(335d) Helium Aggregation Near Grain Boundaries in Plasma-Facing Tungsten

Helium is known to accumulate in plasma-facing materials, forming bubbles. Helium transport is also slowed markedly by the presence of grain boundaries. With this in mind, we report the results of molecular dynamics simulations of helium plasma exposure for intervals of 0.5–1 μs of several tungsten bicrystals with low-Miller-index surfaces exposed to the plasma. These bicrystals have grain boundaries either intersecting the surface at right angles or parallel to the original surface approximately 6 nm below it. As expected, helium accumulates on the grain boundaries and is trapped there, but we also observe a “depleted region” around the grain boundaries up to 3.5 nm away from the grain boundary plane that is largely devoid of helium bubbles over this time interval. The distributions of helium atoms and clusters in the directions parallel to the grain boundary plane are largely indistinguishable from those from single-crystal simulations—it is only the out-of-plane distribution that is significantly different. The presence of grain boundaries does not seem to impede the formation of adatom “islands” generated by prismatic dislocation loops that are annihilated on the surface, but some surface orientations allow the dislocations to traverse the grain boundary plane rather than escaping into the bulk, creating a ridge at the junction of the grain boundary with the surface. Grain boundaries parallel to the surface essentially act as a barrier for helium, trapping any helium atoms and preventing them from making their way deeper into the crystal. This results in a very abrupt change in the helium distribution as a function of depth at the grain boundary, with only a few stray atoms making their way into the grain facing away from the plasma.