India - UK Civil Nuclear Collaboration: Development of Radiation Damage Resistant High Entropy Alloys for Advanced Nuclear Systems

High entropy alloys (HEAs) are a novel and recently developed class of materials that do not contain one dominant element. Rather, four or more elements are combined together in equal or near equal measures with each element being randomly arranged. They have been reported to have a large number of desirable properties, including high strengths at high temperatures, good corrosion resistance and ability to withstand irradiation damage. These properties make HEAs strong candidate materials for use as structural materials in future nuclear fission and fusion reactors. These reactors are being designed to operate at higher temperatures, use less fuel and can be made safer and more efficient than current reactors.

For HEAs to be utilised in this highly harsh and demanding environment there are three key needs that must be addressed.

First, we must identify promising alloys, have them manufactured on a small scale, and characterise their mechanical behaviour and the stabilities of their microstructures. This will be carried out using advanced microscopes capable of studying the chemistry and structure of these alloys at the atomic level.

Second, the underlying materials physics of HEAs that provide the excellent resistance to irradiation damage need to be fully understood. To assess radiation damage resistance, we will perform ion implantations rather than using neutrons. This allows us to perform a large number of experiments and the samples are not radioactive so can be easily handled in a laboratory. The damage caused will be studied using mechanical testing methods capable of probing the small damage regions produced by these heavy ions. Results from experiments, combined with computational modelling of damage effects in these novel alloys, will allow us to describe radiation damage accumulation and recovery mechanisms in these materials, which can be used to design even more advanced alloys.

Finally, we must determine how well the materials will behave in service by assessing how they react when placed under load at high temperatures for long periods of time. We will work with Indian partners, utilising their unique facilities, to perform these tests on our most promising compositions. These compositions will be manufactured in large-scale quantities, tested, and returned to the UK for further study.

The next generation of nuclear reactors (either Gen-IV fission or fusion) will be more efficient and considerably safer than the current generation of gas or water-cooled systems. Current designs are hampered by a lack of suitable materials to use in structural reactor components that experience high temperatures and neutron fluxes. Examples of such components include fuel claddings in fission reactors, or blankets in tokomak fusion reactors. Current generation materials such as zirconium alloys, which are widely used in fuel cladding for PWRs, cannot be used in advanced reactor systems, as the operational temperatures are too high. Whilst steels have been used in several test systems, and there continues to be development in this area, they suffer from swelling under irradiation and a significant deterioration of mechanical performance after irradiation. This is of a primary concern as the future reactors are designed to have longer lifetimes and are likely to have higher neutron fluxes (hence higher radiation damage levels in structural components). The so-called high entropy alloys (HEAs) are a relatively new and novel class of alloys where no single element dominates, as is the case in traditional metallic alloys. Instead four or more elements are used in near equal amounts. Preliminary work carried out by partners in this project has shown some HEAs are exceptionally resistant to the radiation damage which compromises conventional nuclear materials. This grant will accelerate the development of HEAs in the UK and India and assess their ability to act under the extreme environments envisaged for future reactors, studying both their properties after high levels of radiation damage and their creep behaviour at operational temperatures. By developing safer nuclear materials this grant will allow the deployment of more advanced nuclear power plants, in the UK and India, in an effort to reduce the emissions of greenhouse hence reducing the effect of climate change. This will have a positive impact on not just the collaborators countries, but also on the wellbeing of the global population.

HEAs are of interest to a wide range of end users across the nuclear, aerospace and automotive industries. In particular, the UK Atomic Energy Authority (UKAEA) and the Culham Centre For Fusion Energy (CCFE) are investigating them as potential structural materials in future nuclear fusion devices, and Rolls Royce plc are pursuing them for potential aero engine applications. Researchers in this project already have existing projects with these industrial partners and we will disseminate all key finding with them, by means of our regular meetings as well as larger public workshops. By doing so we will aim to find synergistic links across the nuclear and aero industries and accelerate the design of novel alloys in this developing area of research. We will work with the innovation arms of all three universities and our Indian partners, to maximise the use of any commercial aspects of our research.