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

Authors: 
Hammond, K. D., University of Missouri
Naeger, I. V., University of Missouri
Hu, L., University of Massachusetts Amherst
Blondel, S., University of Tennessee
Wirth, B. D., University of Tennessee, Knoxville
Maroudas, D., University of Massachusetts, Amherst
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.