Process design of new reduced activation ferrite martensite (RAFM) steels for nuclear fusion reactors

To achieve the UK zero carbon emission target by 2050, alternative energy generation with zero CO2 emission, such as wind, solar, and nuclear energy, is now the target of urgent development to completely replace the use of fossil fuels such as coal, oil, and natural gas. However, the widely used nuclear fission reactors have many issues, for example, the difficulty of nuclear waste treatment and storage and the risk of uncontrolled chain reactions. On the other hand, nuclear fusion energy has many potential advantages, for example, four times higher energy than fission, abundant hydrogen and its isotopes as the fuel, and the short lifespan of the radioactive waste products. However, the development of fusion reactors puts a high demand on materials, as these must withstand high energy levels, high transmutation rates, high temperatures, and high thermomechanical stresses. This brings major material design challenges and requires the design and development of superior materials, along with innovative, facile, manufacturing routes, especially for the first wall structures and breeder blanket of fusion reactors. The structure is not only irradiated by the plasma but also undergoes neutron bombardment from the plasma, as well as high loadings of helium and hydrogen, which causes serious damage to the structural materials. Currently, one of the potential materials designed for the first wall and blanket structures on the fusion reactors is the reduced activation ferritic/martensitic (RAFM) steels, due to the superior thermal conductivity, relatively low thermal expansion, and resistance to radiation-induced swelling and helium embrittlement, as well as the easy commercial process, compared to other materials. However, the properties of these RAFM steels restrict their maximum operating temperature to only 550C, which is much lower than the service temperature of 650C. Moreover, irradiation induces the hardening of these steels at lower service temperatures (250-350C) and embrittlement at high temperatures (450-550C), which also restricted their application. Thus, the 3rd generation oxide dispersion strengthened (ODS) RAFM steels have been developed through nanoparticle and ultra-fine grains, which successfully increase the operating temperature to 650C. However, the limitation of the ODS RAFM steels is the obvious difficulty in powder manufacturing at a sufficient scale to be used in the first wall and blanket structures in fusion reactors. ODS steels also have a problem with a high ductile to the brittle transition temperature. This severely limits their applicability. Thus, there is still an urgent need to develop new RAFM steels for the structure materials on fusion reactors with a service temperature of 650C and easy manufacturing to various scales and structures.
In this project, according to ODS RAFM steels, the guiding principles of a fine structure and a high-temperature stable precipitate phase will be used to design new, processable, RAFM steels. For example, the intermetallic precipitates and carbonitrides, which have a lower coarsening rate than carbides at high temperatures, will be the target precipitates; these can be achieved through alloy design with corresponding heat treatment. Moreover, grain refinement can be achieved through the modification of the manufacturing process, for example, by using ausforming, which will produce an extremely high dislocation density. Subsequently, during heat treatment, these dislocations will form nanoscale subgrains through recovery and recrystallization. Thus, the ultimate goal of the research will be to produce new RAFM steels for supply to the spherical tokamak (STEP). This requires advances to allow materials selection between 2023 to 2025 and provision to produce net electricity from fusion in 2040. It will also support the UK to be the world leader in fusion materials design and develop this prominent position through cutting-edge research on groundbreaking material systems