Tungsten is widely considered a promising material for components in fusion reactors, but new billion-atom simulations suggest that extreme-energy collisions may create more radiation damage in the metal than existing models predict. The simulations reveal a previously unaccounted-for shift in how damage increases as collision energy rises, giving researchers a more detailed picture of what happens to tungsten at the highest energies.
Fusion reactors contain superheated, electrically charged plasma where nuclear fusion reactions take place. The extreme conditions can damage the materials surrounding that plasma.
One source of this damage is neutrons, which are electrically neutral particles released during fusion reactions. When a high-energy neutron strikes an atom, it can knock that atom out of its usual position. The displaced atom can then hit other atoms, causing them to move as well. This chain of collisions creates what is known as primary radiation damage.
Understanding how this process unfolds is important for predicting how materials such as tungsten behave under intense irradiation.
A team at the University of Helsinki used molecular dynamics simulations to examine those atomic collisions in tungsten at extremely high energies. The work was made possible by a machine-learning model designed to reproduce the material’s molecular dynamics and physical processes efficiently enough to run very large simulations.
A billion atoms in one simulation
The project originally began as an effort to adapt the team’s machine-learning simulation model so it could run efficiently on graphics processing units, or GPUs.
That change allowed the researchers to perform much larger simulations than before. They then used the approach to investigate the damage produced when tungsten is struck by extremely high-energy ions.
One common way to measure primary radiation damage is to count how many atoms are knocked out of their normal positions after an atom is hit by a high-energy neutron.
Existing models for metals assume that the number of defects initially increases sublinearly with recoil energy and then becomes linear. In other words, the number of defects does not rise as quickly at first, before eventually increasing in direct proportion to the energy.
The simulations produced a different pattern for tungsten.
The researchers found that the relationship changes from sublinear to superlinear and then finally to linear as recoil energy increases. The superlinear stage means that the number of defects increases faster than the energy during that part of the range.
Capturing these changes required simulations on an unusually large scale. The researchers reached one billion atoms in a single simulation.
Extreme energies produced more damage
The simulations suggest that tungsten components exposed to the extremely high energies expected inside fusion reactors could deteriorate more than previously anticipated.
The researchers described two main outcomes from the work. One is technical: the simulations demonstrate that accurate calculations involving a billion atoms are possible and can be used to explore physical behavior that was not previously accessible at this scale.
The other is scientific. The simulations produced a model of primary radiation damage in tungsten that can be used in the broader study of fusion materials, including for making predictions about radiation damage over longer time periods relevant to fusion reactors.
The simulations focus on only the first stage of damage
The study examined primary radiation damage, meaning the damage created by a single atomic recoil.
Actual reactor operation involves much more irradiation over longer time and length scales. Understanding how materials respond under those conditions remains an active area of research.
The researchers are continuing to develop their simulation tools with the goal of increasing both their accuracy and the size of the systems they can model. Their work is also moving toward higher irradiation doses and larger length scales.
They are interested in how features of a material’s microstructure, including different grain sizes, grain boundaries and alloying elements, affect its behavior.
The approach used for tungsten could also eventually be applied to other materials exposed to high-energy environments.
The study was published in Physical Review Letters.
