Diffusion and transformation kinetics of small helium clusters in bulk tungsten

Abstract

The production of energy through nuclear fusion poses serious challenges related to the stability and performance of materials in extreme conditions. In particular, the constant bombardment of the walls of the reactor with high doses of He ions is known to lead to deleterous changes in their microstructures. These changes follow from the aggregation of He into bubbles that can grow and blister, potentially leading to the contamination of the plasma, or to the degradation of their mechanical properties. We computationally study the behavior of small clusters of He atoms in W in conditions relevant to fusion energy production. Using a wide range of techniques, we investigate the thermodynamics of the clusters and their kinetics in terms of diffusivity, growth, and breakup, as well as mutation into nano-bubbles. Our study provides the essential ingredients to model the early stages of He exposure leading up to the nucleation of He bubbles. arXiv:1406.6418v1 [cond-mat.mtrl-sci]

Effects of temperature and surface orientation on migration behaviours of helium atoms near tungsten surfaces

“…The Finnis-Sinclair type potential proposed by Ackland and Thetford [21], which has been widely used in the literature [10,12,13,[15][16][17], was adopted for the interactions between W atoms. For the interactions between the He and W atoms, we used a pairwise potential that was constructed by fitting its long range part to ab initio data and smoothly connecting the long range part to Ziegler-Biersack-Littmark (ZBL) potential in the short range part [19].…”

“…There is also chance for a helium atom to dissociate from a trapping site and to return to diffusion status [15]. For the diffusion behaviours of the He atoms and the small He clusters in the bulk W, MD studies were conducted by Wang et al [19], Zhou et al [15] and Perez et al [16]. These studies found the non-Arrhenius diffusion of He atoms at high substrate temperatures and some counter-intuition features for the migration of small He clusters (e. g., He 5 cluster migrates faster than He 4 cluster).…”

“…There have been a number of MD simulations that deal with the interaction of helium atoms with tungsten [7][8][9][10][11][12][13][14][15][16][17][18]. The He atoms, starting from their incidence on a surface, are generally considered to experience two major phases.…”

Molecular Dynamics in the Multicanonical Ensemble: Equivalence of Wang–Landau Sampling, Statistical Temperature Molecular Dynamics, and Metadynamics

“…Indeed, as interstitial He cluster grow, they reach a point where they are able to eject W atoms from the lattice and condense into the resulting vacancies, creating the nuclei of a bubble that can then grow and disrupt the structure of the material (see Refs. [36,37] for further details and results on that system). The reference temperature is set to T 0 = 3500 K (upper temperature boundary of right plot in Fig.…”

Object kinetic Monte Carlo simulations of irradiated tungsten for nuclear fusion reactors

“…Although small clusters can migrate [201,202], the assumption is justified because cluster diffusion slows down with the cluster size and is strongly suppressed by impurities and interstices [67,195,203]. In addition, trap mutation reactions would trap the pure He n clusters [157,204]. Trap mutation consists of the growth of He n (in the case of pure He atoms in interstitial positions is also named self-trapping [157]) and He n V m clusters by trapping He.…”