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.

Szpunar, Barbara
Malakkal, Linu
Rahman, Jahidur
Szpunar, Jerzy A.

SiC is an important multifunctional material with application in electronics and as a structural material. Many investigations of SiC have been done using both classical molecular dynamics and first principles methods. However, they are of limited scope and, in particular, SiC properties at finite temperatures have not been adequately evaluated. The good mechanical, thermal, and chemical properties of SiC such as high stiffness, high hardness, high mechanical strength at high temperature, and high thermal conductivity, make SiC a candidate for various applications in nuclear industries. In this work, we evaluated thermomechanical properties at finite temperatures obtained by LAMMPS code with traditionally used Tersoff potential (TR89 with PRB 41 correction), and the newer GW 2002 (GW02) potential. We compared them with the calculations made using MEAM 1995 (MEAM 95) and with our first principles DFT predictions. It is demonstrated that the thermal expansion and mechanical properties calculated as a function of temperature for classical potentials TR89 and GW02 do not agree well with first principles calculations while better agreement is found for the MEAM95 potential. Classical molecular dynamics calculations made with the use of two earlier potentials under-predict thermal conductivity by one order of magnitude for the TR89 potential and by more than 30% for the GW02.

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.

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.

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.

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.

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.

This study investigates the optical properties of selected metal oxides due to their high dielectric constants. The local-spin-density approximation plus Hubbard U (commonly called LDA+U) is used in a study of the structural, mechanical and optical properties of UO2. The inclusion of a Hubbard U correction to 5f electrons of uranium changes UO2 from a metal to an insulator and, therefore, has a dramatic effect on the localisation of the electron spin and charge density of uranium. However although the band gap can be reproduced using the effective U parameter, which is equal to 3.5 eV and optical properties were calculated in our previous work, it is difficult to calculate ionic contribution to the static dielectric constant within LDA+U formalism for this compound. It is shown in the present work that the electronic structures of both ceria and thoria exhibit similarities to urania within LDA or PBE functional implementations. Within this functional and linear response theory one can easily calculate static dielectric permittivity and it is shown that in agreement with experiment the predicted values are an order of magnitude larger than the dielectric constant of SiO2. In this work, high accuracy, first-principles calculations are also used to compare properties of urania versus ceria and thoria and how these similarities can help in understanding these compounds. It is also shown that the B3LYP functional predicts slightly overestimated band gaps for ceria and thoria as well as smaller than experimentally observed electronic contribution to the static dielectric constant, while the index of refraction is well reproduced for thoria. (C) 2013 Elsevier Ltd. All rights reserved.

Choudhary, Aditya
Malakkal, Linu
Siripurapu, Ravi Kiran
Szpunar, Barbara
Szpunar, Jerzy

Transition metals (TM) such as palladium (Pd) and copper (Cu) atom were introduced on defect-free graphene and graphene with a single vacancy (SV) defect, to investigate their potential hydrogen storage using first principles calculation. All calculations were performed using plane wave based density functional theory (DFT) as implemented in Quantum ESPRESSO (QE). Generalized gradient approximation (GGA) exchange-correlation function as described by Perdew-Burke-Ernzerhof (PBE) was used. The interaction between the valence electron and the ionic core was represented by ultrasoft pseudopotentials. In this study, the structure and geometry of complex hydrides were investigated, and the effect of a SV defect in graphene on hydrogen storage was explored. It was concluded that the introduction of a SV defect prevents the TM atom clustering and enables hydrogen molecule adsorption within the ideal usable range. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The local-spin-density approximation plus Hubbard U (commonly called LDA+U) is used in a comprehensive study of the structural, mechanical and optical properties of urania. It is shown that using the new LDA+U implementation in CASTEP calculations, the symmetry constraints, imposed in agreement with both the symmetry of the unit cell and the present magnetic interaction, can be used to predict the ground state with the occupancy of the orbitals (5f in urania) in agreement with Hund's rules. The inclusion of a Hubbard U correction to 5f electrons of uranium changes urania from a metal to an insulator and, therefore, has a dramatic effect on the localisation of the electron spin and charge density on uranium. The value of the adjustable parameter U, which predicts the optical band gap in agreement with the experiment, leads to a slightly larger lattice constant than experimentally observed. The real and imaginary dielectric constants, reflectivity and energy loss functions are calculated using independent particle excitation approximations. The 5f intraband transitions are described adequately by the LDA and the effective U parameter, which is equal to 3.5 eV. However, additional corrections are needed to account for the 1-eV shift in the 2p-6d transitions. (C) 2012 Elsevier Ltd. All rights reserved.

Smith, Reginald W.
Scott, Paul J.
Szpunar, Barbara

We have been engaged in examining the influence of gravity on the results of experiments to measure the variation of solute diffusion coefficients (D) with temperature (T) in fused metals and semimetals since our first STS flights in 1992. These early experiments, conducted with the in situ g-fitter of the shuttle, showed the near-parabolic variation of D with T reported by others. However, with the aid of the Canadian Space Agency's microgravity isolation mount (MIM) to isolate the diffusion facility from the existing g fitter of the Russian space station MIR, we showed that in all the alloy systems and over the temperature range studied, D increased linearly with T. If the isolating system was deactivated, then the more familiar parabolic relationship appeared. We have always assumed that the values of D measured using the MIM would be closer to the intrinsic values for the alloy system considered; to test this contention, we have been involved in two modeling activities. The first has been to estimate the effects of g fitter-level disturbances on solute distributions in long capillary diffusion couples. The second has been to conduct various molecular dynamics modeling studies of solute diffusion. This paper presents results of these studies.

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.

Szpunar, Barbara
Lewis, Laurent J.
Swainson, Ian
Erb, Uwe

Nanocrystalline materials have a proportion of atoms at grain boundaries that can be as high as 50% and are thus expected to have properties quite different from those of bulk or large-grained polycrystalline materials. In this article, we study the influence of the presence of grain boundaries on thermal expansion, mean-square amplitudes of vibration (MSAV's), and hydrogen diffusion using classical molecular-dynamics simulations with embedded-atom-method potentials, for the particular case of the special [100] Sigma = 5, 13, and 17 twist grain boundaries. We find that in the presence of grain boundaries, thermal expansion increases only slightly while both hydrogen diffusion and MSAV's are enhanced significantly, in accord with neutron-diffraction measurements. The presence of pores, or voids, likewise, seem to have very little effect on thermal expansion. The diffusion of hydrogen is found to proceed mainly in the plane of the grain boundaries, where the electron density is lowest. [S0163-1829(99)05438-7].