Irradiation Damage in Tungsten alloys

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Materials suitable for use in the divertor of a nuclear fusion reactor need to be able to withstand, high heat loads (upto 20MW/m2), high transient stress levels, and high levels of irradiation damage from neutrons. This makes it one of the most extreme environments for any material to have to survive in. The leading candidate material is tungsten due to its high melting point, and good resistance to plasma erosion. However its mechanical properties are poor and there is a lack of knowledge on how they are degraded by neutron irradiation. It is well know that neutron irradiation will have two major effects on the tungsten. 1) damage to the crystal lattice leading to defects such as vacancies, dislocation loops and voids and 2)transmutation in to different elements which leads to upto 5% of the tungsten becoming rhenium, osmium, tantalum and other minor elements. The clustering and evolution of the microstructure due to the original and transmuted elements interacting with the crystallographic defects determines important properties like ductility and thermal conductivity. The interaction of multiple elements is relatively unknown as most research to now has focused on pure tungsten or binary alloys. In particular how the different transmutation products interact with each other is unclear. Some are known to clusters but others may antisegregate. A knowledge of this is need if models of the irradiation damage process are to be developed that can include realistic transmutation microstructures. Two sorts of irradiated samples will be studied 1) ion irradiated samples which are pre-alloyed to have WReTaOs levels predicted to be produced in future fusion reactors. These have been irradiated at LANL in the USA and Surrey in the UK and 2) neutron irradiated samples from fission reactors (HFR Holland, SCK-CEN Belgium and Maria, Poland). How well the ion irradiations mimic the neutron irradiation is an open question and one we will answer with this project.This project will use atom probe tomography (APT) to perform chemical analysis at the single atom length scale and relate the chemical structures observed to crystallographic defects imaged in the transmission electron microscope. There will be a focus on developing correlative techniques to directly image the same defects in TEM and APT. Neutron irradiated samples will be studied using newly installed equipment at CCFE and Oxford for the study of active materials. This includes the countries first active atom probe. The effect the microstructural damage has on mechanical properties will be studied using newly developed nanoindentation methods to map large areas (several mm2). This will then allow an understanding of the differences between ion and neutron damage in tungsten alloys and to develop methods to predict neutron irradiated microstructures using surrogate irradiations. The student will be exposed to a wide range of experimental techniques and data analysis methods and learn to work on active materials. This project is aligned with the energy and nuclear fusion EPSRC areas.