As fusion technology progresses, identifying materials that can withstand the extreme conditions within reactors is essential—particularly for components like divertors, which bear the brunt of plasma exposure. Researchers at Nicola Marzari’s MARVEL lab at EPFL recently took on this challenge.
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In a study published in PRX Energy, the team presented a cutting-edge computational method for screening potential plasma-facing materials. This large-scale approach allowed them to identify a shortlist of promising candidates that could significantly advance fusion technology.
Fusion offers the possibility of nearly limitless, clean energy without greenhouse gas emissions, yet substantial technological challenges remain, particularly regarding materials. Fusion reactors need components that can endure intense conditions where plasma and reactor structures interact.
For example, the ITER reactor in France includes a divertor—a critical part that channels heat and particles away from the plasma to protect the reactor. The materials facing the plasma in this divertor must handle extreme temperatures and constant exposure to high-energy particles and radiation.
Although tungsten is currently used in ITER for its heat resistance, alternatives like carbon fibers and ceramics have been explored. The ideal material for future reactors is still under investigation, leading scientists at EPFL’s MARVEL lab to apply a fresh computational approach. Their method screened a wide range of materials, bringing us closer to finding optimal solutions for the future of fusion energy.
First, the scientists needed to develop a method to make the computations manageable.
A realistic simulation of the dynamics at the plasma-material interface would require simulating the behavior of thousands of atoms over several milliseconds, that would not be feasible with ordinary computational power. So we decided to select a few key properties that a plasma facing material needs to have, and use them as an indication of how well the material may perform on the divertor.
Andrea Fedrigucci, PhD Student and Study First Author, THEOS Lab, National Centre for Computational Design and Discovery of Novel Materials
The scientists began by examining the Pauling file database, an extensive resource of known inorganic crystal structures. They developed a workflow to identify materials with sufficient heat resistance for reactor conditions, focusing on properties like thermal capacity, thermal conductivity, melting point, and density.
As surface temperature varies with material thickness, they calculated the maximum thickness each material could sustain before reaching its melting point, ranking them accordingly. For materials where maximum thickness could not be determined, they applied a Pareto optimization technique, prioritizing the other key properties.
This process yielded an initial shortlist of 71 candidates. At this point, however, a hands-on, traditional method was required.
I patiently looked up the literature on each of them to check if they had already been tested and discarded, of if there were properties that would prevent their use in a fusion reactor and that were not in the database, such as a tendency to erosion or degradation of their thermal properties under plasma and neutron bombardment.
Andrea Fedrigucci, PhD Student and Study First Author, THEOS Lab, National Centre for Computational Design and Discovery of Novel Materials
Interestingly, this phase of the study led to the exclusion of certain innovative materials recently proposed for use in fusion reactors, including high-entropy alloys, from consideration as divertor materials.
Ultimately, 21 materials remained. For these, the researchers applied a DFT (density functional theory) workflow to calculate two critical properties essential for plasma-facing materials in fusion. First, they measured surface binding energy, which indicates how easily atoms can be removed from the surface. Second, they assessed the formation energy of a hydrogen interstitial, a property linked to tritium solubility within the crystal structure.
If a divertor material is excessively eroded during its operational lifetime, the released atoms disperse into the plasma, leading to a reduction in its temperature. In addition, if the material is chemically reactive with tritium, it can subtract the tritium available for fusion and cause an accumulation of tritium inventory that exceeds the safety limits imposed for this type of technology.
Andrea Fedrigucci, PhD Student and Study First Author, THEOS Lab, National Centre for Computational Design and Discovery of Novel Materials
The final ranking of materials based on all critical properties included familiar candidates extensively tested in fusion settings: metallic tungsten (W) and its carbide forms (WC and W2C), diamond, graphite, boron nitride, and transition metals like molybdenum, tantalum, and rhenium.
However, there were also some unexpected finds, such as an unusual phase of tantalum nitride and ceramics based on boron and nitrogen, which have yet to be tested in fusion applications.
Looking ahead, Fedrigucci notes that the team aims to incorporate neural networks into their research to more accurately simulate real-world reactor conditions, including interactions with neutrons, which couldn’t be fully modeled in this study.
Journal Reference:
Fedrigucci, A., et al. (2024) Comprehensive Screening of Plasma-Facing Materials for Nuclear Fusion. PRX Energy. doi.org/10.1103/PRXEnergy.3.043002.