Molecular dynamics simulations of the diffusion and coalescence of helium in tungsten

Molecular dynamics (MD) simulations are performed on the diffusion and coalescence of helium in tungsten. A new method for determining the effective capture radii (ECRs) and the dissociation energies of helium-related defects is proposed in this work. It is observed that the ECR of an interstitial helium atom trapping helium interstitials (denoted as He-He-n, n = 1-3) decreases with increasing temperature, except for He-He-2 at T < 400 K. The traditional view that the ECR is approximately equal to the lattice constant, which has been widely used in kinetic Monte Carlo (KMC) and rate theory (RT) models, is only valid in some cases. However, the ECR between an interstitial helium atom and a substitutional helium atom (denoted as He-Hey) always approximates the third nearest-neighbor tetrahedral positions of the Hey. The diffusion coefficients D-n for helium clusters are also investigated. He-2 migrates more quickly than a single He atom does at T < 400 K, whereas the diffusion path of He-2 changes at higher temperatures. Another counterintuitive observation is that D-5 > D-3 > D-4 at T < 500 K, which can be attributed to the disordered structure of He-5. The Arrhenius relation describes the diffusion of He-n well in the temperature range from 300 K to 550 K, whereas the diffusion is not a standard thermally activated process at higher temperatures. Taken together, these results help elucidate the initial stage of helium bubble formation in tungsten as well as the requirements of long-term evolution methods such as KMC or RT models. (C) 2013 Elsevier B.V. All rights reserved.