Performance of Tungsten-Molybdenum Alloys in Fusion Tokamak Environments

Abstract

Tungsten has been chosen as the primary Plasma Facing Material to be used in the current generation of experimental fusion reactors, and is a leading candidate for future demonstration reactors. It exhibits a combination of properties that make it well suited to this function, including high strength at elevated temperatures, good thermal conduction, and low sputtering yield. At the same time however, there remain significant issues with its use, particularly its response to irradiation by fusion reaction products and its brittleness. In light of these challenges, alloying presents a means of modifying tungsten’s properties to better meet the demands placed on it by the fusion environment, either by altering those characteristics which make it challenging to use, or by improving its response to fusion loadings. An alloying addition that has received limited attention is molybdenum, which exhibits many of the same properties that make tungsten a favourable choice. Further to this, first principles simulations have found that molybdenum improves tungsten’s ductility, its irradiation damage tolerance, and its response to helium irradiation, which has significant implications for both the service life of Plasma Facing Components and for fuel retention characteristics. Despite this, experimental evidence for molybdenum’s effect on tungsten is very limited. The effect of Mo alloying has therefore been experimentally investigated to provide a baseline characterisation for W-Mo alloys in the context of fusion loadings. Thermo-mechanical properties of tungsten-molybdenum alloys have been studied at dynamic, quasi-static and creep relevant strain rates at temperatures up to 800 °C, using both Resonant Frequency Damping Analysis and conventional tensile testing. molybdenum was found to reduce the stiffness of tungsten, but also to increase ductility with no detriment to high temperature mechanical properties. The response of tungsten-molybdenum alloys to helium irradiation has been investigated in-situ up to 1 × 1020 He/m2 between 800 °C and 1200 °C. Dilute additions of molybdenum were found to significantly delay the fluence at which helium bubble formation occurs, and also to inhibit the increase in bubble population with increasing temperature. Finally, molybdenum’s contribution to the fuel retention properties of tungsten were studied using image plate and Beta-Induced X-ray Spectrometry analysis. Results showed that molybdenum-alloying significantly reduced tritium uptake when the irradiation effects of a fusion plasma were taken into account. In combination, these results present a broad characterisation of W-Mo alloy performance in the fusion environment, and provide a compelling argument for the re-assessment of molybdenum’s potential as an alloying element.

Date of Award	2 Sept 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Grace Burke (Co Supervisor) & Kun Yan (Main Supervisor)