Radiation-material interactions and thermal performance of W-Ta alloys for a tokamak fusion device

Abstract

With net-zero targets relying on significant decarbonisation of energy production, industrial processes, and a new hydrogen economy, nuclear fusion has emerged as a promising technology to meet these needs. The high temperatures available for process heat and the high energy density of fusion fuel make it an attractive option. However, critical challenges exist in finding materials that can withstand the harsh environment inside a fusion reactor. One such challenge is the divertor region, which deals with the exhaust of hot, charged plasma particles, and is the focus of this thesis. The divertor faces multiple challenges, including plasma-surface interaction, neutron irradiation, and heat fluxes exceeding 10 MW/m². Tungsten is currently the leading candidate material due to its high melting point, resistance to sputtering, and good thermal conductivity. However, tungsten has several limitations, such as a high Ductile-to-Brittle Transition Temperature (DBTT), embrittlement from transmutation, degradation of thermal properties under irradiation, and recrystallization embrittlement. Solid solution alloying has emerged as a potential solution to strengthen tungsten. Refractory metal alloys, in particular, show promise in retaining the beneficial properties of tungsten while addressing its weaknesses. Tantalum, located next to tungsten in the periodic table, has the potential to alloy with tungsten and improve its radiation resistance. Tantalum additions have already shown improvements in the behavior of solid and gas transmutation products, plasma-surface interactions, and some mechanical properties. This thesis further investigates the tungsten-tantalum system by examining alloyed samples with 6 and 11wt.% tantalum. Plasma-surface interaction studies have explored high-fluence regimes and demonstrated that tantalum helps mitigate high-fluence helium plasma blistering. Ion-irradiation studies, combined with Transient Grating Spectroscopy (TGS) measurements, have been used to examine the effects of dislocation structure on thermal properties. Molecular Dynamics (MD) simulations have also been employed to understand the changes in thermal properties, identifying a more radiation-tolerant alloy at the cost of initial thermal performance. Finally, in-situ Transmission Electron Microscopy (TEM) dual-beam ion-irradiations have provided further insight into the importance of synergistic effects in fusion material systems. Overall, this thesis highlights both the advantages and drawbacks of this alloy system and offers recommendations for more refined experimental designs to aid in the selection of materials for future fusion devices.

Date of Award	1 Aug 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Paul Mummery (Supervisor) & Enrique Jimenez-Melero (Supervisor)