decrease was attributed to uranyl ion-specific chelation by calixarene within the formulation, since no significant effect was obtained after application of the same emulsion without calixarene. Thus, calixarene cleansing emulsion could be considered as a promising treatment in case of accidental contamination of the skin by highly diffusible uranium compounds.

High-temperature interaction of uranium mononitride with refractory metals. Interaction with rhenium

The interaction of uranium mononitride UN with a refractory metal (rhenium) is studied in the temperature range 1773–2273 K. The coefficients of uranium diffusion from UN into polycrystalline and single-crystal rhenium and their temperature dependences are determined. The diffusion mobility of uranium in rhenium is shown to be lower than that in molybdenum and to be higher than in tungsten.

A molecular dynamics study of the thermal properties of thorium oxide

There is growing interest in the exploitation of the thorium nuclear fuel cycle as an alternative to that of uranium. As part of a wider study of the suitability of thorium dioxide (thoria) as a nuclear fuel, we have used molecular dynamics to investigate the thermal expansion, oxygen diffusion, and heat capacity of pure thoria and uranium doped (1-10%) thoria between 1500 K and 3600 K. Our results indicate that the thermal performance of the thoria matrix, even when doped with 10%U, is comparable to, and possibly better than, that of UO2.

Diffusive parameters of tritiated water and uranium in chalk

The Cretaceous Chalk of North-western Europe exhibits a double porosity (matrix and fracture) providing pathways for both slow and rapid flow of water. The present study aims at understanding and predicting the contaminant transfer properties through a significant section of this formation, with a particular emphasis on diffusion. This requires to study the nature of porosity and to perform diffusion experiments in representative samples using uranium and tritiated water (HTO), respectively taken as a reactive tracer and an inert one. The diffusive parameters, i.e. the accessible porosity and the effective diffusion coefficient were determined. Additional information was obtained with mercury porosimetry, gravimetric water content, textural and mineralogical characterization. The diffusion tests performed with HTO appear to be the best method to measure the total accessible porosity in any type of porous media, especially those having large pore size distributions. Our study demonstrates that classical gravimetric water content measurements are not sensitive to the reduction in pore size as opposed to HTO diffusion tests because capillary water is not extracted by conventional gravimetric method but can still be probed by diffusion experiments. We found effective diffusion coefficients D{sub e}(U(VI)) near 4 x 10{sup -10} m{sup 2}s{sup -1}). The slower migration ofmore » U(VI) compared to HTO indicates sorption, with R{sub d}(U(VI)) from 100 to 360 mL g{sup -1}. These values are one order of magnitude larger than other determinations of the U(VI) sorption coefficient because only the matrix porosity is concerned here. The migration of U(VI) in chalk is only limited by sorption on ancillary Fe-Pb-bearing minerals. Transport of HTO and U(VI) is independent of the porosity distribution. Uranium diffusion in the chalk matrix porosity is fast enough to allow the total invasion of the pore space within characteristic time scales of the order of 1000 years. This results in a partitioning of uranium velocities in fracture flow and matrix flow proportionally to the respective fracture and matrix porosities. (authors)« less

Fuel components and fission products diffusion on the surface of communicating cavities of a thermionic electricity-generating channel

The diffusion of fuel components and fission products on the surface of communicating cavities of a thermionic electricity-generating channel of a nuclear power facility is studied analytically. The Arrhenius constants are determined for the electricity-generating channel. A model is developed for the diffusion of the atoms of the fuel components and fission products on the surface of fuel element cladding. The results are used to determine the negative effect of the diffusion of uranium, as the main component of the fuel, and the uranium fission products molybdenum, niobium, and zirconium on the emission-adsorption characteristics of the electrodes of the electricity-generating channel and the electrical properties of the nuclear power facility. Proposals are made for reducing the effect of the phenomenon analyzed.