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Molecular dynamics (MD) simulations have been performed to investigate the diffusion behavior of helium (He) clusters in tungsten (W), because their diffusion properties provide basic knowledge in understanding the He bubble formation. The binding energy between He and He cluster is shown to be positive, and thus, He is easy to form bubbles by self-trapping. The mean squared displacements (MSDs) were employed to determine the diffusivities of He clusters with different sizes at different temperatures. The He bubble formation at different temperatures with 1% He was also investigated. It is revealed that the formation of He bubbles is strongly associated with the temperature and the diffusivities of the He clusters in W. The results demonstrate the initial stage of the He bubble formation and growth in W. [ABSTRACT FROM AUTHOR]

Abstract: We have constructed W–He and H–He analytical bond-order potentials for a W–H–He system. In combination with the previously self-developed W–H potential [X.-C. Li, X. Shu, Y.-N. Liu, F. Gao, G.-H. Lu, J. Nucl. Mater. 408 (2011) 12] and the Hartree–Fock-dispersion pair potential (Aziz-potential) for He−He interactions, we demonstrate that such potentials behave well for reproducing various properties of the W–H–He system such as defect formation energies, structural properties, and diffusion barriers. Such potentials can be employed to model both the He behaviours and the H–He synergetic effects in the W–H–He system. [Copyright &y& Elsevier]

Abstract: We construct modified W–H and W–W analytical bond-order potentials for a W–H system. In combination with Brenner’s H–H potential, we demonstrate that such potentials can reproduce energetics and structural properties of W and W–H systems, including defect formation energies, surface energies and diffusion barriers as well as melting point determined from first-principles or experiments. The present potentials can be employed for modelling the behaviour of H in W containing defects such as vacancies and surfaces. [Copyright &y& Elsevier]

In fusion reactors, tritium permeation barrier (TPB) technology is one of the key scientific technologies. α-Al 2 O 3 has been considered an ideal candidate material for TPBs. In this work, a series of first-principles calculations have been performed to investigate the dissolution, clustering and diffusion behavior of hydrogen (H) in bulk α-Al 2 O 3. The calculation results show that the most energetically stable form of hydrogen in a perfect α-Al 2 O 3 crystal under H 2 gas annealing treatment is the H 2 molecule. This can also be confirmed by the electron localization function results. The attraction between two H atoms located in first and second nearest octahedral interstitial sites (OISs) is so strong that if multiple H atoms are dissolved in α-Al 2 O 3 , these atoms can migrate toward their adjacent H atoms and form clusters, which will prevent further diffusion of H. The most stable configuration of H cluster in α-Al 2 O 3 is 2H i , with the smallest formation energy and the largest average binding energy. The formation energy and binding energy are similar to those of the gaseous H 2 molecule. We have derived the temperature-dependent diffusivity of the H 2 molecule in α-Al 2 O 3 as D (T) = (3.65 × 10 − 7 m 2 / s) e x p (− 2.27 e V / K T) , which is in good agreement with the experimental values. In addition, both the dissolution energy and migration barrier of the H 2 molecule are so high that dissolution and diffusion of the H 2 molecule in α-Al 2 O 3 are very difficult, resulting in low hydrogen permeability. [ABSTRACT FROM AUTHOR]

Abstract Atomic-level studies have been performed to investigate the hydrogen behavior near a 1/2<111>{110} edge dislocation and a 1/2<111> screw dislocation in α iron. Molecular statics analysis has been carried out to determine the stress distributions and hydrogen binding energy around the dislocation cores. The results reveal that the hydrogen binding is more sensitive to the hydrostatics stress than the shear stress around the edge dislocation, while the shear stress plays a leading role on the hydrogen binding around the screw dislocation. In addition, nudged elastic band (NEB) calculations have been applied to explore different migration paths and the corresponding migration energy barriers of a single hydrogen atom at the dislocation cores. It is of interest to note that both edge and screw dislocations are not able to offer fast pipe diffusion of hydrogen atoms along the dislocation line, as normally considered. Instead, hydrogen atoms prefer to diffuse slowly along oblique paths crossing the edge dislocation line on the slip plane and spiral paths surrounding the screw dislocation line. Furthermore, molecular dynamics simulations have been performed to study the diffusion behavior of hydrogen at the dislocation cores, which verifies the results of NEB calculations. Highlights • The distribution of the stress and hydrogen binding energies around the dislocations together with their correlations. • Neither the edge nor screw dislocations can provide fast diffusion pipes for hydrogen in α iron. • The hydrogen diffusion mechanism near the dislocation cores at different temperatures. [ABSTRACT FROM AUTHOR]

Permeation behavior of deuterium (D) through various tungsten (W) materials, including rolled, annealed, recrystallized W and ZrC/La 2 O 3 -dispersion strengthened W, has been studied. Experiments were performed using a gas-driven permeation facility in the temperature range of 700–1000 K. The permeability of D was not sensitive to the dislocations/vacancies or ZrC/La 2 O 3 phases in W, and the measured permeation values are comparable to Frauenfelder and Zahkarov’s data. The diffusivity of D, however, is microstructure dependent. The higher the density of defects was, the lower the effective diffusivity was measured. These results imply that dislocations/vacancies or small amounts of ZrC/La 2 O 3 phases in W will not influence the hydrogen isotopes steady state permeation flux through the first wall. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations have been performed to investigate the diffusion properties of hydrogen (H) atoms in body-center-cubic (bcc) iron (Fe) with point defects, i.e., vacancies (Vs) and self-interstitial atoms (SIAs). Firstly, the binding energies of a H atom to a H/SIA-H cluster have been calculated by molecular statics simulations. The results show that single-H diffusion should be the major diffusion mechanism in perfect bcc Fe and bcc Fe with SIAs, but for bcc Fe with Vs, results from literatures show that multi-H diffusion should be taken into account. Further, the mean squared displacements method has been employed to determine the diffusivities and diffusion energy barriers of single-H at different temperatures in perfect bcc Fe. The diffusion energy barrier varies with the temperature. Furthermore, the H diffusion properties with different point defect concentrations have also been demonstrated, which showed that diffusivities reduce as the point defect concentration increases and the influence of point defects reduces as the temperature increases. Especially, SIAs exhibit negligible effect when the temperature is higher than 600 K. [ABSTRACT FROM AUTHOR]

The diffusion behavior of hydrogen (H) in tungsten (W) has been studied by molecular dynamics simulations. The diffusivities from 200 K to 3000 K are calculated and the diffusion equation is fitted to D = 5.13 × 10 −8 exp(−0.21 eV / kT) m 2 /s . The diffusion equations in different temperature ranges of 200–500 K, 500–2400 K and 2400–3000 K are also given separately and the results imply different H diffusion modes in different temperature ranges, which is proved by analyzing equilibrium H positions at low and high temperatures. The H atom jumps between different tetrahedral interstitial sites (TISs) at lower temperatures, but the transition to octahedral interstitial sites (OISs) is also observed at high temperatures. Moreover, with a vacancy present in the W system, vacancy trapping of H is observed, and it is shown that the vacancy trapping reduces with the increasing temperature. The H binding energy to monovacancy is obtained using three different methods including NEB and fitting both H diffusivity and time for H to detrap from the vacancy, which provides more information of the H behaviors with the existence of defects. [ABSTRACT FROM AUTHOR]