(389c) Analysis of Mobile Helium Cluster Dynamics Near Surfaces and Grain Boundaries of Plasma-Exposed Tungsten

Plasma facing components (PFCs) as the first wall materials in nuclear fusion reactors are exposed to intense plasma heat and particle fluxes. The implantation of helium (He) atoms into these materials impacts significantly their surface morphological and near-surface structural evolution. Tungsten (W) is a promising PFC material because of its thermomechanical properties. In tungsten, such interstitial He atoms are very mobile and aggregate to form clusters of various sizes. The smallest of these helium clusters, containing n He atoms with n = 1-7, also are mobile and their diffusional transport mediates the evolution of surface morphology and the sub-surface helium gas bubble structure and dynamics.

In this presentation, we report the results of a systematic atomic-scale analysis of the dynamics of small mobile helium clusters near low-Miller-index tungsten surfaces, aiming at a fundamental understanding of the near-surface transport and kinetics of helium-carrying species in plasma-exposed tungsten. These small mobile helium clusters are attracted to the surface and migrate to the surface by Fickian diffusion and drift due to an elastic interaction that generates a thermodynamic driving force for surface segregation. As the clusters migrate toward the surface, trap mutation (TM) and cluster dissociation reactions are activated at rates higher than in the bulk. TM produces W adatoms and immobile complexes of helium clusters surrounding W vacancies located within the lattice planes at a short distance from the surface. These reactions are identified and characterized in detail based on analysis of a large number of molecular-dynamics (MD) trajectories for each such mobile cluster near W(100), W(110), W(111), and W(211) surfaces. TM is found to be the dominant cluster reaction for all cluster and surface combinations, except for the clusters with n = 4 and n = 5 near W(100) where cluster partial dissociation following TM dominates. We find that there exists a critical cluster size, n = 4 near W(100), W(111), and W(211) and n = 5 near W(110), beyond which formation of multiple W adatoms and vacancies in TM reactions is observed.

In addition, we have carried out a systematic atomic-scale analysis of the dynamics of small mobile helium clusters near a prototypical, symmetric tilt grain boundary (GB) in tungsten based on MD simulations. The mobile clusters are attracted to the GB by an elastic interaction, similar to that with the surfaces, which generates a drift flux toward the GB and leads to helium segregation on the grain boundary. We find that TM reactions are activated in the vicinity of the GB, they are the dominant kinetic processes for clusters with n = 4-7, and only a single vacancy is formed in each of the TM reactions identified regardless of cluster size n. The displaced W atom generated from the TM reaction near the GB forms an extended W interstitial configuration on the GB. This interstitial configuration is characterized by mobility that depends on the location where the TM reaction occurs: it is immobile when the vacancy produced by the TM reaction is located a few lattice planes away from the GB plane and highly mobile along a specific direction when the produced vacancy is located on the GB. The latter mechanism initiates a potentially fast migration path for W atoms along the GB toward a free surface, which may influence significantly the surface morphology of plasma-exposed tungsten.

The identified cluster reactions in the vicinity of surfaces and grain boundaries in tungsten are responsible for important structural, morphological, and compositional features in plasma-exposed tungsten, including surface adatom populations, near-surface immobile helium-vacancy complexes, and retained helium content in the tungsten, which are observed in our large-space-scale MD simulations of He implantation and resulting near-surface evolution in tungsten. These features are expected to strongly influence the amount of hydrogen re-cycling and tritium retention in fusion tokamaks. We have also analyzed the effects of helium segregation on surfaces and grain boundaries in tungsten on their elastic interactions with mobile helium clusters and on the resulting cluster reaction rates. The results of our study contribute significantly to the parameterization of continuum transport-reaction models for the computationally efficient, coarse-grained modeling of such cluster dynamics.