Mechanics and radiation tolerance of nanostructured steels for fusion plant structures (materials strand projects)

Supervisor/s – Dr. Enrique Jimenez-Melero & Prof. Grace Burke (University of Manchester). Dr. Huw Dawson (UKAEA)

Structural materials and components in proximity to the D-T plasma fuel in tokamak-type reactors will be subjected simultaneously to environmental degradation effects from high thermal and radiation fluxes together with variable mechanical loads. Structural steels based on body-centred cubic (bcc) structures are a preferential option for first-wall components due to their enhanced resistance to radiation-induced void swelling and high-temperature mechanical strength. Such is the case for example of reduced activation ferritic/martensitic (RAFM) steels (e.g. EUROFER97 RAFM steel). The future DEMO plant is envisaged to comprise a first wall with a tungsten armour joint to a RAFM steel, with predicted surface heat fluxes of 0.5-1.2 MW m-2 under normal operation.

Unfortunately, those traditional RAFM steels are thermally unstable at temperatures > 550 °C and their mechanical & creep strength deteriorates, potentially not fulfilling their minimum performance requirements. This limits the high temperatures allow locally for safe reactor operation, and consequently also the thermal efficiency of the reactor. Alternative approaches have been searched over the years to increase the thermal stability of bcc steels. One such approach that has regained interest in the last decade is the use of powder metallurgy to produce bcc steels with nm-size oxide particles, namely Oxide Dispersion Strengthened (ODS) steels. The fine distribution of nano-particles acts as effective barriers for dislocation and grain boundary mobility, and simultaneously as recombination sites for radiation-induced lattice defects. However, ODS steels cannot be produced to date in relatively large components for their adoption in the fusion community. ODS steel welding causes non-desirable consequences to the original particle distribution, leading to weak points, both radiation and mechanically, for any reactor components.

The focus of this project is on a new promising alternative to traditional RAFM steels and ODS steels, namely nano-structured steels. Those new FM nano-steels base their appeal on the presence of a thermally-stable population of fine MX particles in larger volume fractions, opening the door to extended operational limits of ~650 °C and potentially beyond that temperature. These steels also offer the unique advantage in this field of being castable into large components. The development of nano-structured steels is still in its very infancy, but they have recently been flagged by the fusion community as a frontrunner for structural components in tokamaks.

The aim of this project is to accelerate the adoption of nano-structured steels in fusion technology by a systematic approach to their characterization and performance testing in environmental conditions relevant for the first wall of tokamak designs. A down-selection of steel chemistries and microstructures will be characterised in-depth using analytical electron microscopy in Manchester. The most promising steels will be radiation tested using ion irradiation in combination with a transmission electron microscope at MIAMI-2 (Huddersfield), in order to visualise in situ the atomic migration, local chemical segregations, nano-particle stability 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 such as Diamond (UK) or ESRF (France). Their facilities allow only in very recent days to reconstruct the bulk multi-grain structure in real time as plasticity evolves. These results will be retro-fed into steel processing, aiming to accelerate the nano-steel deployment, offering optimised performance under the radiation and thermo-mechanical loads in first-wall fusion structures.  

The student will acquire a unique set of transferrable skills ranging from programming to design of complex sample environments, data mining, and effective communication skills. He/she will also gain in-depth knowledge about steel metallurgy, plastic deformation mechanisms in polycrystalline materials and grain reconstruction techniques. 

The project will be mainly based in Manchester, but there is opportunity to travel to conferences and visits to UKAEA. There will also be short trips (max. 1 week each) to (inter-)national large scale user facilities to perform experiments related to the project.

This project is offered by The University of Manchester. For further information please contact: Dr. Enrique Jimenez-Melero (enrique.jimenez-melero@manchester.ac.uk)

This project may be compatible with part time study, please contact the project supervisors if you are interested in exploring this