As the focus on energy resiliency and competitiveness increases, the development of advanced materials for next-generation, commercial fusion reactors is gaining attention. A recent paper published in the journal Current Opinion in Solid State & Materials Science examines a promising candidate for these reactors: ultra-high-temperature ceramics, or UHTCs. Researchers from the Department of Energy's Oak Ridge National Laboratory, the University of Tennessee, Knoxville, and Stony Brook University collaborated on this study, highlighting the unique qualities of UHTCs and their potential to serve as plasma-facing components.
UHTCs are notable for their extremely high melting points, which make them excellent candidates for withstanding the conditions within fusion reactors. They also boast adjustable ability to transfer heat (thermal conductivity) and impressive mechanical properties, including fracture toughness that rivals current candidate fusion materials such as tungsten.
However, understanding how these materials react when exposed to both high-energy plasma and intense neutron radiation is crucial for their effective use in such demanding environments. High-energy plasma is a super-hot, ionized gas made up of charged particles, while intense neutron radiation refers to the extremely high flux of neutrons that can damage materials.
Yan-Ru Lin, a materials scientist in the Radiation Effects and Microstructural Analysis group within the Materials Science and Technology Division at ORNL, shares the latest research findings on UHTCs. Lin earned his doctorate in materials science and engineering from the University of Tennessee, Knoxville, and holds degrees in nuclear engineering from National Tsing Hua University in Taiwan. His expertise focuses on the effects of radiation damage in nuclear materials, where he employs leading-edge analysis techniques, including transmission electron microscopy.
This discussion addresses key questions about UHTCs, including their advantages over traditional materials, the role of plasma-facing components in fusion energy, the challenges in their application, and the critical effects of exposure to neutron radiation on their performance. Moreover, Lin shares perspectives on ongoing research that could enhance the capabilities of UHTCs in fusion reactors. The insights provided will illuminate the realm of UHTCs and their potential to transform fusion technology.
Funding support from DOE's Office of Fusion Energy Sciences, along with contributions to the UHTC research from collaborators Takaaki Koyanagi and Yutai Kato of ORNL; Steven Zinkle, an ORNL-UT Governor's Chair Professor for Nuclear Materials; and Lance Snead of Stony Brook University was instrumental. The support from colleagues in the ORNL Nuclear Materials groups and the technicians at ORNL's Low Activation Materials Development and Analysis laboratory likewise played an essential role.
Q: Will you explain what plasma-facing components are and the role they play in fusion energy?
Plasma-facing components are materials and structures located at the inner walls of a fusion reactor, directly exposed to a large amount of heat energy transferred to a surface in a short period of time (high heat flux), energetic plasma and intense neutron irradiation. Notably, no existing material can simultaneously withstand all these extreme conditions for long-term operations. The commercial viability of fusion energy depends on developing plasma-facing components capable of enduring these harsh environments.
Q: What are ultrahigh-temperature ceramics, and why are they being considered for use in fusion reactors?
UHTCs are generally defined as ceramics with melting points above 3,000 degrees Celsius. The upper operating temperature limit of nuclear structural materials is closely tied to their melting points. Given this constraint, the UHTCs with high melting points show great potential for significantly extending the upper temperature limit in fusion reactor applications. This extension aligns with a strategy for developing radiation-resistant materials by selecting those in which radiation-produced vacancies remain immobile at operating temperatures, while promoting vacancy-interstitial recombination during irradiation. Vacancies are locations in the material's structure of crisscrossed bars or strips (lattice) in which atoms are missing. Vacancy-interstitial recombination is the process in which a vacancy and an interstitial (an extra atom positioned between lattice sites) come together and eliminate each other, thereby restoring the regular lattice structure. This process helps maintain the stability of UHTCs under the extreme conditions present in fusion reactors.
Volumetric swelling in irradiated materials can compromise mechanical integrity and reduce their ability to withstand the extreme conditions inside a reactor over time. Traditional steels typically exhibit significant volumetric swelling under neutron irradiation at 400 to 600 degrees Celsius. In contrast, UHTCs are expected to experience such swelling, only at much higher temperatures, above 1,000 degrees Celsius. This is because of their high melting points and low vacancy mobility at elevated temperatures. In addition, compared with traditional ceramics, which typically exhibit purely ionic or covalent bonding, many UHTCs possess a mixture of bonding types, including metallic bonding. This mixed bonding gives UHTCs the potential for enhanced thermal and mechanical properties compared with conventional ceramics.
Q: What advantages do UHTCs offer over traditional materials currently used in fusion reactors?
Currently, no mature high-performance plasma-facing material solutions exist for fusion reactors. UHTCs present several advantages over traditional materials currently used in fusion plasma devices, including exceptional material performance at elevated temperatures, wide potential operation temperature and time windows, compositional flexibility, and tailorable thermomechanical properties.
Q: What are the major challenges or limitations you see in using UHTCs as plasma-facing materials?
UHTCs possess exceptionally high melting points and demonstrate significant resilience to radiation damage. However, their use as plasma-facing materials is restricted because of the absence of fusion-relevant radiation data. The major challenges include the uncertain effects of neutron irradiation and plasma interactions with plasma-facing materials, inherent trade-offs between thermal conductivity and toughness, and expected difficulties in fabrication and scalability for commercial deployment.
Q: How does neutron irradiation affect UHTCs, and why is it important to study this effect for future fusion applications?
Studying effects of neutron irradiation on UHTCs is essential to validate their suitability as plasma-facing materials. ORNL has a long history in radiation damage research, with unique capabilities for neutron irradiation experiments at the High Flux Isotope Reactor (a DOE Office of Science user facility) and for characterizing neutron-irradiated radioactive materials at the Low Activation Materials Development and Analysis lab, resources rarely found elsewhere. Key issues observed in other candidate fusion materials under neutron irradiation - such as irradiation-induced changes in thermal and mechanical properties, tritium retention and subsurface blistering - must also be thoroughly investigated in UHTCs before they can be considered for future fusion applications.
Q: What are some exciting areas of ongoing research and development that could improve the performance of UHTCs in fusion reactors?
The development of multicomponent UHTCs is an interesting area. These materials offer enhanced oxidation resistance, thermal conductivity and toughness through tailored transition metal compositions - specific mixtures of transition metals that are specially designed to achieve desired properties or performance in materials.
Although near-term fusion research should prioritize binary UHTCs, advancing multicomponent systems may help address some irradiation and plasma-related challenges. For instance, significantly improved fracture toughness in multicomponent UHTCs could compensate for radiation causing a decrease in the ability of the material to conduct heat, mitigating concerns over thermal stress resistance. Fiber-reinforced ceramic matrix composites based on UHTCs are also a promising avenue.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science . - Scott Gibson
