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Now showing items 1 - 15 of 15

  • Thermophysical properties of (UxAm1-x)O-2 MOX fuel

    Jossou, Ericmoore   Malakkal, Linu   Ranasingh, Jayangani   Szpunar, Barbara   Szpunar, Jerzy  

    The effect of the addition of americium on the thermophysical properties of uranium dioxide (UO2) has been systematically studied by molecular dynamics (MD) simulation technique in the whole concentration range of americium and the temperature range from 300 K to 3200 K. The predicted thermophysical properties for (UxAm1-x)O-2 solid solutions agree well with the available experimental data. The lattice parameters decreased with the increase in americium concentration and obey Vegard's law up to 2000 K. There is no significant change in the enthalpy, heat capacity, lattice expansion, and thermal conductivity as we increased the concentration of americium. Overall, a series of empirical models are derived for the thermophysical properties of (UxAm1-x)O-2 MOX fuel based on the MD data.
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  • Anisotropic thermophysical properties of U3Si2 fuel:An atomic scale study

    Jossou, Ericmoore   Rahman, Md Jahidur   Oladimeji, Dotun   Beeler, Benjamin   Szpunar, Barbara   Szpunar, Jerzy  

    Due to renewed interest in uranium silicide compounds as a candidate for nuclear reactor fuels, there is a need for extensive investigations of their thermophysical properties as a function of temperature. In this work, we calculate the thermophysical properties of the U3Si2 compound within the framework of molecular dynamics (MD) using a semi-empirical modified Embedded-Atom Method (MEAM) potential and density functional theory (DFT). Thermal expansion, thermal conductivity, heat capacity, and elastic properties are presented as a function of temperature from 300 to 1800 K. The thermal conductivity of U3Si2 increases with temperature due to the electronic contribution while the phonon contribution decreases with increasing temperature. The phonon contribution to the thermal conductivity at 300 K is estimated at 2.03 W/mK and 1.41 W/mK using non-equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD), respectively. The electronic contribution is estimated to be 8.56 W/mK using the semi-classical Boltzmann transport theory at 300 K. Furthermore, we compared the thermal conductivity in two different crystallographic directions to shed light on the spatial anisotropy using NEMD and EMD methods. The inherent anisotropic thermophysical properties can be used to parametrize phase field models to incorporate anisotropic thermal conductivity and thermal expansion, allowing for a more accurate description of microstructural evolution under variable temperature and irradiation conditions. (c) 2019 Elsevier B.V. All rights reserved.
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  • Accident tolerant composite nuclear fuels

    Szpunar, Barbara   Malakkal, Linu   Chung, Seanne   Butt, Momina Mateen   Jossou, Ericmoore   Szpunar, Jerzy A.  

    Investigated accident tolerant nuclear fuels are fuels with enhanced thermal conductivity, which can withstand the loss of coolant for a longer time by allowing faster dissipation of heat, thus lowering the centerline temperature and preventing the melting of the fuel. Traditional nuclear fuels have a very low thermal conductivity and can be significantly enhanced if transformed into a composite with a very high thermal conductivity components. In this study, we analyze the thermal properties of various composites of mixed oxides and thoria fuels to improve thermal conductivity for the next generation safer nuclear reactors.
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  • Accident tolerant composite nuclear fuels

    Szpunar, Barbara   Malakkal, Linu   Chung, Seanne   Mateen Butt, Momina   Jossou, Ericmoore   Szpunar, Jerzy A.   Kida, Katsuyuki  

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  • Comprehensive study on the electronic and optical properties of alpha-U3O8

    Ranasinghe, Jayangani, I   Malakkal, Linu   Jossou, Ericmoore   Szpunar, Barbara   Szpunar, Jerzy A.  

    Triuranium octoxide (U3O8), the most stable form of uranium oxide, is an important material that finds application in nuclear and semiconductor industry. The understanding of electronic, magnetic and optical properties of U3O8 is essential for these advanced applications. Therefore, in this work, we have performed the density functional theory (DFT) study to investigate the structural, electronic, magnetic and optical properties of the low-temperature orthorhombic phase of U3O8 (alpha-U3O8) within the generalized gradient approximation (GGA) by using WIEN2k software package. To capture the highly correlated nature of 5f electrons in uranium atoms, an on-site Coulomb repulsion term (Hubbard-U) of 4.5 eV, was considered. Further, the effect of spin orbital coupling (SOC) on the electronic structure and band gap of alpha-U3O8 is demonstrated. Work functions (phi) were evaluated for the planes [0 0 1], [1 0 0], [0 1 0] and [1 1 1] using the first principles code QUANTUM ESPRESSO (QE). This study verifies the importance of SOC on structural, electronic and optical properties of alpha-U3O8 and claim for indirect theoretical band gap-E-g of 2.03 eV verifying the semiconductor behaviour. The optical anisotropy is analyzed through the frequency-dependent optical properties, namely, the real and imaginary parts of the dielectric tensor (epsilon(1) (omega) and epsilon(2) (omega)), absorption coefficient (alpha (omega)), optical conductivity (sigma (omega)), refractive index (n(omega)) and Loss-function (L(omega)). By comparing the Fermi energy and the vacuum level energy, the work functions for the planes (1 0 0), (0 0 1), (0 1 0) and (1 1 1) are predicted as 6.31, 6.73, 7.01 and 7.03 eV respectively. Furthermore, in this article, we present our experimental investigation of E-g based on diffuse reflectance spectra (DRS) method. Three powder samples, namely 1 wt%, 2 wt% and 4 wt% of U3O8 diluted with KCl show that U3O8 exhibits semiconducting behaviour with indirect band gaps of 1.86, 1.81 and 1.72 eV respectively.
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  • Thermal conductivity of bulk and porous ThO2:Atomistic and experimental study

    Malakkal, Linu   Prasad, Anil   Jossou, Ericmoore   Ranasinghe, Jayangani   Szpunar, Barbara   Bichler, Lukas   Szpunar, Jerzy  

    Thorium dioxide (ThO2) is proposed to play a vital role in the world's future energy needs and is considered a better and safer alternative to the currently used nuclear fuel, uranium dioxide (UO2). Thermo-physical properties of ThO2 are superior to UO2, but the fundamental physics governing the heat transport in ThO2 is still ambiguous, and the available data for the thermal conductivity (k) of ThO2 was scattered. In this article, a systematic investigation regarding the lattice thermal conductivity (k(L)) of the bulk and porous ThO2 is carried out theoretically and validated with experiments. The phonon transport calculations were done using two different methods; ab-initio calculations combined with the Boltzmann transport equation (BTE) and the equilibrium molecular dynamics (EMD) simulations using Green Kubo (GK) approach. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space modes, Grtineisen parameter, and mean free path (MFP) distributions were analyzed to understand the underlying physics in the thermal transport of ThO2. The effect of porosity on the k(L) by measurements and molecular dynamics (MD) simulations was explored. The measurements were performed on specimens with different porosity, that were prepared by spark plasma sintering (SPS) using the laser flash (LFA) technique. The results obtained demonstrated that the k(L) values predicted by both the BTE and the EMD simulations were in excellent agreement with our experimental measurements. Moreover, the model to simulate the 95% theoretical density (TD) using MD simulations also captured the decrease in thermal conductivity with porosity and agreed well with the measured results for 95% TD dense sintered pellets. (C) 2019 Elsevier B.V. All rights reserved.
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  • Oxidation behaviour of U3Si2:an experimental and first principles investigation

    Jossou, Ericmoore   Eduok, Ubong   Dzade, Nelson Y.   Szpunar, Barbara   Szpunar, Jerzy A.  

    Uranium-containing metallic systems such as U3Si2 are potential Accident Tolerant Fuels (ATFs) for Light Water Reactors (LWRs) and the next generation of nuclear reactors. Their oxidation behaviour, especially in oxygen and water-enriched environments, plays a critical role in determining their applicability in commercial reactors. In this work, we have investigated the oxidation behaviour of U3Si2 experimentally and by theoretical computation. The appearance of oxide signatures has been established from X-ray diffraction (XRD) and Raman spectroscopic techniques after oxidation of the solid U3Si2 sample in synthetic air (oxygen and nitrogen). We have also studied the changes in the electronic structure as well as the energetics of oxygen interactions on the U3Si2 surfaces using first principles calculations in the Density Functional Theory (DFT) formalism. The detailed charge transfer and bond length analyses revealed the preferential formation of mixed oxides of UO2 and SiO2 on the U3Si2{001} surface as well as UO2 alone on the U3Si2{110} and {111} surfaces. The formation of the peroxo (O-2(2-)) state confirmed the dissociation of molecular oxygen before U3Si2 oxidation. Core experimental analyses of the oxidized U3Si2 samples have revealed the formation of higher oxides from Raman spectroscopy and XRD techniques. This work is introduced to further a better understanding of the oxidation of U-Si metallic fuel compounds.
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  • First-principles study of defects and fission product behavior in uranium diboride

    Jossou, Ericmoore   Oladimeji, Dotun   Malakkal, Linu   Middleburgh, Simon   Szpunar, Barbara   Szpunar, Jerzy  

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  • Atomistic and experimental study on thermal conductivity of bulk and porous cerium dioxide

    Malakkal, Linu   Prasad, Anil   Oladimeji, Dotun   Jossou, Ericmoore   Ranasinghe, Jayangani   Szpunar, Barbara   Bichler, Lukas   Szpunar, Jerzy  

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  • Atomistic and experimental study on thermal conductivity of bulk and porous cerium dioxide

    Malakkal, Linu   Prasad, Anil   Oladimeji, Dotun   Jossou, Ericmoore   Ranasinghe, Jayangani   Szpunar, Barbara   Bichler, Lukas   Szpunar, Jerzy  

    Cerium dioxide (CeO2) is a surrogate material for traditional nuclear fuels and an essential material for a wide variety of industrial applications both in its bulk and nanometer length scale. Despite this fact, the underlying physics of thermal conductivity (k(L)), a crucial design parameter in industrial applications, has not received enough attention. In this article, a systematic investigation of the phonon transport properties was performed using ab initio calculations unified with the Boltzmann transport equation. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space, mode Gruneisen parameter and mean free path (MFP) distributions were also conducted. To further augment theoretical predictions of the k(L), measurements were made on specimens prepared by spark plasma sintering using the laser flash technique. Since the sample porosity plays a vital role in the value of measured k(L), the effect of porosity on k(L) by molecular dynamics (MD) simulations were investigated. Finally, we also determined the nanostructuring effect on the thermal properties of CeO2. Since CeO2 films find application in various industries, the dependence of thickness on the in-plane and cross-plane k(L) for an infinite CeO2 thin film was also reported.
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  • A first principles study of the electronic structure, elastic and thermal properties of UB 2

    Jossou, Ericmoore   Malakkal, Linu   Szpunar, Barbara   Oladimeji, Dotun   Szpunar, Jerzy A.  

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  • Stable BaCl solid at high pressure:Prediction and characterization using first principles approach

    Adeleke, Adebayo A.   Jossou, Ericmoore   Yao, Yansun  

    Industrial processes involving the manufacture of heat treatment salts such as BaCl at high pressures are becoming possible. Hence, there is a need to search for a specific form of BaCl with excellent thermal properties. Motivated by this, the potential energy surface of BaCl is extensively explored using the unbiased particle swarm-intelligence optimization algorithm to uncover a global minimum enthalpy phase of BaCl within the pressure range that was recently experimentally explored. Previously predicted phases were confirmed during the structure search. Furthermore, the orthorhombic Pnma form of BaCl is predicted to be more stable and energetically more favorable than the previously predicted R-3m phase in the pressure range of similar to 10-15 GPa. The electronic and thermal properties of the newly discovered phase are extensively studied using first principles calculations. In the pressure range of interest, Pnma BaCl is metallic and nonmagnetic. More so, the solution of the Boltzmann Transport Equation unravels promising thermal properties, which make Pnma BaCl a good candidate for heat management in high temperature systems. We found the overall Gruneisen parameters in Pnma BaCl to range between 0.963 and 0.995 and the lattice thermal conductivity at 300K to be 53.7W m(-1)K(-1). We also found that Pnma BaCl exhibits anisotropy that we observed is constant in all directions explored. Published by AIP Publishing.
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  • Study on Radial Temperature Distribution of Aluminum Dispersed Nuclear Fuels:U3O8-Al,U3Si2-Al,and UN-Al

    Ranasinghe, Jayangani I.   Jossou, Ericmoore   Malakkal, Linu   Szpunar, Barbara   Szpunar, Jerzy A.  

    The understanding of the radial distribution of temperature in a fuel pellet, under normal operation and accident conditions, is important for a safe operation of a nuclear reactor. Therefore, in this study, we have solved the steady-state heat conduction equation, to analyze the temperature profiles of a 12 mm diameter cylindrical dispersed nuclear fuels of U3O8-Al, U3Si2-Al, and UN-Al operating at 597 degrees C. Moreover, we have also derived the thermal conductivity correlations as a function of temperature for U3Si2, uranium mononitride (UN), and Al. To evaluate the thermal conductivity correlations of U3Si2, UN, and Al, we have used density functional theory (DFT) as incorporated in the Quantum ESPRESSO (QE) along with other codes such as Phonopy, ShengBTE, EPW (electron-phonon coupling adopting Wannier functions), and BoltzTraP (Boltzmann transport properties). However, for U3O8, we utilized the thermal conductivity correlation proposed by Pillai et al. Furthermore, the effective thermal conductivity of dispersed fuels with 5, 10, 15, 30, and 50 vol%, respectively of dispersed fuel particle densities over the temperature range of 27-627 degrees C was evaluated by Bruggman model. Additionally, the temperature profiles and temperature gradient profiles of the dispersed fuels were evaluated by solving the steady-state heat conduction equation by using Maple code. This study not only predicts a reduction in the centerline temperature and temperature gradient in dispersed fuels but also reveals the maximum concentration of fissile material (U3O8, U3Si2, and UN) that can be incorporated in the Al matrix without the centerline melting. Furthermore, these predictions enable the experimental scientists in selecting an appropriate dispersion fuel with a lower risk of fuel melting and fuel cracking.
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  • DFT plus U Study of the Adsorption and Dissociation of Water on Clean,Defective,and Oxygen-Covered U3Si2{001},{110},and {111} Surfaces

    Jossou, Ericmoore   Malakkal, Linu   Dzade, Nelson Y.   Claisse, Antoine   Szpunar, Barbara   Szpunar, Jerzy  

    The interfacial interaction of U3Si2 with water leads to corrosion of nuclear fuels, which affects various processes in the nuclear fuel cycle. However, the mechanism and molecular-level insights into the early oxidation process of U3Si2 surfaces in the presence of water and oxygen are not fully understood. In this work, we present Hubbard-corrected density functional theory (DFT + U) calculations of the adsorption behavior of water on the low Miller indices of the pristine and defective surfaces as well as water dissociation and accompanied H-2 formation mechanisms. The adsorption strength decreases in the order U3Si2{001} > U3Si2{110} > U3Si2{111} for both molecular and dissociative H2O adsorption. Consistent with the superior reactivity, dissociative water adsorption is most stable. We also explored the adsorption of H2O on the oxygen-covered U3Si2 surface and showed that the preadsorbed oxygen could activate the OH bond and speed up the dissociation of H2O. Generally, we found that during adsorption on the oxygen-covered, defective surface, multiple water molecules are thermodynamically more stable on the surface than the water monomer on the pristine surface. Mixed molecular and dissociative water adsorption modes are also noted to be stable on the {111} surface, whereas fully dissociative water adsorption is most stable on the {110} and {001} surfaces.
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  • Ceria/Acrylic Polymer Microgel Composite:Synthesis,Characterization,and Anticorrosion Application for API 5L X70 Substrate in Chloride-Enriched Medium

    Eduok, Ubong   Jossou, Ericmoore   Tiamiyu, Ahmed   Omale, Joseph   Szpunar, Jerzy  

    Ceria/poly(acrylic acid) microgel composite with anticorrosion potential has been synthesized via an in situ polymerization method. This polymer composite matrix has demonstrated significant reduction in API 5L X70 steel corrosion in 0.5 M HC1. In-depth studies of the anticorrosion properties of this microgel have been conducted by corrosion electrochemistry, and its adsorption on steel altered both anodic dissolution and cathodic hydrogen evolution in the acid medium. Increment of CeO2 content within the PAA/CeO2 hybrid composite improved its surface protective performance; 1 and 5 g of CeO2 within the composite recorded 82 and 90% inhibition efficiency, respectively, compared to PAA alone (62%), at equal concentration. PAA formed a protective polymeric film on steel upon molecular adsorption, but in the presence of the PAA/CeO2, the metal surface protection was enhanced by the adhesion of compact hybrid films. PAA/CeO2 microgel composite may have a future as an anticorrosive paint component for metal surface treatments.
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