arXiv subjects

Brian D. Wirth

Publications and source records attributed to Brian D. Wirth.

8 recordsLinked to original sources

arXiv2025

Influence of temperature, initial grain-boundary bubble density and grain structure on fission gas behaviour in UO$_2$: a 3D hybrid multiscale study

Fission gas swelling and release in UO$_2$ are governed by the coupled evolution of intragranular clusters and bubbles, migrating grain boundaries (GBs), triple junctions (TJs), and their eventual connection to a free surface (FS). We extend a hybrid multiscale framework that couples cluster dynamics (Xolotl) with a phase-field model (MARMOT) to large 3D polycrystals with heterogeneous GB and surface diffusion and evolving GB networks. We simulate 10- and 100-grain UO$_2$ microstructures at 1200 and 1600 K, with and without a FS, to interrogate bubble growth, coalescence, GB/TJ coverage, gas arrival at interfaces, and fission gas release (FGR). At 1200 K, both GB mobility and gas transport are low, yielding negligible bubble and GB evolution. At 1600 K, intergranular bubbles rapidly become lenticular and coalesce into networks while unpinned GBs migrate; fewer initial bubbles reduce coalescence but enhance GB migration due to less pinning and produce spikes in interfacial gas arrival rate due to GB sweeping. Bubble density versus mean projected area agrees with White's (2004) coalescence trend and remains on the left side of the analytical curve, in contrast to several prior simulations, likely due to the inclusion of GB migration. In domains with a FS, early release is rapid and bubbles near the FS collapse to form a denuded zone, suppressing local network connectivity; GB coverage rises and approaches but does not exceed 50%. TJ coverage remains low without preferential nucleation at TJs. To our knowledge, these are the first large-scale 3D mesoscale simulations of intergranular fission gas behavior that provide mechanistic insight and quantitative metrics to inform engineering-scale FGR models.

cond-mat.mtrl-sci↗

arXiv2025

Simulations of grain growth in tungsten armor materials under ARC plasma edge operation conditions using an integrated plasma-edge/materials model

An integrated model of grain growth deuterium-exposed tungsten polycrystals, consisting of a two-dimensional vertex dynamics model fitted to atomistic data, has been developed to assess the grain growth kinetics of deuterium-exposed polycrystalline tungsten (W). The model tracks the motion of grain boundaries under the effect of driving forces stemming from grain boundary curvature and differential deuterium concentration accumulation. We apply the model to experimentally-synthesized W polycrystals under deuterium saturated conditions consistent with those of the ARC concept design, and find fast grain growth kinetics in the material region adjacent to the plasma (at 1400 K, <100 seconds for full transformation), while the microstructure is stable deep inside the material (several days to complete at a temperature of 1000 K). Our simulations suggest that monolithic W fabricated using conventional techniques will be highly susceptible to grain growth in the presence of any driving force at temperatures above 1000 K.

cond-mat.mtrl-sci↗

arXiv2019

Data-driven Material Models for Atomistic Simulation

The central approximation made in classical molecular dynamics simulation of materials is the interatomic potential used to calculate the forces on the atoms. Great effort and ingenuity is required to construct viable functional forms and find accurate parameterizations for potentials using traditional approaches. Machine-learning has emerged as an effective alternative approach to develop accurate and robust interatomic potentials. Starting with a very general model form, the potential is learned directly from a database of electronic structure calculations and therefore can be viewed as a multiscale link between quantum and classical atomistic simulations. Risk of inaccurate extrapolation exists outside the narrow range of time- and length-scales where the two methods can be directly compared. In this work, we use the Spectral Neighbor Analysis Potential (SNAP) and show how a fit can be produced with minimal interpolation errors which is also robust in extrapolating beyond training. To demonstrate the method, we have developed a new tungsten-beryllium potential suitable for the full range of binary compositions. Subsequently, large-scale molecular dynamics simulations were performed of high energy Be atom implantation onto the (001) surface of solid tungsten. The new machine learned W-Be potential generates a population of implantation structures consistent with quantum calculations of defect formation energies. A very shallow (<2nm) average Be implantation depth is predicted which may explain ITER diverter degradation in the presence of beryllium.

physics.comp-ph↗

arXiv2016

Object Kinetic Monte Carlo Simulations of Radiation Damage in Bulk Tungsten Part-II: With a PKA Spectrum Corresponding to 14-MeV Neutrons

Object kinetic Monte Carlo was employed to study the effect of dose rate on the evolution of vacancy microstructure in polycrystalline tungsten under neutron bombardment. The evolution was followed up to 1.0 displacement per atom (dpa) with point defects generated in accordance with a primary knock-on atom (PKA) spectrum corresponding to 14-MeV neutrons. The present study includes the effect of grain size (2.0 and 4.0 $μ$m) but excludes the impact of transmutation or pre-existing defects beyond grain boundary sinks. Vacancy cluster density increases with dose rate, while the density of vacancies decreases. Consequently, the average vacancy cluster size and the fraction of vacancies in visible clusters decrease with increasing dose rate. The density of vacancies and vacancy clusters decrease with grain size such that the average size of the clusters remains similar. However, the average size is larger for larger grains at dose rates < 4.5 x 10-7 dpa/s. The trend of vacancy accumulation as a function of dose, dose rate, and grain size is similar to that obtained with the High Flux Isotope Reactor (HFIR) PKA spectrum. However, the amount of vacancy accumulation and the vacancy microstructure are quite different. Compared to the HFIR case, we find that even though the dose rates are 2.5 times higher, the density of vacancies and the average vacancy cluster sizes are lower. In addition, a void lattice forms only for the lowest two dose rates (4.5 x 10-8 and 4.5 x 10-9 dpa/s). In contrast, a void lattice formed at all dose rates studied using the HFIR PKA spectrum. We discuss in detail the factors that lead to these different microstructures.

cond-mat.mtrl-sci↗

arXiv2016

Object Kinetic Monte Carlo Simulations of Radiation Damage in Neutron-Irradiated Tungsten Part-I: Neutron Flux with a PKA Spectrum Corresponding to the High-flux Isotope Reactor

Object kinetic Monte Carlo simulations were performed to study the impact of varying dose rate and grain size up to a dose of 1.0 dpa in pure, polycrystalline tungsten, subjected to a neutron irradiation having a PKA spectrum corresponding to the High Flux Isotope Reactor. The present study models defect cluster accumulation in tungsten, but does not consider the impact of transmutation or pre-existing defects beyond the grain boundary sinks, with varying grain size. With increasing dose rate, the vacancy cluster density increases, while the number density of vacancies decreases. Accordingly, the average vacancy cluster size and the fraction of vacancies that are part of visible clusters decreases with increasing dose rate. With increasing grain size, both the number densities of vacancies and vacancy clusters decrease, while both the fraction of vacancies in visible clusters and the average vacancy cluster size increase. This is caused by the pseudo-ripening of the vacancy clusters due to the longer-lived self-interstitial clusters in larger grains. The spatial ordering of vacancy clusters along {110} planes was observed for both grain sizes and all dose rates studied. Interplanar spacing increases with grain size; however, no clear dependence on dose or dose rate was observed. The results of this study show that 1D diffusion of self-interstitial clusters, while necessary, is not sufficient to form a void lattice, and that the diffusion of vacancies is also required. A methodology is suggested for choosing the simulation box dimensions so as to represent more faithfully the effects of one-dimensional migrating self-interstitial-atom clusters.

cond-mat.mtrl-sci↗

arXiv2014

Cascade morphology transition in bcc metals

Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, $b$, in the defect production curve as a function of cascade energy ($N_F \sim E_{MD}^b$). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, $μ$, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of $μ$ as a function of displacement threshold energy, $E_d$, is presented for bcc metals.

cond-mat.mtrl-sci↗

arXiv2014

Displacement cascades and defects annealing in tungsten, Part I: defect database from molecular dynamics simulations

Molecular dynamics simulations have been used to generate a comprehensive database of surviving defects due to displacement cascades in bulk tungsten. Twenty one data points of primary knock-on atom (PKA) energies ranging from 100 eV (sub-threshold energy) to 100 keV ($\sim$780$\times E_d$, where $E_d$ is the average displacement threshold energy) have been completed at 300 K, 1025 K and 2050 K. Within this range of PKA energies, two regimes of power-law energy-dependence of the defect production are observed. A distinct power-law exponent characterizes the number of Frenkel pairs produced within each regime. The two regimes intersect at a transition energy which occurs at approximately 250$\times E_d$. The transition energy also marks the onset of the formation of large self-interstitial atom (SIAs) clusters (size 14 or more). The observed defect clustering behavior is asymmetric, with SIA clustering increasing with temperature, while the vacancy clustering decreases. This asymmetry increases with temperature such that at 2050 K ($\sim 0.5 T_m$) practically no large vacancy clusters are formed, meanwhile large SIA clusters appear in all simulations. The implication of such asymmetry on the long-term defect survival and damage accumulation is discussed. In addition, rare $<$100$>${110} SIA loops are observed.

cond-mat.mtrl-sci↗

arXiv2014

Displacement cascades and defects annealing in tungsten, Part II: Object kinetic Monte Carlo Simulation of Tungsten Cascade Aging

We describe the results of object kinetic Monte Carlo (OKMC) simulations of the annealing of primary cascade damage in bulk tungsten using a comprehensive database of cascades obtained from molecular dynamics [1] as a function of primary knock-on atom (PKA) energy and direction, and temperatures of 300, 1025 and 2050 K. An increase in SIA clustering but decrease in vacancy clustering with temperature combined with disparate mobilities of SIAs versus vacancies causes an interesting temperature effect on cascade annealing, which is quite different from what one would expect. The annealing efficiency (ratio of number of defects after and before annealing) exhibits an inverse U-shape curve as a function of temperature. In addition, we will also describe the capabilities of our newly developed OKMC code; KSOME (kinetic simulations of microstructure evolution) used to carryout these simulations

cond-mat.mtrl-sci↗