Double, Double Toil and Trouble: Tungsten Burns and Helium Bubbles

New models reveal the impact of competing processes on helium bubble formation in plasma-exposed tungsten.

Image courtesy of Los Alamos National Laboratory
Snapshots of a helium bubble just before bursting when grown at slow versus fast rates. At slow rates, the bubble experiences the influence of the surface more readily and grows more asymmetrically toward the surface. At fast rates, the bubble grows more symmetrically and is larger when it bursts, creating more surface debris. The colors indicate helium atoms (blue) and tungsten atoms (red).

The Science

When simulated helium (He) bubbles grow quickly, the surrounding tungsten (W) cannot respond, leading to over-pressurized bubbles that burst violently when they reach the surface. When the bubbles grow more slowly, the tungsten atoms pressed against the bubble’s surface can diffuse around it, leading to a smaller bubble when it ultimately bursts.

The Impact

Helium bubbles are detrimental to plasma-facing materials such as tungsten in fusion reactors, which could serve as a possible new power source. Thus, understanding how helium bubbles form and grow is important for predicting large-scale material response to the extreme fusion environment. This study presents the first atom-based simulation of helium bubble growth at a rate appropriate for understanding bubble formation in fusion plasma facing materials in ITER, a large-scale experiment focused on the viability of fusion energy. The helium simulations find a qualitatively different growth mode when helium arrival rates approach experimental values. These simulations reveal rate effects on bubble size, shape, pressure, and surface damage.

Summary

Accelerated molecular dynamics was used to extend the time scale of atomistic simulations beyond conventional molecular dynamics while retaining full atomic fidelity. The deployment of these methods on the Titan supercomputer at Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory has enabled, for the first time, the simulation of the evolution of helium bubbles at helium implantation rates appropriate for the conditions at ITER. The simulations revealed that at high helium implantation rates, the tungsten atoms surrounding the bubble simply did not have time to respond to the accumulated pressure, resulting in highly over-pressurized bubbles that grew to large sizes and burst violently upon reaching the material surface. In contrast, once the helium implantation rate was reduced to more reasonable values, possible via the use of accelerated molecular dynamics methods on up to 50% of the supercomputer, the time between the arrivals of helium atoms to the bubble was much longer, allowing the tungsten atoms to respond to the pressure within the bubble. In particular, tungsten interstitial atoms at the surface of the bubble can diffuse around it and feel the nearby surface. This interaction leads to bubble growth directed toward the surface and results in a smaller bubble size when it ultimately bursts. These results highlight the importance of accounting for all relevant kinetic processes and how these kinetic processes enhance the interaction of, in this case, the helium bubble with the local microstructure. The results further have consequences for the nucleation of surface morphology on the tungsten, which is ultimately the source of fuzz, a nanostructured “steel wool”-like structure that causes significant degradation in performance of the material.

Contact

Luis Sandoval, Danny Perez, Blas P. Uberuaga, and Arthur F. Voter
Los Alamos National Laboratory
luisand@lanl.gov, danny_perez@lanl.gov, blas@lanl.gov, afv@lanl.gov

Funding

LS, DP, and BPU acknowledge support by the DOE, Office of Science, Office of Fusion Energy Sciences, and Office of Advanced Scientific Computing Research through the Scientific Discovery through Advanced Computing (SciDAC) project on Plasma-Surface Interactions. AFV was supported by the DOE, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. This research used resources at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory and the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory.

Publications

L. Sandoval, D. Perez, B.P. Uberuaga, A.F. Voter, “Competing kinetics and He bubble morphology in W.” Physical Review Letters 114, 105502 (2015).

Related Links

Fusion Researchers Use Titan to Burst Helium Bubbles

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