Kinetic Monte Carlo Simulations of Helium Cluster Nucleation in Tungsten with Preexisting Vacancies

The object kinetic Monte Carlo code Kinetic Simulations Of Microstructure Evolution (KSOME) was used to study the subsurface helium clustering behavior in tungsten as a function of temperature, helium implantation rate, and vacancy concentration. The simulations evaluated helium implantation fluxes from 10²² to 10²⁶ m⁻² · s⁻¹ at temperatures from 473 to 1473 K for 100-eV helium ions implanted below tungsten surfaces and for vacancy concentrations between 1 and 50 parts per million. Such vacancy concentrations far exceed thermodynamic equilibrium values but are consistent with supersaturated concentrations expected during concurrent, or preexisting, neutron irradiation. The thermodynamics and kinetic parameters to describe helium diffusion and clustering are input to KSOME based on values obtained from atomistic simulation results. These kinetic Monte Carlo results clearly delineate two different regimes of helium cluster nucleation, one dominated by helium self-trapping at high implantation rates and lower temperatures and one where helium–vacancy trapping dominates the helium cluster nucleation at lower implantation rates and higher temperatures. The transition between these regimes has been mapped as a function of implantation rate, temperature, and vacancy concentration and can provide guidance to understand the conditions under which neutron irradiation effects may contribute to subsurface gas nucleation in tungsten plasma-facing components.