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The recombination of one Frenkel pair in tungsten has been investigated through first-principles simulation. Two different recombination types have been identified: instantaneous and thermally activated. The small recombination barriers for thermally activated recombination cases indicate that recombination can occur easily with a slightly increased temperature. For both of the two recombination types, recombination occurs through the self-interstitial atom moving towards the vacancy. The recombination process can be direct or through replacement sequences, depending on the vertical distance between the vacancy and the 〈1 1 1〉 line of self-interstitial atom pair. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated diffusion and permeation behaviors of hydrogen (H), deuterium (D), and tritium (T) in bulk molybdenum (Mo) by a first-principles calculations combined with simplified models. The H diffusion energy barrier with quantum correction is shown to be 0.12eV, in good agreement with the value of 0.11eV obtained from experiment. According to the diffusion theory presented by Wert and Zener, the diffusion coefficients of H, D and T are estimated, respectively. The H diffusion coefficient is found to be , which is also basically consistent with the experiment. Permeability Φ can be approximately evaluated in terms of Φ = SD, where S and D are the solubility and the H diffusion coefficient, respectively. We found that H has the low solubility in bulk Mo, consistent with the results at the temperature from 900K to 1500K in the earlier experiment. The permeability of H isotopes are calculated to be one order of magnitude larger than those of the experimental values. Such discrepancy should stem from that the “defect-free” Mo in the present calculations is considered. The permeation flux of H isotopes are finally examined. At the temperature of 1200K, the fluxes of H, D, and T are calculated to be 1.27×10−6 molm−2 s−1, 8.95×10−7 molm−2 s−1, and 7.30×10−7 molm−2 s−1, respectively. [Copyright &y& Elsevier]

We investigate the stability, diffusion, and impurity concentration of nitrogen in intrinsic tungsten single crystal employing a first-principles method, and find that a single nitrogen atom is energetically favourable for sitting at the octahedral interstitial site. A nitrogen atom prefers to diffuse between the two nearest neighboring octahedral interstitial sites with a diffusion barrier of 0.72 eV. The diffusion coefficient is determined as a function of temperature and expressed as D(N) = 1.66 × 10-7 exp(-0.72/kT). The solubility of nitrogen is estimated in intrinsic tungsten in terms of Sieverts' law. The concentration of the nitrogen impurity is found to be 4.82 × 10-16 A-3 at a temperature of 600 K and a pressure of 1 Pa. A single nitrogen atom can easily sit in an off-vacancy-centre position close to the octahedral interstitial site. There exists a strong attraction between nitrogen and a vacancy with a large binding energy of 1.40 eV. We believe that these results can provide a good reference for the understanding of the behaviour of nitrogen in intrinsic tungsten. [ABSTRACT FROM AUTHOR]

Molecular dynamic simulation is utilized to study the nanocluster and the fuzz structure on the PFM surface of tungsten. The polyhedral and linear cluster structures based on the icosahedron, cuboctahedron and rhombic dodecahedron are built up. Three interatomic potentials are used in calculating the relationship between the cluster energy and the number of atoms. The results are compared with first-principles calculation to show each potential’s best application scale. Furthermore, the transition between the icosahedral and the cuboctahedral clusters is observed in molecular dynamic simulation at different temperatures, which follows a critical curve for different numbers of atoms. The linear structures are proved to be stable at experimental temperatures by thermodynamics. The work presents a selection of interatomic potentials in simulating tungsten cluster systems and helps researchers understand the growth and evolution laws of clusters and the fuzz-like structure formation process in fusion devices. [ABSTRACT FROM AUTHOR]

Abstract: Using a first-principles method, we have investigated dissolution and diffusion properties of oxygen (O) in tungsten (W). Single O atom prefers to occupy the tetrahedral interstitial site (TIS). Two interstitial O atoms are attractive and tend to be paired up at two neighboring TIS with a distance of 0.228nm and a large binding energy of 1.60eV, which indicates a strong tendency of O clustering in W. O is preferred to diffuse between the most nearest neighboring TIS with a diffusion barrier of 0.17eV. By the estimation of pre-exponential factor according to an empirical theory, the diffusion coefficient as a function of temperature has been determined, which is 1.50×10−9 m2/s at a typical temperature of 500K. The results provide a good reference to understand the behavior of O in intrinsic W. [Copyright &y& Elsevier]

We apply the Object Kinetic Monte Carlo method to simulate the hydrogen (H) clustering behaviour with large vacancy (V) clusters in tungsten. The main advantage of this method is to consider the temperature effects, large V clusters, and long-time evolution. It is thus a very useful tool to study the microstructure evolution of defects in irradiated materials covering from the atomic scale to mesoscale. Simulation results show that the number of trapped H atoms by V and V clusters decreases with the increase of temperature. This is expected because the de-trapping ability of H atoms increases when the temperature is increased. With the correction of the zero-point-energy in calculating binding energies, the present results show that the number of H atoms trapped by monovacancy decreases for all studied temperatures. We also found that the trapping ability of V clusters for H decreases with the increase of the sizes of V clusters, independent on temperature. The H concentration can largely affect the clustering behaviour of H m V n. When the H saturation concentration is reached, the clustering behaviour of H m V n is mainly determined by the sizes of V clusters. [ABSTRACT FROM AUTHOR]