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| Fig. 1: Deuterium-Tritium fusion reaction. (Source: Wikimedia Commons.) |
Nuclear fusion is the chemical process by which two atoms collide and fuse to become a single atom. This process naturally occurs in the cores of the Sun and other stars, where temperatures pressures are high enough to overcome the repulsion between positively-charged nuclei. The fusion of light elements is an exothermic reaction; the total mass of the fused product is smaller than that of the reactants, and this mass difference is manifested as a release of energy. [1]
The easiest fusion reaction to achieve is the deuterium-tritium fusion reaction, which is a reaction between two heavy isotopes of hydrogen. As seen in Fig. 1, this reaction produces a Helium atom with an energy of 3.5 MeV, and a lone neutron with an energy of 14.1 MeV. Just like in traditional nuclear fission power generators, this lone neutron can be used to heat up water to power a steam turbine. [1] Nuclear fusion power is still a theoretical form of power conversion - despite successfully reaching conditions to sustain plasma of tens of millions of degrees, several instabilities and problems have deterred the success and commercialization of power from fusion. [2]
No material container can withstand temperatures exceeding millions of degrees. Magnetic confinement fusion is one approach to overcome this obstacle; it takes advantage of the electrical conductivity of the hot plasma to contain it with magnetic fields. The most competitive version of this method is the tokamak, a machine that spins plasma in a torus as in Fig. 2. The ITER is a fusion plant that uses the tokamak design, scheduled to open in the next decade. [3]
Even though the design of tokamaks maintains distance between the hot plasma and its container, the environment inside the fusion device will still be one of the most brutal ever envisioned, and the plasma-facing materials will be subjected to high temperature, high heat flux, high irradiation from neutrons, and bombardment by deuterium, tritium, and helium. [3] Next, I will summarize two of many materials challenges with the deuterium-tritium tokamaks design.
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| Fig. 2: Schematic of a superconducting tokamak. (Source: Wikimedia Commons.) |
The ITER uses tungsten as the plasma-facing material because of its high melting point and low sputter yield, meaning that fewer tungsten atoms will sputter out when it is bombarded with ions compared with other potential materials. However, tungsten has been shown to grow nanotendrils, or fuzz, under tokamak-relevant conditions. This is due to a phenomenon called trap mutation, where several helium atoms accumulate and force themselves into a tungsten lattice site. The tungsten atom that is pushed out of its lattice site is now an interstitial defect which is then attracted to the surface, where there is space for this is the incipient fuzz. [4] Generally, though, the detailed mechanisms of growth are unknown. Due to lack of accurate modeling, it is difficult to determine if this will pose a problem to the fusion reactor in the long run, and there is continued research on this topic to fully understand and predict the its consequences before implementation in ITER.
High-temperature superconducting magnets, materials that have exactly zero resistivity above 30 K, are necessary to economically produce the high magnetic fields required by tokamaks and other magnetic confinement fusion devices. However, high flux levels of neutrons cause significant structural change to superconductors, and their critical currents, fields, and temperatures are negatively impacted. This is preventable with shielding; however, increasing shielding of these superconductors will greatly increase materials cost. [5] Therefore, it will be extremely important to research and construct high-temperature superconductors that are more radiation resistant.
The extreme conditions necessary for fusion reveal uncharted territory for materials scientists and engineers, and significant research must be completed to understand how materials respond to the plasma environment in order to estimate lifetime and fluence of tokamaks.
© Teresa Dayrit. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Stayner, "Is Nuclear Fusion a Proliferation Risk?," Physics 241, Stanford University, Winter 2017.
[2] D. J. Rose, "On the Feasibility of Power by Nuclear Fusion," Oak Ridge National Laboratory, 1968.
[3] P. Schiller and J. Nihoul, "Projected Requirements for the Next European Torus and for Long-Term Tokamak-Based Demo Fusion Devices," J. Nuc. Mat. 155-157, 41 (1988).
[4] K. Wang et al., "Morphologies of Tungsten Nanotendrils Grown Under Helium Exposure," Sci. Rep. 7, 42315 (2017).
[5] M. A. Abdou, "Radiation Considerations for Superconducting Fusion Magnets," J. Nuc. Mater. 72, 147 (1978).
