Understanding Radiation Induced Transmutation in Tungsten Alloys for Nuclear Fusion

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The development of viable nuclear fusion power plants relies on materials capable of withstanding the combined effects of high neutron irradiation and elevated temperatures in an extreme environment around the fusion plasma. Few materials proposed for plasma-facing components are capable of meeting these demands. Molybdenum, carbon composites and beryllium have all been examined but issues with either activation, tritium retention or sputtering rate exist for all these. Tungsten is now considered the leading candidate for the divertor and possibly all first wall armour. During irradiation with 14MeV neutrons, transmutation effects will alter the composition of any materials, further complicating accurate predictions of lifetime assessment. Previous work has shown that binary tungsten -rhenium and tungsten -tantalum alloys can form nanosized precipitates under irradiative exposures, which harden then ultimately embrittle the materials. However ternary W-Re-Ta and W-Re-Os alloys have been shown to behaviour quite differently under irradiation which raises important questions about the best route for mimicking transmutation pathways using pre-alloyed materials and fundamental questions regarding the mechanisms of precipitation formation in both irradiated and unirradiated alloys. In addition there have been no studies of tungsten-rhenium-tantalum-osmium alloys despite this being the terminal composition, previous studies focused on binary systems only and sometimes with overly high solute contents. This can lead to precipitates being observed that are not expected under operational conditions. In this project, higher-order (both ternary and quaternary alloys from the of tungsten-rhenium-tantalum-osmium systems) alloys representing more accurate transmutation products will be exposed to heavy ion-irradiation, then examined using a multi-technique approach of Atom Probe Tomography, TEM and Nanoindentation Measurements, aiming to link 3D chemical information at the atomic-scale to mechanical properties of these alloys. In particular how thermally stable these clusters are will be studied using thermal aging treatments. There has been no studies performed on this system with respect to this and this will be the first time the dissolution or growth will be observed. Nanoindentation will be used to study the interaction of dislocations with clusters and how this lead to hardening. These experiments are key to understanding if thermal treatments can be used to prolong divertor lifetime in service by annealing out radiation damage.In this manner we hope to obtain a much deeper understanding of interactions between solute additions in these materials following irradiation, better understand the kinetics and thermodynamics of irradiation assisted precipitation and how this alters the mechanical behaviour. This will provide valuable data needed to underpin atomistic simulations being performed at CCFE and eventually lead to better lifeing predictions for the divertor.EPSRC research theme is Energy.