(332a) Helium Transport and Bubble Growth Near Grain Boundaries in Plasma-Facing Tungsten

We conduct molecular dynamics simulations to study helium behavior near surfaces and grain boundaries in tungsten. Grain boundaries perpendicular to the surface and parallel to the surface are considered. The net attraction of mobile helium to the grain boundary results in a "denuded zone" within approximately 3.5 nm of the grain boundary plane; helium transport slows considerably once helium clusters encounter a grain boundary. Helium retention is also strongly affected by grain boundaries and grain size: grain boundaries parallel to the plasma-facing direction tend to reduce helium retention due to the attraction of mobile clusters to the grain boundary, which reduces the concentration of helium near the surface, where it is being implanted. Grain boundary planes perpendicular to the surface tend to increase helium retention due to retention of helium on the grain boundary plane. For grain boundaries parallel to the surface, the strong attractive force of the grain boundaries results in essentially no helium transport across the grain boundary plane during at least the first microsecond. It is not known how long such grain boundaries are able to prevent helium from making its way deep into the metal, or whether such a barrier would have any measurable impact on observable phenomena such as the growth of tungsten fuzz.