(388c) Thermal Gradient Effect on Helium and Intrinsic Defect Transport in Tungsten

Plasma-facing materials (PFMs) in nuclear fusion reactors are expected to withstand stringent conditions, with high heat and particle fluxes that modify the materials microstructure. These fluxes can create strong gradients of temperature and concentration of diverse species in the PFM. In addition to the helium (He) ash and hydrogenic species, neutron particles generated in the fusion reaction will be implanted in the material creating intrinsic point defects, such as vacancies and self-interstitials atoms (SIAs), and their clusters. Additional vacancies and SIAs are generated by trap mutation reactions undergone by helium clusters in the PFM bulk and in regions near the plasma-exposed surfaces. These defects as well as He atoms and small mobile helium clusters will then migrate in the presence of the afore-mentioned gradients.

In this work, we use nonequilibrium molecular-dynamics (NEMD) simulations to study the transport of He, vacancies, and SIAs in the presence of a thermal gradient in tungsten used as PFM. We find that, in all cases, the intrinsic point defects and impurity atoms tend to migrate toward the hot regions of the material and calculate their concentration profiles in the direction of the temperature gradient. We also analyze thermal and species transport in tungsten within the framework of irreversible thermodynamics. The resulting concentration profiles from the NEMD simulations are in agreement with the predictions of irreversible thermodynamics. We compute a negative heat of transport for each species analyzed, which indicates that the respective driven species fluxes are directed opposite to the heat flux. These results have important implications for PFMs in fusion environments, mostly when abnormal operation (edge-localized modes or disruptions) occurs in the plasma, which increases the heat flux toward the material and intensifies the thermal gradients. We demonstrate that when thermomigration, i.e., drift species transport driven by the thermal gradient also known as Soret effect, is considered, the resulting steady-state profiles differ significantly from those when species transport is decoupled from heat transport.