Thermal gradient effect on the transport properties of helium and intrinsic defects in tungsten

Plasma-facing materials (PFMs) in a fusion reactor are expected to withstand stringent conditions, with high heat and particle fluxes that modify the materials microstructure. These fluxes create strong gradients of temperature and concentration of diverse species. Besides the He ash, neutron particles will create intrinsic point defects, such as vacancies and self-interstitials atoms (SIAs), and their clusters. These species will then migrate in the presence of the afore-mentioned gradients. In this work, we use nonequilibrium molecular dynamics simulations to study the transport properties of He, vacancies, and SIAs in the presence of a thermal gradient in tungsten. In all cases, the defects and impurity atoms tend to migrate toward the hot regions of the material. The resulting concentration profiles are in agreement with irreversible thermodynamics. We compute a negative heat of transport for all species, which indicates that the respective driven species fluxes are directed opposite to the heat flux. We demonstrate that, when the mass-heat transport coupling is considered, the resulting steady-state profiles vary significantly from those when species transport is decoupled from heat transport.