Radiation re-solution of fission gas in non-oxide nuclear fuel

Renewed interest in fast nuclear reactors is creating a need for better understanding of fission gas bubble behavior in non-oxide fuels. Collisions between fission fragments and their subsequent cascades can knock fission gas atoms out of bubbles and back into the fuel lattice, resulting in a loss term for the bubble. By assuming these collisions can be treated as binary collisions, we calculated a re-solution parameter as a function of bubble radius. The calculations showed that there is a sharp decrease as bubble size increases until about 100 nm when the re-solution parameter stays nearly constant. The bubble size dependence may explain the large bubble size distribution found in some uranium carbide and nitride fuels. Furthermore, our model shows ion cascades created in the fuel result in many more implanted fission gas atoms than collisions directly with fission fragments. Utilization of our calculated re-solution parameter can be used to find a re-solution rate for future bubble behavior simulations

Strukturelle und elektronische Eigenschaften von Spuren schneller schwerer Ionen in amorphem Kohlenstoff

An verschiedenen Kohlenstoffmaterialien wurden Bestrahlungseffekte im elektronischen Energieverlust-Bereich diskutiert. Die Spurbildung in amorphem Kohlenstoff wurde experimentell analysiert unter Verwendung verschiedener Mikroskopietechniken (AFM, TEM) und durch ortsaufgelöste elektrische Messungen. Ein Modell wurde entwickelt, dass die Simulation der Spurbildung mit klassischen Molekulardynamikmethoden ermöglicht

Atomistic simulations of swift ion tracks in diamond and graphite

We have used molecular dynamics simulations to study ion tracks in diamond and graphite. Tracks are included using a thermal spike model, i.e. a certain number of atoms within an initial track radius are given an initial excitation energy. The total energy given to the excited atoms and the length of the track determine an "effective" stopping power dE/dx. Electronic excitations in semiconductors and semimetals like diamond and graphite can diffuse far from each other or be quenched before they couple to the lattice. This effect is included by varying the number of atoms that are effectively energized within the track. We use an initial track radius of 3 nm and we find that full amorphization of this region during the first few ps only occurs when the "effective" dE/dx is larger than 6 +/- 0.9 keV/nm for graphite and 10.5 +/- 1.5 keV/nm for diamond. Since the "effective" dE/dx depends on the electron-phonon coupling, our simulations set bounds on the efficiency of the coupling between the electronic excitations and the lattice in this highly non-equilibrium scenario. (c) 2006 Elsevier B.V. All rights reserved

Formation Path of δ Hydrides in Zirconium By Multiphase Field Modeling

A multiphase field model is developed to study the effects of metastable ζ and γ hydrides on the nucleation and growth of the stable δ hydrides in α zirconium matrix. The model incorporates all the possible phases using the Gibbs free energies of formation for each phase and their available material properties. The multiphase field model is constructed by utilizing one conserved phase-field variable to represent the concentration of hydrogen, and six non-conserved phase-field variables to represent the α phase, ζ phase, three orientation variants of γ phase, and δ phase. The evolution equations are coupled with the mechanical equilibrium equations and solved using the Multiphysics Object Oriented Simulation Environment (MOOSE). Nucleation of hydrides is controlled using classic nucleation theory, inserting nuclei randomly with a probability dependent on the competition between the hydride\u27s volume free energy and the interface\u27s area free energy to form critical sized nuclei. The comparison between the results of the multiphase model and a two-phase model (without metastable phases) indicate that the intermediate phases are influential in the initial formation and evolution of δ phase hydrides. Random seed simulations, both in the basal plane and the (101̄0) plane, also indicate that the intermediate metastable phases play a key role in the shape evolution of δ hydrides. Results suggest that quantitative phase field models of δ hydride growth need to include intermediate phases in order to accurately predict the morphology of hydrides

A Modified Embedded-Atom Method Potential for a Quaternary Fe-Cr-Si-Mo Solid Solution Alloy

Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system—i.e., four major elements of T91—using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments

A modified embedded-atom potential for Fe-Cr-Si alloys

We developed a modified embedded atom method (MEAM) potential for Fe-Cr-Si ternary systems. These alloys have superior corrosion and crack resistance, making them candidate materials for several engineering applications such as accident-tolerant fuel cladding. We used a multiobjective optimization approach to match Fe-Cr-Si's elastic constants, ground-state energies, and structural parameters with ab initio calculations. The potential has been parameterized by fitting to a set of literature values obtained using density functional theory (DFT) or experimental studies. The developed potential was used in molecular dynamics (MD) simulations to extract mechanical and thermal properties. We obtained the calculated elastic constants for Fe-Cr-Si binary interactions using the proposed potential, agreeing with ab initio calculations. Our calculated self-diffusion coefficient values and defect formation energy using this potential are in good agreement with the previous literature. Therefore, the developed potential can investigate the fundamental behaviors on an atomic scale under harsh conditions like elevated temperature and irradiation.This project is partly supported by DoE-ARPA-E OPEN (DE-AR0001066) and the NSF-CAREER under NSF cooperative agreement CBET-2042683

Electronic properties of graphite-like ion tracks in insulating tetrahedral amorphous carbon

We investigated the formation of quasi one-dimensional conducting filaments in tetrahedral amorphous carbon (ta-C) films created by swift heavy ion irradiation. Various ta-C films with thicknesses of about 100 nm were grown using mass-separated ion beam deposition on highly conducting Si and Ni substrates. After deposition, the films were irradiated with 1 GeV U-238 ions at fluences between 109 and 10(11) ions/cm(2). Due to their high electronic energy loss of about 40 kev/ nm, the swift heavy ions graphitize the predominantly (70%) sp(3)-bound tertahedral amorphous carbon film (ta-C) along their trajectories, yielding conducting nanowires embedded in an insulating matrix. Using atomic force microscopy (AFM) with conducting cantilevers and an applied bias voltage the presence of conducting tracks was confirmed and their conductivities were determined to be several orders of magnitude higher than that of the host matrix. Temperature-dependent electrical measurements were performed on the irradiated samples at 300K - 15K with fields of up to 5V/mu m