Multiscale high-temperature mechanical performance of materials for nuclear fusion

Nuclear fusion is now being seriously considered as future power source for post 2050, as demonstrated by the recent announcement of the STEP construction in Nottinghamshire. Understanding how irradiation damage from neutrons affects the mechanical properties of structural materials is a key step towards realising nuclear fusion as a sustainable power source. Without understanding the effect that neutron damage has on the materials there is not realistic method of lifing components hence reactors. However we cannot just build a reactor to test materials and however, working on irradiated materials is costly, and generating mechanical data from them is difficult.

Neutron damage can be simulated with ion irradiations but the damage layers are thin - 200 nm to 100 um. As such traditional mechanical testing methods cannot be used and novel micro-mechanical tests must be conducted. This leads to difficulties in interpreting the results due to size effects inherent in testing small material volumes. Mechanical models are being developed that will include the effect of irradiation damage on the evolving mechanical properties, but these require substantial experimental input in the form of characterisation and mechanistic understanding of how different microstructural features (voids, precipitates, loops) control irradiation induced hardening.

This project will aim to use micromechanical testing on a series of model alloys to deconvolute the effects of different microstructural features. The alloys will be cast with controlled levels of chromium, carbon, and vanadium, to allow control of solid solution, interstitial and carbide strengthening. Samples will be irradiated with both heavy ions and helium to generate voids and dislocations loops. Hardening will be studied using nanoindentation at both room and operational temperatures. Nanoindentation at room temperature is a standard method of studying irradiation hardening but much less work has been carried out on high temperature irradiation hardening using nanoindentation. We expect that using newly developed computational plasticity finite elements methods developed in oxford we will see the effect off irradiation damage not just on yield tress but also work hardening. To understand both the starting microstructure, the microstructural evolution under irradiation and the interaction of the glissile dislocations with microstructural features advanced electron diffraction imaging will be used. This will include diffraction contrast TEM which is well established for studying such microstructures and also transmission Kikuchi diffraction which is much less applied to studying radiation damage but has the advantage of simpler and cheaper imagining equipment. We expected that byt the end of the project we will have gnateatered a range of microstructures, irradiated them, mechanically tested them and related the hardening to the observed microstructures. This will then be inputed to newly developed models at UKAEA.

The project is in collaboration with the UKAEA/Culham Centre for Fusion Energy and the DPhil student will be part of the EPSRC CDT on the Science & Technology of Fusion. The research programme aligns with the EPSRC portfolio themes of both 'Energy' and the sub-themes 'Fusion' and 'Nuclear Power'. The 'Engineering' theme is also relevant through the 'Materials Engineering - Metals and Alloys' research area.

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.