Location preferences of fission products in high density U(Mo) dispersion fuel element

In the frame of the RERTR (Reduced Enrichment for Research and Test Reactors) program a fuel element is being developed with the concept of high density bcc uranium that can remain stable during fabrication and later irradiation, dispersed in aluminum powder. The whole constitutes a compact material which is later rolled with an aluminum-silicon clad plate. Under further irradiation, an interaction layer (IL) grows through a diffusion process around the fuel element particle, leading to the swelling of the fuel element and formation of pores. This behavior can lead to catastrophic failure of the disperse fuel. Therefore it is our great interest to gain knowledge about the influence the fission products (FP) have over the IL formation and swelling. The stable compounds that have been observed in the IL of U(Mo)/Al(Si) tested in diffusion pair experiments are U(Al, Si)3, USi2, U1+xSi2-x, U3Si5 UMo2Al20 and U6Mo4Al43. Among them, U(Al, Si)3 has been observed to remain stable when subject to irradiation, delaying or stopping the IL swelling. Compositional analysis shows that La, Ce, Pr and Nd are some of the FP present in the burned dispersed fuel. Hence, these are the considered elements for a first evaluation of the problem that we performed in this work by means of computational methods.Fil: Alonso, Paula Regina. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Forti, Mariano D.. Universidad Nacional de San Martín; ArgentinaFil: Kniznik, Laura. Universidad Nacional de San Martín; ArgentinaFil: Rubiolo, Gerardo Hector. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Torres, Dario Nelson. No especifíca;Fil: Gargano, Pablo Hugo. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Ab-Initio Studies on Carburization of Fe3Al Based Alloys

AbstractFe-Al based alloys exhibit excellent properties but suffer metal dusting in carburizing atmospheres. Surface composition can be a determinant factor in the solution of this problem. We calculate in this work the C adsorption energies in the L21 Fe2AlX (X=Ti,V,Nb) structures and we study the influence of surface covering. Our results show the beneficial effect of Ti, suggesting that there could exist an activation energy to promote the incorporation of C in the subsurface layers when the surface is covered enoug

First-principles study for comparison of the electronic and optic bandgaps of the CH3NH3Pb1-xYxI3 (Y=Bi, x=0.00, 0.125) and CH3NH3Pb1-xYxI3 (Y=Ca, Sr, x=0.125, 0.250) perovskites

We computed the optical bandgaps of the CH3NH3Pb(1-x)Y(x)I3 (Y=Bi, x = 0.00, 0.125) and CH3NH3Pb(1-x)Y(x)I3 (Y=Ca, Sr, x = 0.125, 0.250) perovskite crystal structures using optical arguments such as the imaginary part of the dielectric function ε2(ω), absorption coefficient (α), the zero-crossing point of the (αhυ)2 versus (hυ). Density Functional Theory (DFT) calculations and the Vienna ab-initio simulation package (VASP) are used in theoretical calculations. The optical bandgap of the stoichiometric CH3NH3PbI3 phase calculated as 1.694 eV is in agreement with both experimental and theoretical studies. The optical bandgaps of the non-stoichiometric phases are calculated as 1.358 eV, 1.493 eV, 1.537 eV, 1.503 eV, and 1.588 eV, respectively. These optical bandgaps results are reported for the first time in this study. © 2021 Elsevier B.V

Role of Composition and Structure on the Properties of Metal/Multifunctional Ceramic Interfaces

The formation of intermetallic secondary phases, such as Pt3Pb, has been observed experimentally at PbTiO3/Pt and Pb(Zr,Ti)O3/Pt, or PZT/Pt, interfaces. Density functional theory calculations are used here to calculate the work of adhesion of these interfacial systems with and without the secondary intermetallic phase. The charge density maps of the interfaces reveal the electronic interactions at the interface and the impact of the secondary phase. In addition, Bader charge analysis provides a quantitative assessment of electron transfer from the perovskites to the Pt. Analysis of the band diagrams indicates an increase of the potential barrier associated with electron transfer due to the formation of the Pt3Pb at PZT/Pt interfaces

Electronic properties of Β-TaON and its surfaces for solar water splitting

This is the final version of the article. Available from Elsevier via the DOI in this record.Recently, oxynitrides materials such as β-TaON has been using as a photoanode material in the field of photocatalysis and is found to be promising due to its suitable band gap and charge carrier mobility. Computational study of the crystalline β-TaON in the form of primitive unit cell, supercell and its N, Ta, and O terminated surfaces are carried out with the help of periodic density functional theory (DFT). Optical and electronic properties of all these different species are simulated, which predict TaON as the best candidate for photocatalytic water splitting contrast to their Ta 2 O 5 and Ta 3 N 5 counterparts. The calculated bandgap, valence band, and conduction band edge positions predict that β-TaON should be an efficient photoanodic material. The valence band is made up of N 2p orbitals with a minor contribution from O 2p, while the conduction band is made up of Ta 5d. Turning to thin films, the valence band maximum; VBM (−6.4 eV vs. vacuum) and the conduction band minimum; CBM (−3.3 eV vs. vacuum) of (010)-O terminated surface are respectively well below and above the redox potentials of water as required for photocatalysis. Charge carriers have smaller effective masses than in the (001)-N terminated film (VBM −5.8 and CBM −3.7 eV vs. vacuum). However, due to wide band gap (3.0 eV) of (010)-O terminated surface, it cannot absorb visible wavelengths. On the other hand, the (001)-N terminated TaON thin film has a smaller band gap in the visible region (2.1 eV) but the bands are not aligned to the redox potential of water. Possibly a mixed phase material would produce an efficient photoanode for solar water splitting, where one phase performs the oxidation and the other reduction.We acknowledge the financial support of Engineering and Physical Science Research Council, UK (EPSRC)under the research grant Nos. EP/P510956/1, EP/P003435/1 and EP/R512801/1. S.K acknowledges the Notur Norwegian supercomputing facilities through project nn4608k and the HyMatSiRen project 272806 by the Research Council of Norway. We also acknowledge Prof. Neil Allan and Dr. Sergio C. Espindola for their help in completing this work

Pressure-Induced Structural Phase Transition and Superconductivity in NaSn5

The structural and electronic properties of the tin-rich compound NaSn5 were investigated under pressures of up to 10 GPa on the basis of the evolutionary algorithm (EA) technique coupled with first-principles total energy calculations. Upon compression, the known metallic tetragonal P4̅21m phase transforms into a metallic hexagonal P6/mmm phase at 1.85 GPa accompanied by an unusual change in the existing form of Sn atoms. The P6/mmm phase can be interpreted as a quasi-layered sandwich structure with two Sn layers and one sodium layer. The presence of softening phonon modes and the existence of Fermi pockets together with the obvious Fermi surface nesting indicate a strong electron-phonon coupling (EPC) and thus potential superconductivity in the P6/mmm phase. The strong EPC in the P6/mmm phase is mainly attributed to the phonons from Sn1 atoms together with electrons from the Sn1 p y and Sn1 p z states. The calculated superconducting critical temperature Tc of the P6/mmm phase is 5.91 K at 1.85 GPa. This study provides a new clue for designing intercalated compounds with superconductivity

High-Performance Solvers for Dense Hermitian Eigenproblems

We introduce a new collection of solvers - subsequently called EleMRRR - for large-scale dense Hermitian eigenproblems. EleMRRR solves various types of problems: generalized, standard, and tridiagonal eigenproblems. Among these, the last is of particular importance as it is a solver on its own right, as well as the computational kernel for the first two; we present a fast and scalable tridiagonal solver based on the Algorithm of Multiple Relatively Robust Representations - referred to as PMRRR. Like the other EleMRRR solvers, PMRRR is part of the freely available Elemental library, and is designed to fully support both message-passing (MPI) and multithreading parallelism (SMP). As a result, the solvers can equally be used in pure MPI or in hybrid MPI-SMP fashion. We conducted a thorough performance study of EleMRRR and ScaLAPACK's solvers on two supercomputers. Such a study, performed with up to 8,192 cores, provides precise guidelines to assemble the fastest solver within the ScaLAPACK framework; it also indicates that EleMRRR outperforms even the fastest solvers built from ScaLAPACK's components

Comparison of the effects of Sr2+ and Ca2+ substitution on the structural and electronic properties of the perovskites CH3NH3Pb1-xYxI3 (Y[dbnd]Sr, Ca) by using the Density Functional Theory

The Vienna ab-initio simulation package (VASP) and Density Functional Theory (DFT) calculation method are used to study the structural and detailed electronic properties through atomic substitution in the CH3NH3Pb(1-x)Y(x)I3 (Y[dbnd]Sr, Ca x = 0.125, 0.25, 0.50, 0.75, and 1.0) perovskites. We determined that the non-stoichiometric crystal structures were calculated as a distorted orthorhombic phase as predicted by the tolerance factor range 0.7 < t < 0.9. The bandgaps of the stoichiometric CH3NH3SrI3 and CH3NH3CaI3 compounds are calculated 3.261 eV (Q → Γ indirect) and 3.144 eV (Q → Γ indirect), respectively and they are very high for ideal photo absorbers. We were determined that the bandgap (Γ → Γ direct) of the CH3NH3Pb0.875Ca0.125I3, CH3NH3Pb0.750Ca0.250I3, and CH3NH3Pb0.875Sr0.125I3 compounds are calculated 1.44 eV, 1.54 eV, and 1.525 eV respectively and are more suitable for ideal photo absorbers. It was seen that Ca2+ substitution was more successful than Sr2+ substitution. © 2020 Elsevier B.V