Modelling the Oxidation of Tungsten and Tungsten Alloys for Fusion Reactor First Walls

Kerr, Ryan and Murphy, Samuel (2025) Modelling the Oxidation of Tungsten and Tungsten Alloys for Fusion Reactor First Walls. PhD thesis, Lancaster University.

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Abstract

To address the growing demand for low-carbon energy, the United Kingdom has committed to building the world’s first nuclear power plant by 2040, the Spherical Tokamak for Energy Production (STEP). One of the challenges faced by the STEP project concerns the oxidation of the reactor’s tungsten-based first wall, which may occur during a loss of coolant accident or remote maintenance handling. The oxidation of tungsten begins at 673 K. This ultimately leads to the formation of tungsten trioxide, a volatile phase which may be a vector for the distribution of radioactive isotopes into the surroundings. This poses a safety hazard for site staff and first responders, and therefore, the suppression of oxide formation stands as one of the key motivations in first wall research, including the development of Self-passivating Metal Alloys with Reduced Thermo-oxidation materials, or `SMART' materials. This thesis explores the transport of oxygen through the observed tungsten oxide phases, including the sub-stoichiometric Magnéli phases, using density functional theory. Therefore, the relationships between the physical and electronic structures of the stoichiometric and sub-stoichiometric oxide phases are presented. In the second part of this thesis, the roles of Cr and Y in the SMART alloy W-11.4Cr-0.6Y are elucidated using density functional theory and classical mechanics. The results show that yttrium acts as an oxygen scavenger at the grain boundaries of the alloy, whilst chromium diffuses via a vacancy exchange mechanism which is sensitive to the presence of oxygen. Crucially, it is demonstrated that, in the absence of yttrium, impurity oxygen reduces the availability of vacancies in the tungsten-chromium grains, inhibiting the self-passivating action of the alloy by preventing the migration of chromium to the surface.