Mechanics and radiation tolerance of nanostructured steels for fusion plant structures

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Structural steels based on body-centred cubic (bcc) structures are a preferential option for first-wall components in tokamak-type fusion reactors 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. 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. One alternative approach is the use of powder metallurgy to produce bcc steels with nm-size oxide particles, namely Oxide Dispersion Strengthened (ODS) steels. However, ODS steels cannot be produced to date in relatively large components for their adoption in the nuclear community. Besides that, 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 as a frontrunner for structural components in fusion 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 characterized in-depth using analytical electron microscopy. The most promising steels will be radiation tested using ion irradiation in combination with a transmission electron microscope, in order to visualize in situ the atomic migration, local chemical segregations, nano-particle stability and the lattice defect nucleation and evolution. The plastic deformation mechanisms will be monitored in real time by performing in situ mechanical testing at high-brilliance synchrotron facilities. Those facilities allow only in very recent days to reconstruct the bulk multi-grain structure in real time as the material is being strained. These results will be retro-fed into steel processing, aiming to accelerate the nano-steel deployment, offering optimized performance under the radiation and thermo-mechanical loads in first-wall fusion structures. During this project, 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. The student will also gain in-depth knowledge about steel metallurgy, plastic deformation mechanisms in polycrystalline materials and grain reconstruction techniques.