Self-diffusion near symmetrical tilt grain boundaries in UO2 matrix: A molecular dynamics simulation study

Molecular dynamics simulations have been carried out to study the influence of grain boundaries in stoichiometric UO₂ on uranium and oxygen self-diffusions over a large range of temperature varying from 300 K to 2100 K. The study was carried out on two symmetrical tilt grain boundaries, Σ5 and Σ41, which have respectively two different atomic structures. Firstly, the study of the temperature effect on the grain boundary core structure is presented. With the raise of temperature, the grain boundary core grows with an increase of disorder. Secondly, self-diffusion near both grain boundaries is studied. It has been found that grain boundaries accelerate the uranium and oxygen self-diffusion rates over several nanometres from the grain boundary interface. Uranium and oxygen self-diffusion are anisotropic, with a high acceleration along the grain boundary interface. Using the self-Van Hove correlation functions, hopping mechanisms were identified for Σ41 in all directions while for Σ5 hopping mechanism takes place along the grain boundary interface and random diffusion appears in the perpendicular direction of the grain boundary plane.     

Large molecular dynamics simulations of collision cascades in single-crystal, bi-crystal, and poly-crystal UO2

Classical molecular dynamics simulations have been carried out to study the primary damage due to α-decay self-irradiations in single-, bi-, and poly-crystal UO₂ matrices. In all the cases no amorphization has been found, only the creation of few point defects is observed. However, in all grain boundary systems numerous point defects are created along the interfaces. Furthermore, cascade morphologies depend strongly on the grain boundary structure. For symmetrical tilt grain boundaries with small misorientation angles (lower than 20°) the structure at the grain boundaries is composed of edge dislocations, whereas for higher misorientation angles is formed by Schottky defects. The grain boundary structure in the poly-crystal is found to be highly disordered. For the last two systems, cascades seem stopped by the interfaces unlike those with edge dislocation grain boundaries. These types of interface act like sink which traps moving atoms.     

Effects of the density of collision cascades: Separating contributions from dynamic annealing and energy spikes

We present a quantitative model for the efficiency of the molecular effect in damage buildup in semiconductors. Our model takes into account only one mechanism of the dependence of damage buildup efficiency on the density of collision cascades: nonlinear energy spikes. In our three-dimensional analysis, the volume of each individual collision cascade is divided into small cubic cells, and the number of cells that have an average density of displacements above some threshold value is calculated. We assume that such cells experience a catastrophic crystalline-to-amorphous phase transition, while defects in the cells with lower displacement densities have perfect annihilation. For the two limiting cases of heavy (500 keV/atom ²⁰⁹Bi) and light (40 keV/atom ¹⁴N) ion bombardment of Si, theory predictions are in good agreement with experimental data for a threshold displacement density of 4.5 at.%. For intermediate density cascades produced by small 2.1 keV/amu PF_n clusters, we show that dynamic annealing processes entirely dominate cascade density effects for PF₂ ions, while energy spikes begin contributing in the case of PF₄ cluster bombardment.     

Analyzing the effect of displacement rate on radiation-induced segregation in 304 and 316 stainless steels by examining irradiated EBR-II components and samples irradiated with protons

Recent studies have indicated that, at temperatures relevant to fast reactors and light water reactors, void swelling in austenitic alloys progresses more rapidly when the radiation dose rate is lower. A similar dependency between radiation-induced segregation (RIS) and dose rate is theoretically predicted for pure materials and might also be true in complex engineering alloys. Radiation-induced segregation was measured on 304 and 316 stainless steel, irradiated in the EBR-II reactor at temperatures near 375 °C, to determine if the segregation is a strong function of damage rate. The data taken from samples irradiated in EBR-II is also compared to RIS data generated using proton radiation. Although the operational histories of the reactor irradiated samples are complex, making definitive conclusions difficult, the preponderance of the evidence indicates that radiation-induced segregation in 304 and 316 stainless steels is greater at lower displacement rate.     

Temperature accelerated dynamics study of migration process of oxygen defects in UO2

We studied the migration dynamics of oxygen point defects in UO₂ which is the primary ceramic fuel for light-water reactors. Temperature accelerated dynamics simulations are performed for several initial conditions. Though the migration of the single interstitial is much slower than that of the vacancy, clustered interstitial shows faster migration than those. This observation gives us important insight on the formation mechanism of high-burnup restructuring, including planar defects and grain sub-division (the rim structure), found in UO₂.     

Modification of a shell model for the study of the radiation effects in BaTiO3

In order to study the radiation effects in BaTiO₃ ferroelectric crystal, a previously developed shell model is modified. The modifications include adding the ZBL universal potentials at short distances and distance-dependent spring constants for core-shell interactions. The phase transition sequences in BaTiO₃ were correctly reproduced using molecular dynamics simulations with this modified shell model. Also, the calculated Frenkel pair formation energies agree well with results obtained by first principles calculations, which suggests that this model is suitable for the simulation of the radiation effects in BaTiO₃. The dependence of polarization on the number of oxygen vacancies was also studied.     

Establishing the isotropy of displacement cascades in UO2 through molecular dynamics simulation

Simulation of displacement cascades is a valuable approach in furthering our understanding of how the physical properties of nuclear fuel evolve. Molecular dynamics simulations of displacement cascades in uranium dioxide have been performed at three different primary knock-on atom energies. Various properties of the cascade (such as the spatial extent and total number of defects) are monitored as the cascade progresses. Both the statistical variation of these properties and the dependence on the crystallographic direction of the primary knock-on atom are investigated in order to determine the isotropy of these events.     

Computer study of intrinsic defects in CaWO4

A computer simulation has been performed to study the intrinsic defects in CaWO₄. The inter-atomic interaction potentials are empirically fitted to the known crystal properties. The results reveal that the predominant intrinsic defects existed in the crystal should be oxygen Frenkel-type. Calculated formation energies of electronic defects suggest that the hole is easier to be trapped by oxygen ion than by calcium ion, and oxygen would prior to occupy the oxygen vacancy during the annihilation under oxidation atmosphere. An analysis of activation energy shows that oxygen vacancy migration is movable ionic defects in CaWO₄.     

First-principles modeling of He-clusters in UO2

We have investigated the behavior of He in UO₂, using the projector-augmented-wave (PAW) method and the generalized gradient approximation (GGA) based on the density functional theory. Total energy calculations with atomic relaxation included have been performed in a 96-atom large supercell. We have found that He has a strong tendency to form a cluster in vicinity of an octahedral interstitial site (OIS) in the UO₂ matrix. In addition, the strain energy produced by a He-cluster was found to be sufficient to create point defects of the host atoms in UO₂. Our study suggests that He-clusters and He-induced point defects play an important role for the local mechanical properties of UO₂.     

Atomic diffusion mechanism of Xe in UO2

We have investigated vacancy-assisted diffusion of Xe in uranium dioxide (UO₂) calculating incorporation, binding, and migration energies. All the energy values have been obtained using the density functional theory (DFT) within the generalized gradient approximation (GGA) and the projector-augmented-wave (PAW) method. Considering spin-polarization effect, we find that the computed migration energy is reduced by and agrees well with experimental data compared to those obtained from non-magnetic calculations. We also find that an oxygen vacancy lowers the migration energy of a uranium vacancy by about 1 eV, enhancing an effective movement of vacancy clusters consisting of both uranium and oxygen vacancies. Furthermore, the strain energy of Xe is large enough to contribute to the clustering of vacancies making it the driving force for the vacancy-assisted diffusion of Xe in UO₂. In summary all the calculated results suggest that the trivacancy is a major diffusion pathway of Xe in UO₂.     

First-principles study of helium effect in a ferromagnetic iron grain boundary: Energetics, site preference and segregation

Using a first-principles method based on density functional theory, we have investigated energetics and site preference of helium (He) in a ferromagnetic bcc-iron (Fe) grain boundary (GB). We calculate the binding energies of He atom in the GB, which show that the substitutional He is energetically favored in comparison with the interstitial He with a small energy difference of 0.06 eV. The segregation energy is calculated to be ∼1.4 eV for the energetically favorable GB substitutional and interstitial sites, which is large enough for the He atoms to segregate to these sites, independent of the temperature and the bulk He concentration. This leads to the conclusion that all the He atoms will segregate into the GB at a typical temperature range of 573-1173 K.     

Behavior of crystalline silicon under huge electronic excitations: A transient thermal spike description

Recent experimental works devoted to the phenomena of mixing observed at metallic multilayers Ni/Si irradiated by swift heavy ions irradiations make it necessary to revisit the insensibility of crystalline Si under huge electronic excitations. Knowing that Ni is an insensitive material, such observed mixing would exist only if Si is a sensitive material. In order to extend the study of swift heavy ion effects to semiconductor materials, the experimental results obtained in bulk silicon have been analyzed within the framework of the inelastic thermal spike model. Provided the quenching of a boiling (or vapor) phase is taken as the criterion of amorphization, the calculations with an electron-phonon coupling constant g(300 K) = 1.8 × 10¹² W/cm³/K and an electronic diffusivity D_e(300 K) = 80 cm²/s nicely reproduce the size of observed amorphous tracks as well as the electronic energy loss threshold value for their creation, assuming that they result from the quenching of the appearance of a boiling phase along the ion path. Using these parameters for Si in the case of a Ni/Si multilayer, the mixing observed experimentally can be well simulated by the inelastic thermal spike model extended to multilayers, assuming that this occurs in the molten phase created at the Ni interface by energy transfer from Si.     

Oxygen defects created in CeO2 irradiated with 200 MeV Au ions

CeO₂ films were irradiated with 200 MeV Au ions in order to investigate the damages created by electronic energy deposition. In the Raman spectra of the ion-irradiated films, a broad band appears at the higher frequency side of the F_2g peak of CeO₂. The band intensity increases as ion fluence increases. Furthermore, the F_2g peak becomes asymmetric with a low-frequency tail. In order to understand the origin of these spectral changes, an unirradiated CeO₂ film was annealed in vacuum at 1000 °C. By comparing the results for the irradiation and for the annealing, it is concluded that the broad band obtained for irradiated samples contains the peak observed for the annealed sample. The F_2g peak becomes asymmetric with a low-frequency tail by the irradiation as well as the annealing. Therefore, the above-mentioned changes in the Raman spectra caused by 200 MeV Au irradiation is closely related to the creation of oxygen vacancies.     

Desorption of nanoclusters from gold nanodispersed layers by 72 keV Au400 ions: Experiment and molecular dynamics simulation

The interaction of 72 keV Au₄₀₀ ions (with a diameter of approximately 2 nm) with nanodispersed gold targets has been studied. These interactions are dominated by elastic collisions. The gold nanodispersed target with 2-12 nm nanoislets was bombarded with a fluence of 1.7 × 10¹² ions/cm². The desorbed nanoclusters were collected on carbon foils supported by TEM-grids. Intact 29 nm gold nanoclusters were found on the collectors. The desorption yield (normalized to the total cross-section of the projectile-cluster interaction) was estimated to be 0.62 nanocluster/projectile. Preliminary estimates were made using molecular dynamic simulations for comparison with the experimental results.     

Theory of He trapping, diffusion, and clustering in UO2

We have performed ab initio total energy calculations to investigate the behavior of helium and its diffusion properties in uranium dioxide (UO₂). Our investigations are based on the density functional theory within the generalized gradient approximation (GGA). The trapping behavior of He in UO₂ has been modeled with a supercell containing 96-atoms as well as uranium and oxygen vacancy trapping sites. The calculated incorporation energies show that for He a uranium vacancy is more stable than an oxygen vacancy or an octahedral interstitial site (OIS). Interstitial site hopping is found to be the rate-determining mechanism of the He diffusion process and the corresponding migration energy is computed as 2.79 eV at 0 K (with the spin-orbit coupling (SOC) included), and as 2.09 eV by using the thermally expanded lattice parameter of UO₂ at 1200 K, which is relatively close to the experimental value of 2.0 eV. The lattice expansion coefficient of He-induced swelling of UO₂ is calculated as 9 × 10⁻². For two He atoms, we have found that they form a dumbbell configuration if they are close enough to each other, and that the lattice expansion induced by a dumbbell is larger than by two distant interstitial He atoms. The clustering tendency of He has been studied for small clusters of up to six He atoms. We find that He strongly tends to cluster in the vicinity of an OIS, and that the collective action of the He atoms is sufficient to spontaneously create additional point defects around the He cluster in the UO₂ lattice.     

Dependence of cluster ranges on target cohesive energy: Molecular-dynamics study of energetic Au402 cluster impacts

It has long been known that the stopping and ranges of atoms and clusters depends on the projectile-target atom mass ratio. Recently, Carroll et al. [S.J. Carroll, P.D. Nellist, R.E. Palmer, S. Hobday, R. Smith, Phys. Rev. Lett. 84 (2000) 2654] proposed that the stopping of clusters also depends on the cohesive energy of the target. We investigate this dependence using a series of molecular-dynamics simulations, in which we systematically change the target cohesive energy, while keeping all other parameters fixed. We focus on the specific case of Au₄₀₂ cluster impact on van-der-Waals bonded targets. As target, we employ Lennard-Jones materials based on the parameters of Ar, but for which we vary the cohesive energy artificially up to a factor of 20. We show that for small impact energies, E₀ ? 100 eV/atom, the range D depends on the target cohesive energy U, D ∝ U^−β. The exponent β increases with decreasing projectile energy and assumes values up to β = 0.25 for E₀ = 10 eV/atom. For higher impact energies, the cluster range becomes independent of the target cohesive energy. These results have their origin in the so-called ‘clearing-the way’ effect of the heavy Au₄₀₂ cluster; this effect is strongly reduced for E₀ ? 100 eV/atom when projectile fragmentation sets in, and the fragments are stopped independently of each other. These results are relevant for studies of cluster stopping and ranges in soft matter.     

Molecular simulations of cluster ejection by CO2 cluster impact on carbon-based surfaces

Single (CO₂)_N (N = 1-20) cluster impact on three different carbon-based surfaces of fullerite (1 1 1), graphite and diamond (1 0 0) has been investigated by MD simulations with the cluster collision energy from 5 to 14 keV/cluster as a first step toward the general modeling of the reactive sputtering by cluster impact of a solid surface. A crater permanently remained on the fullerite and graphite surfaces while it was quickly replenished with fluidized carbon material on the diamond surface. In spite of the smaller crater size as well as the crater recovery resulting in the reduction of the surface area, the sputtering yields were the highest on diamond. The effective energy deposition near the surface contributes to the temperature rise and consequent sputtering seemed highly reduced due to the collision cascades especially on the fullerite target.     

Study on the behavior of oxygen atoms in swift heavy ion irradiated CeO2 by means of synchrotron radiation X-ray photoelectron spectroscopy

To study the effects of swift heavy ion irradiation on cerium dioxide (CeO₂), CeO₂ sintered pellets were irradiated with 200 MeV Xe ions at room temperature. For irradiated and unirradiated samples, the spectra of X-ray photoelectron spectroscopy (XPS) were measured. XPS spectra for the irradiated samples show that the valence state of Ce atoms partly changes from +4 to +3. The amount of Ce³⁺ state was quantitatively obtained as a function of ion-fluence. The relative amount of oxygen atom displacements, which are accompanied by the decrease in Ce valence state, is 3-5%. This value is too large to be explained in terms of elastic interactions between CeO₂ and 200 MeV ions. The experimental result suggests the contribution of 200 MeV Xe induced electronic excitation to the displacements of oxygen atoms.     

Radiation tolerance in a nanostructure: Is smaller better?

While previous experimental studies suggest that the presence of nanostructure is detrimental to the phase-transition resistance under an irradiation environment, we have recently found an opposite effect - nanostructure enhances phase-transition resistance. Here we analyze the change in free energy of an irradiated single-phase nanocrystalline material and explain the radiation tolerance (characterized by the resistance to phase-transition) in terms of two competing effects: (i) a smaller grain size tends to lower the free energy because the accumulation of point defects (mainly vacancies) in the grain interior is suppressed and (ii) a smaller grain size tends to increase the free energy because the area fraction of grain boundary is larger. For a two-phase nanocrystalline material, the heat of mixing between the two-phases often needs to be sufficiently positive so that the ion-beam mixing is avoided. Our analysis explains all previous experimental results where a nanostructure is found to either enhance or lower the phase-transition resistance.