(332f) Analysis of Soret Diffusion of Helium, Hydrogen, and Intrinsic Defects in Tungsten

Plasma-facing components (PFCs) in a nuclear fusion reactor are expected to withstand harsh conditions, with high particle fluxes and extreme heat loads that modify the PFC materials microstructure. These fluxes will create strong gradients of temperature and concentration of diverse species, including He and H atoms and small mobile helium clusters. In addition to these impurity species, neutron particles generated in the fusion reaction will penetrate the PFC material creating intrinsic defects, such as vacancies, self-interstitial atoms (SIAs), and clusters of such point defects. These defects and impurity species will then migrate in the PFC material in the presence of the aforementioned gradients.

In this study, we use nonequilibrium molecular-dynamics (NEMD) simulations to analyze the transport of He, mobile helium clusters, H, and SIAs in the presence of a thermal gradient in tungsten. We find that all the species examined tend to migrate toward the hot regions of the tungsten sample. The resulting species concentration profiles are exponential distributions in the direction of the imposed temperature gradient, rising toward the hot regions of the sample, in agreement with irreversible thermodynamics analysis. For all the species examined, intrinsic point defects and impurities, we find that the resulting species flux is directed opposite to the heat flux, indicating that species transport is governed by a Soret effect, namely, thermal-gradient-driven diffusion, characterized by a negative heat of transport that drives species transport uphill, i.e., from the cooler to the hot regions of the tungsten sample. The findings of our thermal and species transport analysis have been implemented in our cluster-dynamics code, Xolotl, which has been employed to compute temperature and species profiles over spatiotemporal scales representative of plasma-facing tungsten under typical reactor operating conditions, including extreme heat loads at the plasma-facing surface characteristic of plasma instabilities that induce edge localized modes (ELMs). We demonstrate that the steady-state species profiles obtained accounting for the Soret effect vary significantly from those where temperature-gradient-driven transport is not accounted for and discuss the implications of such a Soret effect on the response to plasma exposure of plasma-facing tungsten.