Simulations of Hydrogen and Helium Near Grain Boundaries in Plasma-Facing Tungsten*

We conduct molecular dynamics simulations of hydrogen and helium transport in plasma-facing tungsten in the presence of grain boundaries. These simulations serve as benchmarks for simulations based on continuum mechanics or kinetic Monte Carlo and also reveal and explore mechanisms of gas transport and surface evolution in plasma-facing materials. Helium and hydrogen are both strongly attracted to and immobilized on the grain boundaries, and grain boundaries serve as barriers that generally prevent hydrogen or helium from crossing them for times up to hundreds of nanoseconds. As it does in single-crystal systems, helium forms bubbles beneath the surface, but the attractive forces near the grain boundaries draw helium to them, sweeping out a helium-depleted region several nanometers in each direction from the plane of the grain boundary. Hydrogen, which does not form bubbles in the bulk without defects present, also collects along the grain boundary, and the regions on either side of the grain boundary are also depleted of hydrogen. We see little evidence, even at half a microsecond (fluences on the order of 10²⁰ m⁻² s⁻¹), of any phenomena that would lead to hydrogen blister formation, though we cannot rule out the possibility that it will occur at much later times.