(551f) Defect Interaction Energetics and Stress State of Plasma-Exposed Tungsten

Tungsten (W) is a promising plasma-facing component (PFC) material in nuclear fusion reactors because of its excellent thermomechanical properties and low sputtering yield. In tungsten, interstitial He atoms are very mobile and aggregate to form clusters of various sizes. Small, mobile helium clusters (He_n, 1 ≤ n ≤ 7) are attracted to the tungsten surface due to an elastic interaction force that drives surface segregation, and their diffusional transport mediates the dynamics of surface morphology and near-surface microstructure.

Here, we report results for helium-related defect interaction energetics near surfaces of tungsten that has been exposed to different levels of He ion irradiation based on atomistic simulations according to a reliable many-body interatomic potential. At higher helium fluence, mobile helium clusters are subjected to cluster-defect interactions in addition to cluster-surface interactions, which complicate cluster dynamics beyond the dilute limit of helium content in the PFC material. Based on configurations generated by large-scale molecular-dynamics (MD) simulations of implanted helium evolution in plasma-exposed tungsten, we examine the effects of varying helium fluence due to increased plasma exposure byconducting systematic molecular-statics (MS) computations of small helium cluster energeticsnear the W surface as a function of distance (depth) of the cluster center from the surface on a grid of lateral locations on the surface. We analyze the defect interactions that mediate the energetics of small helium clusters migrating to the surface, taking into account that the migrating cluster also is subjected to the stress fields generated by larger helium bubbles, as well as other small clusters. The outcome of this analysis is the systematic parameterization of mobile helium cluster energetics at varying levels of He irradiation through functional forms that include contributions from cluster-cluster and cluster-bubble interactions as well as cluster-surface interactions. Such parameterizations of the energetics of helium cluster interactions with tungsten defects and surfaces are subsequently incorporated into hierarchical multiscale models of helium cluster dynamics in PFC materials.

Furthermore, based on detailed characterization of the configurations generated from the large-scale MD simulations, we construct models of plasma-exposed tungsten with a near-surface helium nanobubble region represented by regular arrays of over-pressurized nanobubbles of proper sizes and separation distances. We carry out MD simulations of the deformation state of such model nanobubble regions near tungsten surfaces of various crystallographic orientations. In all cases examined, we identify the state of stress in the near-surface nanobubble layer as equibiaxial compressive stress. The MD-computed biaxial stress level is in good quantitative agreement with that predicted by a heterogeneous elastic inclusion model in conjunction with an equation of state for the pressure in the helium bubbles. Our findings for the state and level of stress in PFC tungsten constitute an important component in the development of atomistically-informed, continuum-scale models of surface morphological evolution in PFC materials.