Understanding Radiation Induced Transmutation in Tungsten Alloys for Nuclear Fusion

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

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.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1802461 Studentship EP/N509711/1 01/10/2016 30/09/2020 Matthew Lloyd
 
Description Test specimens made of pure tungsten were irradiated in a test fission reactor in the Netherlands. The samples had a composition after irradiation that closely matches the predicted composition of the components after being in a fusion power station for several years. By looking at these samples we hoped to determine whether the impurities produced by irradiation (Re, Os and Ta) had formed clusters within the material, which are known to reduce the performance of the components. Using atomic resolution microscopes we detected the presence of these clusters, however they appeared different to previous work that had used alternative irradiation techniques. The internal structure of the clusters showed Os at the core surrounded by Re. This indicates that there is an underlying mechanism for their formation that has not yet been identified. The hope is that by comparing with our modelling work we will be able to hypothesise how this structure is forming and propose a method of addressing the formation of these clusters. Further investigation with additional techniques have partially confirmed this hypothesis and led to further investigation.
Exploitation Route The experimental work gives computational modellers data with which they can validate their models. The experimental findings also give experimental researchers doing ion implantation (a common surrogate for neutron irradiation) a point of reference with which to compare. Our findings also raise questions within the atom probe tomography community as to how voids in the sample affect reconstruction, as this is something we detected in our work.

This result has led to further investigation and a new collaboration between Oxford and UKAEA has begun to study this using a combined TEM/APT approach
Sectors Energy,Environment

 
Description This work has contributed towards the development of materials for extreme environments. The materials studied in this project are not commonly used engineering materials and are being developed for an application that uses a higher temperature, and a higher dose of radiation than any other reactors.The project is part of the wider programme for the development of fusion energy, a novel energy source that could be used to generate a near unlimited supply of clean, emission free electricity, whilst not producing large quantities of nuclear waste, as is the case with conventional fission reactors. This project has so far identified key flaws in the current favourite material for first wall applications, through the study of irradiated test specimens. These finding are helping to develop our understanding of how he degradation of their properties occur, in the hope that reactor designers can develop strategies for mitigation of damage, and create new materials to test and screen for use in a reactor or power station.
First Year Of Impact 2018
Sector Energy,Environment
Impact Types Policy & public services