Alloy design and high-temperature mechanics of plasma-facing materials

Tungsten stands as the main plasma-facing material for fusion reactors due to its melting point (3420C), low sputtering yield, and good high-temperature strength and thermal conductivity. Unfortunately, tungsten is brittle by nature and its ductile-to-brittle transition temperature increases under neutron irradiation, due to the formation of lattice defects such as dislocations and voids. The limited data available on W-based binary alloys reveals that alloying elements such as Ti, V or Ta improves the material's ductility. Unfortunately, alloy selection has to date been done by trial-and-error, and the existing data on high-temperature mechanics of irradiated tungsten alloys reduces mainly to nano-indentation studies in W-implanted specimens.

The aim of this project is to accelerate the development of the next generation of binary and ternary W alloys, by adopting upfront a systematic design approach using thermodynamic and ab initio calculations, coupled where relevant with density functional theory and Monte Carlo simulations of the phase stability under irradiation. The most promising alloys will be produced by powder metallurgy, and characterised in-depth using analytical electron microscopy in Manchester, including the Titan ChemiSTEM microscope with atomic resolution. The key to reliable predictions of in-reactor alloy performance is to monitor in situ the damage formation and the plastic deformation mechanisms at elevated temperatures (<1000C). The former will be done by using ion irradiation in combination with a transmission electron microscope, in order to visualise in situ the atomic migration, local chemical segregations, and the lattice defect nucleation & evolution as a function of damage level and temperature. The plastic deformation mechanisms will be monitored in real time by performing in situ mechanical testing at high-brilliance synchrotron facilities. Their experimental capabilities allow only in very recent days to reconstruct the bulk multi-grain structure in real time as plasticity evolves. These results are these days in very much need to validate and dictate multiscale modelling campaigns in the nuclear fusion technology community, and are therefore expected to support safety cases and steer alloy design strategies for DEMO and future commercial fusion power plants.

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.