Could High H₉₈-Factor Commercial Tokamak Power Plants Use Nb–Ti Toroidal Field Coils?

In large engineering applications, materials that can fail by brittle fracture are avoided if there are practical, ductile alternatives. In recent years, advances in the experimental control and shaping of fusion energy plasmas have produced confinement times that are longer than the accepted IPB98(y,2) values (i.e., higher H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> -factors). Detailed understanding of these enhancements in H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> -factor is not available, but values as large as 1.5-1.8 may be possible. If such high values are reliably realized, they will enable such a large reduction in the magnetic field required from the toroidal field (TF) coils that ductile Nb-Ti becomes a possible superconducting materials choice for TF fusion energy magnets. In this paper, we investigate what values of enhanced H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> -factor are required to enable the commercial use of Nb-Ti TF coils in tokamaks. We have investigated the use of Nb-Ti TF coils in an ITER-like geometry, for a 500 MW net electricity producing tokamak using the PROCESS systems code. If we use present day Nb-Ti conductors, the minimum H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> -factor required for practical power plants is 1.5. For Nb-Ti cable with a critical current density increased by a factor of 5, the minimum falls to H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> ≈ 1.4. With this improvement for an H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> = 1.5, aspect ratio 3.1 (i.e., ITER-like geometry) tokamak, we find the cost of base-load electricity is ~42% greater than if Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn is used and about 1.4 times that of a typical fission power strike price (scaled up to 2.5 GWe net electricity).