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

为了实现英国到2050年的零碳排放目标，风能、太阳能和核能等零二氧化碳排放的替代能源发电是目前迫切发展的目标，以完全取代煤炭、石油和天然气等化石燃料的使用。然而，广泛使用的核裂变反应堆存在许多问题，例如，核废物处理和储存的困难以及不受控制的连锁反应的风险。另一方面，核聚变能源具有许多潜在的优势，例如，比裂变高四倍的能量，丰富的氢及其同位素作为燃料，以及放射性废物的寿命短。然而，聚变反应堆的发展对材料提出了很高的要求，因为这些材料必须能够承受高能级、高嬗变率、高温和高热机械应力。这带来了重大的材料设计挑战，并要求设计和开发上级材料，沿着创新的、简便的制造路线，特别是用于聚变反应堆的第一壁结构和增殖包层。该结构不仅受到等离子体的照射，而且还受到来自等离子体的中子轰击，以及高负载的氦和氢，这对结构材料造成严重损伤。目前，设计用于聚变反应堆上的第一壁和包层结构的潜在材料之一是低活化铁素体/马氏体（RAFM）钢，这是由于与其它材料相比，其具有上级导热性、相对低的热膨胀、对辐射诱导的膨胀和氦脆化的抗性以及容易的商业化工艺。然而，这些RAFM钢的性能将其最高工作温度限制为仅550 C，远低于650 C的使用温度。此外，辐照导致这些钢在较低的使用温度（250- 350 ℃）下硬化和在高温（450- 550 ℃）下脆化，这也限制了它们的应用。因此，第三代氧化物弥散强化（ODS）RAFM钢已经通过纳米颗粒和超细晶粒开发，成功地将工作温度提高到650 ℃。然而，ODS RAFM钢的局限性是在足够规模的粉末制造中的明显困难，以用于聚变反应堆中的第一壁和包层结构。ODS钢还具有高延性到脆性转变温度的问题。这严重限制了它们的适用性。因此，迫切需要开发出适用于650 ℃高温、易于加工成各种尺寸和结构的新型RAFM钢，本课题将根据ODS RAFM钢的组织细化和高温稳定析出相的指导思想，设计出可加工的新型RAFM钢。例如，在高温下粗化速率低于碳化物的金属间沉淀物和碳氮化物将成为目标沉淀物;这些可以通过合金设计和相应的热处理来实现。此外，可以通过修改制造工艺来实现晶粒细化，例如，通过使用形变热处理，这将产生极高的位错密度。随后，在热处理过程中，这些位错将通过回复和再结晶形成纳米级亚晶。因此，研究的最终目标将是生产新的RAFM钢供应的球形托卡马克（STEP）。这需要在2023年至2025年之间进行材料选择，并在2040年通过聚变产生净电力。它还将支持英国成为聚变材料设计的世界领导者，并通过对开创性材料系统的前沿研究来发展这一突出地位

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