Modelling the Oxidation of W and W-Alloys in Fusion Reactor First Walls

To address the growing demand for clean energy, the UK government has committed to fusion through the Spherical Tokamak for Energy Production (STEP) project, which is planning to build a prototype fusion reactor capable demonstrating net generation of electricity by 2040. One of the challenges faced by the STEP project concerns the oxidation of the reactor’s tungsten-based first wall, which may occur due an extreme loss of coolant accident or when performing remote handling maintenance. The tungsten-oxide, once formed, is volatile and will pose a severe hazard – especially since the oxide will be radioactive. Therefore, it is essential to understand the atomic level mechanisms underpinning the oxidation of tungsten and tungsten-based smart alloys to enable the operation of STEP and other fusion reactors to plan for and mitigate the oxide formation.<br/><br/>The oxidation of tungsten is a complex process due to the large number of WxOy phases, including WO₂ and WO₃, as well as some of the sub-stoichiometric ‘Magnéli’ phases. Therefore, any model must consider the thermodynamic stability of these phases at the temperature of interest as well as the ability of the oxygen to diffuse through them. Here we use density functional theory (DFT) to calculate the formation energies of the different WxOy phases, allowing creation of a convex hull showing their relative stabilities. While the differences in formation energies are shown to be very small, the convex hull enables the identification of oxides that may play a key role in the oxidation of tungsten. For these phases, we calculate the activation energies for oxygen diffusion, again using DFT. This will allow a multi-phase Stefan problem to be parameterised, from which the kinetics of tungsten and tungsten-alloy oxidation can be quantified and understood.