The development of fusion energy is recognized as one of the twelve Engineering Grand Challenges for the 21st century by the National Academy of Engineering. Although fusion energy offers many potential benefits as a long-term solution to the world’s energy needs (including low-cost and widely available fuel and environmentally attractive safety and waste disposal attributes), the process of harnessing the energy from a miniature sun poses numerous engineering science challenges ranging from development of high performance plasma-facing materials immediately adjacent to the high temperature plasma to the design and development of advanced high performance structural materials.
The plasma-facing materials are exposed to intense steady state heat fluxes (comparable to ~10% of the heat flux at the sun surface) along with high fluxes of particles (H isotopes and He) that can induce pronounced surface sputtering and erosion and energetic neutron bombardment. The structural materials beyond the plasma-facing region must endure high thermomechanical cyclic stresses, corrosive high temperature coolants, and be resistant to dimensional, mechanical and physical property changes for extremely high displacement damage levels where each atom may be knocked off its lattice site hundreds of times during the structure lifetime. Specialized materials and components within the blanket region adjacent to the plasma-facing components are also needed for replenishing the tritium fuel needed to power the fusion reaction.
The study of Plasma-Materials Interactions (PMI) focuses on the challenges of designing a reactor vessel wall that must withstand the extreme fluxes of heat, charged particles, and high energy neutrons emitted from the core fusion plasma. At the same time, the plasma must be protected from any particles eroded from the wall because these impurities will create parasitic radiative power losses as the impurities drain energy from the plasma and waste it by emitting photons. PMI research requires the use of boundary plasma physics, material science, computational modeling, and diagnostic development.
These tools are used to understand how the plasma behaves as it is exhausted from the core and moves toward the wall, as well as how the plasma- facing material reacts to the influx of heat and particles. PMI research endeavors to identify plasma operating configurations that minimize damage to the walls while maintaining high core temperatures, as well as developing robust material science solutions to create plasma-facing components that can enable long term operations.
Fusion Structural Materials
To enable the safety and environmental attractiveness of fusion energy, a central requirement is the development of high-performance so-called reduced activation materials that ensures public safety during all conceivable accident scenarios and also do not produce long-lived radioactive waste (accomplished by judicious selection of alloying compositions from a handful of suitable elements in the periodic table).
Research at UT is focused on advanced multi-scale computational modeling and experimental studies to explore the fundamentals of radiation effects in materials and the influence of neutron transmutation-induced gases such as H and He on the overall microstructural evolution of materials during irradiation. Utilization of advanced manufacturing techniques such as additive manufacturing is also being explored to fabricate geometrically complex, high performance structural materials with superior neutron irradiation resistance. Both ion beam and neutron irradiation studies are being performed. Strong collaborations exist with researchers at Oak Ridge National Laboratory (ORNL) and numerous other national and international research institutions.
Fusion Blanket and Fuel Cycle R&D
Concepts for replenishing the tritium fuel that would be consumed during operation of a fusion reactor are based on neutron-induced transmutation of lithium compounds in the blanket region adjacent to the plasma-facing components. Both solid ceramic and liquid concepts are under investigation worldwide. Key scientific issues include the permeation and trapping of hydrogen isotopes in these materials, as well as advanced technologies to efficiently and reliably extract the generated tritium from hot flowing fluids so that it can be processed into fuel pellets to sustain the fusion reaction.
A comprehensive set of experimental and computational facilities are utilized to perform fusion technology research. Extensive materials characterization facilities including scanning transmission electron microscopes, scanning electron microscopes, X-ray diffraction facilities, atomic force microscopy, Raman spectroscopy, and nanoindentation mechanical property testing are available at the Joint Institute for Advanced Materials (JIAM) user center facilities.
The state-of-the art UT Ion Beam Materials Laboratory is frequently utilized to perform ion irradiations on materials at temperatures from cryogenic to ~800 C. Specialized facilities for precision cutting, mechanical polishing, optical microscopy, high temperature heat treatment, and mechanical property testing (microindentation hardness testing, vacuum or inert environment tensile and thermal creep testing up to 800C) of materials are available in faculty research labs located in the Science and Engineering Research Facility as well as JIAM.
An ultrahigh vacuum system for thermal desorption spectroscopy as well as a dedicated positron annihilation spectroscopy facility developed by the Wirth group is available at the low activation materials development and analysis (LAMDA) laboratory at ORNL. Access to an extensive set of advanced microstructural characterization, mechanical property and physical property test equipment is available via ongoing research collaboration agreements at ORNL.
On-campus laboratory space in the SERF contains the UT Plasma Exposure Stage, which utilizes a compact electron cyclotron resonance (ECR) plasma source to expose samples heated up to one thousand degrees Celsius to a directed beam of ions able to accelerated to 1 keV. This laboratory space is also used to design, assemble, and test prototypes for advanced plasma and heat flux diagnostics that are being prepared for use on multiple fusion reactors around the world.