(335c) Effects of Elastic Softening and Surface Hole Formation on Surface Morphological Evolution in Plasma-Facing Tungsten

Tungsten is the plasma-facing component (PFC) material for the divertor region of the International Thermonuclear Experimental Reactor (ITER) due to its exceptional thermomechanical properties, such as high melting point, mechanical strength, and thermal conductivity, as well as its low sputtering yield and low tritium retention. A large body of experimental evidence has established that PFC tungsten suffers severe surface degradation as a result of exposure to high fluxes of helium (He) and extreme heat loads. Specifically, experimental studies have shown that, under typical operating conditions expected in ITER, He implantation above a threshold of incident ion energy of approximately 35 eV causes formation of a fuzz-like nanostructure on the tungsten surface over the temperature range from 900 K to 2000 K. The formation of such surface nanostructure has adverse effects on the mechanical behavior and structural response of PFC tungsten as well as on the reactor performance.

Here, we report results on the surface morphological evolution of PFC tungsten and examine a number of factors that impact such evolution. Our analysis is based on self-consistent dynamical simulations according to an atomistically-informed, continuum-scale surface evolution model that has been developed following a hierarchical multiscale modeling strategy and can access the spatiotemporal scales of relevance to fuzz formation. The model accounts for PFC surface diffusion driven by the compressive stress originating from the over-pressurized helium bubbles in a thin nanobubble region, which forms in the near-surface region of PFC tungsten as a result of He implantation, in conjunction with formation of self-interstitial atoms in tungsten that diffuse toward the surface. The model also accounts for the softening of the elastic moduli of PFC tungsten, both thermal softening at high temperature and softening due to He accumulation in tungsten upon implantation, and the elastic moduli are time dependent and evolve until the He content in tungsten reaches its steady state. The dependence of the elastic moduli on the He content follows an exponential scaling relation predicted by molecular-dynamics simulations, while the He content in the near-surface region of PFC tungsten evolves according to a first-order saturation kinetics, consistent with experimental and simulation results reported in the literature. Our analysis establishes that this elastic softening accelerates nanotendril growth on the PFC surface and the onset of fuzz formation. We also introduce the concept of an incubation time as a kinetic metric for nanotendril growth on the PFC surface, which is equivalent to that of incubation fluence discussed in the literature, in order to predict and explain the minimum exposure time required to observe fuzz formation on PFC tungsten surfaces.

Furthermore, our simulations account, in an empirical fashion, for two types of subsurface bubble dynamical phenomena in the nanobubble region of PFC tungsten during He plasma irradiation, involving bubble bursting and surface crater formation. We demonstrate that the bubble-bursting-mediated surface hole formation effect on the PFC tungsten surface further accelerates the growth rate of nanotendrils and the onset of fuzz formation. As a result, the predicted incubation time for surface nanotendril growth is shortened, in agreement with experimental data of incubation fluence at comparable plasma exposure conditions. We also explore systematically the dependence of the PFC surface morphological response on the areal density of holes introduced at regular time intervals onto the He-implanted tungsten surface, a parameter in our analysis that serves as a proxy for the rate of He bubble bursting. More importantly, our simulations capture fine surface features in the PFC tungsten surface morphology and predict that the average spacing between nanotendrils is on the order of 100 nanometers, consistent with experimental findings.