(457f) Atomistic and Continuum Drift-Diffusion Simulations of Helium Transport in Tungsten

Helium from linear and tokamak-like plasma devices has been known to cause "fuzz" and other nanometer-scale features on the surface of tungsten and other metal surfaces after a few hours of plasma exposure. The mechanisms involved occur on multiple length and time scales, and understanding all of those details is the subject of significant multi-scale modeling efforts. In particular, the spatial and temporal scales necessary to simulate even a small portion of the tungsten divertor in a real fusion device make atomistic simulations intractable, so we have sought a continuum-based description that is informed by atomistic simulations of more accessible systems. This effort comes in four parts: (1) large-scale atomistic simulations of helium diffusion in tungsten, which aims to discover mechanisms and provide a benchmark against which more coarse-grained models can be tested; (2) small, targeted molecular dynamics and statics calculations that quantify the rates of specific mechanisms, such as drift forces near surfaces and grain boundaries, and (3) coarse-grained, drift-reaction-diffusion models that incorporate information from small-scale molecular dynamics simulations and use large-scale simulations as a benchmark for still-larger, more realistically-sized simulations of plasma-facing tungsten. The orientation of the surfaces, the presence of grain boundaries, and the presence of other bubbles all have a strong impact on the rate of local helium transport, resulting in a spatially-dependent rate of helium transport in the divertor. These tools aim to provide a tractable and reasonably accurate discription of helium bubble distributions over time, an important step in the process of understanding tritium retention and material fatigue in nuclear fusion reactors.