Irradiation Damage in Tungsten alloys

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.

Identifying a sustainable energy supply is one of the biggest challenges facing humanity. Fusion energy has great potential to make a major contribution to the baseload supply - it produces no greenhouse gases, has abundant fuel and limited waste. Furthermore, the UK is amongst the world leaders in the endeavour to commercialise fusion, with a rapidly growing fusion technology and physics programme undertaken at UKAEA within the Culham Centre for Fusion Energy (CCFE). With the construction of ITER - the 15Bn Euro international fusion energy research facility - expected to be completed in the middle of the 2020's, we are taking a huge step towards fusion power. ITER is designed to address all the science and many of the technology issues required to inform the design of the first demonstration reactors, called DEMO. It is also providing a vehicle to upskill industry through the multi-million pound high-tech contracts it places, including in the UK.
ITER embodies the magnetic confinement approach to fusion (MCF). An alternative approach is inertial fusion energy (IFE), where small pellets of fuel are compressed and heated to fusion conditions by an intense driver, typically high-power lasers. While ignition was anticipated on the world's most advanced laser fusion facility, NIF (US), it did not happen; the research effort is now focused on understanding why not and the consequences for IFE, as well as alternative IFE schemes to that employed on NIF.

Our CDT is designed to ensure that the UK is well positioned to exploit ITER and next generation laser facilities to maximise their benefit to the UK and indeed international fusion effort. There are a number of beneficiaries to our training programme: (1) CCFE and the national fusion programme will benefit by employing our trained students who will be well- equipped to play leading roles in the international exploitation of ITER and DEMO design; (2) industry will be able to recruit our students, providing companies with fusion experience as part of the evolution necessary to prepare to build the first demonstration power plants; (3) Government will benefit from a cadre of fusion experts to advise on its role in the international fusion programme, as well as to deliver that programme; (4) the UK requires laser plasma physicists to understand why NIF has not achieved ignition and identify a pathway to inertial fusion energy.

As well as these core fusion impacts, there are impacts in related disciplines. (1) Some of our students will be trained in low temperature plasmas, which also have technological applications in a wide range of sectors including advanced manufacturing and spacecraft/satellite propulsion; (2) our training in materials science has close synergies with the advances in the fission programme and so has impacts there; (3) AWE require expertise in materials science and high energy density plasma physics as part of the national security and non-proliferation strategy; (4) the students we train in socio-economic aspects of fusion will be in a position to help guide policy across a range of areas that fusion science and technology touches; (5) those students involved in inertial fusion will be equipped to advance basic science understanding across a range of applications involving extreme states of matter, such as laboratory astrophysics and equations of state at extreme pressures, positioning the UK to win time on the emerging next generation of international laser facilities; (6) our training in advanced instrumentation and control impacts many sectors in industry as well as academia (eg astrophysics); (7) finally, high performance computing underpins much of our plasma and materials science, and our students' skills in advanced software are valued by many companies in sectors such as nuclear, fluid dynamics and finance.